General considerations regarding atmospheric and biological models for aquaponic production systems Atmospheric and water data model for aquaponics… [307339]
TABLE OF CONTENTS
Part I
General considerations regarding atmospheric and biological models for aquaponic production systems
Atmospheric and water data model for aquaponics production system…….3
Biological model……………………………………………………………………….15
General considerations…………………………………………………………15
The concept of aquaponics modeling for a recirculating aquaculture system……………………………………………………………………………… 20
References…………………………………………………………………………… 28
[anonimizat], based on aquaponics techniques
Aquaculture production systems classification……………………….33
Recirculating aquaculture systems SWOT analysis…………………………..35
Examples of configurations for recirculating integrated systems which uses aquaponics techniques………………………………………………………………36
[anonimizat] ……………………………………………………………..38
Sediments removal……………………………………………………………………38
Introduction………………………………………………………………………38
Gravitational separation…………………………………………………………39
Mechanical filtration……………………………………………………………..42
Physico-chemical processes…………………………………………………..50
Applications for solid particle control systems……………………………….55
Ratings of different mechanical filters…………………………………………57
Biological Filtration…………………………………………………………………58
Introduction………………………………………………………………………58
Biofiltration and nitrification……………………………………………………..58
Configuration of the nitrification filter…………………………………………..59
Factors influencing the biofiltering process…………………………………….65
Ratings of different biological filtration……………………………………………73
Aeration and Oxygenation…………………………………………………………74
Introduction……………………………..………………………………………..74
Gas transfer……………………………………………………………………….74
Ratings of different Aeration and oxygenation……………………………………80
Carbon Dioxide Control………………………………………………………………81
Introduction……………………………………………………………………….81
Carbon balance and carbon dioxide control by pH management……………83
Carbon dioxide control by gas exchange………………………………………84
CO2 control by gas transfer combined with kinetics of chemical reactions…86
Ratings of different Carbon dioxide control………………………………………87
pH Control………………………………………………………………………………88
Introduction……………………………………………………………………..88
Alkalinity and pH control………………………………………………………..88
Nitrification………………………………………………………………………..89
Management of alkalinity and pH……………………………………………….90
Ratings of different pH control……………………………………………………..92
Water Disinfection Methods……………………………………………………….93
Introduction……………………………………………………………………..93
Chlorination………………………………………………………………………94
Thermal treatment……………………………………………………………….95
Ultraviolet Radiation Treatment…………………………………………………96
Ozone………………………………………………………………………….106
Ratings of different water disinfection…………………………………………113
Aquaponics………………………………………………………………………….114
Introduction…………………………………………………………………….114
Media grow beds………………………………………………………………115
Deep water culture (DWC)……………………………………………………117
Nutrient film technique (NFT)…………………………………………………119
Lighting………………………………………………………………………….120
Ratings of different aquaponics…………………………………………………121
Water quality in RAS systems……………………………………………………122
References……………………………………………………………………………133
Part I
General considerations regarding atmospheric and biological models for aquaponic production systems
Atmospheric and water data model for aquaponics production system
Biological significance of environmental parameters
INTRODUCTION
Agricultural and livestock activities are considered the biggest consumers of fresh water. Estimations reveal that 85% of the global fresh water consumption is for agriculture (Hoekstra and Chapagain, 2007) and nearly one-third of the total water footprint of agriculture in the world is used for livestock products (Mekonnen and Hoektra, 2012).
In the last 30 years, the increase in the income of the population in developing countries, led to an increase in fish consumption from 25.0 to 104.3 million ton fish per year (FAO, 2014). Due to the depletion of marine resources the FAO predicts that in the future the supply of fish for the population will be entirely dependent on fish production in aquaculture systems.
The increased demand for fish, water and fertilizer for crop production and the concerns about environment and health are motivations to test innovative farming systems such as “aquaponics” as viable systems for sustainable fish and crop production (FAO, 2014).
Aquaponics has ancient roots. Aztec cultivated agricultural islands known as chinampas in a system considered by some to be the first form of aquaponics for agricultural use (Boutwelluc, 2007 and Rogosa, 2013) where plants were raised on stationary islands in lake shallows and waste materials dredged from the chinampa canals and surrounding cities were used to manually irrigate the plants (Boutwelluc, 2007 and Rogosa, 2013). Also, South China, Thailand, and Indonesia who cultivated and farmed rice in paddy fields in combination with fish are cited as examples of early aquaponics systems (FAO, 2001). These aquaponic farming systems existed in many far eastern countries, in USA, and Canada.
Recent advances by researchers and growers alike have turned aquaponics into a working model of sustainable food production. The integration of fish and plants results in a polyculture that increases diversity and yields multiple products.
Aquaculture development as a whole in the country in combination with production technology, favorable socioeconomic condition and culture environment has already proven successful in terms of increasing productivity, improving profitability and maintaining sustainability (Toufique and Belton, 2014).
Aquaponics is, farming technique in which water from aquaculture is used to grow crops and extra water returns back to the fish tank. When this water circulated near root zone, nitrogen fixing bacteria (manly nitrosomonas and nitrobactor) convert ammonia (NH4+) into nitrite (NO2-) and then to nitrate (NO3-) form. By these, plants get nutrients as fertilizer and nitrates been less toxic to fish; fish grows better than normal aqua farming. By this integration of fish farming and agriculture, one can get maximum output.
Fishes produces nitrogenous compound mainly ammonia which is hazardous to fish, even in small quantities and toxicity increases in relation to pH and temperature in the water column. On the other hand, Nitrosomonas bacteria break down ammonia to NO2- and Nitrobacter convert the nitrite into nitrate which is food for the plants. By contrast, NO3- is less harmful to fish. Decaying organic matters can help to fertilize ponds, at the same time provides good environment for growing plants which are less prone to disease unlike soil. Raft aquaponics is one of the ways to use aquaculture site for vegetable production and can help to overcome nutritional demand for the growing population.
Green leafy vegetables with low to medium nutrient requirements are well adapted to aquaponic systems, including lettuce, basil, spinach, chinese cabbage, chives, herbs, and watercress (www.backyardaquaponics.com ).
The selection of plant species in aquaponics system is important. Lettuce, herbs, okra and especially leafy greens have low to medium nutritional requirements and are well suitable to aquaponics system. Plants yielding fruits like tomato, bell pepper and cucumber have higher nutritional requirement and perform better in a heavily stocked and well established aquaponics system (Adler et all, 2000).
Research conducted at University of Florida showed that cucumber crop can be successfully adopted with aquaponics system. This is estimated that 45.300 Kg of fish will produce sufficient nitrogen for 4050 lettuce or 540 tomato plants when they are fed with 3 % of their body weight.
Freshwater fish are the most common aquatic animal raised using aquaponics, although freshwater crayfish and prawns are also sometimes used (Drive, 2006).
A few fish species are adapted to recirculating aquaculture which includes tilapia, trout, perch, arctic char and bass. Most commercial aquaponics system in North America is based on tilapia. Furthermore, tilapia is tolerant of fluctuating water conditions such as pH, temperature, oxygen and dissolved solids (Rakocy, 1999). Tilapia is the fish species which is very hardy, can tolerate wide range of environmental parameters, can live with versatile of feed and are fast growthing fish species (Salam, M.A., 2012).
MATERIALS AND METHODS
The hydroponic greenhouse production system requires a high degree of environmental control including supplemental lighting and moveable shade to provide a target amount of light which, in turn, results in a predictable amount of daily growth. Computer technology is an integral part in the production of hydroponic. A computer control system should be used to control the abiotic environment. Different sensors are used to monitor greenhouse environment parameters. These parameters include temperature of greenhouse air and nutrient solution, relative humidity and carbon dioxide concentration of greenhouse air, light intensities from sunlight and supplemental lighting, pH, Dissolved Oxygen (DO) levels, and Electrical Conductivity (EC) of the nutrient solution. Sensors will communicate the environmental conditions to the control computer which will activate environmental control measures such as heating, ventilation, and lighting.
Atmospheric and water data model for aquaponic production system:
Temperature
Relative Humidity
Carbon Dioxide or CO2
Lights
Dissolved Oxygen
pH
Electrical Conductivity
Temperature controls the rate of plant growth. Generally, as temperatures increase, chemical processes proceed at faster rates. Most chemical processes in plants are regulated by enzymes which, in turn, perform at their best within narrow temperature ranges. Above and below these temperature ranges, enzyme activity starts to deteriorate and as a result chemical processes slow down or are stopped. At this point, plants are stressed, growth is reduced, and, eventually, the plant may die. The temperature of the plant environment should be kept at optimum levels for fast and successful maturation. Both the air and the water temperature must be monitored and controlled.
The relative humidity (RH) of the greenhouse air influences the transpiration rate of plants. High RH of the greenhouse air causes less water to transpire from the plants, which causes less transport of nutrients from roots to leaves and less cooling of the leaf surfaces. High humidities can also cause disease problems in some cases. For example, high relative humidity encourages the growth of botrytis and mildew.
The CO2 concentration of the greenhouse air directly influences the amount of photosynthesis (growth) of plants. Normal outdoor CO2 concentration is around 390 parts per million (ppm). Plants in a closed greenhouse during a bright day can deplete the CO2 concentration to 100 ppm, which severely reduces the rate of photosynthesis. In greenhouses, increasing CO2 concentrations to 1000-1500 ppm speeds growth. CO2 is supplied to the greenhouse by adding liquid CO2. Heaters that provide carbon dioxide as a by-product exist but we do not recommend these because they often provide air contaminants that slow the growth.
Light measurements are taken with a quantum sensor, which measures Photosynthetically Active Radiation (PAR) in the units μmol/m2/s. PAR is the light which is useful to plants for the process of photosynthesis. Measurements of PAR give an indication of the possible amount of photosynthesis and growth being performed by the plant. Foot-candle sensors and lux meters are inappropriate because they do not directly measure light used for photosynthesis.
Dissolved oxygen (DO) measurements indicate the amount of oxygen available in the pond nutrient solution for the roots to use in respiration. If no oxygen is added to the pond, DO levels will drop to nearly 0 ppm. The absence of oxygen in the nutrient solution will stop the process of respiration and seriously damage and kill the plant. Pure oxygen is added to the recirculation system in the ponds. Usually the level is maintained at 8 (7-10, no advantage to 20) ppm. For sufficiently small systems, it is possible to add air to the solution through an air pump and aquarium air stone but the dissolved oxygen level achieved will not be as high as can be achieved with pure oxygen.
The pH of a solution is a measure of the concentration of hydrogen ions. The pH of a solution can range between 0 and 14. A neutral solution has a pH of 7. That is, there are an equal number of hydrogen ions (H+) and hydroxide ions (OH-). Solutions ranging from pH 0-6.9 are considered acidic and have a greater concentration of H+. Solutions with pH 7.1-14 are basic or alkaline and have a greater concentration of OH-.The pH of a solution is important because it controls the availability of the fertilizer salts.
Electrical conductivity (EC) is a measure of the dissolved salts in a solution. As nutrients are taken up by a plant, the EC level is lowered since there are fewer salts in the solution. Alternately, the EC of the solution is increased when water is removed from the solution through the processes of evaporation and transpiration. If the EC of the solution increases, it can be lowered by adding pure water, e.g., reverse osmosis water). If the EC decreases, it can be increased by adding a small quantity of a concentrated nutrient stock solution.
Water quality parameters such as NH4+/NH3+, NO3-, NO2-, PO4-, pH and dissolved oxygen were measured fortnightly using test kits. Number of flowers, fruits and fruits weight were recorded. All the sampling data were recorded in the Microsoft Excel 7 for analysis.
The production of the lettuce crop is started in a germination area where they germinate and grow for 11 days. They should be shaded from full sun on the first day after germination, but can then be exposed to full light (17 mol/m2/d) or slightly greater. On Day 11, the plants are transported to the greenhouse and transplanted into the pond area where they are grown until re-spacing on day 21 and finally harvested on Day 35.
Lettuce will grow satisfactorily at a DO level of at least 4 ppm.
A pH of 5.8 is considered optimum for the described lettuce growing system, however a range of 5.6-6.0 is acceptable. Nutrient deficiencies may occur at ranges above or below the acceptable range.
Letuce atmospheric and water data model for hidroponic production system:
Air Temperature: 24 0C Day/19 0C Night (75 0F/65 0F)
Water Temperature: No higher than 250C, cool at 260C, heat at 240C
Relative Humidity: minimum 50 and no higher than70%
Carbon Dioxide: 1500 ppm if light is available, ambient (~390 ppm) if not
Light: 17 mol m-2 d-1 combination of solar and supplemental light
D O: 7 mg/L or ppm, crop failure if less than 3 ppm
pH: 5.6-6 upH
To understand the environmental condition of pond, the physico-chemical parameters of water were needed to be measured. The main parameters including temperature, pH, dissolved oxygen, nitrate and ammonia were measured before starting the experiment.
For determine the growth parameters, length, weight and number of leaves and branches were taken into consideration. The percent gain of growth parameters of the aquaponic plan were measured using the following formula.
Final stage- Initial stage
%gain = –––––––––––––- x 100
Initial stage
The recorded data were entered into the spreadsheet in MS Excel 2010 and then summarized properly before statistical analysis. After entering the data, the descriptive statistical analyses were done by MS Excel.
RESULTS AND DISCUSSION
Aquaponics water data model
Aquaculture production depends on physical, chemical and biological qualities of pond water to a greater extent. The successful pond management requires an understanding of water quality. Intensification of pond makes the water quality undesirable with a number of water quality parameters. Pond water quality is largely defined by temperature, transparency, turbidity, water color, carbon dioxide, pH, alkalinity, hardness, unionized ammonia, nitrite, nitrate, primary productivity, biological oxygen demand and plankton population (Bhatnagar and Devi, 2013).
The accepted level of ammonia should be under the range of 0.05 to 0.10 mg/l (Shoko et al., 2014) and above range it is toxic to the cultured fish (Francis-Floyd et al., 2009).
According to Mizanur et al. (2004), intensive aquaculture ponds sediments has various fertilizing components such as nitrogen, phosphorous, sulphur etc. which are very useful for growth and production of aquaponic plants. Moreover, water spinach is an efficient plant having clustered roots that can absorb nutrients from the water very efficiently (Kibria and Haque, 2012).
The length-weight relationship of water spinach depends on the fertility of media from where nutrients are supplied. The plant’s length-weight relationship is attributed to a variety and concentration of nutrients, of which nitrogen is the dominating factor. Waste water of stinging catfish ponds supplied various nitrogenous components of which ammonia has considerable fertilizing supports to the plant under floating condition on the pond surface (Kibria and Haque, 2012).
Aquaponics biological data model
Aquaponics is an integrated and intensive fish-crop farming system under constant recirculation of water through interconnected devices. It is considered a promising technology, which is highly productive under correct set up and proper management (Lal 2013; Orsini et al., 2013). First, fish feed is eaten by fish and converted into ammonia (NH3+). Some ammonia ionizes in water to ammonium (NH4+). Then, bacteria (Nitrosomonas) convert ammonia into nitrite (NO2-) and consequently bacteria (Nitrobacter) oxidize nitrite into nitrate (NO3-) (Tyson et al., 2011). Finally, the water delivers nutrients and oxygen to promote plant growth. Graber and Junge (2009), found similar yields between hydroponic systems and aquaponics systems. Finally, it is important to establish systems under “smart water” use and to balance nutrient concentrations in water to ensure maximum fish and plant growth.
Aquaponics is considered a method where water and nutrients are efficiently used and maintained within the system (Liang & Chien, 2013). In aquaponics it is possible to reduce daily water loss to 2% of the total water volume of the system. Due to the constant recirculation of water it is also possible to maintain evenly distributed high nutrient concentrations in the water (nitrate) as the small addition of water to compensate the daily loss will not dilute the nutrients (Rakocy, 2006). The “water smart” approach makes aquaponics an alternative systems to produce food under sustainable practices in areas where water is scarce (Essa et al., 2008).
Developing an accurate and practical tool to predict plant and fish growth and monitor nutrient concentrations in water, will improve the adoption and implementation small or commercial scale of aquaponic systems as urban farming or as a business model for household food security.
The objective of this model is to test and predicts plant and fish growth and net ammonium and nitrate concentrations in water in an aquaponic system. This is done by comparing the model outputs with measurements under controlled conditions in order to assess the accuracy of the tool to simulate nutrient concentrations in water and fish and plant biomass production of the system.
An accurate prediction of the model, with low radiation and temperature in our experiment compared to the experiments with which the model was calibrated, will enhance the value of the tool to monitor nutrient concentrations in water and the ability to determine crop and fish production under diverse environmental conditions.
Aquaculture production can potentially cause environmental pollution due to the nutrients content in the water discharged to the soil, underground water and other water sources (Edwards, 2015). By adding the plant component, the nutrients concentrated in the water will be taken up through the plant roots and enhance plant growth, reducing the need of fertilizer. Furthermore, the constant recirculation of water through interconnected devices, maintains and delivers resources such as nutrients and water to all system components. Finally, the fact that aquaponics systems do not need soils, makes them suitable to be built in small household areas in developing countries or within the cities as urban farming (De Bon, 2010).
The nutrients such as nitrogen in particular, start the flow from feed intake by the fish and excretion into the water. The feaces are rich in ammonia (NH3) and dependent on parameters such as pH and temperature, this is partly or completely converted into ionized ammonium (NH4+). The combination of both forms is referred to as total ammonia nitrogen (TAN) (Francis-Floyd et al., 2010). In reality, water is filtered through bio-filters containing bacteria which nitrify the TAN into nitrite (NO2-) and afterwards into nitrate (NO3-) according to the following equations:
Equation 1: NH3+ + H2O ↔ NH4+ + OH-
Equation 2: NH4+ + 1.5O2 => NO2- + 2H+ + H2O
Equation 3: NO2- + 0.5 O2 => NO3-
At high pH the balance in equation 1 lies at the left (NH3+) and with low pH at the right (NH4+). Both active forms of nitrogen (NH4+ and NO3-) can be absorbed by the plant, with preference to nitrate as the active form to enhance plant growth (Andriolo et al., 2006). Additionally, waste water from fish contains macronutrients such as phosphorous, potassium and micronutrients such as iron that are important through the growing cycle of the crop (Diver, 2006). Nevertheless, previous studies report nutrient deficiency in plants grown in the aqua pool after the use of commercial fish feed over long periods (Roosta, 2014). Therefore, addition of amendments such as Iron (Fe) is a common practice to supply the nutrient deficit caused by the fish feed. Additionally, water exchange is adapted according to nutrients concentrations to avoid any toxicity (ammonia) and salinity such as sodium (Na) above 50 mg/l, and to minimize denitrification (Ako and Baker, 2009).
In aquaponics systems the ratio between fish feed delivered per day and the area covered by crops is essential to provide enough nutrients for plants and avoid toxicity levels from nitrate and ammonia for fish. Additionally, accurate amounts of fish feed per day will avoid accumulation of organic matter in the systems, reducing potential denitrification sources by anaerobic conditions (Seawright et al., 1998). Rakocy et al. (2006) established a ratio between 60-100 g feed per m2 of crop area for leafy crops such as lettuce, spinach, basil and cabbage.
The approach of aquaponics can also be seen as a weakness of the system. The susceptibility of the fish to chemical compounds such as pesticides, increases the complexity of aquaponics and forces it to rely on integrated pest management practices to avoid any negative effect on the crop yield. Finally, according to the definition of Lehman et al., (1993) aquaponics are considered sustainable food production systems which do not compromise any natural resource and are free of any potential harmful chemical for humans and the environment (Somerville, 2014).
Fish are dependent on the external temperature to regulate their metabolic functions and rate of activity that affect feed intake, digestion and oxygen consumption. However, fish have a range of temperatures they can tolerate according to species (warm and cold water fish). Therefore, assessing the environmental conditions (temperature) of a region is essential to choose the fish species to be produced.
Aquaponics systems are complex and sensible food productions system which demand daily maintenance and monitoring. Furthermore, parameters such as water temperature, dissolved oxygen (DO), pH, and nutrient levels must be monitored frequently to avoid inefficient performance (Bernstein 2011). Rakocy et al. (2006) recommended DO concentration in water of 6 mg/L in order to provide enough oxygen for plants, fish and bacteria. Moreover, a high ammonia concentration in water is toxic for fish, decreasing their growth (feeding and digestion) and eventually can cause death. Therefore, the suggested maximum concentration of ammonia (1mg/L) should be maintained in water (Ebeling et al. 2012). Additionally, The European Inland Fishery Advisory Commission (EIFAC) established a maximum ammonia (NH3+) concentration of 0.25 mg/L in water. Nitrate concentrations should be kept lower than 50 mg/L to avoid negative effect in the fish immune system and prevent algae bloom in the system in order to avoid the reduction in the oxygen concentrations in water (Watson and Hill, 2006). Finally, other macro and micronutrients are not often measured individually in aquaponics. Any concentration above 200 mg/L of total dissolved solid in water should be avoid (Rakocy et al. 2006).
In order to avoid accumulation of toxic compounds in water such as ammonia and nitrite, which can cause fish death, it is necessary to decompose these compounds into more favorable compounds and promote the growth of plant and fish. The main purpose of the bacteria (Nitrosomonas and Nitrobacter) present in the bio filter is to convert (TAN), essentially the -ionized fraction (NH3), into nitrate (NO3-). Additionally, the larger the amount of oxygen and ammonium present in the water, the higher the expected nitrification rate (Lucas and Southgate, 2003).
The productivity of water plants and the quality of water for culturing are influenced, among others, by the stocking density ratio in the culture media, in order to achieve an ideal combination for a useful biological control. The results of fish metabolism in forms of ammonium (NH4+), nitrate (NO3-) and phosphate can be utilized by the water plants to reduce the percentage of nitrogen in the media (Rakocy 2007), as well as to increase the growth level of the water plants.
Aquaponics growing model
Lettuce is the most common leafy crop grown in aquaponics systems, due to its low nutrient demand and short growing cycle (5 weeks). Moreover, the constant recirculation of water in aquaponics systems provides a permanent supply of nutrients to the root zone and therefore no depletion on nutrients is visible (Tyson et al., 2011). Seginer (2003) suggests to maintain a pH between 7.5 and 8.0 to promote nitrification and availability of nutrients as phosphorous, calcium and magnesium. Nevertheless, Rakocy et al., (2006) state that such high pH values affect negatively the solubility of essential micronutrients such as iron, manganese, copper, zinc and boron. Therefore, a pH between 6.5 and 7.0 is acceptable for the three main components of the aquaponic system (plant, fish and bacteria).
Lettuce is considered a cold season crop and temperatures below 7˚C and above 25˚C will result in physiological disorders and unmarketable quality products. Providing the crop with a proper environment will achieve greater biomass production.
Based on the volume of crops produced, it appears that the productivity of the plant in the aquaponics system is lower compared to hydroponics. According Bittsanskzky et al. (2016), most plant nutrients were at significantly lower concentrations in the research aquaponics systems as compared with the standard hydroponic solutions. The differences were highest in the case of Fe2+ and Mn2+ (with ratios of 68.5 and 138.7, respectively). Fish density also affects the availability of nutrients for plants in aquaponics system. The results of the study Villarroel et al (2011) states that the total amount of feed required per mEq at the low biomass (2 kg fish/m3) ranged from 1.61-13.1 kg for the four most abundant ions (NO3-, Ca2+, H2PO4- and K+), suggesting that it is feasible to integrate fish culture (at low densities) to reduce the cost of the hydroponic solution supplementation for strawberries. The nutritional requirements vary with variety, life cycle stage, day length, and weather conditions. However, the use of lettuce and kale in this study had an appropriate selection. Provided that the system is stocked with enough fish, it is not necessary to add nutrients for plants with short cropping cycle which do not produce fruits (e.g. lettuce). In contrast to, for example, lettuce, tomatoes which need to bear fruit, mature and ripen, need supplemental nutrients.
The results showed that the frequency of crops is higher than the fish harvest. This is in line with the Love et al (2015) found that fish production was less profitable than production plant in the aquaponic units analyzed. That may help to explain why we found more than fish production plant production in the current study. Also plant growing cycles are shorter, and the area used for their production tends to be larger than for fish. According to Hepher and Pruginin (1981), this decline of dissolved oxygen and increase of ammonium in the water are caused by the number and size of the cultured fish. Boyd and Linchtkoppler (1982) explained that the higher the stocking density ratio, the higher the demand for oxygen due to the increasing number of fish.
Along with the increase in fish weight, the oxygen consumption rate and metabolism waste of each fish also increase. The decrease in the dissolved oxygen concentration may further lead to fish’s loss of appetite which stunts the fish growth. Tilapia in farms can tolerate lower concentrations (ranging from 3 to 4 mg/L), the optimum levels of dissolved oxygen are higher, and so the desirable range is usually above 5 mg/L (Pillay 2004). Meanwhile dissolved oxygen that sufficient for the growth of the catfish larvae is above 1 mg/L (Durborow et al. 1985).
Treatment with water plants can heighten the concentration of dissolved oxygen because, in the daytime, the plants are undergoing assimilation and add oxygen to the water. At night, the plants use the oxygen available in the water for respiration. As a result, the increase of dissolved oxygen concentration due to the treatment with water plants will further improve the appetite of the fish that in turn leads to better fish growth. Dissolved oxygen in water can affect the activity of tilapia and metabolism in the body of the fish (Ardita et al. 2015).
Water plants ensure the nitrification process runs well due the interaction among fish, water plants and nitrifying bacteria. Protein coming from the feed is decomposed into a simple compound by converter bacteria, such as Nitrosomonas that can convert ammonia into nitrite and Nitrobacter that converts nitrite into nitrate; this nitrate is further used by the water spinach and lettuce as a nutrient, thus balancing the content of nitrogen in the aquaponics system (Graber and Junge 2009). This is a good thing for us because nitrate happens to be the favorite food of plants. Also the fish will tolerate a much higher level of nitrate than ammonia or nitrite (Blidariu and Grozea 2011). In this condition, the quality of water remains adequate to support fish growth through the good use of feed.
In terms of water quality, and the concentrations of salts and minerals needed for the production of sweet basil (or general guidelines), Racozy (2003) noted: ‘Our general guideline is to feed fish at a ratio of 57 grams per m2 of plant growing area per day. This ratio provides good nutrient levels. We supplement with equal amounts calcium hydroxide and potassium hydroxide to maintain pH near 7.0. Every three weeks we add 2 mg/L of iron in the form of a chelated compound. In a commercial-scale aquaponic system at UVI (University of the Virgin Islands) that was to produce lettuce continuously for 2.5 years, nutrient concentrations varied within the following ranges (mg/L) that would have produced excellent sweet basil growth’:
According to Effendi (2003) phosphorus in the form of phosphate is a necessary macronutrient and very essential for aquatic organisms including aquatic plants. Lack of phosphate may stunt the growth of phytoplankton which may further affect the aquatic balance in the water (Bahri 2006). At the end of the study, the treatments with water plants result in a lower concentration of phosphate, since the phosphate has been used as a nutrient by plants. On the other hand, in the control treatment without water plants, the phosphate concentration increases due to it not being used by plants. In a low water exchange, the accumulation of phosphate, As and Cu led to higher mortality and reduced larvae length and body weight in the culture of carp (Okemwa 2015). Phosphorus is one of the essential nutrients due compound will be absorbed by phytoplankton and get into the food chain (Hutagalung and Rozak 1997).
Orthophosphate is a form of phosphorus that can be directly utilized by aquatic plants, while polyphosphate orthophosphate should be reduced first before being used. Maintain the levels of phosphorus in the aquaponics system is very important because if its levels are too high can cause algae or microorganisms in the water to grow uncontrolled in aquaculture ponds. The deficiency of phosphorus will cause some signs of deficiency include slow growth and leaf stem (Diver 2006).
Biological model
2.1 General considerations
A major global trend nowadays is urbanization and its degree reached 70% in various European countries (dos Santos 2016). The challenges brought by this trend include population rise, food insecurity and climate change (dos Santos 2016). According to FAO (2014) aquaculture is one of the fastest-growing food production sector, which provides approximately 50% of fish and fish products for human consumption. However, traditional aquaculture production in natural ponds have a negative impact on the environment, due to the use of high amount of fresh water and the load with nutrients of waste water (Suhl et al., 2016). Aquaculture integrated systems (such as aquaponics) have many advantages like enhanced productivity, environmental sustainability and water use efficiency (Cerozi and Fitzsimmons, 2017). In developing countries aquaponics may resolve issues such as water scarcity, soil degradation and climate-related challenges (Wongkiew et al. 2017; Shete et al. 2016). Aquaponics is the fusion of recirculating aquaculture and soilless vegetable production, within a complex closed-loop system (Cerozi and Fitzsimmons, 2017). Being a strictly controlled system, it combines a high level of biosecurity with a low risk of disease and external contamination (FAO 2016). Aquaponics integrates aquaculture and hydroponic techniques in a one merged system. It is a soilless agriculture system that synergistically combines aquaculture and hydroponics (Wongkiew et al., 2017). The aquaponics system has the capability to raise fish in high densities, produce profitable vegetables, reduce the use of pesticides and chemical fertilizers, minimize water exchange and sustain an adequate water quality (Cerozi and Fitzsimmons, 2017). According to dos Santos (2016) aquaponics could represent a new integrated agricultural system from producers to consumers in an integrated manner, due to the short supply chains and organic fresh food. As well, aquaponics could relieve environmental pressure by the double use of water and nutrients and increased profit by producing two crops (Suhl et al., 2016). This technological method has the potential for higher yields of produce and protein with less labor, less land, fewer chemicals and a fraction of the water usage (FAO 2016). The integration of the aquaculture and hydroponic systems reduces water discharge into the environment (Bailey and Ferrarezi, 2017). Therefore, aquaponics is environmental friendly, suitable for agriculture practice and it is expected to become widely used for sustainable food production in the future (Forchino et al., 2017).
Water is one of the most important medium to understand in an aquaponic system and the main water quality parameters are dissolved oxygen, pH, water temperature, nitrogen compounds and electrical conductivity (Thorarinsdottir 2015).
Temperature
Plants have different temperature requirement, such as 15-19°C for salads and higher temperature and humidity need for tropical plants. In terms of optimum temperature for the nitrifying bacteria, the acceptable temperature range is 17-34°C. Nitrobacter group is less tolerant to lower temperature, compared to the Nitrosomonas group, therefore during colder periods nitrite should be monitored more carefully to avoid harmful accumulations (Thorarinsdottir 2015).
Dissolved oxygen
To obtain good fish growth, dissolved oxygen levels should be maintained at saturation and at least above 5 mg/L. Low dissolved oxygen can cause potentially irreversible damage to fish gills and reduce the efficiency of the nitrifying bacteria (Thorarinsdottir 2015).
pH
Plants prefer a pH< 6.5 and nitrifying bacteria perform optimally at pH> 7.5 The ideal pH value for the aquaponic system ranges between 6.8-7.0 (Thorarinsdottir 2015).
Nitrogen compounds
All four forms of nitrogen (NH3, NH4+, NO2-, NO3-) can be used by plants and stimulate growth, however the form quickly absorbed by plants is nitrate (Thorarinsdottir 2015).
Macro- and micronutrients
Plants cultivated in an aquaponic system need several nutrients that are required for the enzymes that facilitate photosynthesis for both growth and reproduction (Thorarinsdottir 2015). The nutrients are categorized as macro- and micronutrients. The 6 macronutrients are: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulphur (S). The range of micronutrients is much bigger and it includes iron (Fe), copper (Cu), boron (B), manganese (Mn), molybdenum (Mo) and zinc (Zn) (Thorarinsdottir 2015).
In the young stages of plants, particularly during the plant’s vegetative growth and before fructification, nitrogen need is high. During maturity, the nitrogen need decreases in order to avoid difficulties to blooming and fall of young fruits (Thorarinsdottir 2015). Excess of nitrogen fertilization makes also plants more prone to pests and diseases, due to the tenderness of the vegetable tissues (Thorarinsdottir 2015). The yellowing of older leaves is a main indicator that the system lacks N. In plants, nitrogen can be reallocated within plant tissues because it is a mobile element. In case of N deficiency, N gets transferred from older leaves to new growth areas, which is the reason why N deficiency can be mainly observed in old leaves (Thorarinsdottir 2015). The limit of N in the aquaponic system ranges between 150-1000 PPM and the average is 250 ppm.
In the aquaponic system, plants require optimum conditions in comparison to light, oxygen, carbon dioxide, pH, temperature and nutrients (Thorarinsdottir 2015). The aquaponic technology can produce different vegetable crops but in general, leafy vegetables grow well with the abundant nitrogen in the system, have a short production period, and are in high demand. Even though fruiting crops have longer production periods and produce less marketable yield, their value is often higher than the value of leafy produce ((Bailey and Ferrarezi, 2017). The main plant species grown so far in aquaponic systems is lettuce, which has been grown under different densities (16 to 44 plants/m2) and crop lengths (21-28 days) (Thorarinsdottir 2015).
Phosphorus deficiency causes stunted plant growth, whereas phosphorus excess may lead to antagonistic interactions with micronutrients, especially zinc (Cerozi and Fitzsimmons, 2017).
Cerozi and Fitzsimmons (2017) observed in their study that plants are negatively affected by the high concentration of nitrite in the water. When plant based diets were fed to fish in aquaponics systems, low amounts of dissolved orthophosphate were produced in the nutrient solution, which constrained growth and quality of lettuce (Cerozi and Fitzsimmons, 2017).
In order for plants to fulfill their metabolic requirements, usually aquaponics nutrient solutions are added in the technological water as dissolution. Cerozi and Fitzsimmons, 2017 applied the necessary nutrients as foliar spraying in order to avoid the unwanted interaction between these supplements and dissolved phosphorus, such as the formation of insoluble salts (magnesium and iron phosphates).
A percent of 50% of the phosphorus input is retained by the fish, while 20% occurs in the particulate form and 30% is dissolved as orthophosphates (d'Orbcastel et al., 2008).
According to Cerozi and Fitzsimmons (2017) lettuce growth has a direct effect on the phosphorus dynamics in aquaponics systems and in the timeline between first day of transplanting and ten days after the lettuce experienced a growth lag, which allowed a linear increase of dissolved phosphate in the aquaponics solution. At day 21 after transplanting the lettuce reached exponential phase, therefore a dramatic decrease in dissolved phosphate was noted due to high phosphorus extraction rate.
Aquaponics can maximize phosphorus utilization 71.7% of total P input, with fish and plants assimilating 42.3% and 29.4% of the phosphorus input in the feed, respectively. 13.1% of the phosphorus input occurred in the unavailable form to fish and plants (Cerozi and Fitzsimmons, 2017).
In aquaponic systems emissions are mainly constituted by nitrogen and phosphorous released in the environment, due to suspended solids and dead lettuces removal and disposal (Forchino et al., 2017). However, these wastes can be easily recycled within the farm. Low potassium (K), sulfur (S), iron (Fe), and manganese (Mn) have been reported in aquaponic plants that received nutrition only from fish waste (Saha et al., 2016).
Fish in an integrated recirculating aqua-hydroponic system can be reared with 8 times higher density than the recommended fish stocking density (Diem et al., 2017). As a follow up of the experiment, Diem et al. (2017) proposed an optimal fish density in an aquaponic system as follows: 114-125 fish/m3 or 6-7 kg/m3, a feeding rate of 176-282 g/m3/day, a 4.2 m2 surface of hydroponic trenches per m3 (with canna and water spinach plants) and a water recirculation of 200-400% per day.
Canna is a very suitable plant for the aquaponic system, mostly for the use in wastewater purification. Besides the potential for nutrient removal canna has aesthetical values providing nice-looking treatment systems (Diem et al 2017). Diem et al (2017) demonstrated in their study that canna (C. glauca L) had the highest growth rate compared to water spinach and lettuce. Even though water spinach (Ipomoea aquatica Forssk.) produced less biomass than canna, it has more economic value as a leafy vegetable (Diem et al 2017).
Basil (Ocimum basilicum L.) is an annual herb with high commercial importance, both for its fresh and dried leaves. It has culinary and medicinal purpose (Saha et al., 2016). According to Saha et al. (2016) basil produces higher yield in aquaponics compared with conventional systems.
Lettuce (Lactuca sativa) is higly recommended as an aquaponics crop due to its fast growth and low nutritional requirement (Pinho et al., 2017). Lettuce growth can be limited (inhibited) by the use of sponge material to hold the plants in position. Diem et al. (2017) demonstrated that the sponge material has a high water holding capacity and thus, creating a too moist environment around the plant stems.
Aquaponics may be more efficient in the presence of a diverse microbial community (Pinho et al., 2017) fact confirmed by Cerozi and Fitzsimmons (2017). In their study, the authors concluded that the addition of a commercial mixture of Bacillus spp. enhanced plant growth, increased the P accumulation in plant tissues, and increased the chlorophyll content in the leaves. As well, systems that received the Bacillus mixture demonstrated higher concentrations of dissolved orthophosphate in the water than untreated systems.
In aquaponic systems the alkalinity should be above 100 mg/L for a proper ammonia assimilation and nitrification process by heterotrophic bacteria (Pinho et al., 2017). Pinho et al. (2016) measured in their aquaponic system a concentration of 20.6 ± 1.6 mg/L orthophosphates and noted to be within the recommend values for leafy vegetables. The pH is a parameter which is difficult to adjust in an aquaponic system due to the different optimal requirements of fish and plants. In aquaponics the ideal pH ranges between 6.5 and 7.0, providing the best nutrient absorption by the plants (ideal pH 5.5–6.5) and reduction the levels of un-ionized ammonia for fish (Pinho et al., 2017). Plant production would be higher if pH levels were lower.
In traditional agriculture lettuce has an optimum temperature range between 4 and 27°C. In aquaponic systems the temperature variate from 22°C in the winter season and 30°C in the summer season and from the growth performance point of view, lettuce had higher productivity in the hot season compared to the cold one. Lettuce tolerance to higher temperatures may be due to the direct contact with water and thus reducing the negative effects (Pinho et al., 2017).
2.2 The concept of aquaponics modeling for a recirculating aquaculture system
Aquaponic systems are characterized as being complex systems, that involves the presence of two different growing technologies, for fish and respectively, for plants, with the purpose of increasing the sustainability and to limit the environmental impact. Thus, as it is a complex system, there are many variables that must be taken into consideration in order to make a prediction model that will estimate the productivity of an aquaponics system.
Integrated aquaponics systems have two major components, fish and plant biomass. Therefore, it is important to analyze both those components apart, in order to observe their connection to other factors that interfere in the multi-trophic system production cycle. At the end of this activity, the results obtained must be correlated, finding the links that ensures the direct relation of dependency between fish and vegetal biomass.
Various studies regarding aquaponics integrated systems were conducted in the past decade (Cristea et. al 2013, Petrea et. al, 2013a, Petrea et. al, 2013b, Blidariu, 2013, Roosta, 2011, Savidov et al. 2007, Licamele, 2009), having different objectives, from analyzing the growth performance of fish biomass, till identifying the nutrients balance inside a multi-trophic integrated aquaponics system. However, a lot of researches that take into study the growth of different fish species under various technical and technological conditions were made (Adler et al, 2003, Timmons et al, 2013, Leoni, 2003, Meade, 2002, Lacheta, 2010, Wahome et al, 2011, Graber et al, 2009, Wilson et al 2006, Leannard et al 2004, Aquaponic Research Project, 2013). On the other part, studies that imply the growth of plants under hydroponic conditions are conducted in order to evaluate the nutritional requirements of different plant species, growth by using various technologies.
Thus, two approach methods can be distinguished in order to collect the data present in the scientific literature till nowadays, in order to elaborate a correlation model for aquaponics integrated systems.
The first way of approach is to consider an aquaponics system as an own identity and to study the direct and indirect correlations between the main elements presented in the system, meaning plants and fish. In this case, only the aquaponics research studies must be taken into consideration for the incipient data processing.
The second way of approach is to consider aquaponics systems as complex systems, composed of two major parts (plants and fish biomass), each corresponding to a production technology. Thus, in this case, the research studies that must be taken into consideration are divided in fish biomass researches and plant biomass researches. The scientific studies regarding fish biomass, that will be analyzed, must have as a goal the test of different technological aspects as fish stocking density, feeding ration, as well as various nutritional aspects. On the other hand, the plant biomass studies that will be considered as useful are the ones that target the nutritional requirements of different species of plants, grow in hydroponic systems, emphasizing especially the nutrients retention rates kinetics and dynamics.
The integrated recirculating aquaculture systems can be analyzed by using a mathematical model, also by dividing the concept into three parts, as follows: biological part, microbiological and chemical part and in the end, technical part.
Therefore, by taking into consideration the biological part, the main characteristics that must be followed are the ones leading to both fish and plants biomass. Both fish and plants growth rate is depended on atmospheric and technologic factors. Thus, among the atmospheric factor that can influence the plants growth, air temperature, humidity, light intensity and wavelength are determinant variables that must be taken into account in order to establish the prediction growth model for plant biomass. Each plant has its own requirements when it comes to the above mentioned factors. However, plants requirements may vary, depending on its growth stage. In order to determine the proper growth prediction model, a large series of variants, corresponding to the same variable, must be tested.
The microbiological and chemical model creates a rather close bond between fish biomass and plant biomass, comparing with atmospheric model, but it correlates perfect with the technological factors from the previous biological model.
The microbiological issues are popular nowadays, when it comes to integrated multi-trophic aquaculture systems and namely integrated recirculating aquaculture systems, where aquaponics techniques were used. There is a strong debate related to product security and safety for consumers, since in aquaponics systems, both fish and plants are safety from the chemical point of view, but the lack of pesticides can involve microbiological problems. It is obvious that the accumulation of organic matter inside the production system can cause microbiological issues as well. This accumulation is related with applying a proper operative management inside the system, but also with using a proper technique and production technology.
It is known that the nutrients requirements of plants different significantly, both in terms of dynamics and quantity. As fish are feed mostly with a fix feeding ration, the dynamic of their digestive process is relatively constant in time, fact which implies a constant dynamics of the main nutrients in the technological water, on a long term production cycle. Plants have many upward tendencies, followed by suddenly downward tendencies, such dynamics being the cause of lack of nutrients required by them, at a certain moment. This fact makes very difficult the attempts of finding a perfect microbiological and chemical prediction model in integrated recirculating aquaculture systems, based on using aquaponics techniques.
The correlation between water chemistry parameters in different research studies made in recirculating aquaculture systems are presented in table 1.
TABLE 1. The interdependence between fish weight gain and technological water nutrients concentration in different studies made in recirculating aquaculture systems
The technical model perspective is the most important data model and implies the engineering part of the integrated production system. As a start-up data, the configuration of the production system and also, the type of equipment used and the operational management applied are required. The start-up data will generate a certain hydraulic inside the production system, characterized by indicators as hydraulic loading rate and hydraulic retention time. Those indicators will influence most of the parameters taken into account in the previous two models that were described.
Generally, most of the integrated recirculating aquaculture systems, based on aquaponics techniques, follow certain water treatment processes, as follows: mechanical filtration, biological filtration (nitrification), denitrification, phytoremediation, oxygenation, pH buffering, sterilization. Each integrated production system must be projected starting from the amount of maximum biological material that can be produced, at a certain moment.
Considering that this requirement has been proper accomplished, the prevision model will therefore analyze the efficiency of the equipment during a long term use. This action is generating an operational management model that must be used in order to keep the performances of the production system at an optimal level.
The biological filtration module performance, on a long term, can be influence by the hydraulic regime of the production system. Therefore, the fluctuation of hydraulic loading rates and hydraulic retention time values will influence the nitrification capacity of the technological water bio-filtration unit. Also, the specific surface of the bio-balls inside the bio-filter and also their design, not to mention the bio-filter dimensions, are important variable since they influence the oxidation process and therefore, the removal rates of ammonia and nitrites.
The technical type of the bio-filter has a very high impact on the production performances and the dynamics of the biological and chemical model. The performances of different bio-filters are presented in the second part of this report.
Barbu et al. 2016 described a mathematical modeling and analysis of trickling bio-filter and experimentally noticed that the bio-filter aeration in countercurrent with respect to the flow of the processed water has a practical negligible effect, so that the trickling bio-filter does not offer control means of the nitrification process. In these conditions, the main possibility to control the nitrification process is the control of recirculated flow.
Regarding the mechanical filtration units, Barbu et al. 2016 mentioned that disturbances are produced by the washing processes of mechanical, sand and active carbon filters of recirculating aquaculture systems. Their presence makes difficult to discern the effects of control applied to the recirculating flow by the variations induced through internal disturbances. Mechanical filtration still remains the main problem inside a recirculating aquaculture system.
When elaborating a prediction model for aquaponics integrated recirculating systems, it must be start from the assumption that we are dealing with close systems, with minimal to none relation with the environment. Therefore, all the inlets and outlets of the system must be quantified with a high precision. Also, the second step in elaborating a model, after analyzing the production capacity of the projected system, is to take into account the possible mortalities manifested on both fish and plants biomasses, during a single production cycle.
When referring to the chemical model mentioned above, it must be accepted the assumption that all the main nutrients presented in the technological water, especially the ammonia are due to fish waste discompose. On the other hand, the ammonia reduction is exclusively generated by the oxidation processes that convert it into nitrite.
The oxidation process of ammonia to nitrite and then nitrate will normally generate a nitrate accumulation in technological water, on the long term. Therefore, it must be considered that plant biomass is the only source of nitrate uptake from the analyzed closed system.
It is quite important that the system to have a high mixing capacity of the technological water even from the projection and design phase, fact mentioned also by Bobak and Kunze (2017). This will ensure an almost equal concentration of each nutrient, in the technological water volume and will offer equal possibility for the plants to absorb the required elements, corresponding to their nutritional demands.
Loria (2017) mentioned that an aquaponics model presents a way to combine several components (bacteria, plant and fish) and relate them to environmental factors such as radiation, temperature, feed given, nitrate and ammonium concentrations, in order to assess the performance within the system and achieve optimal plant and fish growth. The function in the fish component relating water temperature to feeding ratio as a percentage of the fish mass had a subroutine adapted to lower temperature factors which was used to fit with the measured water temperature (Loria, 2017).
Each model should be based also on the characterization of final products. Therefore, the quality of both fish and plants biomass must be determinants factors for establishing new growing conditions or maintaining the current ones from a certain model. For example, for an atmospheric model, light intensity and wavelength is considered a main factor that influence the growth performances and nitrate and nitrogen concentrations in the plant, while for a chemical model, the same parameters must by influence by considering the water nutrients concentration as a main dependent factor. However, for a technological model, fish to plants ratio is the must determinant factor for influence growth performances and nitrate and nitrogen concentrations in the plant.
Therefore, as a conclusion, it must be mentioned that aquaponics modeling is used as a tool for a better understanding of the relations between plants – bacteria – fish. Also, the models give predictions and information that will clarify the way in which different parameters can positively or negatively affect the nutrients balance in the technological water and the productivity of the integrated system.
REFERENCES
A.H. Al-Harbi And A.Q. Siddiqui Effects of Tilapia Stocking Densities on Fish Asian Fisheries Science 13(2000): 391-396 Asian Fisheries Society, Manila, Philippines Growth and Water Quality in Tanks
Adler, P., R., Summerfelt, S., T., Glenn, D., M., and Takeda, F., 2003, Mechanistic approach to phytoremediation of water. Ecol. Eng., 20, Pages 251–264.
Adler, P.R., J.K. Harper, E.W. Wade, F. Takeda and S.T. Summerfelt, 2000. Economic analysis of an aquaponic system for the integrated production of rainbow trout and plants. International Journal of Recirculating Aquaculture. 1(1): 15-34.
Ako, H., & Baker, A., 2009. small scale lettuce production with hydrophonics or aquaponics. Sustaianable Agriculture, 2.
Andriolo, J.L., Godoi, R.D.S., Cogo, C.M., Bortolotto, O.C., da Luz, G.L., Madaloz, J.C., 2006. Growth and devlopment of lettuce plants at high NH4+:NO3- ratios in the nutrient solution. Horticultura brasileira, Volume 24, Issue 3: 352-355.
Aquaponics Research Project The relevance of aquaponics to the New aid programme, particularly in the Pacific Commissioned Report, Hambrey Consulting – New Zealand Aid Programme Ministry of Foreign Affairs and Trade – December 2013, Pages 1-96.
Ardita N, Budiharjo A, Sari SLA. 2015. Growth and feed conversion ratio of tilapia fish (Oreochromis niloticus) with addition of probiotics. Bioteknologi 12: 16-21.
Bahri FA. 2006. Analysis of Nitrate and Phosphate Content in Recovered Mangrove Sediments in Sub-District Mallusetasi, District Barru. Asosiasi Konservator Lingkungan, Makassar, Indonesia.
Bernstein, S. (2011). Aquaponic Gardening: A Step-by-Step Guide to Raising Vegetables and Fish Together; New Society Publishers: Gabriola Island, BC, Canada, p. 256.
Bhatnagar A and P Devi, 2013. Water quality guidelines for the management of pond fish culture. International Journal of Environmental Science, 3: 6.
Bittsanskzky A, Uzinger N, Gyulai G, Mathis A, Junge R, Villarroel M, Kotzen B, Komives T. 2016. Nutrient supply of plants in aquaponics systems. Ecocycles 2 (2): 17-20.
Blidariu F, Grozea A. 2011. Increasing the economical efficiency and sustainability of indoor fish farming by means of aquaponics-Review. Sci Pap: Anim Sci Biotechnol 44 (2): 1-8.
Blidariu, F. C., 2013. Sporirea eficienșei economice și a sustenabilității creșterii peștilor în sisteme recirculante prin intermediul culturilor acvaponice’’, Teza de doctorat, USAMVB Timisoara,
Boutwelluc, Juanita, 2007. "Aztecs' aquaponics revamped". Napa Valley Register.
Boyd CE, Linchtkoppler F. 1982. Water Quality Development Series No 22. International Center for Aquaculture. Aquaculture Experiment Station, Auburn, Alabama.
Carly Bobak and Herb Kunze, 2017, An inverse problem for a mathematical model of aquaponic agriculture Citation: AIP Conference Proceedings 1798, 020180 (2017); doi: 10.1063/1.4975642 View online: http://dx.doi.org/10.1063/1.4975642 View Table of Contents: http://aip.scitation.org/toc/apc/1798/1 Published by the American Institute of Physics
Cerozi B.S. and Fitzsimmons K., Phosphorus dynamics modeling and mass balance in an aquaponics system, Agricultural System, 153, 94-100, 2017, doi.org/10.1016/j.agsy.2017.01.020
Cristea V., Dicu Maria Desimira , Dediu Lorena , Măereanu Marilena , Coadă M.T. 2012, The influence of feeding intensity on growth performance of Acipenser Stellatus (Pallas 1771) juvenils Lucrări Științifice – Seria Zootehnie, vol. 58
De Bon, H., Parrot, L., & Moustier, P., 2010. Sustainable urban agriculture in developing countries. A review. Agronomy for sustainable development, 30(1), 21-32.
Diem N.T., Konnerup D., Brix H., Effects of recirculation rates on water quality and Oreochromis niloticus growth in aquaponic systems, Aquacultural Engineering, 2017, doi.org/10.1016/j.aquaeng.2017.05.002
Diver, S., 2006. Aquaponics-Integration of hydroponics with aquaculture. ATTRA: 1-28.
Diver, Steve, 2006. "Aquaponics — integration of hydroponics with aquaculture" (PDF). ATTRA – National Sustainable Agriculture Information Service (National Center for Appropriate Technology). www.backyardaquaponics.com/guide-toaquaponics/fish
d'Orbcastel E.R., Blancheton J-P., Boujard T., Aubin J., Moutounet Y., Przybyla C., Belaud A., 2008Comparison of two methods for evaluating waste of a flow through trout farm, Aquaculture 274, 72-79,
dos Santos M.J.P.L., 2016, Smart cities and urban areas- Aquaponics as innovative urban agriculture, Urban Forestry & Urban Greening, 20, 402-406,.
Durborow RM, Avault Jr JW, Johnson WA, Koonce KL. 1985. Differences in mortality among full-sib channel catfish families at low dissolved oxygen. Progress Fish-Cult 47:14-20.
Ebeling, J. M., Timmons, M. B., 2012. Recirculating Aquaculture Systems, Chapter 11, Recirculating Aquaculture Systems. ISBN: 9780813801261.
Edwards, P., 2015. Aquaculture environment interactions: past, present and likely future trends. Aquaculture, 447, 2-14.
Effendi H. 2003. Assessing Water Quality for Resource Management and Environmental Water. Kanisius , Yogyakarta. [Indonesian]
Essa, M.A., Goda, A.M.A.S., Hanafy, M.A., El-Shebly, A.A., Mohamed, R.A., El-Ebiary, E.H., 2008. Small-scale fish culture: guiding models of aquaponics and net-enclosures fish farming in Egypt. Egyptian Journal of Aquatic Research, Volume 34, Issue 3: 320-337.
FAO, 2014. Make agriculture, forestry and fisheries more productive and sustainable. http://www.fao.org.
FAO, 2014b. Urban agriculture in the Gaza Strip through vertical gardens and aquaponics. http://www.fao.org/fileadmin/templates/FCIT/PDF/Fact_sheet_on_aquaponics_Final.pdf.
FAO, The state of World Fisheries and Aquaculture- Contributing to food security and nutrition for all, Food and Agricultural Organization of the United Nations (FAO), Rome, 2016.
FAO, The state of World Fisheries and Aquaculture- Opportunities and challenges, Food and Agricultural Organization of the United Nations (FAO), Rome, 2014
Forchino A.A., Lourguioui H., Brigolin D., Pastres R., 2017, Aquaponics and sustainability: The comparison of two different aquaponic techniques using the Life Cycle Assessment (LCA), Aquacultural Engineering, 77, 80-88, doi.org/10.1016/j.aquaeng.2017.03.002 .
Francis-Floyd R, C Watson, D Petty and DB Pouder, 2009. Ammonia in aquatic systems (Univ. Florida, Dept. Fisheries Aquatic Sci., Florida Coop. Ext. Serv. FA-16), <http://edis.ifas.ufl.edu/FA031>. Date of Access 01 January, 2015.
Francis-Floyd, R., Watson, C., Petty, D., Pouder, D.B., 2010. Ammonia in aquatic systems. FA1. UF/IFAS, University of Florida.
Graber, A., Junge, R., 2009. Aquaponic Systems: Nutrient recycling from fish wastewater by vegetable production. Desalination, Volume 246, Issue 1:147-156.
Graber, A., Junge, R.; Aquaponic Systems: Nutrient recycling from fish wastewater by vegetable production. Science Direct, Desalination 246, 2009;: Pages 147–156
Hepher B, Pruginin Y. 1981. Commercial Fish Farming: With Special Reference to Fish Culture in Israel. John Wiley and Sons, New York.
Hoekstra, A.Y. and Chapagain, A.K., 2007. Water footprints of nations: water use by people as a function of their consumption pattern, Water Resources Management 21(1): 35–48.
Hutagalung HP, Rozak A. 1997. Methods Analysis of Sea Water Sediment and Biota. Book 2nd. LIPI, Jakarta. [Indonesian]
Kibria ASM and MM Haque, 2012. Integrated Multi-Trophic Aquaculture (IMTA) Systems in Freshwater Ponds in Bangladesh: Initial Understanding. Department of Aquaculture, Bangladesh Agricultural University, Mymensingh, Bangladesh.
Lacheta, A., 2010The future of food, WellBeing Natural Health & Living News,
Lal R, 2013. Beyond Intensification. In: Paper presentation at the ASA, CSSA, & SSSA international annual meetings, Tampa, Florida, USA.
Lehman, H.; Clark, E.A.; Weise, S.F., 1993. Clarifying the definition of Sustainable agriculture. J. Agric. Environ. Ethics, 6, 127–143.
Lennard, W. and Leonard, B. 2004, A comparison of reciprocating flow versus constant flow in an integrated, gravel bed, aquaponic test system, Aquaculture International 12, Kluwer Academic Publishers, Pages. 539–553.
Leoni, S., 2003, Colture Senza Suolo in Ambiente Mediterraneo. Le Nuove Tecniche per L’orticoltura e la Floricoltura da Serra. Edagricole, Bologna, Italy. Page 278.
Liang, J.Y., Chien, Y.H., 2013. Effects of feeding frequency and photoperiod on water quality and crop production in a tilapia–water spinach raft aquaponics system. International Biodeterioration and Biodegradation, Volume 85: 693-700.
Licamele, J., 2009.Biomass production and nutrient dynamics in aquaponic systems. PhD. Dissertation Submitted to the Faculty of the Department of Agriculture and Biosystems Engineering – The University of Arizona,
Lorena Dediu (Sfetcu), Angela Docan, V. Cristea, Iulia Grecu, 2010, considerations regarding the rearing of european catfish, Silurus glanis l. In a flowthrough production aquaculture system Lucrări Științifice – vol. 53, Seria Zootehnie
Lorena Dediu, Victor Cristea, Mirela Mocanu, Desimira Dicu, Angelica Docan, and Iulia Grecu The effect of feeding frequency on growth performance of rainbow trout fingerlings reared in recirculating system AACL Bioflux, 2011, Volume 4, Issue 2.
Love DC, Uhl MS, Genello L. 2015. Energy and water use of a smallscale raft aquaponics system in Baltimore, Maryland, United States. Aquac Eng 68: 19-27.
Lucas, J. S., Southgate, P.C., 2003. Aquaculture: Farming aquatic animals and plants. ISBN: 9781405188586. 276-294.
Luis Gerardo Rosado Loría, 2017, MSc Thesis Plant Production Systems ”Modelling nitrogen uptake, fish growth, plant dry matter and water quality in an aquaponic system producing Lactuca sativa and Leuciscus idus” Wageningen University
Marian Barbu, Emil Ceangă and Sergiu Caraman, 2016, Water Quality Modeling and Control in Recirculating Aquaculture Systems, http://dx.doi.org/10.5772/62302 pg. 110
Meade, A., Soilless lettuce in the South Pacific, The Growing Edge 13(4), 2002, Pages 40-43.
Mekonnen, M. M., & Hoekstra, A. Y., 2012. A global assessment of the water footprint of farm animal products. Ecosystems, 15(3), 401-415.
Mirela Crețu , Victor Cristea, Lorena Dediu, Stefan Mihai Petrea The Influence of Different Stocking Densities on Biochemical Composition of Rainbow Trout Meat Reared in a Recirculating Aquaculture, System Animal Science and Biotechnologies, 2014, 47 (1)
Mirela Mocanu (Crețu), V. Cristea , Lorena Dediu , Elena Bocioc , Rodica Iulia Grecu , Sandița (Plăcintă) Ion , I. 2013, Vasilean The effect of probiotic diet on growth and hematology parameters of rainbow trout (Oncorhynchus mykiss, walbaum 1792) Lucrări Științifice-Seria Zootehnie, vol. 59
Mizanur R, A Yakupitiyage and SL Ranamukhaarachchi, 2004. Agricultural use of fish pond sediment for environmental amelioration. Thammasat International Journal of Science and Technology, 9(4):1-12.
Okemwa E. 2015. Effectiveness of aquaponic and hydroponic gardening to traditional gardening. Intl J Sci Res Innov Technol 2 (12): 2313-3759.
Orsini F., Kahane R.C., Nono-Womdim R., Gianquinto G., 2013. Urban agriculture in the developing world: a review. Agron Sustain Dev 33(4):695–720.
Petrea, St. M., Cristea, V., Dediu, L., Contoman, M., Ion (Placinta), S., Mocanu (Cretu), M. and Antache, A. 2013b,; Study of nitrogen cycle in an integrated aquaponic system with different plant densities; 2013; Bulletin of University of Agricultural Sciences and Veterinary Medicine Cluj-Napoca. Animal Science And Biotechnologies Volume. 70(1), Pages 55-64.
Petrea, St. M., Cristea, V., Dediu, L., Contoman, M., Antache, A., Coada, M. T., Ion, S., Bandi, Al. C., 2014, A study of phosphorus and calcium dynamics in an integrated rainbow trout and spinach (Nores variety) aquaponic system with different crop densities; Bioengineering of animal resources Symposium; Animal Science and Biotechnologies, Volume 47 (2), Pages 196-206.
Petrea, St.M., Cristea, V., Dediu, L., Contoman, M., Lupoae, P., Mocanu (Cretu), M. and Coada, M.T. 2013; Vegetable production in an integrated aquaponic system with rainbow trout and spinach;; Bulletin of University of Agricultural Sciences and Veterinary Medicine Cluj-Napoca. Animal Science and Biotechnologies, Volume 70(1), , Pages 45-55.
Petrea, Șt M., Cristea, V., Dediu, L, Contoman, M., Lupoae, P., Ion (Placinta), S., Coada, M. T., Enache M. 2013a, A comparison of nitrate level in spinach grown both under different densities in aquaponic system and under natural growth conditions; The Annals of the University Dunarea de Jos of Galati Fascicle VI – Food Technology, Volume 37(2), Pages 47-58.
Pillay TVR. 2004. Aquaculture and the Environment,” 2nd ed, Blackwell, Oxford.
Pinho S.M., Molinari D., de Mello G.M., Fitzsimmons K.M., Emerenciano M.G.C., Effluent from a biofloc technology (BFT) tilapia culture on the aquaponics production of different lettuce varieties, Ecological Engineering, 103, 146-153, 2017, doi.org/10.1016/j.ecoleng.2017.03.009
Rakocy J. 2003. Question and Answer. Aquaponics J 7 : 29.
Rakocy JE. 2007. Ten guidelines for aquaponics systems. Aquaponics J 46: 14-17.
Rakocy, J. E., Masser, M.P., Losordo, T.M., 2006. Recirculating aquaculture tank production systems: aquaponics-integrating fish and plant culture. SRAC , Volume 454: 1-16.
Rakocy, J.E., 1999. Aquaculture engineering – the status of aquaponics, part 1. Aquacult. Magaz, 25(4): 83-88.
Raluca C. (Guriencu) Andrei, Victor Cristea, Lorena Dediu, Mirela Crețu, Angela I. Docan, Iulia R. Grecu, Marian Tiberiu Coadă, Ira A. (Chihaia) Simionov The influence of different stocking densities on growth performances of hybrid bester (Huso huso ♂ x Acipenser ruthenus ♀) in a recirculating aquaculture system AACL Bioflux, 2016, Volume 9, Issue 3. http://www.bioflux.com.ro/aacl
Rogosa, Eli. "How does aquaponics work?". Retrieved April 24,2013.
Roosta, H. R., Hamidpour, M., Effects of foliar application of some macro- and micro-nutrients on tomato plants in aquaponic and hydroponic systems, Scientia Horticulturae 129, 2011, Pages 396–402.
Roosta, H.R., 2014. Comparison of the Vegetative Growth, Eco-Physiological Characteristics and Mineral Nutrient Content of Basil Plants in Different Irrigation Ratios of Hydroponic: Aquaponic Solutions. Journal of Plant Nutrition, Volume 37, Issue11: 1782-1803.
Saha S., Monroe A., Day M.R., Growth, yield, plant quality and nutrition of basil (Ocimum basilicum L.) under soilless agricultural systems, Annals of Agricultural Sciences, 61 (2), 181-186, 2016, doi.org/10.1016/j.aoas.2016.10.001
Salam, M.A., 2012, Raft Aquaponics for sustainable fish and vegetable production from high density fish pond. 5th Bi-annual Fisheries Conference and Research Fair 2012, Bangladesh Fisheries Research Forum, 18-19 January, 2012 at BARC, Dhaka.
Savidov, N.A., Hutchings, E., Rakocy, J.E., 2007, Fish and plant production in a recirculating aquaponic systems: a new approach to sustainable agriculture in Canada, Proc. Int. Conf. & Exhibition on Soilless Culture, Ed. K. K. Chow, Acta. Hort. 742, Pages 209-221.
Seawright, D. E., Stickney, R.R., Walker, R.B., 1998. Nutrient dynamics in integrated aquaculture-hydroponics systems. Aquaculture, Volume 160, Issues 3-4: 215-237.
Seginer, I., 2003. A Dynamic Model for Nitrogen stressed Lettuce. Annals of Botany, Volume 91, Issue 6: 623-635.
Shete A. P. , Verma A. K., Tandel R. S. , Chandra Prakash , Tiwar V. K. i & Tanveer Hussain Optimization of Water Circulation Period for the Culture of Goldfish with Spinach in Aquaponic System Journal of Agricultural Science; Vol. 5, No. 4; 2013 ISSN 1916-9752 E-ISSN 1916-9760
Shete A.P., Verma A.K., Chadha N.K., Prakash C., Peter R.M., Ahmad I., Nuwansi K.K.T., Optimization of hydraulic loading rate in aquaponics system with Common carp (Cyprinus carpio) and mint (Mentha arvensis), Aquacultural Engineering, 72-73, 53-57, 2016.
Shoko AP, SM Limbu, HDJ Mrosso and YD Mgaya, 2014. A comparison of diurnal dynamics of water quality parameters in Nile tilapia (Oreochromis niloticus, Linnaeus, 1758) monoculture and polyculture with African sharp tooth catfish (Clarias gariepinus, Burchell, 1822) in earthen ponds. Aquaculture Research, 6: 1-13.
Somerville, C., Cohen, M., Pantanella, E., Stankus, A., & Lovatelli, A., 2014. Small-scale aquaponic food production. Food and Agriculture Organization of the United Nations. Rome: FAO.
Sri Wahyuningsih, Hefni Effendi, Yusli Wardiatno Nitrogen removal of aquaculture wastewater in aquaponic recirculation system AACL Bioflux, 2015, Volume 8,
Suhl J., Dannehl D., Kloas W., Baganz D., Jobs S., Scheibe G., Schmidt U, Advanced aquaponics: Evaluation of intensive tomato production in aquaponics vs conventional hydroponics, Agricultural Water Management, 178, 335-344, 2016, doi.org/10.1016/j.agwat.2016.10.013
Timmons, M.B., Ebeling, J.M., Wheaton, F.W., Summerfelt, S.T., Vinci, B.J., Recirculating Aquaculture Systems -2nd Edition, Pub. No 01-002, Cayaga Aqua Ventures – N. Y.
Toufique, KA and B Belton, 2014. Is Aquaculture Pro-Poor? Empirical Evidence of Impacts on Fish Consumption in Bangladesh, World Development, 64: 609-620.
Tyson, R. V., Treadwell, D.D., Simonne, E.H., 2011. Opportunities and challenges to sustainability in aquaponic systems. Horttechnology, Volume 21, Issue 1: 6-13.
Villarroel M, Alvariño JMR, Duran JM. 2011. Aquaponics: integrating fish feeding rates and ion waste production for strawberry hydroponics. Spanish J Agric Res 9 (2): 537-545.
Wahome, P.K., Oseni, T.O., Masarirambi, M.T. and Shongwe, V.D., Effects of Different Hydroponics Systems and Growing Media on the Vegetative Growth, Yield and Cut Flower Quality of Gypsophila (Gypsophila paniculata L.), World Journal of Agricultural Sciences Volume 7 (6), 2011, Pages 692-698.
Watson, C.A., Hill, J.E., 2006. Design criteria for recirculating, marine ornamental production systems. Aquacultural Engineering, Volume 34, Issue 3: 157-162.
Wilson, A., Lennard, Ć., Brian, V. Leonard, A comparison of three different hydroponic sub-systems (gravel bed, floating and nutrient film technique in an Aquaponic test system; Aquacult Int Springer Science, Business Media B.V., Volume 14, 2006, Pages 539–550.
Wongkiew S., Hu Z., Chandran K., Lee J.W., Khanal S.K., Nitrogen transformations in aquaponic systems: A review, Aquacultural Engineering, 76, 9-19, 2017, doi.org/10.1016/j.aquaeng.2017.01.004
Part II
Start-up guide for recirculating integrated systems, based on aquaponics techniques
Aquaculture production systems classification
Aquaculture production systems and technological facilities can be classified based on a multitude of aspects, of a technic, technological and ecological nature. It is known that the main technological facilities encountered in an aquaculture production system are the rearing units, and these are classified into the following categories:
Ponds (diked, excavated, dammed);
“Raceways” – canal-like, long rearing units, with an intense water flow (based on the design, placement and water flow, these are the following types: single water passage “raceways”, parallel or in-series “raceways”, floating/”in pond” “raceways”);
Tanks of different shapes (circular, octagonal, rectangular) made of concrete, fiber glass, plastics, metal, etc.;
Mesh holding structures (floating cages, enclosures/pens);
Shellfish aquaculture specific installations (floating pontoons, floating grates, floating trays, etc.).
Regarding production systems, the literature mentions numerous criteria for classification.
By type of rearing unit within the production system:
Pond aquaculture;
“Raceway” aquaculture;
Recirculating aquaculture systems (RAS);
Floating cages aquaculture;
Mesh pens aquaculture.
Relative to the rearing intensity level:
Extensive, semi-intensive;
Intensive;
Very intensive.
By the placement of the technological facilities:
Terrestrial;
Aquatic;
Transition.
depending on the complexity of technological management and water quality control the production systems are:
Opened, semi-controlled (ponds, “raceways”, mesh retention structures: cages and pens);
Controlled (systems with serial water reuse, systems with partial water reuse, recirculating systems).
after the production-environment system relationship, respectively the possibility of impact control of the production system on the environment:
Opened (mesh retention structures, shellfish aquaculture specific installations);
Semi-closed (ponds, “raceways”, other tank-type rearing units);
Closed (recirculating systems).
according to the way of water management in a production system:
flow-through aquaculture systems (the water passes once through the rearing units and it is entirely discarded in the waterway from whence it was collected);
Partial water reuse aquaculture systems – PRAS (a part of the rearing units effluent is retained and recycled to reuse the water);
Recirculating aquaculture systems – RAS (the production system effluent is entirely recycled and reused).
By water salinity:
Freshwater aquaculture systems/continental aquaculture;
Saltwater aquaculture systems/marine aquaculture;
Brackish aquaculture systems/brackish aquaculture.
By the reared species:
Cyprinids aquaculture;
Salmonids aquaculture;
Sturgeon aquaculture;
Mollusks aquaculture;
Crustaceans aquaculture;
Algae aquaculture, etc.
Considerations related to the profitability of aquaculture and/or reduction of the environmental impact has led to its integration/association with different plant and animal systems. As such, the following production systems can be identified:
Aquaponics systems;
Multi-trophic integrated aquaculture systems;
Partitioned aquaculture systems;
Plant growing and aquaculture integrated systems;
Animal rearing and aquaculture integrated systems;
Substrate-based aquaculture systems;
Periphyton-based aquaculture systems.
Thus, the industrial aquaculture activity operational management differs mainly depending on the production system that is being used.
Recirculating aquaculture systems SWOT analysis
Examples of configurations for recirculating integrated systems which uses aquaponics techniques
First design (Recommended for hobbyist/family production systems)
Figure 1. Recirculating integrated aquaculture system with for hobbyist/family use
Second design (Recommended for small scale production systems)
Figure 2. Recirculating integrated aquaculture system with aquaponics module placed after the biological filtration unit
First design (Recommended for medium scale production systems)
Figure 3. Recirculating integrated aquaculture system with aquaponics module placed above fish growing units and communicating directly with them.
Third design (Recommended for large scale production systems)
Figure 4. Large scale recirculating integrated aquaculture system with aquaponics module placed after the biological filtration unit
Processes in recirculating integrated aquaculture systems, based on aquaponics techniques
Sediments removal
Introduction
Suspended solids removal represents a primary objective in the design of water treatment schemes within recirculating systems. Numerous organic particles that can be found in the rearing environment strongly influence the noxious substances content and the oxygen content, leading to serious diseases of the biological material within the rearing systems, especially in the case of those with a high level of water reuse.
The impact caused by the suspended solids within the water consists in the following: causing lesions to fish, mechanical clogging of the biological filters, increasing ammonia levels due to the nitrification processes, and the increase in oxygen consumption needed for organic matter decomposition. As a consequence, solid wastes are an important factor in limiting productivity, especially in the case of systems those with a high level of water recirculation. The EIFAC recommended maximum allowed concentration for solid particles content is 15 mg/L.
There are three ways of approach in regard to the suspended solids control within recirculating systems:
Sedimentation and micro-screen filtration – insures the removal of large particles, but are inefficient for fine particles (< 50 µm); are recommended for systems that function with limited water reuse;
Granular media filtration – is efficient for the control of suspended solids under 50 µm and is recommended for systems with a high degree of water reuse or for systems with special demands regarding water transparency; just like the micro-screens, the granular media filters require a small space, but, in general, lead to a much higher loss of hydraulic load than in the case of settling tanks (sumps) or micro-screen filters;
Special physical and chemical processes – consist in the use of special installations (hydrocyclones, activated carbon filters, foam fractionators, ozonizers, porous filters); these are not used separately, they, usually, constitute complementary filtration to the processes previously presented.
Within a recirculating system, the total suspended solid (TSS) particles with a diameter greater than 1 µm, are an important parameter for environment quality assessment. The concentration of these TSS must be maintained, through an adequate management, within optimum limits for each reared species. For example, for salmonids, it is appreciated that fine particles (5-10 µm) accumulation can have lethal effects.
From a chemical point of view, the suspended solids are both of an organic and inorganic nature. The organic part, known as volatile suspended solids (VSS) determine oxygen consumption. The inorganic part, contributes to the forming of sludge deposits, with negative effects on the habitat.
From a physical point of view, the suspended solids can also be categorized as sedimentable (> 100 µm) and unsedimentable (< 100 µm). As the size of the particles decreases, the efficiency of the removal processes is reduced.
The removal of the suspended solid particles within an aquaculture production system is realized through known processes of solid/liquid phase separation, namely gravitational separation, mechanical filtration and flotation.
The gravitational separation is based on the sedimentation principle in settling tanks (sumps) or from case to case in hydrocyclones.
The mechanical filtration consists in the retention of the suspended solid phase as water passes through screens, granular media or porous media.
Through the flotation procedure, the solid particles are attached to the surface of air bubbles introduced within the water under pressure and later removed from the system through specific processes.
In all these processes, the solid phase separation consists in the passing of these particles through a separation surface. The separation surfaces are represented, according to the previously presented processes, by the bottom of the settling tank, the external surface of the filtration medium, respectively by the air bubbles.
4.1.2. Gravitational separation
Gravitational separation consists in the gravitational removal of the solid particles from the water of an aquaculture production system. There are three gravitational separation procedures, namely: sedimentation, centrifugation and hydrocyclone separation.
Sedimentation
This represents the simplest method used for solid macroparticles control. The procedure consists in the passing of the suspended solids loaded water, at a low flow rate, through a specially set tank (sump) with the purpose of decanting it.
The settling tanks can function in a continuous or discontinuous regime, depending on the degree of suspended solids accumulated in the water, the granulometric composition of the solid particles, and the technologic flow rate. For RAS, continuous flow rate settling tanks are most common.
Continuous flow rate settling tanks, usually rectangular in shape, are compartmented in four zones (fig. 5) each of these having specific functions regarding the access of technological “dirty” water, assuring the settling conditions, accumulation of the settled solids and evacuation of the technological “clean” water.
Figure 5. Continuous flow rate settling tank (Timmons et al., 2002)
The inlet zone assures an even distribution of the suspended solids along the entire surface of the tank. The settling zone, through its morpho-dimensional characteristics, insures the actual solid phase separation process through settling. From the sludge accumulation zone, solids are removed periodically. The settled water is collected at the tank outlet zone level, over its entire cross-section and is discharged through the top.
Centrifugation
Centrifugation is a procedure that, mainly, consists in the separation of solid particles from a system with the help of centrifugal forces induced by the system’s rotation around its own axle.
In this way, the separation process is substantially intensified, the dimensions of the solid particle control system and the time needed for suspended solids removal being considerably reduced.
From a constructive and functional point of view, the centrifugation separation installations can have a continuous or intermittent flow rate. For the treatment of aquaculture wastewater, the continuous flow rate centrifuges are used (fig. 6).
Figure 6. Conical scroll-type continuous centrifuge (Wheaton, 1985)
The parameters that influence the functioning and the efficiency of the centrifugal installation are the rotation speed, solid material (TSS) concentration, the system’s clean water discharge section size, and the thickness of the water layer maintained along the wall of the installation. The clean water outlet section is adjustable according to the degree of solid wastewater loading and the TSS concentration required for the cleaned water.
Hydrocyclone separation
Hydrocyclones also function on the principle of centrifugal separation of the suspended solid particles in a liquid medium as a result of amplification of the weight difference between liquid and solid phase under the action of centrifugal force. The separation efficiency is determined by the difference between solid particle density and water density.
The figure (fig. 7) shows the design schematic of a continuous flowing centrifugal installation and the principle of its operation. It is noticed that wastewater penetrates tangentially at a certain speed at the top of the installation. The specific way of the inlet water determines the general vertical spiral motion of the water in the installation.
The circular motion of the water causes the heavier solid particles to move toward the hydrocyclone wall. The specific spiral centrifugal motion of the mass of water within the hydrocyclone causes a downward current in the peripheral area adjacent to the walls and an upward current in the central area. Thus, the suspended solids, centrifuged in the wall area, will be trained towards the base of the hydrocyclone and discharged.
The efficiency of a hydrocyclone used to control solid particles in a recirculating system depends on its constructive characteristics, the speed of inlet water in the hydrocyclone and the suspended solids (TSS) concentration in the water.
Hydrocyclones, relatively small size equipment, are widely used in industry due to their low cost. The main drawback, which restricts their use, is the high electricity consumption needed to drive pumps that have to provide high pressure and high water speeds in the installation.
Figure 7. Operation of hydrocyclone (Wheaton, 1985)
4.1.3. Mechanical filtration
Mechanical filtration is a basic process used to control solid particles in recirculating systems. The mechanical filters separate the solid phase from the liquid phase when water passes through a filter medium based on the difference between the particles size of the solid phase components.
Mechanical filters provide an easy way to operate and are relatively easy to maintain under the conditions of careful design and exploitation. Mechanical filters can be made in various types of variants depending on the capacity of the growing systems and the loading degree of solids in water (TSS).
Mechanical filtration does not ensure total removal of solid particles, very small ones are to be eliminated by applying specific procedures.
Operating costs of mechanical filters are appreciable if the concentration in the TSS is high, which is why mechanical filtration must be preceded by a gravitational separation.
Depending on the nature of the filter medium, the following mechanical filtration processes are most commonly used in aquaculture:
screen filtration;
granular media (GM) filtration;
porous media (PM) filtration.
Screen filtration
The process consists in the passage of wastewater through a site system that retains and eliminates most of the solid particles. The screens are sized according to the solids water loading (TSS) and granulometric composition.
The main advantage of mechanical screen filter is the recording of low load losses at the passage of water through the filter medium comparable to those recorded for gravitational separation.
Mechanical screen filters also show some inconveniences that must be known for a careful exploitation. Thus, small particles cannot be retained, requiring removal by other processes. Also, at a high water flow through the filter, large particles fragment, resulting in fine particles not being retained by the filter. Theoretically, it is possible to remove these fine particles if a proper mesh size is chosen. However, the use of fine webs is limited by some impediments, such as high pressure losses and rapid clogging of filters. In this case, filter cleaning costs can greatly reduce the efficiency of their use.
Depending on the way of operation and the constructive solution, there are three types of screens: stationary, rotatory and vibratory.
Stationary screen filters
They are the simplest mechanical filters. In the simplest form, the screen is placed perpendicular to the liquid flow (fig. 8). In this way, particles larger than mesh size are retained and collected by it.
Stationary screens are seldom used for particles smaller than 1.5 mm in diameter, or when the concentration of water in the TSS is too high because there is a risk of rapid clogging in this case.
The maintenance of stationary sites consists of their periodic removal and their countercurrent washing with a strong water jet; if the degree of clogging is high and the adhesion of the solid particles to the screen is strong, the screens are mechanically cleaned with the brush or by other means.
The screens can be made of various materials resistant to corrosive water (steel, brass, stainless steel, textile fabrics, and plastics) in a wide mesh size dimension ranging of microns or millimeters.
Figure 8. Stationary screen filter (Wheaton, 1985)
Rotary screen filters
Rotary screen filters are designed to reduce the clogging potential which is a major disadvantage of stationary screens.
Constructively, such a filter is composed of a drum provided with a screen on the outer surface. The rotary drum is partially immersed in the wastewater which flows through a prismatic enclosure.
Continuous rotation of the drum causes the immersed part of the screen to filter solids, and the upper part periodically passes in front of a mounted flushing system. The washing process, permanently and automatically, ensures continuous operation, with minimal hydraulic system losses and low labor need.
Specific to rotary screen filters is the fact that large amounts of water with a significant amount of solids must be recycled before being discharged from the system.
Depending on the way of water access to the filter section, there are two types of rotary filter filters, namely axial filters and radial filters.
Axial rotary screen filters
An axial filter with a rotary screen is made up of two chambers, the screen being at the level of the septum. The wastewater, enters the first chamber, passes axially through the screen and arrives, in filtered form, in the second chamber where it is discharged. The rotation of the screen causes the partially immersed portion to intermittently face a washing mechanism where a pressurized downstream water jet washes the retained solid particles; the water loaded with washed material is taken up by a trough placed on the upstream side of the screen and discharged (fig. 9).
Figure 9. Axial flow rotary screen (Wheaton, 1985)
Rotary screen axial filters are relatively inexpensive, easy to operate and maintain. They can be washed automatically and efficiently filter out wastewater with a higher concentration of solid suspension (TSS) than stationary screen filters.
The screens used in an axial rotary filter are circular in shape. This is a major disadvantage because, given that the water level in the first chamber cannot exceed, for functional reasons, the axis of the screen, the active surface available for flow is determined by the diameter of the screen. In the case of high flows, screens of appreciable diameters and large filtration enclosures are required, also more complicated constructive solutions for washing and discharging solids are required.
Radial rotary screen filters
The radial rotary screen filter consists of a cylindrical drum that rotates around its horizontal axis, being partially immersed in filtered water that is passing through a specially designed tank. The side surface of the drum is the active part of the installation and is a screen.
The suspended solids (TSS) water stream penetrates axially into the drum and is discharged, filtered, in a radial direction through the mesh network of the screen.
Depending on the system of washing and evacuating solid particles retained in the screen, there are several constructive variants of rotary screen radial filters.
Radial rotary screen filters have the same advantages as axial flow filters. In addition, they are not so restrictive in terms of capacity, as is the case with other filters.
Screens are made from a wide variety of materials, from galvanized steel to fabrics. The choice of the material from which the screen will be made is based on the characteristics of the treated water, the adopted construction solution and the screen mesh size.
Figure 10. Radial flow rotary screen filter with backwash (Wheaton, 1985)
Chain-type rotary screen filters
Constructively, this type of filter consists of a screen in the form of a funicular strip made up of an assembly of articulated panels (fig. 11).
The screen is made in articulated variant to be rotated by the wheels mounted on the drum spindle. Since it is difficult to obtain a perfect joint between the screen panels and between the drive and screen chains, the use of this system is limited, being effective in the case of coarse materials. The advantages of this type of filter consist in the fact that the size of the filter surface is appreciable and the operating costs are reasonable. The functional characteristics of chain-type rotating screen filters recommend them to equip the water intakes of aquaculture rearing systems.
In most constructive solutions, the cleaning of rotary screens is accomplished by continuous flushing with a water jet. For this reason, water losses used for washing can reach significant costs, which is a major drawback of these types of filters. Rotary screen filters are recommended for retaining small solid particles up to 3 μm. The minimum size that can be retained under effective conditions is limited by the complexity of the plant and the amount of water required for washing. The yield of the filter is determined by the mesh size, the size of the active surface, and the amount and characteristics of solid particles in wastewater.
Figure 11 – Chain-type movable screen filter (Wheaton, 1985)
Vibratory screen filters
Vibratory screen filters are special installations where liquid and solid phase separation is carried out on filter surfaces to be printed in order to intensify the filtration process, a vibrational movement in the horizontal plane.
Depending on the direction of the water inlet into the installation, the vibratory screens are of two types: axial drainage and radial drainage. The most commonly used in the aquaculture of the recirculating systems are those with axial discharge (fig. 12).
Figure 12 – Axial flow vibratory screen (Wheaton, 1985)
Wastewater loaded with solid particles moves at a certain speed along the vibratory screen where the separation of the two phases occurs which are collected and discharged from the system. The vibration motion is performed with electromagnetic vibrators having amplitudes and fixed or adjustable frequencies that are determined by the concentration and characteristics of the suspended solids.
In the case of axial vibratory filters, it is important to ensure an optimal correlation between the feed rate and the kinematics of the site so that the moisture content of the retained and removed solids is small enough to reduce the water loss in the system as much as possible.
Granular media filtration
This type of filtration involves passing the waste water stream through a layer of granular material (medium) and retaining the solid particles on its contact surface; the most commonly used granular material is sand, but under certain conditions, other filtering agents (most often plastic floating beads) may be used. Granular filters work gravitationally or under pressure and the direction of water circulation through the filter medium can be upward or downward ((fig.13).
Sand filters
Sand filters consist of a layer of sand or other mineral granular material (gravel) through which water passes. Filtration is a mechanical process that results in retaining solid particles at the surface or pores of the filter medium, depending on their size.
The maximum particle size that can be retained in the filter is conditioned by the size of sand grains generally ranging from 2.0 to 0.02 mm. In order to ensure the retention of smaller particles in the order of the microns, very fine grained clay, clay or similar materials may be used; In this case, the flow rate of the water through the filter medium is very low, reducing the efficiency of the filter. As a rule, sand filters completely remove only particles larger than 30 μm in diameter.
Figure 13. Gravity flow sand filter (Wheaton, 1985)
The discharge rate through the filter and its clogging intensity are dependent on the particle size of the filter medium and the concentration or characteristics of the solid particles in the wastewater. Filtering media with fine grain yields low drainage speeds. The higher the concentration of TSS, the faster the filter will clog, and more frequent washing and, implicitly, high washing water demand will be required.
Sand filters can work gravitationally or under pressure; the choice of one of the two functional types is based on the concentration of the TSS in the waste water and the technologically clean water flow (fig.14).
Figure 14 Pressure sand filter (Wheaton, 1985)
Bead filters
The filtration medium for these filters is a layer made up of 3 to 5 mm diameter plastic floating beads. Due to the subunit specific weight, the plastic, lighter than the water, forms a compact bed at the top of the filtration chamber which, by complex adsorption processes, retains the solid particles in the wastewater.
Depending on the TSS water load and the construction features of the growing system, the bead filters can operate free or under pressure and the water flow direction in the filter can be descending or ascending.
The size and density of the beads are determined by the nature of the solid particles and the concentration in the TSS.
The plastic bead filters have the advantage of a high reliability and the possibility of applying some easy washing procedures.
Porous media filtration
Porous media filters are used to remove smaller solid particles that cannot be effectively retained by other processes.
The operating principle of these filters is similar to that of the screen filters and consists in passing the waste water through a microporous structure filtering medium in which suspended solids are retained. Unlike screen filters, porous agents are characterized by lower filtering speeds and higher pressure losses.
In porous media filters, the thickness of the filtering layer is higher than that of the screen filters, and the pore size is smaller than for regular granular filters.
The most commonly used porous agent is diatomaceous earth (DE).
Figure 15. Diatomaceous earth filter system (Wheaton, 1985)
Diatomaceous earth filters (fig. 15) are mechanical filters that are used for rearing systems that require a high technological requirement in terms of clarity and degree of microorganism load in the water. There are a multitude of diatomite earth categories used as filter media, the finest of which is capable of retaining solid particles with diameters up to 0.1 μm.
Physico-chemical processes
The physico-chemical processes accomplish the retention of fine solids as well as dissolved solids, mainly through adsorption processes. Adsorption can be defined as a process of accumulating or concentrating a substance on a separation between two phases. In waste water treatment, adsorption usually takes place at the level of the separation between a liquid and a solid or the separation surface between a liquid and a gas.
The most commonly used methods for treating waste water in recirculation systems by physico-chemical processes are: adsorption at the level of the separation surface between a liquid and a solid (active carbon filters and resin ion exchange filters) and adsorption to the separation surface between a liquid and a gas (foam separator).
Active carbon filtration
The process consists in the passage of waste water through an active carbon layer where adsorption resolves the dissolved substances. The dynamics of the adsorption process is directly proportional to the decrease in surface tension at the level of the solid-liquid separation surface, which in turn is directly proportional to the concentration of surfactants represented by the solutes dissolved in water.
The efficiency of chemical filtration depends mainly on the size of the contact surface available for the adsorption process. Active carbon is the material that satisfies this demand. The carbon activation process consists in the formation of a very large number of cracks in its structure by specific (thermal or chemical) processes, to which corresponds a significant contact surface.
Factors influencing adsorption on carbon
Adsorption on carbon depends on many factors whose influence is often simultaneous and difficult to quantify. The main factors that influence this process are: the size of the contact surface, the solvent's characteristics, the water reaction, the water temperature, the degree of diversity of the solvents.
The contact surface is the most important factor influencing the adsorption process. As a surface phenomenon, the adsorption level is determined, as mentioned, by the size of the contact surface available to the adsorption process. The size of the contact surface depends on the starting material from which the activated carbon is obtained, the type of activation process used to produce it and the size of the constituent particles. Thus, activated carbon can be obtained from various materials: bones, coal, wood, walnut shells (especially coconut and peanuts), sawdust, processing waste and agricultural waste.
The solubility properties, the molecular weight and the ionic characteristics are the characteristics of the solvent that influences the adsorption dynamics (as velocity and efficiency). The law of Lundelius, the first of the qualitative laws defining the influence of solvation characteristics on the adsorption process, establishes that the degree of adsorption of the solution in a solution is inversely proportional to its solubility.
The water reaction (pH) frequently influences the adsorption of certain ions, partly because the hydrogen and hydroxide ions tend to be strongly adsorbed, and on the other hand because the pH influences the degree of ionization of the acid solutions and basic. The ionization state of these solutions influences adsorption. In general, organic pollutants in water are better adsorbed to lower pH.
The effects of the temperature on the adsorption process are both direct and indirect. Since the adsorption process is exothermic, low temperatures tend to favor higher adsorption rates. Indirect influences refer to the influence of temperature on viscosity and water density. However, the temperature in ordinary aquaculture systems has a relatively insignificant influence on adsorption on activated carbon.
The degree of diversity of solvates in water from aquaculture systems is also a factor influencing the dynamics of carbon adsorption. It has been shown that the presence of more than one dissolved in water decreases the adsorption rate of each of them, but increases the total adsorption capacity of the active carbon over the value that would have been achieved in the case of a single solvite substance.
Types of activated charcoal filters
From a constructive and functional point of view, charcoal filters can be intermittent or continuous.
The activated charcoal filter with intermittent flow regime consists of a granular activated charcoal tank flooded with wastewater. Typically, the water-coal mixture is agitated to speed up the adsorption process. For a specified period of time, called contact time, the water-charcoal mixture is left at rest, after which the purified water is drained and the sedimentary coal is either removed or reactivated. Active charcoal filters with intermittent flow mode function properly for relatively small systems. It has the disadvantage of requiring high operating expenses to have a limited flow.
Active charcoal filters with continuous flow mode can be classified according to flow direction in ascending flow filters and descending flow filters.
The first system uses a granular activated charcoal pool similar to the intermittent system. Waste water, introduced continuously at the bottom of the basin, passes through the charcoal bed and is discharged to the top of the tank. In this case, contact time is dependent on the ascent water speed as well as on the height of the coal bed. Upward flow filters can use either granular or powdered charcoal.
The second type of continuous flow filter exposes an activated charcoal bed (usually a column) to a descending stream of wastewater. This system eliminates oversized basins and is frequently used to treat municipal waters.
Filters with ion exchangers
Ion exchange is an electrochemical process that consists of exchanging ions between two substances, usually a solution and an insoluble solid in that solution. Ion exchange is not only an adsorption phenomenon on a surface, but also involves the three-dimensional internal structure of solid phase molecules. Ion exchange is, in these conditions, a complex process of both adsorption and absorption.
Ion exchange resins can be classified, depending on the mode of production and their characteristics, into one of the following four groups: very acidic cations, weak acid cations, very basic anions, weak basic anions. There are also some materials (clays and zeolites) in which ionic exchange occurs naturally. Choosing the right resin to retain a particular component depends on its properties.
Foam Fractionation (separation)
Foam fractionation is a process of separating or concentrating the dissolved materials by adsorbing one or more solvents to the surface of the air bubbles passed through the solution to be treated. Thus foam forms on the surface of which are dissolved both dissolved substances and solid particles (TSS) which are fixed on the surface of the bubble.
By removing the foam from the surface of the liquid, solvates and solid particles concentrated in the respective area are eliminated at the same time.
There are several separation techniques based on foam fractionation, some of which apply with good results in aquaculture, such as bubbling and flotation.
Bubble fractionation is similar, with respect to the operating principle, to the foam fractionation process which differs in that no foaming occurs. Bubble fractionation occurs where a surfactant is present but, for various reasons, the solution does not foam. As the solution near the surface of the liquid is enriched with solvent, removing the surface fluid leads to a decrease in the solvent content in the waste water introduced at the bottom of the column.
Flotation-based processes are used for waste disposal, ore purification and microorganisms concentration. These processes consist of bubbling, under a certain pressure, the air into the wastewater which, from a physical point of view, can be considered a liquid-solid mixture. In this case, the air bubbles attach to the solids, the process continuing until the solid – bubble combination has a specific apparent weight less than that of the water, at which point the solids become floating and can be easily removed from the system. The physical principle of flotation consists in the transfer of dissolved substances and fine solid particles from wastewater by adsorption to the gas bubbles.
The operating principle of the installation (fig. 16) consists in pumping the waste water (solvent and solvite) into a circular section column, where air is introduced as a small bubble through a diffuser tube at the lower part.
Figure 16 Basic foam fractionation unit (Wheaton, 1985)
In their ascension move, the bubbles collect the solution they concentrate on the surface of the water. Thus, at the surface of the liquid the bubbles pass into the foam phase, carrying with them the solvite charge and fine solid particles together with a small amount of solvent. The process of continuous foam formation causes it to be lifted into the column until it is forced to pass into the collector from where it is discharged.
The filtered water is collected at the base of the column and the level of the liquid in the column is controlled by a tap placed on the path of the filtered water outlet.
Ozonation
Ozonation is not a proper procedure for separating solvents from wastewater. Ozone helps to control solid particles indirectly by modifying particle size. Ozone, a highly unstable reactive gas, has the ability to fragment high weight molecular organic substances into simpler substances that can be readily biodegraded or retained by precipitation or adsorption.
Ozone is used in many aquarium recirculation systems to reduce turbidity and water color. The effect of ozonation on the change in solid particle size is not yet clearly defined.
Particular attention should be paid to residual ozone in water, which may cause dysfunctions at the gill level and, in certain situations, even the death of fish or other organisms coming into contact with ozonated water.
For these reasons additional research is needed before recommending the use of this procedure for controlling solid particles in aquaculture recirculation systems.
4.1.5. Applications for solid particle control systems
The main criteria in choosing a particular process for TSS control in a recirculating system is to ensure an optimum water quality with respect to the suspended solids content with minimum capital and running costs.
Typically, the processes and installations presented are integrated into complex technological schemes that satisfy the above mentioned need.
A complete technology and related facilities for TSS control are shown schematically in fig.17. Depending on the particle size and concentration, the applied technology may comprise all three phases (pretreatment, main treatment and finishing treatment) or only a part thereof.
The most used pre-treatment installations are sedimentation basins. The main problem to be considered when applying this process is the limited possibility of removing fine particles and the relatively high space requirement.
Also, it is necessary to know that with the removal of sludge from the pre-precipitator a significant solubilization of the suspended solids occurs in the water mass, contributing to the degradation of the water quality in the system. Therefore, sedimentation pretreatment is a recommended procedure for resistant species.
Pretreatment of water in tubular decanters has the advantage that it greatly reduces the technological space and limits the water loss to an appreciable extent, compared to the usual sedimentation basins. However, the high degree of accumulation of fine particles and, implicitly, the degradation of recirculated water constitutes an important restrictive factor for the applicability of these technologies. Biofilter filling, capable of reducing organic matter loading, makes it possible to use these separators to pre-treat waste water in a recirculating system.
The efficiency of solid particle separation is obtained by associating the pretreatment processes with the actual treatment processes. Among the main treatment processes encountered in the practice of recirculating systems, most are based on granular media (GM) filtration.
GM filters are used after the removal of coarse particles by sedimentation, so that the wastewater reached in this treatment stage has a relatively low concentration of TSS. This avoids the clogging of the filters, reduces the frequency of backwashing and, implicitly, the water and energy consumption associated with these operations. Combining pre-settling with the proper filtration with granular agents ensures optimal wastewater treatment under conditions of significant TSS associated with high biomass density in aquaculture rearing systems.
Figure 17 – Solids removal processes and the particle size range in micron over which the processes are most effective (Timmons et al., 2002)
The upward flow filters eliminate the clogging and compaction problems that occur in filters where the flow is descending. The downside of the upward filters is that it involves a difficult and long-lasting, more expensive cleaning. Wet sand filters partially solve the systems with moderate load in the TSS, the deficiencies mentioned above.
For fish species susceptible to TSS concentration, or for some more sensitive stages of their development, a final treatment of waste water, namely “finishing”, is required.
Filters with porous media (PM) and different physicochemical processes can be used for this purpose, the quality of treated water justifying the high cost of these equipment.
Knowing the mechanisms of each solid particle removal process is essential to designing integrated treatment schemes that meet the requirements of each rearing technology.
The wastewater treatment scheme for a recirculating system is primarily conditioned by its carrying capacity, implicit by the nature and level of concentration in the TSS.
NOTE: Regarding the scoring system: Space requirements (where is large and is small); Operational management (Ease of use – where is hard and is easy); Operational performance (where is low and is high); Acquisition and implementation cost (where is expensive and is cheap); Maintenance and operation costs (where is expensive and is cheap). All scoring was done by comparing a technology/equipment with another (within the same category).
4.2. Biological Filtration
2.1. Introduction
Like all living organisms, fish require a clean environment for optimal growth and survival. As fish respire and metabolize feed, toxic metabolites are released into the water column. Metabolite accumulation increasingly degrades system water quality. If inorganic or organic toxins within the water surpass biologically critical levels, fish growth may become inhibited and mortality increased.
To maintain a clean environment in recirculating systems, a combination of mechanical and biological filtration techniques must be employed. Although nitrification can occur throughout the culture system (e.g., in biofilms on pipe and tank walls) (Losordo, 1991), the majority of biochemical reactions pertaining to heterotrophic and autotrophic bacteria occur within biofilters. Biofilters are specifically designed for concentrated bacterial attachment and nitrification via fixed-film processes.
4.2.2. Biofiltration and nitrification
Autotrophic bacteria are credited for performing nitrification (Wedemeyer, 1996). Nitrification is a two-step process, where Nitrosomonas sp. oxidize ammonia to nitrite, and Nitrobacter sp. oxidize nitrite to nitrate. Although less toxic than ammonia, nitrites also are considered toxic to fish, while nitrates (NO3–N), the final oxidized form in nitrification, are considered relatively nontoxic to fish unless high concentrations are sustained for an extended period of time (Spotte, 1979), (fig.18). Since biofiltration is the principal unit process used for treating fish metabolites, biofilters can be considered major components in intensive recirculating aquaculture systems (Libey and Miller, 1985).
To ensure prolonged fish survival, high levels of sustained nitrification must be achieved. Therefore, ecological requirements of the bacteria (Malone et al., 1993) must be met within biofilters for effective nitrification to occur. System water quality and filter design characteristics affect filter environmental conditions. Although a larger number of water quality parameters affect nitrification kinetics, Kaiser and Wheaton (1983) stated that dissolved oxygen, pH, water temperature, ammonia-N concentrations, and filter flow rate are the dominant factors affecting a filter’s nitrification efficacy.
Figure 18. Nitrogen cycle
Nitrification Denitrification
1. Heterotrophic bacteria; 2. Nitrosomonas sp.; 3. Nitrobacter sp.; 4. Plants
4.2.3. Configuration of the nitrification filter
There is a variety of nitrification filters in aquaculture. Depending on the construction solution and the operating mode, the following types of nitrifying filters are distinguished: submersible, drum, disk, fluidized bed and sand.
Submersed (submerged) filters
The sketch of a submerged filter is shown in fig. 19. Its operating principle consists in the passage of water through a filtering layer made up of different materials, the usual mineral aggregates (sand, gravel, broken stone) or granular plastic structures. A distinctive feature of these filters is that the filtering agent is permanently immersed.
Water flow through the filter can be upward or downward. Since the population of nitrifying and heterotrophic bacteria is permanently below the water level, the oxygen required for metabolic activities is represented by oxygen in the water, the atmospheric being inaccessible. The possibility of exclusive use of dissolved oxygen in water is one of the main limits of submerged nitrification filters. Operation of the submerged filter involves making a column of waste water over the filter layer. Filtration speed and filtered water flow, respectively filter yield, are determined by the height of the water column that causes water to flow through the filter at a certain speed. When determining the height of the water column, account shall be taken of the pressure losses occurring in the filter bed which, in the case of this type of filter, are relatively small (5÷60 cm/m).
Figure 19. Submerged filter (Timmons and Losordo, 1994)
Trickling filters
The operating principle of these filters consists in passing water through the filter medium in the form of drops. Water circulation in the trickling filter is descending, similar to that of the submerged filter. In contrast, however, the submerged filter where the water flows into forced mode, the trickling filter circulates gravitationally under the action of its own weight. As a result of this specific flow of water, the filter agent is not flooded, being permanently kept wet. Under these conditions, the filter medium is well aerated, providing enough oxygen for the bacterial population. Airflow through the filter provides most of the oxygen required for the filter (as opposed to saturated water containing up to 15 ppm O2, the air contains 210000 ppm O2). The energy required for the operation of these filters is mainly represented by the energy needed to pump the water at the heights and the required technological flows. There are situations when energy is also consumed and for the introduction of pressurized air into the filter medium by means of compressors.
The thickness of the filter layer on a trickling filter varies between 0.15÷5 m, depending on the nature of the agent used, the design solution, the flow rate and the degree of waste water loading.
Periodically, the trickling filters are counter-current washed to eliminate the solids deposited and restore the physical structure of the filtering agent. The washing frequency is as low as possible, as with submerged filters, as long as the restoration time of the bacterial population (biological film) is appreciable (20÷30 days).
Figure 20 presents the constructional drawing of a trickling filter in which water is uniformly distributed over the filter media.
Figure 20 – Trickling filter (Losordo et al., 1999)
Drum filters (Biodrums)
They consist of a perforated cylindrical drum disposed in a vat through which waste water passes (fig. 21). The drum is filled with a certain type of granular agent having high porosity and/or active surface values. The rotation of the drum is made with an axial shaft. The water level in the drum ensures immersion of half the diameter of the drum. As a result of the rotating motion of the drum, the biological film formed on the granular agent passes alternately, at a certain frequency, both through air and water.
The rotation speed of the drum is such that the duration of the biological film, represented by the heterotrophic and autotrophic bacteria populations, does not affect its development due to lack of oxygen. It is also important that the drum speed does not lead to the maintenance of the biological film longer than necessary to avoid dehydration (drying) of the biological film. In the drum filter due to the specific mode of operation, the oxygen content of the water is no longer as important as in the case of the submerged filter, the concentration in ammonia and nitrite being the one that determines the spin speed of the drum (a lower content in ammonia and nitrite assumes a lower speed, a stronger load of water in nitrogen compounds requires a higher rotational speed).
The dynamics of oxygen consumption in the water is different, depending on the speed of the drum. Thus, in the case of lower rotation speeds, the oxygen in the water is consumed by bacteria before they are exposed to air. At too high a rate of erosion of the biological film occurs through the entrainment of bacteria due to the hydrodynamic action of the water stream subjected to filtration. The rotating motion of the drum causes a turbulent water regime inside the filter, which is why the filtrating agent's clogging rate is quite low. With regard to the energy consumption required for operation, the drum filters have a specific energy consumption higher than the disk filters because of the higher turbulence of the water. In a drum filter the load losses along it are usually 1-5 cm.
Figure 21. Drum filter (Timmons and Losordo, 1994)
Rotating Biological Contactor (RBC)
The operating principle of RBC nitrifying filters is similar to that of drum filters, namely periodically bringing into contact with air the biologically active film. The active surface, on which the nitrifying biological film develops, is represented by the lateral surface of disks disposed spaced on a rotating shaft at a certain speed, in a continuous rotation motion (Figure 22). The optimum distance between discs is, as a rule, approx. 20-30 mm. This distance results in spaces between discs that are small enough to provide an active filter surface as large as possible. The aforesaid distance also provides the vital space necessary for the formation and stabilization of the biologic film on both sides of each disc and optimum hydraulic conditions for the circulation of water between the discs covered with bacteria.
Figure 22. Rotating Biological Contactor (RBC) (Losordo et al., 1999)
The rotational speed of the RBC filters depends, as with the drum filters, primarily on the water loading in ammonia and nitrogen and, to a lesser extent, on the oxygen content of the water.
The turbulence created by the binoculars in the filtering mode of the wastewater is lower than in the case of the drum filter, and consequently the power required to drive the shaft, or the specific power consumption that ensures the operation of this type of filter, are lower than in the case of the rotary drum filter.
For RBC filters, the load losses registered in the water flow direction are almost inexistent, so RBCs are also more effective from this point of view.
When the design of these filters is judicious, especially with regard to the distance between the disks and the speed of rotation, the clogging rate is very low and the washing frequency is low.
Fluidized beds filter
From a constructive point of view, these filters consist of a cylindrical basin whose dimensions (height and diameter) are determined by the waste water flow and its loading into toxic nitrogen compounds (ammonium and nitrates). In both the lower and the upper part, the basin is provided with specific reinforcements that provide access to the wastewater and the effluent. On a perforated septum placed at the bottom of the unit, the filtering agent is placed, on which the biological film will be formed and developed (fig. 23).
Figure 23. Fluidized beds filter (Timmons and Losordo, 1994)
Medium specific gravity filtering agents are used to easily bring in the float and with the finest granularity so that the active surface is as large as possible. The filter agent that corresponds to the highest requirements is fine sand. Waste water is pumped into the bottom of the filter by pumping pressure. In its ascending motion, due to the hydrodynamic drive force, the waste water brings and maintains the floating agent in the form of a fluidized bed. The fluidized bed thickness depends on the physical-mechanical characteristics of the filter agent and the water velocity. As a rule, the fluidized bed occupies a smaller or larger part of the filter height. The water velocity in the filter is set so that the filter agent is kept permanently suspended in the water mass; at low water velocities, there is a danger that the filter agent deposits on the perforated septum and at higher speeds it can be removed from the filter. On the active surface of the fluidized bed a certain biomass is developed which performs the nitrification process. In the case of fluidized bed filters, the oxygen required for the biological film is supplied exclusively by the water stream. Under these conditions, it is important to ensure a certain water velocity depending on its oxygen content. A fluidized bed filter operates efficiently when the dissolved oxygen concentration corresponds to the saturation limit for a particular temperature.
The fluidized bed filters generally require large flow rates on the surface unit and are small in size, being compact. It balances hard and once balanced they have good efficiency. Most of the energy required to ensure their functioning is used for the fluidization of the filtering agent and depends on the flow rate and the amount of load losses.
Bead filter
From a constructive and functional point of view, plastic ball filters are similar to those previously described (fig. 24). The difference between them lies in the nature of the filtering agent, which in this case is represented by plastic balls having a specific subunit weight, thus lighter than water. For the filter layer to have a specific surface area as large as possible, the size of the plastic balls is as small as millimeters (usually 2-4 mm).
Figure 24. Bead filter (Timmons and Losordo, 1994)
The water flow in the filter is upward. The speed of the ascending stream of water to a plastic ball filter may be lower than that of the fluid bed filter since its floating condition is given by the specific subunit weight of the beads, and a certain hydrodynamic drive force or a certain speed is not required minimum limit. The water flow speed in a plastic ball filter is mainly conditioned by the wastewater flow.
Both at the bottom and the top, the filter tank has two perforated plate screens, disposed at different heights. The top perforated plate screen holds the filter balls while running, while the perforated bottom plate screen forms support for the filter agent when the filter does not work or when the water flow is discontinuous.
Periodically, plastic ball filters are washed to restore granularity, or the specific active surface of the filter medium. There are several washing methods. One of these is to periodically discontinue the filter operation and drain it; after restarting, for a short period of time, the effluent is discharged out of the system due to its high suspension content. A second method, which does not require the interruption of the filter function, consists in periodically shaking the ball bed with a mechanical device to displace the excessively developed biological film as well as the retained solid particles; as with the previous method, for some time the effluent is eliminated outward, the filter being connected to the growing system when the water is clean.
It results that plastic ball filters provide mainly biological filtration (control of nitrogen compounds), but also perform a mechanical filtration of solid particles due to the granular structure of the filtering agent.
The plastic ball bed filters are relatively small in size, so they are more economical under certain conditions than other biological filters because the small diameter of the filter agent provides a large active surface.
4.2.4. Factors influencing the biofiltering process
Chemical factors
Water Reaction (pH). The optimal pH value for the nitrification process is 6÷9. Under certain conditions, for a particular type of filter, the optimal range may be narrower, depending on the degree of adaptation of the bacteria from the filters to the water reaction. Nitrification may also occur at pH values that do not fall within that range, as adaptation of bacterial colonies is slow. Experimentally, it has been found that at a pH=6, the intensity of the nitrification process is considerably reduced and at pH=5.5, the nitrifying activity of the biological film usually ceases; for practical reasons it is recommended to maintain the pH of the water to values higher than 6. Increasing the pH value to the maximum limit of the optimal range (pH=9) is also contraindicated since the concentration in non-ionized ammonia (NH3), an extremely toxic product for fish, increases directly in proportion to the degree of alkalinity. For these reasons, the efficient operation of a biological filter requires maintaining the pH of the water to the lower limit of the optimum range for nitrifying bacteria.
Alkalinity. The process of transformation of ammonium into nitrates leads to the decrease of alkalinity. Carbonates and bicarbonates, compounds that make alkaline water, are the nutrient support for nitrifying bacteria. Experimentally, it has been shown that lowering the alkalinity of the carbonate and bicarbonate water can become critical for the development of nitrifying bacteria and biological filter functions, respectively.
Also, it is recommended to maintain a certain minimum alkalinity threshold taking into account that due to acid production in the nitrification process, the pH of the water in the filter tends to decrease.
Oxygen. The rate of nitrification in the filters decreases when the available oxygen for nitrifying bacteria is insufficient. The oxygen concentration limit is dependent on a plurality of variables that refer to temperature, concentration in the organic substance of the influent (wastewater) and the amount of bacteria present in the filter at a given time. The filters contain both heterotrophic bacteria and nitrifying bacteria. The first phase of the nitrogen circuit consists in the decomposition of the organic compounds into ammonia, and in the subsequent steps it is oxidized successively into nitrites and nitrates respectively. This succession of microbiological processes determines that in the first stage of nitrification, heterotrophic bacteria are found to be higher. Subsequently, the Nitrosomonas population participating in the ammonium oxidation will become dominant in the filter. In the last phase the population of Nitrobacter tends to increase, in proportion to nitrite concentration. The dynamics of the above mentioned reactions determine that the oxygen is initially used by the heterotroph and then, successively, by Nitrosomonas and Nitrobacter. Under these conditions, in the case of filters in which oxygen originates only from the supply water, it is quite common that Nitrosomonas and Nitrobacter bacteria no longer have the necessary oxygen, reducing the rate of nitrification. Scientific information on the minimum limit of oxygen concentration in nitrifying filters used in aquaculture recirculating systems is insufficient; the literature indicates that values higher than 2 mg O2/L provide an acceptable level of safety for the operation of nitrifying filters.
In connection with the influence of oxygen on the intensity of the nitrification process, it was found experimentally that it is significantly lower than the influence of the concentration in ammonia and nitrites. So, in the filter activity, the limiting factor will not be the amount of oxygen, but the concentration in ammonium and nitrites, which is the nutrient support of nitrifying microorganisms.
Concentrations in ammonium/nitrites. Experimentally, it has been demonstrated that certain concentrations of ammonium and/or nitrite, considered excessively high, are toxic to the activity of the microorganisms involved in the nitrification process. Thus, non-ionized ammonia (NH3) inhibits the activity of the bacteria of the Nitrosomonas genus at concentrations of 10÷150 mg/l and that of the bacteria of the Nitrobacter genus at much lower concentrations of 0.1÷1 mg/l. As regards nitric acid (HNO2), it is considered as inhibitor at concentrations of 0.22÷218 mg/l.
Low concentrations of ammonia and nitrite, specific to the culture water in the growth units of a recirculating system, are limiting factors of the nitrification rate in the filter since the biological film has insufficient nutrient support.
Organic macroparticles. Organic macroparticles influence the operation of nitrifying filters in two aspects, namely, clogging and providing the specific surface for the support of the biological film.
Particles larger than the pore size of the filter agent will clog the filter over time. Consequently, there will be pressure losses and flow cuts that may lead to the formation of less oxygenated, even anaerobic areas, in the filter, which are unsuitable for the nitrification process. In the case of submersed or "trickling" filters, it is recommended, in order to avoid clogging them with organic particles, that the average size of the pellets of the filtering agent is at least 2 cm. The porosity of the filter agent in this case ensures a specific active surface, available to form the biological film, sufficiently large. This recommendation on the granulometric characteristics of the filter agent does not apply to fluid bed or plastic ball filters, which are not vulnerable to clogging by their specific mode of operation.
Organic macroparticles are a growth substrate for heterotrophic bacteria that compete with nitrifying bacteria for growth. For this reason, it is necessary to keep the organic charge within the filter at an optimum level.
Dissolved organic substances. It has been noted in the waste water treatment process that the removal of ammonia is influenced by organic matter loading in the sense that the rate of ammonia removal decreases with the increase in organic water load. In this regard, it was emphasized that the nitrification rate was maximum at a BOD5/TAN ratio of 0.25; at higher values of this ratio, the rate of nitrification is reduced. It is appreciated that the population of nitrifying organisms becomes stable when the organic load of water expressed by BOD5 is less than 30 mg/l; at higher BOD 5 index values, nitrifying organisms cannot compete with heterotrophic organisms whose growth rate is faster.
Salinity. The filters can operate at almost any salinity regime, from freshwater to salty waters of 40 ppt, provided that the population of nitrifying bacteria is adapted to the specific salinity operating conditions. It is known that nitrifying organisms in freshwater are strongly inhibited by salty waters and vice versa; to a rapid change in water salinity (more than 5 ppt in a few minutes) the nitrification rate decreases. On the contrary, a constant salinity maintains a high rate of nitrification. Experimentally, it was established that the maximum specific rate of growth of ammonium oxide oxidation decreases when the water salinity gradient is higher than 0.04 ppt/h in the case of freshwater, respectively, 0.0028 ppt/hr in the case of salt water.
Rate of gas diffusion. The filter medium of an operational nitrification filter is coated with a biofilm (biological film) as a result of bacterial growth. Bacterial cells develop within this biofilm. Under these conditions, oxygen, ammonia and nitrites must diffuse inside the film and nitrites, nitrates and carbon dioxide must diffuse outward through this film. Adjacent to the biofilm there is a stagnant water layer through which the substances must diffuse to penetrate or exit the cells. Thus, the diffusion process may, under certain circumstances, control the rate of nitrification. The rate of diffusion increases or decreases in proportion to the temperature.
Other chemicals. The literature offers a variety of information on the influence of other minerals and chemicals on the nitrification process. Thus, it is considered that a high level of calcium is necessary for an optimal activity of the bacteria of the genus Nitrosomonas and a high magnesium concentration is essential for the bacteria of the Nitrobacter genus. Phosphates, magnesium, molybdenum, iron, calcium, copper and sodium are considered as stimulants in the nitrification process. On the contrary, copper (over 0.56 mg/l), chromium, nickel, zinc, mercury, silver and some organic compounds (vitamins, amino acids, alcohols, etc.) are known for their inhibitory effect in the nitrification process. Treatment of fish growth systems for therapeutic purposes and in judicious concentrations established with formalin, copper sulphate, potassium permanganate and sodium chloride does not affect nitrification. However, other therapeutic agents may diminish or even interrupt the nitrification process.
Physical factors
Temperature. Regarding the influence of temperature on the nitrification process, there is a wealth of information in the literature, some of them contradictory. There is a consensus about the possibility of gradual adaptation of bacteria to a wide range of temperatures. Thus, there are data demonstrating that bacteria that oxidize ammonium have been isolated, able to withstand -5 °C; optimal growth for these bacteria occurs at 22 °C, while temperatures above 29 °C are fatal. In other cases, the bacteria adapted to 25 °C develop optimally at 30 °C and their lethal temperature is 38 °C. The data suggested suggests that nitrifying bacteria can adapt to a wide variety of temperatures over appropriate periods of time. To deepen these aspects, the effect of temperature in the range 7÷35 °C on nitrification in a drum filter was studied (Timmons and Losordo, 1994).
Reynolds number and RBC rotation speed. It is difficult to characterize and quantitatively describe the flow regime in any type of nitrification filter. The Reynolds number, which expresses the ratio of inertial forces to viscosity forces, is frequently used for hydraulic characterization of flow. The Reynolds numbering relationship contains a characteristic length term that is currently difficult to quantify for the specific flow pattern of a nitrifying filter. Modeling the operation of a nitrification filter implies a good understanding of the Reynolds number for filter flow.
The rotational speed of RBC filters determines the magnitude of the tangential force acting on the biofilm, and in this way the contact between the biofilm and the substrate is influenced. Elimination of ammonia increases as the filter speed increases to a certain point when the relative peripheral speed (relative to water) of the discs is 0.305 m/s; increasing the speed above this level does not improve the efficiency of the RBC type nitrifying filters.
Porosity of the filter. The porosity of a filter is the ratio of the air volume to the volume of the filter medium. A high porosity of the filter agent reduces clogging; filter media with a high porosity coefficient have large, open spaces where solids can easily pass. Also, a large porosity causes low water pressure losses when passing through the filter. It is concluded that a good hydraulic operation of the filter, in terms of clogging dynamics and that of load losses, recommends filtering media with a higher porosity. From the point of view of the specific active surface available, the surface constituting the support of the biological film, fine-grained filler agents are indicated, hence the ones with low porosity. In determining the optimal diameter of the granules of the filter medium, in addition to the above mentioned considerations, it will also be intended to provide the best aeration of the filter, which is particularly important in the case of submersed filters.
Nature and granularity of the filter medium. The filtering agent is a solid material added to the filter to provide growth support for nitrifying bacteria.
In terms of their nature, there are several types of filtering agents used in the nitrification process. Typical filtering agents are various mineral aggregates (sand, gravel, crushed stone, limestone rocks, etc.) as well as various plastic media. The type of filter agent is chosen according to certain criteria, the most important being granularity, specific surface area, specific cost and weight. The mineral aggregate is not recommended as a filter media for drum biofilters, as well as those that require frequent handling due to washing operations. The use of the mineral aggregate as a filter medium is advisable primarily because of the generally low cost. An important argument in the use of carbonate rocks as a filter medium is their buffering capacity, which is appreciable until full coverage by the biological film. Compared with mineral filtering agents, the plastics provide some technological advantages, namely reduced specific weight and porosity, respectively, the specific surface area. Plastic structures are more expensive and have no water buffering capacity, but the advantages mentioned are important, justifying their widespread use in aquaculture. In order to ensure proper use during filter operation, plastic filter media should not be exposed to sunlight.
The size of the granules of the filter media (granularity) determines the clogging rate and the amount of pressure loss. This requires a minimum limit below which the particle diameter should not decrease so that the clogging rate is as low as possible and the pressure losses are not excessive.
The nature of the filter medium and its granularity are chosen, differentiated for each type of filter, according to their mode of operation. Thus, in the fluidized bed filters, sand is commonly used as a filtering agent; the small size of the granules provides a high specific surface area of the filter medium and its high specific weight prevents sand from filter removal. With regard to drum and disc filters (RBC), they can theoretically be used with any type of filtering agent. In their case, the main practical problem to consider when choosing the filtering agent is to have a specific weight as small as possible to limit the mechanical stresses that occur during operation. This condition is met in the case of drum filters with plastic shapes of various shapes (balls, plastic rings and so on); For RBC filters, for the same reason, the biodisks will be made of lightweight materials (fiberglass, wood, plastic, etc.).
Active specific area. The specific surface area represents the surface area of the filter agent relative to the volume unit. From a constructive and functional point of view, for a nitrifying filter, the specific surface is an essential parameter as it determines the degree of development of the biological film. The specific surface area of a filter agent depends on its granularity characteristics, the most important being the porosity and the diameter of the constituent particles. Filtering media of low porosity filtering agents, i.e. small granule diameters, provide higher specific surfaces; under these conditions, more bacteria can be developed per volume unit of the filter medium and, implicitly, increases the rate of ammonia removal. However, the diameter of the filter media granules cannot be reduced below a certain amount to limit load losses and clogging rate.
Hydraulic Loading. In the case of submersed, "trickling" or fluidized bed filters, hydraulic loading has the meaning of unit flow being a measure of the amount of water added to the filter per surface unit and time; is usually expressed as a m3/(m2 x day) ratio. For drum or disk filters, the hydraulic load most frequently expresses the volume of water passed through the filter relative to the area of the filter active surface area.
There are two limit values for the hydraulic charge rate of the nitrifying filters, namely a minimum value and a maximum value.
Minimum hydraulic load is determined by the lowest flow rate that maintains the entire filter wet. The lower value of the leakage hydraulic load is more restrictive for submersed filters and "trickling" filters than for drum or biodisks filters. In "trickling" filters, the filter medium must always be wet for bacteria to survive. For submersed filters, the minimum hydraulic charge is imposed by the smallest water flow that ensures the oxygen demand of the filter. For drum or biodisks filters, the minimum hydraulic load is given by the water flow that provides sufficient nutrient intake for the nitrifying microorganisms.
The maximum hydraulic loading of a nitrifying filter is determined by the condition that the flow rate of the water through the filter does not cause the biodegradation of the film or cause excessive loss of pressure (loss of pressure). When determining the maximum hydraulic load, a number of additional restrictive conditions imposed by the specific filter mode may occur. Thus, in fluid bed filters the speed of the ascending stream of water in the filter must not exceed a certain value in order to maintain the filter agent within the filter; drum, or bio-distribute, the hydraulic charge will be set according to the rotational speed so that the biofilm is not dislodged by the centrifugal force.
Depth and submergence. Depth is a constructive feature specific to submersed and "trickling" filters, and submersion is a feature that refers to drum filters and discs that express the depth of immersion in the vat.
For functional reasons, the submergence is considerably less than half the diameter of the drum or discs; this allows an optimal exposure of the drum or discs to the air and at the same time ensures the operation of the shaft of the plant under unsupervised conditions. The literature indicates that a submergence of 35÷50% of the diameter of the drum or discs ensures efficient nitrification; for the mentioned interval there are relations of direct proportionality between the yield of nitrification and the value of the submergence, relations used in the design and exploitation of biological filters.
The depth of the submersed and "trickling" filters depends on a multitude of variables. Following the specific mode of operation or water circulation, the submersed filters require a sufficient depth so as to have the necessary time for oxidation of the organic compounds by the heterotrophic bacteria, a precursor to the nitrification, respectively for the oxidation of ammonia to the nitrate phase by Nitrosomonas and Nitrobacter respectively. Of the plurality of variables contemplated for determining the depth of a submerged filter so as to provide the necessary conditions for carrying out said conversions, the most important are: the nature and granularity of the filtering agent, the hydraulic loading rate, the ammonia loading rate and the Reynolds number. Depending on the variables presented, the depth of the submersed filters and the "trickling" filters is variable, for the same load in organic matter and ammonia, from a few centimeters when the filter medium is sand to 4÷5 m when the filter medium is made of plastic rings of 8 cm. As a general recommendation, it is estimated that the minimum depth for submersed and "trickling" filters cannot be less than 1 m, except for filters where the fine sand filter medium is used. Filters whose filter medium has a particle diameter of 7÷10 cm and a high hydraulic charge can have a depth of 4÷5 m.
Area of cross section. This constructive parameter expresses the cross-sectional area of the upper part of the filter in which no filter medium is found. Its value is important to ensure designed hydraulic load. For drum and disc filters, the cross-sectional area is of lesser importance; for these, the diameter of the drum and discs is important.
Thickness of film. At the level of the biological film there is a thin (thin) film of stagnant water. The optimal metabolic activity of bacteria requires this layer of water to have a certain thickness. Film thickness depends on water speed, viscosity and water temperature. By modeling the water drainage phenomenon through different filtering media, it has been determined that the thickness of the film may vary between 1÷100 µm, most frequently 50÷60 µm.
Light. The results of the application of various nitrification equipment in the aquaculture of the recirculating systems show that bacterial activity is more intense in dark conditions; an intensity of light of even 1% of sunlight inhibits nitrifying bacteria. The negative effect of light on the metabolism of nitrifying bacteria is related to the cytochrome C oxidation phenomenon in both Nitrosomonas and Nitrobacter genes. Nitrobacter bacteria are more sensitive to light than those of the Nitrosomonas because they contain less cytochrome C. Regarding the influence of light on the activity of the decaying bacteria, accumulated experience indicates that complete darkness is preferable to diurnal cyclical light.
Biological factors
Density of biomass. This parameter is a measure of the mass of bacterial cells per unit volume of biofilm. Biomass density depends on availability in nutrients, water speed over biological film, cell characteristics, and so on. The density of the biomass varies considerably from one area to another within the same filter. Also, this parameter is very different from one filter to another. For the different types of nitrifying filters used in aquaculture, an average biomass density of 886 g cells/m3 was determined by appropriate modeling.
Cell production. This biological factor expresses the amount of nitrifying bacterial cells produced by the ammonia-converted nitrate unit. A biomass of 0.17 g of nitrifying bacterial cells is formed per gram of nitrate oxide ammonia. The said rate of growth of nitrifying bacteria, relative to the substrate unit, is lower than the rate of growth of heterotrophic bacteria; this is a major advantage in the biofilters' operation due to the lower rhythm of clogging. A lower value of cell production than indicated indicates a decrease in the efficiency of the nitrification process.
NOTE: Regarding the scoring system: Space requirements (where is large and is small); Operational management (Ease of use – where is hard and is easy); Operational performance (where is low and is high); Acquisition and implementation cost (where is expensive and is cheap); Maintenance and operation costs (where is expensive and is cheap). All scoring was done by comparing a technology/equipment with another (within the same category).
4.3. Aeration and Oxygenation
4.3.1. Introduction
Dissolved Oxygen (DO) is one of the most important water quality parameters that determines, in a limitative sense, the level of the carrying capacity and the density of the population in a recirculating system. The limitation is due to the relatively high oxygen requirement of aerobic organisms within the system, the relatively low solubility of oxygen in the water, the absence of photosynthesis, and the reduced water retention rate.
In order to avoid oxygen becoming a limiting factor, it is necessary to supplement it with respect to the feed ration.
Depending on the size of the carrying capacity of the system, there are a variety of oxygen enrichment methods
Thus, in the case of systems designed for reduced load capacities, water oxygenation is achieved by processes whose principle of operation is to provide a maximum contact surface between atmospheric air and water. For this purpose, specific installations and equipment are used, such as waterfalls in water basins, water jets, paddlewheels surface aerators, etc.
In the case of recirculating systems with a very high load capacity, the supplementation with DO is frequently carried out by passing water through an oxygen-enriched gas installation. This is designed to provide a large contact surface between the gaseous phase and the liquid phase. This type of equipment has as main quality overloading of water with oxygen, while avoiding water loading with dissolved nitrogen (DN). The use of supersaturated water with DO allows for increased stocking density and, implicitly, lower production costs by reducing basin volume, water flow rate through the system, and the capacity of the water treatment unit. Functionally, these systems have the advantage of not introducing nitrogen into the water, thereby providing the possibility of dissolving more oxygen in the water while the total dissolved gas pressure (TGP) is constant.
The management of dissolved oxygen from water to aquaculture farming systems involves knowing a multitude of aspects, namely: the properties of dissolved gases in water; the operating principle, the performance indicators and the design methods of the various types of aeration or oxygenation equipment and installations.
4.3.2. Gas transfer
In the design and operation of aeration – oxygenation equipment it is necessary to know a series of basic notions regarding gas transfer theory, such as: gas solubility, transfer speed (rate) and specific performance indicators.
Gas solubility
The saturation concentration of a water dissolved in water (C*) influences both the direction and the rate of gas transfer at the gas-water interface. The value of the saturation concentration (C*) of a gas is determined by its partial pressure in the gaseous phase, the water temperature and its composition.
Transfer speed
The driving force to which the transfer of molecular oxygen from the gas phase to the liquid is mainly determined by the difference between the saturation concentration of the dissolved oxygen in water (C*) and the oxygen concentration of the environment (C).
In principle, the transfer of oxygen from the gaseous to the liquid phase takes place in several stages. In a first step, the oxygen molecules pass from the gas phase to the gas-liquid separation interface. Further, the oxygen molecules diffuse through the laminar gas and liquid layers of the interface, and ultimately, their permeation into the liquid phase.
Upon their transfer from the gaseous to the liquid phase, the oxygen molecules (nitrogen, carbon dioxide, etc.) have a certain resistance; the maximum value of this resistance is recorded in the gas diffusion phase in the liquid.
Under these conditions, the net transfer rate (absorption or elimination) of the oxygen in the gaseous phase in the liquid phase is directly proportional to the difference between the saturation concentration of the dissolved oxygen in water (C*) and the oxygen concentration of the environment (C).
Performance indicators of the aeration – oxygenation systems
In the practice of aquaculture rearing systems, a variety of equipment is used to enrich the water content of oxygen, depending on the degree of production intensity and the ecotechnological particularities of the crop species. Depending on these, there are, in principle, two systems of equipment to ensure optimum oxygen content of water, namely: air-water contact equipment and pure oxygen input equipment in the water mass; each of these systems is characterized by specific performance indices.
Air – water contact aeration equipment
Air-water contact equipment (air-water contactors) are used in rearing systems with a limited degree of production intensity where the oxygen demand is lower in proportion to the amount of biomass. The main aeration equipment used in aquaculture are: floating surface stirrers, submersible air diffusers, surface stirrers with ejector tube, water jet stirrers, padding aeration columns, air-lift equipment, waterfall perforated trays.
Air-water contact systems are designed to accelerate the absorption of oxygen in the air by various processes, the most commonly used of which consist principally in increasing the water-air contact surface and/or the introduction of air under pressure into the mass of water.
By contacting the water with the air, the partial gas pressures in the air or the partial stresses of the same gases dissolved in the water tend to a state of equilibrium that is reached by the gas transfer between the two phases.
Air stone. Air stones are very inefficient oxygen transfer devices (3–7%), but very inexpensive in terms of capital and operating costs. At low stocking densities and high exchange rates, they work very well at maintaining adequate oxygen levels. One disadvantage is the maintenance requirements due to clogging and biofouling, especially in very hard water (Timmons et al., 2002).
Oxygen-water contact equipment
Pure oxygen-water contact equipment (water-oxygen contactors) is used in densely populated growth systems where the rate of oxygen consumption is very high.
The performance of water-oxygen contactors is assessed using several indicators, namely: oxygen transfer rate (OTR), oxygen transfer efficiency (OTE), oxygen absorption (OA), and oxygen transfer costs (OTC).
When using these types of contactors, it is possible to over-saturate the water with dissolved oxygen.
U-tubes. The U-tube aerator operates by increasing the gas pressure, thus increasing the overall gas transfer rate. It consists of either two concentric pipes or two pipes in a vertical shaft 9 to 45 m deep (fig.25). Oxygen is added at the upper end of the down-leg of the U-tube and as the water/gas moves downward through the contact loop, an increase in hydrostatic pressure increase the oxygen transfer rate. The overall oxygen transfer efficiency is a function of the depth of the U-tube, inlet gas flow rate, water velocity, diffuser depth and inlet DO concentration. Concentrations of dissolved oxygen ranging from 20–40 mg/L can be achieved, but the overall oxygen transfer efficiency if only 30–50%. Off-gas recycling can improve the absorption efficiency to 55–80%. Two advantages of the U-tube are the low hydraulic head requirements that allow operation with no external power if sufficient head is available, and that it can be used with water containing high levels of particulates or organics. Its chief disadvantages are that it does not vent off gasses such as nitrogen or carbon dioxide very efficiently and construction costs can be high, particularly if bedrock is present (Timmons et al., 2002).
U-tubes are designed for flows where the downflow velocity is between 1.8 to 3.0 m/s. A particularly unique problem with U-tubes is that if too much oxygen is added a gas bubble blockage can occur that results in flow interruption. This will tend to happen if gas-liquid ratios exceed 25%. Be careful when adding oxygen (Timmons et al., 2002).
Figure 25 – U-Tube aeration system
Packed Columns. Packed column aerators (PCA) consist of a vertical column filled with media having a high specific surface area. Water is uniformly distributed over the top of the media with a perforated plate or through a spray bar, and trickles down through the media. Oxygen is injected into the column and is transferred into the passing water through the large gas/liquid interface on the media. The column may be either open or closed at the top. Packed columns are efficient nitrogen strippers, but require a higher gas/liquid ratio (forced aeration) for carbon dioxide stripping. Packed columns are simple to build and easy to retrofit into existing facilities. Performance design characteristics include the water distribution method, media characteristics, media bed depth, gas/liquid loading rates, inlet DO concentration and operating pressure. Their main disadvantage is fouling due to the accumulation of organics and particulates on the media over time (Timmons et al., 2002).
Packed columns have two additional advantages (Timmons et al., 2002):
Provide nitrification;
Provide CO2 gas stripping.
LHO’s. Low Head Oxygenators (LHO) are being used more frequently, particularly because of their adaptability to high flows using minimal hydraulic head, hence their name Low Head Oxygenator. The original LHO design was developed and patented by Watten (1989). LHO’s vary in configuration, but all are fundamentally similar in operation. These units consist of a distribution plate positioned over multiple (5 to 10) rectangular chambers (fig. 26). Water flows over the dam boards at the end of a raceway or is pumped upwards from an indoor fish tank, through the distribution plate, and then falls through the rectangular chambers. These chambers provide the gas-liquid interface needed for mixing and gas transfer. The streams of falling water impact a collection pool at the bottom of each chamber where the effluent water flows away from each chamber equally in parallel. All of the pure oxygen is introduced into the outer or first rectangular chamber. The mixture of gases in the first chamber, which now has a diluted oxygen concentration passes, sequentially through the remaining chambers. The gaseous mixture will decrease in oxygen concentration from chamber to chamber as the oxygen is continued to be absorbed. Finally the gaseous mixture will exit from the last chamber. This gas is referred to as off-gas. Each of the rectangular chambers is gas tight and the orifices between the chambers are properly sized and located to reduce back-mixing between chambers (Timmons et al., 2002).
Figure 26 – Low head oxygen (LHO) unit. A typical LHO configuration and components are shown: water flowing into a collection trough or plate (A), through a perforated distribution plate (B), and is oxygenated in the chambers (C), as gas flows from inlet gas port (D), through holes between chamber to chamber (E), to the off gas port (F), where excess gas is bubbled off under water. Water exits at the bottom of the unit (G) (Timmons et al., 2002).
Aeration Cone/Downflow Bubble Contactors. The aeration cone, bicone, or downflow bubble contact aerator consists of a cone-shaped cylinder or a series of pipes with reducing diameters. Water and oxygen enter at the top of the cone, flow downward, and out. As the cone’s diameter increases, the water velocity decreases, until the downward velocity of the water equals the upward buoyant velocity of the bubbles. Thus, the bubbles are held in suspension, until they dissolve into the water. The performance of aeration cones is determined by gas and water flow rates, influent DO concentration, cone geometry and operating pressure. Absorption efficiency range from 95–100% with effluent concentrations from 30 to 90 mg/L. Commercial units are available that transfer from 0.2 to 4.9 kg of oxygen per hour, at 25 mg/L, at flow rates from 170 to 2,300 L per minute (fig. 27) (Timmons et al., 2002).
Figure 27 – Aerator cone (downflow bubble contact aerator) (Stickney, 2000)
Diffused Aeration (Air Stones). Due to their low absorption efficiency, the use of diffusers or air stones have been limited mainly to emergency oxygenation and fish live-haul systems. Although some of the recent fine-bubble diffusers (bubbles 100 to 500 microns) do perform well in deep tanks (50% oxygen transfer efficiency), they require a high pressure source of oxygen (25–50 psi) and are subject to both chemical and organic fouling (Timmons et al., 2002).
Oxygen Injection. The most widely used form of oxygen injection takes advantage of the increased pressure available when pumping water. Oxygen is injected though a Venturi nozzle or orifice, creating a fine bubble suspension in the pressurized line. Pressures of 30–235 psi (2 to 22 atmospheres) are needed to achieve satisfactory absorption, with contact times of 6–12 seconds. Absorption efficiency ranges from 15 to 70% with effluent DO concentration form 30–50 mg/L (Timmons et al., 2002).
NOTE: Regarding the scoring system: Space requirements (where is large and is small); Operational management (Ease of use – where is hard and is easy); Operational performance (where is low and is high); Acquisition and implementation cost (where is expensive and is cheap); Maintenance and operation costs (where is expensive and is cheap). All scoring was done by comparing a technology/equipment with another (within the same category).
4.4. Carbon Dioxide Control
4.4.1. Introduction
Carbon dioxide management is an essential issue for intensive aquaculture systems. If its concentration is not maintained at an optimum level, important ecotechnological changes are made in the system consisting of the following: reducing the water reaction to stress levels, disturbing the fish's respiration mechanism even in the presence of an optimal oxygen concentration, reducing the efficiency of the nitrifying filters.
In the case of less intensive rearing systems there is no tendency for carbon dioxide to accumulate up to the tolerance limit. In aquaculture farming systems where water is re-used or recirculated, carbon dioxide can accumulate up to stressful concentrations. In addition, the increasing use of pure oxygen as a source of dissolved oxygen in intensive systems has led to a decline in conventional aeration methods (mechanical agitation, water flow through obstruction environments: cascades, stuffing columns, etc.) which, due to their specific mode of operation, also provided effective water degassing, including the removal of excess carbon dioxide.
Carbon dioxide is a product of the respiratory activity of fish and other organisms that may be present in a rearing system, the most important of which is bacteria in the filters. An important difference between carbon dioxide and other dissolved gases, such as oxygen and nitrogen, is that carbon dioxide is part of a chemical balanced system and its concentration in solution is influenced by pH. A result of carbon dioxide dependence on pH is the possibility of reducing its concentration by different physical or chemical processes.
Changing the pH of a solution is the primary chemical method of reducing the concentration of CO2 in water, a method whose efficiency is determined by alkalinity, initial pH and temperature.
Physical removal methods are based on the transfer of carbon dioxide from water to an adjacent gaseous phase, usually air. The removal of carbon dioxide causes the pH and concentration of the inorganic carbon to change and thereby to change the share of inorganic carbon in water as carbon dioxide.
The mechanism of carbon dioxide removal from the water is complex due to the interaction between physical gas and chemical transfer processes on the acid-base reaction.
The accuracy of estimating the rate of carbon dioxide removal depends on the extent to which changes in the composition of the gases and the chemical processes occurring within the carbon system are taken into account. Thus, a crude estimate of carbon dioxide removal implies ignoring changes in gas composition as well as chemical reactions occurring at the carbon system level.
Due to the diversity of known physical processes for the removal of carbon dioxide from aquaculture waste waters, the most common, and with very good results, are the transfer of gas in packed columns.
Mainly, chemical processes for carbon dioxide control and carbonate balance consist in applying a specific pH adjustment to water depending on the transformations that take place within the system.
In intensive aquaculture systems, carbon dioxide is the result of aerobic breathing of aquatic organisms, namely the reared biomass and the bacterial biomass. The amount of carbon dioxide produced is directly proportional to the amount of oxygen consumed. A simplified stoichiometric description of breathing results from the reaction:
According to this simplified stoichiometry, one mole of carbon dioxide is produced by each mole of oxygen consumed, and for each gram of oxygen consumed there are 1.27 grams of carbon dioxide produced (Grace and Piedrahita, 1991 cited by Timmons and Losordo, 1994).
Carbon dioxide in an intensive aquaculture system is mostly produced in growing tanks; In addition, a lower amount of carbon dioxide is also produced in biological filters.
In less intensive crop systems with a low degree of water reuse, the required oxygen is normally provided by the oxygen-rich influent water stream and the resulting carbon dioxide is discharged by the effluent water stream; this avoids the accumulation of carbon dioxide in the system.
In the case of densely populated and high-water reuse systems requiring pure oxygen supply, the balance between the carbon dioxide production rate and its removal rate in the system is disrupted and therefore the carbon dioxide concentration increases, requiring controlling it to keep it within optimal limits.
The system's biomass loading rate (system load capacity, kg/L/min) that does not cause a dangerous level of carbon dioxide is dependent on a multitude of ecotechnological parameters such as: water temperature and alkalinity, feeding rate, tolerance of species to CO2 content, etc. Thus, the literature (Piedrahita and Grace, 1991 quoted by Timmons and Losordo, 1994) indicates a carbon dioxide concentration of 20 mg/L at a system capacity of 4.5 kg/L/min.
High concentrations of carbon dioxide influences a recirculating rearing system in the following two ways:
dysregulation of the fish's respiratory mechanism by reducing the blood's ability to transport oxygen;
decrease in the pH value of the water.
4.4.2. Carbon balance and carbon dioxide control by pH management
Carbon dioxide control by pH management consists in changing the equilibrium concentrations of species from the carbonate system.
The balance of carbonated carbon expresses a dependency relationship between pH and carbon dioxide (CO2), carbonic acid (H2CO3), bicarbonate (HCO3-) and carbonate (CO32-).
Carbon dioxide is not a component of alkalinity. Although carbonate and bicarbonate are reduced when carbon dioxide is removed, the molar sum of the alkaline components remains constant. Thus, alkalinity remains constant in both the addition of CO2 in water and its removal from the solution.
Practically, the reduction of carbon dioxide can be achieved by adding to a water a concentrated base solution, such as sodium hydroxide. The basic solution is introduced into the water with a chemical dispensing pump and with a pH sensor system, the process can be automated.
Addition of sodium hydroxide or a similar strong base leads to an increase in alkalinity without causing changes in total carbon carbonate concentration. Increased alkalinity, accompanied by increasing pH and decreasing carbon dioxide levels, causes the rebalancing of carbonate species to increase carbonate and bicarbonate ions. The total carbonated carbon concentration remains, however, unchanged.
An alternative method of chemical control of carbon dioxide in a growth system consists of using Na2CO3 or calcium-containing substances such as Ca(OH)2, CaMg(CO3)2, or CaO. And in this case, the alkalinity of the solution increases, and in some cases also the total carbonated carbon.
The analytical determination of the change in carbon dioxide concentration of an aqueous solution as a result of the addition of one of these substances is based on the carbonate system equation. With an acceptable accuracy, the concentration in carbon dioxide in an aqueous solution can also be determined graphically
The chemical control of the carbon dioxide concentration of a growth system causes, over time, a steady increase in carbonated carbon concentration and alkalinity. This phenomenon requires a periodic replacement of water in the system in order to keep the two parameters in the optimal technological field.
The chemical process apparently ensures an easy control of the carbon dioxide concentration in the culture water in a recirculating system. The application of this procedure in practice, however, presents a number of inconveniences among which the most important are: the high cost of the chemicals used, difficulties in handling the reagents, the induction of negative technological effects, etc.
4.4.3. Carbon dioxide control by gas exchange
Physical processes have a wide use in carbon dioxide management in aquaculture recirculating systems. The main physical process used to remove carbon dioxide from water consists of exchanging gases between two different physical media, namely water and air. Removing carbon dioxide through gas exchange or aeration is a very complex process. Unlike other dissolved gases (N2, O2), carbon dioxide is present in the solution as part of a chemical equilibrium system. Thus, by removing the carbon dioxide from the solution, the configuration of the chemical equilibrium system, with respect to the concentration of the species in the carbonate system, changes, their sum remaining constant. Under these conditions, the bicarbonate, [HCO3-], is constituted in a carbon dioxide reservoir which completes, by dihydroxylation, the dissolved carbon dioxide (CO2 (water)) as it is removed from the solution. The dihydroxylation reaction of carbonate in carbon dioxide is not instantaneous. Therefore, there is a delay in the dihydroxylation of carbonate in carbon dioxide which develops as it is removed from the solution; the magnitude of the delay is dependent on the kinetics of the carbonate reaction.
Gas exchange.
Supersaturated carbon dioxide from an aqueous solution can be transferred to a gaseous phase. Exposure of water to the gaseous phase can be accomplished in various ways, namely: introducing air or pure oxygen into the water, passing water through U injector tubes, degassing columns (PCA) or cascades, using paddles, etc. Since the carbon dioxide concentration in intensive growth systems is typically 20 times higher than the saturation concentration of the environment, the carbon dioxide transferred has a significant impact on the adjacent gaseous phase. Therefore, the gaseous phase of the liquid must be permanently refreshed (replaced) to prevent the accumulation of carbon dioxide, i.e. to increase its concentration to values that reduce the efficiency of gas transfer between the two media. For this reason, maintaining a proper rate of carbon dioxide transfer from water to air requires that the ratio (G/L) between the flow rate of the gaseous phase (G) and the liquid phase (L) be as high as possible. Under these conditions, the amount of gas available to transport the carbon dioxide transferred from the water is high, eliminating the possibility of accumulating it at the interface of the two media. The G/L ratio is the most important variable that causes the removal of carbon dioxide. In the case of degassing columns (PCAs), most commonly used in aquaculture for carbon dioxide removal, it was determined experimentally that the minimum value of the G/L ratio, volumetric, is 3.0. Aerating systems with a G/L ratio of less than 3.0, such as diffusers and injectors, are not indicated for aquaculture rearing systems characterized by high concentrations of carbon dioxide that require high rates of removal. It is appreciated that only judiciously designed paddle aerators and degassing columns (PCA) expose the water to a sufficient amount of air to ensure efficient carbon dioxide removal. The carbon dioxide removal rate was quantified only for compact degassing columns (Grace, 1992); for other aeration equipment, data on carbon dioxide removal rate are insufficient.
Carbon dioxide transfer coefficient
The global mass transfer coefficient (KLa) is usually determined and, therefore, known for oxygen. The global mass transfer coefficient for carbon dioxide is determined by the global oxygen mass transfer coefficient and by the ratio of the molecular diffusivity coefficients of the two gases. Oxygen diffusivity is 2.5 x 10-5 ± 20% atm-1 while carbon dioxide diffusivity is 1.96 x 10-5 ± 1% atm-1 (Perry, Green and Maloney, 1984 quoted by Timmons and Losordo, 1994). The value range for the ratio of diffusivity coefficients is 0.65÷0.99. Even though this ratio is less than 1.00, a mass transfer coefficient for carbon dioxide equal to that of oxygen is recommended when designing degassing columns (PCAs).
Depending on the availability of the data that characterizes the operation of a degassing column (PCA), there are more or less accurate methods to estimate carbon dioxide effluent loading.
An approximate (gross) method for estimating the carbon dioxide removal by aeration in a degassing column is based on the following simplifying assumptions:
the transfer of gas from the liquid does not significantly affect the composition of the adjacent gaseous phase nor the value of the saturation concentration;
no chemical reactions occur in the carbonate system during the transfer.
Characterization of gas flow
It must be taken into account, in addition to the G/L ratio, the characteristics of the gas flow through the column, which can be in the block (laminar), completely mixed (turbulent) or combined (mixed).
The flow regime significantly influences the saturation concentration in different sections of a PCA column. If the gas flow through the column is turbulent, then the saturation concentration is constant over the entire height of the column. In this case, the rate of carbon dioxide removal is directly proportional to the height of the degassing column.
When the flow of gas inside the column is laminar, the molar fraction of gas or corresponding saturation concentration is variable on column height. The gas flow regime in a laminar or turbulent degassing column (PCA) is conditioned by the column dimensions (height and diameter) and the gas flow rate.
For a more accurate determination of mass transfer, assuming a laminar flow regime, it is necessary to take into account the fact that the saturation concentration is variable on the height of the column.
4.4.4. CO2 control by gas transfer combined with kinetics of chemical reactions
It was found that the removal of carbon dioxide with a degassing column (PCA) is the combined result of gas exchange and chemical reactions, both phenomena being produced simultaneously. The operation of the column involves the entry of waste water at the top of the column, with all carbonate species in balance. In the first section of the column, of the carbonate species, only the carbon dioxide is removed as a result of the gas exchange. The removal of carbon dioxide reduces total carbon dioxide and increases the pH by adjusting the carbonate carbon species balance levels accordingly. While the bicarbonate, carbonate and hydroxyl ions instantly reach the relative balance, the balance between carbon dioxide and bicarbonate is reached at a lower rate after a certain period of time. During the flow of liquid from the top of the section to the bottom of it, part of the bicarbonate ions react to form carbon dioxide. Due to the difference between the rate of carbon dioxide removal by gas exchange and the CO2 production by dihydroxylation, the difference between the actual and the carbon dioxide equilibrium concentration appears in the column, a difference which increases the flow rate of the liquid.
NOTE: Regarding the scoring system: Space requirements (where is large and is small); Operational management (Ease of use – where is hard and is easy); Operational performance (where is low and is high); Acquisition and implementation cost (where is expensive and is cheap); Maintenance and operation costs (where is expensive and is cheap). All scoring was done by comparing a technology/equipment with another (within the same category).
4.5. pH Control
4.5.1. Introduction
The functionality of aquaculture growth systems is dependent on the biological nitrification process by which ammonia and nitrites (toxic compounds), are oxidized into nitrates compounds with much less toxicity. The nitrification process results in H+ ions consuming the alkalinity of the water and reducing the pH of the system. To the extent that the pH is allowed to fall, the nitrification rate, thus the system's fish productivity, will record lower values. Therefore, the management of the design and operation of a recirculating system implies a good knowledge of the fundamental relationship between pH and alkalinity, as well as how to use this relationship in order to maintain the water reaction at the optimum values required by the nitrification process.
4.5.2. Alkalinity and pH control
In most of the natural freshwater sources, the dominant water buffering system is the carbonate system. Therefore, in the recirculating systems, the most suitable pH control method is to buffer the culture water with carbonate products. The carbonation buffer system consists of 3 components, namely: carbonic acid (H2CO3), bicarbonate (HCO3-), and carbonate (CO32-). Regarding H2CO3, this exists only in very low concentrations. The sum of the molar concentrations of all these components is denoted by CT and represents the total concentration of inorganic carbon in the system.
CT = [H2CO3] + [HCO3-] + [CO32-]
The carbonate system can be considered "volatile" or "non-volatile" as the carbon dioxide in the water is allowed or not, as a gas exchange, to balance atmospheric carbon dioxide. Mixing conditions and hydraulic residence times are criteria for assessing whether a closed cycle aquaculture system is "volatile" or "non-volatile" in relation to the atmospheric carbon dioxide balance.
Both in the non-volatile and volatile systems the pH equilibrium is controlled by the alkalinity (ALK); the [ALK]-pH correlation for volatile and non-volatile systems is shown in figure 28. The graphical representation of the [ALK]-pH correlation (fig. 28) indicates that at the same system alkalinity, [ALK] = 0.0007 ech/L, the pH of the volatile system is 8.2 and for non-volatile is 6.7.
Most systems approach the behavior of volatile systems, but the atmospheric balance is never reached. The addition or depletion of any component of alkalinity will result in pH change.
Figure 28 – [ALK]-pH correlation for volatile and non-volatile systems
(Timmons and Losordo, 1994)
4.5.3. Nitrification
It is known that biological nitrification of ammonia-nitrogen is primarily accomplished by two types of bacteria. These genes, Nitrosomonas and Nitrobacter, are classified as autotrophs because they obtain energy from inorganic compounds. Nitrification comprises two steps; the first step is mediated by the Nitrosomonas genus, while the second stage, is mediated by the Nitrobacter genus.
The reactions described by previous equations are accompanied by the production of a certain amount of energy. The energy produced is used by nitrifying bacteria in cellular synthesis processes. In both reactions, oxygen is an electron acceptor, the only one that can be used by Nitrosomonas and Nitrobacter. Therefore, the nitrification process requires an aerobic environment for a continuous and normal deployment.
The biomass synthesis reaction for Nitrosomonas and Nitrobacter is:
where C5H7O2N represents the biomass of Nitrosomonas and Nitrobacter bacteria.
The synthesis reaction requires continuous power supply. During the nitrification this energy is obtained from the oxidation of NH4+ and NO2-. The energy produced by oxidizing one mole of NH4+ is less than the energy required to produce a "mole" of bacterial cells (C5H7O2N). All of the three previous equations should be well proportioned so that, after taking into account the energy transfer efficiency, energy consumption is equal to energy production.
When this condition is satisfied, the overall reaction of the nitrification process (Timmons and Losordo, 1994) is expressed by the relationship:
An important result of this equation is that alkalinity in the form [HCO3-] is consumed in the reaction of HCO3- + H+ = H2CO3. The alkalinity consumption rate is 1.98 moles of HCO3- per mole of NH4+ oxidized.
In order to maintain a relatively constant pH of the system, the alkalinity supplementation rate should be equal to the alkalinity consumption rate; otherwise, the omission of alkalinity will ultimately lead to a rapid pH decrease, and at the same time, interrupting the system.
4.5.4. Management of alkalinity and pH
The alkalinity and pH management of a closed cycle aquaculture system involves assessing the rate of alkalinity consumption (nitrification rate) and determining the type and amount of supplement to be used to restore lost alkalinity.
The rate of alkalinity consumption
The rate of water alkalinity consumption in a recirculating system depends on the rate of nitrification of the nitrogen compounds.
To evaluate the rate of nitrification in a recirculating system, the following hypotheses are assumed:
the nitrification is complete to provide optimum media conditions, corresponding to the technological requirements of the reared biomass;
nitrogen from food that has not been assimilated to the fish protein or has not been removed with the removed macroparticles will be nitrified;
volatilization of ammonia is minimal;
nitrification takes place both in the system's growing tanks and outside in the water treatment plants.
Substances used as alkalinity supplements
The criteria used to choose an alkalinity supplement are the degree of solubility, cost and ease of application; Table 2 lists the main substances used as alkaline supplements and their characteristics.
In terms of solubility, some compounds (MgCO3) should be avoided due to their low solubility under the pH conditions specific to aquaculture systems.
Another aspect of solubility refers to the possibility of precipitation of CaCO3 when calcium-based alkalinity supplements such as CaO or Ca(OH)2 are used. Precipitation of calcium carbonate does not pose a problem because the precipitated CaCO3 precipitate forms the alkalinity reserve of the system, a reserve that contributes to pH stabilization in the event of a sudden increase in the rate of nitrification. In this regard, it is worth mentioning that precipitation of CaCO3 is accompanied by a slight decrease in pH.
The potential for overdose is another important consideration taken into consideration when choosing the alkalinity supplement.
Table 2 – Alkalinity Supplement Properties (Bisogni and Timmons, 1991)
Overdosing causes a rapid, difficult to control, pH of culture water above the desired value. NaOH and Na2CO3, strongly reactive compounds, are highly soluble in water. The risk of overdosing when using these substances as alkaline supplements is very high. In the case of NaHCO3, relatively weak and very soluble base, it is difficult to produce an overdose when using this compound. With regard to the potential for calcium-overdose, it is diminished by the high precipitation rate of CaCO3 as pH increases.
NOTE: Regarding the scoring system: Space requirements (where is large and is small); Operational management (Ease of use – where is hard and is easy); Operational performance (where is low and is high); Acquisition and implementation cost (where is expensive and is cheap); Maintenance and operation costs (where is expensive and is cheap). All scoring was done by comparing a technology/equipment with another (within the same category).
4.6. Water Disinfection Methods
4.6.1. Introduction
The main criteria for the technological management of aquaculture rearing systems is to ensure adequate hygiene and sanitation conditions for the reared biomass. The requirement for monitoring the health status of the fish stock is determined by the degree of intensity of the crop and the extent to which it can be satisfied depends on the technical complexity of the system and the value of the initial capital for its realization.
More and more severe restrictions on discharging wastewater from open systems in aquaculture and capitalizing on the natural water ecosystems for their recreational purposes are key arguments for promoting and developing farming systems with closed or semi-closed cycle. These systems usually involve high population densities and the recirculation of as much as possible of the technological water, which are the main risk factors for the occurrence of illnesses. Since closed systems involve high capital and operational costs, much more rigorous disease control is needed than open systems.
The most serious diseases that can affect crop biomass in a closed system are caused by microorganisms (viruses, bacteria, fungi, unicellular organisms). In the culture water of a recirculating system there may be a multitude of pathogenic organisms at different stages of their life cycle. Generally, spores are more difficult to destroy than organisms found in their unsupportive stages.
Controlling diseases caused by microorganisms involves, first of all, the application of effective methods of disinfection or sterilization of culture water. Sterilization consists in destroying all living organisms in a culture medium while disinfecting only some of these, namely pathogenic microorganisms, which are destroyed. Sterilization is rarely applied in the treatment of water in recirculating systems because this process can destroy some useful microorganisms, such as crop feed crops or nitrifying bacteria in the biofilter.
In principle, disinfection can be achieved by various methods based on the lethal action under certain conditions of heat, ultraviolet (UV) radiation or chemical substances on pathogenic microorganisms. Chlorine and ozone are the main chemicals used to disinfect the culture medium in the recirculating systems. Other chemicals, such as certain metal ions (silver ions), various bases and acids, some surfactants and a wide range of oxidizing chemicals (iodine, bromine) can also be used with good results in certain situations, potassium permanganate and hydrogen peroxide).
4.6.2. Chlorination
Chlorine, in various forms, has been extensively used for a long time to disinfect water, especially the waste water. The most common and good results for disinfection of culture water in the recirculating systems were gaseous chlorine (Cl2) and hypochlorite (OCl) in the form of calcium or sodium hypochlorite.
Gaseous chlorine is very soluble in water. Thus, at a temperature of 20 °C and a pressure of 1013×105 Pa (Rich, 1963 quoted by Wheaton, 1985), the chlorine solubility is 7160 mg/L. Chlorine rapidly hydrolyzes into water to form hypochlorous acid (HOCl) as follows:
At low concentrations of chlorine (<1000 mg/L) and at pH<3, the entire amount of chlorine is virtually converted to HOCl.
Hypochlorous acid dissociates in aqueous solutions with the formation of hypochlorite (OCl-) and hydrogen ion as follows:
The dissociation reaction of hypochlorous acid is highly dependent on the reaction of water.
The hypochlorites, such as, for example, calcium hypochlorite, ionize when dissolved in water, according to the reaction:
The hypochlorite ions are in balance with hypochlorous acid.
Chlorine, hypochlorous acid (HOCl) and hypochlorite ions (OCl-) are strongly oxidizing agents. One of the most important compounds that react quickly with hypochlorous acid is ammonia. Reactions between NH3 and HOCl may give rise to monocloramines, dichloramines or trichloramines (also called nitrogen trichlorides), according to these equations:
Chloramines can also result from reactions of hypochlorous acid with organic amines present in the solution. The equilibrium constants, depend on the pH, the temperature of the solution and the relative concentrations of ammonia and hypochlorous acid. The balance of ammonia (NH3) and ammonium (NH4+) is also highly dependent on pH.
Hypochlorous acid (HOCl) and hypochlorite ions (OCl-) are known in the literature as free chlorine and chloramines as combined chlorine. The toxicity of the two forms, both for microorganisms and for evolved aquatic organisms, is different. Thus, it is known that, in its free form, chlorine is more toxic. The combined form is not eliminated as easily as free form, which is why it is intentionally produced, in many situations, in water to generate residual chlorine, recognized for its high residual capacity.
Water in aquaculture systems contains a multitude of compounds of organic or inorganic nature. The introduction of chlorine into such water leads to the formation of residual chlorine.
The use of chlorine to control the development of micro-organisms in aquatic organisms' systems presents high risks for crop biomass. Thus, most fish species are susceptible even at low concentrations of total residual chlorine (0.1÷0.3 mg/L, Kelly, 1974 quoted by Wheaton, 1985). Chloramines, having a degree of toxicity equal to that of chlorine, also require appropriate precautions when used for water disinfection in aquaculture.
Effective use of chlorine as a disinfectant in aquaculture systems requires sufficient long-term contact with the culture water, followed by the rapid elimination of chlorine and chloramines from the growing basin.
The literature (Liu et al., 1971, quoted by Wheaton, 1985) indicates that the contact time required to obtain a 99.99% destruction of viruses at a temperature of 2 °C and a concentration of 0.5 mg/L residual free chlorine, ranges from 2.7 to 120 minutes.
Chlorine and chloramines can be removed from the water by adsorption techniques or chemical processes. Adsorption on activated carbon seems to be effective. Various reducing agents, such as sodium thiosulphate or iron salts, may also be used to remove chlorine and chloramines from water (Wheaton, 1985). In this case, the potential toxic effects of the chemical substances used and / or the resulting reaction products on the culture biomass should be checked. Chlorine can also be eliminated by aerating the water. A vigorous and complete aeration ensures efficient chlorine removal, but there are reservations about the effectiveness of applying this procedure to eliminate all chloramines, especially from water with significant organic matter content.
Due to the recognized toxic effects of chlorine and chloramines on aquatic organisms, their use as disinfectants in aquaculture requires a great deal of caution. Even if the disinfectant is subsequently removed, typically by adsorption onto activated carbon, before the water enters the culture basin, the risk of crop biomass damage is very high in closed or semi-closed systems, especially when the disinfectant is chlorine.
4.6.3. Thermal treatment
The water can be sterilized or disinfected by heating to a sufficiently high temperature and maintaining this temperature for a period of time. Thus, the process of bacterial destruction is a temperature-dependent function and the time to maintain that temperature, parameters which in turn are conditioned by the species of microorganisms (bacteria, viruses, etc.) to be removed. Milk pasteurization and thermal processing of preserved foods are such processes. For example, milk can be pasteurized by maintaining it at 60 °C for 30 min or 71.1 °C for 13 seconds. Heat is an efficient and convenient means of disinfection, since the rate of elimination of microorganisms can be adjusted according to the technological requirement by changing the temperature and / or the time to maintain it.
The main disadvantage of using heat as a water disinfection process is the specific energy consumption that is quite high due to its high specific heat. An important aspect taken into consideration when applying the thermal treatment for water disinfection is that the crop biomass cannot survive the temperature range required for the disinfection process. Therefore, before being introduced into the growing tank, the disinfected water has to be cooled. Water cooling can be done in a number of ways, depending on the technical characteristics of the technological facilities; in general, water cooling is carried out in regenerative heat exchangers that allow the recovery of a significant part of the energy consumed during heating.
The energy expenditure required for disinfection of heating water is a major criteria for the management of a recirculating system, which is why it needs to be assessed as accurately as possible. Energy consumption for water heating depends on the amount of recirculation flow, the proposed thermal gradient and the heat recovery efficiency for preheating.
Obviously, the cost of energy for water heating is quite high, including the initial capital, repair and maintenance costs, and other heating costs. Therefore, except in exceptional circumstances, the heating process has proven to be extremely costly to use for water disinfection in aquatic recirculating systems.
4.6.4. Ultraviolet Radiation Treatment
Downs and Blunt (1878, quoted by Wheaton, F., 1985) have found that certain bacteria can be destroyed by sunlight. In the last decade of the nineteenth century, Ward (1893) confirmed that the ultraviolet portion of the solar radiation spectrum is responsible for the bactericidal action of light. Ultraviolet (UV) radiation includes wavelengths in the range 150 Å÷4000 Å (150Å represents the upper limit of the X-ray wavelength range, while 4000 Å is the lower limit of the visible wavelength range). The energy of bright (visible, infrared or ultraviolet) radiation exists in discrete or quantum units. The energy of a quantum depends on the wavelength of the radiation and is defined by the equation:
where:
– E = the energy of a single quantum (erg);
– hp = Plank's constant (6.62 x 10-27 erg);
– c = light velocity 3 x 1010 cm/s;
– λ = the wavelength of the radiation (cm).
The equation expresses the inverse relationship between the quantum energy and the wavelength of the light radiation. Thus, the energy of a quantum of UV radiation is greater than the energy of a quantum of visible radiation since the wavelength of the UV light is less than the visible light wavelength.
The destructive or lethal effect of UV radiation on bacteria, fungi, viruses or other small organisms depends on the wavelength of the radiation. The correlation between the relative bactericidal efficiency of UV radiation and its wavelength is illustrated in the curve of fig. 29. The most effective wavelength is 2600 Å, for which the bactericidal effect is 100%. On both sides of this value, the bactericidal effect of UV radiation drops rapidly. Thus, UV light with a wavelength of 3200 Å was found to be only effective at 0.4% against UV light with a wavelength of 2600 Å, and the bactericidal radiation effect of 7200Å (the visible field ) is only 0.002% of the bactericidal effect of UV radiation of 2600 Å (Koller, 1965 quoted by Wheaton, F., 1985).
Figure 29 – Relative bactericidal efficacy of UV light versus wavelength
(Wheaton, F., 1985)
The actual mechanism by which UV light determines the destruction of microorganisms is not yet fully elucidated. There is, however, sufficient evidence to indicate that the bactericidal effect of UV radiation is the result of certain photochemical interactions of UV light with nucleic acids (Phillips and Hanel, 1960 cited by Wheaton, F., 1985).
The destruction of microorganisms, is interpreted as an indication that a single radiation quantum is sufficient to produce a lethal effect (Koller, 1965 quoted by Wheaton, F., 1985). Sometimes, the destruction of microorganisms by treatment with UV radiation can be described by a sigmoidal curve. In these situations, the sigmoid curve explains the need for multiple strokes (more than a single light quantum) on a single organism or a single hit on several organisms (targets). Both curves, exponential and sigmoidal, are analyzed according to the target species, the irradiation technique, the irradiation characteristics and the stage of development of the organism. Since the volume and accuracy of bacterial physiology data is insufficient, it is often difficult to assess the suitability of applying one of the two curves to a particular system.
The degree of survival of irradiated microorganisms depends on the multiplication between the UV intensity and the exposure time (I x t). Thus, the same survival ratio can be achieved with low exposure times and high radiation intensities or high exposure times and reduced radiation intensity.
Phillips and Hanel (1960, quoted by Wheaton, F., 1985), summarizing information on the influence of temperature on the bactericidal action of UV radiation with the wavelength of 2540 Å, concluded for the thermal range 5÷36 °C that once with increasing temperature increases, to a certain extent, the efficiency of microorganisms destruction. Although the literature reports different opinions on the influence of the heat factor on the bactericidal action of UV radiation, it seems that the temperature has a reduced influence on the rate of destruction within the tolerance limits of the target species.
Tables 3÷5 show a series of UV radiation data required to kill 90% or 100% of different microorganisms. Some of the values mentioned in the tables are contradictory. There are several arguments justifying these discrepancies, namely: the diversity of sources of bibliographic information; pigmentation, concentration and / or different age of microorganisms; other variables that characterize media conditions. Therefore, when designing UV light water disinfection systems, the highest amount (indicated in Tables 3 to 5) of the energy required to achieve a certain percentage of microorganisms destruction is adopted.
In order to achieve the same percentage of microorganisms destruction, water disinfection requires a 5 to 10-fold higher UV radiation dose than the required dosage for the destruction of suspended organisms in the air (Phillips and Hanel, 1960 quoted by Wheaton F., 1985).
Also, other researchers have determined that the exposure required to destroy a microorganism is 40÷50 times higher in water than in dry air (General Electric Company, 1953). The requirements for the energy requirement of UV radiation to achieve different percentages of destruction of the Serratia marcescens bacteria are listed in Table 6 and in Table 7 similar data for different species of microorganisms are presented.
Table 3 The UV radiation energy (E) required to destroy 90%
of the microorganisms colonies sown on agar (Wheaton, F., 1985)
It is known that water absorbs the energy of most wavelengths of light radiation. The degree of absorption is dependent on the wavelength of the radiation, first of all, and on the turbidity of the water. The water is relatively transparent for UV radiation with a wavelength of approx. 2600 Å, recognized for their bactericidal action. Percentages of water transmission of UV radiation with a wavelength of 2537 Å calculated for different values of water absorption coefficients.
Table 4 – The UV radiation energy (E) with a wavelength of 2537 Å required to
inhibit 90% and 100% colony formation (Wheaton, F., 1985)
Table 5. The UV radiation energy (E) required to destroy 100% of the microorganisms
(Wheaton, F., 1985)
The absorption coefficient of a water can only be determined experimentally. Distilled water allows good transmission of UV radiation with a wavelength of 2537 Å or greater. UV radiation with wavelengths less than 2537 Å are absorbed to a greater extent by water. Impurities in water (mineral salts and organic matter) significantly reduce the transmission of radiation. In the case of similar concentrations, iron and organic matter reduces to a greater extent the transmission of UV radiation through water than alkaline salts (Phillips and Hanel, 1960 quoted by Wheaton, F., 1985). Thus, seawater absorbs a higher amount of ultraviolet radiation than most freshwaters because of its higher ionic content.
Increased turbidity due to plankton, alluviums or other materials also reduces the transmission of UV radiation.
Table 6 The effectiveness of destroying the Serratia marcescens bacterium
with an UV Wave Cathode 30W lamp (Wheaton, F., 1985)
Table 7. The UV energy required to achieve different levels of destruction efficiency of some microorganisms (Wheaton, F., 1985)
UV lamps
Devices used for water disinfection in the aquaculture of recirculating systems include a variety of lamps capable of emitting significant ultraviolet radiation, most of which contain mercury vapors. The electric current flowing through the vapor excites the mercury atoms it brings in various energetic states. As atoms return to lower energy states, they emit very accurate wavelengths of radiation. Because all atoms have multiple states of excitation, the wavelength of the radiation emitted is variable depending on the energy state at which the transition occurs. The probability of occurrence of any energy state transition can be increased by varying the mercury vapor pressure, the amount and the chemical form of mercury contained in the lamp as well as the electrical emission conditions (Phillips and Hanel, 1960 quoted by Wheaton, F., 1985).
There are three general types of low pressure mercury vapor lamps, namely: cathode lamps, cold cathode lamps and germicidal high intensity lamps.
Warm cathode lamps operate at a low voltage, similar to standard fluorescent lamps. The commissioning of a hot cathode lamp is facilitated by the addition of a gas (argon) and the use of an electrodes preheating device. Electrodes, usually located at the ends of the tube, are tungsten filaments coated with a layer of calcium oxide, barium or strontium. The lifetime of a hot cathode lamp depends on the electrode's reliability and the degree of solar radiation of the lamp glass. Repeated starts and stops shorten the life of hot cathode lamps. Solarization is the process of slow glass darkening, a consequence of prolonged exposure to high-intensity UV radiation. Hot cathode lamps diminish relatively quickly, have a very low efficiency, and have difficulty starting when temperatures are low. The yield of hot cathode lamps is greatly influenced by the temperature of the water into which they are introduced; the effect of the temperature on the yield of a wavelength of 2537 Å emitted by a UV lamp when it is in direct contact with water is shown in fig. 30 (Wheaton, F., 1985).
Figure 30. The correlation between 2537 Ǻ UV radiation and water temperature
(Wheaton, F., 1985)
Cold cathode lamps produce UV radiation whose yield may be the same magnitude as those emitted by hot cathode lamps. Compared to hot cathode lamps, cold cathode lamps have some functional features, which may, on a case-by-case basis, constitute advantages or disadvantages. Thus, starting at low temperatures is not a problem for cold cathode lamps because, for this type of lamps, both start-up and operation require high voltages. As the name suggests, their nickel electrode does not require preheating. Since the electrode is "cold", the lifetime of the lamp is mainly determined by the solar radiation speed (reduction of UV transmission) of the lamp. Cold cathode lamps also contain mercury vapors used in hot, argon and neon cathode lamps (Phillips and Hanel, 1960 cited by Wheaton, 1985).
High-intensity germicidal lamps, also called “Slimline” germicidal lamps, represent a combination of cathode and cold cathode lamps as a constructive and functional solution. High-intensity germicidal lamps use high voltages that allow cold cathode starting, and then start operating with the hot cathode after start-up. Under these conditions, the lifetime of a high intensity germicidal lamp depends, first of all, on the life of the electrode that is inversely proportional to the starting frequency. The main advantage of high intensity germicidal lamps is their high efficiency, superior to hot or cold cathode lamps. Germicidal lamps produce ozone in negligible quantities. To limit, however, the production of ozone, these lamps are made of a special glass.
There are, along with the lamps described above, several types of high pressure mercury vapor lamps. Generally, these lamps are not widely used to disinfect water, being considered inferior to low pressure lamps for the following reasons: the efficiency of the conversion of electric energy into radiant energy is low; the radiation emitted is distributed over a wider band of wavelengths, thus less used for germicidal purposes; have a relatively reduced life expectancy and are more expensive; produce considerable amounts of ozone.
UV radiation disinfection systems
Disinfection systems based on water treatment with UV radiation, widely used in the practice of recirculating systems, can be made in two constructive versions, namely suspended systems (with or without reflectors) and submersed systems (with or without direct contact of the water tube).
Suspended systems
Suspended systems are schematically shown in fig. 31. In this constructive design, UV lamps are installed above a pool through which water flows. Typically, the height at which the lamps are suspended above the water level is 10 ÷ 20 cm. In the absence of a reflector, the intensity of UV radiation is inversely proportional to the distance between the lamp and the free surface of the water. It would be necessary, from this point of view, for lower suspension heights. Practical and functional considerations, however, limit the height at which the radiation source is installed. Thus, if the UV lamps are too close to the surface of the water, there are large variations in the radiation intensity along the pool, and the water that sprays the lamps creates problems, which are difficult to solve, for their maintenance.
Figure 31 – Suspended UV disinfection unit for water using reflectors (Wheaton, 1985)
The main advantage of this constructive design is that the lamps work in air and therefore the water temperature does not significantly affect the emission of the lamp, especially when the area around the lamp is strongly ventilated with air at room temperature. This also presents an important opportunity, namely simple and easy access to UV lamps for maintenance, by reclining the roof of the installation.
The main calculation parameters of a suspension system are water depth, water quality, lamp height, lamp spacing, and flow rate of water flowing through the gutter. Most of these parameters are conditioned, first of all, by the degree of disinfection proposed.
The absorption coefficient of UV radiation is mainly influenced by the turbidity of the water. This parameter determines two main features of the flow, namely water depth and water flow rate. Thus, a water with a high load of suspended solids requires a lower thickness of the water layer so that the UV radiation reaches the bottom of the basin. Also, a high germicidal efficiency of UV radiation requires lower flows in case of high water turbidity. Thus, the literature suggests that for the same suspended system of UV radiation at a water turbidity of 240 ppm the optimal water flow is 5 L/min, while at a lower turbidity of 70 ppm the water flow can reach up to at 15 L/min. (Hill et al., 1967 quoted by Wheaton, F., 1985).
The location of the lamps, both with regard to the suspension height and their spacing, is a major criteria considered when designing a suspension system. The distance between the lamp and the free surface of the water must, as far as possible, be compatible with the lamp maintenance requirements. The distance between the lamps shall be such that the intensity of the radiation is constant, within reasonable limits, throughout the length of the treatment unit. With regard to the two dimensional elements defining the optimal location of the lamps in a Kelly-Purdy UV radiation disinfection system that does not use reflectors over the lamps, the literature (Furfari, 1966 quoted by Wheaton, F., 1985) signals, in the case of a recirculating lamellibranchiate growth system, a suspension height of 15 cm above the water level and a lamp size range of 15 cm. The germicidal yield of UV radiation can be increased by focusing it with a reflector arranged above the lamp. Thus, a 30 W reflective lamp, installed 25 cm above a pool, develops at the free water surface a radiation intensity of 610 μW/cm2, sufficient to provide the germicidal effect. The actual radiation intensity at the free water surface in the case of a suspended structure of UV lamps is dependent on the reflector construction. Therefore, adopting a reflector, irrespective of its constructive solution, requires experimental or mathematical determinations of the intensity of UV radiation, corresponding to the desired distance from the lamp. In the case of a cold cathode UV lamp equipped with a reflector, the radiation distribution or radiation intensity depends on a multitude of variables, such as: reflector construction material, orientation and physical location of the lamp relative to the reflector and not last row, type of lamp used.
With regard to the depth of water in the treatment basin, it shall be so arranged as to ensure that at least 90% of the incident radiation energy is absorbed. A lower absorption rate of UV radiation in the water mass implies a higher degree of absorption at the level of the walls of the enclosure. Since the energy absorbed by the enclosure walls is not available for bacterial destruction, the yield of treatment decreases rapidly when the water depth is too low. As UV penetration enters the water column in the disinfection chamber, its intensity decreases, which is why the water layer near the bottom is exposed to lower intensity radiation. Therefore, when determining the depth and drainage of the water in the disinfection chamber, it will be intended that the entire water mass is exposed to the same UV intensity during the same exposure period. Satisfaction of the two requirements, equal exposure time and constant intensity of UV radiation, requires the creation of a turbulent drainage regime of the deep water layer by placing baffles (thresholds) on the bottom of the basin, perpendicular to the flow direction of the water.
The flow of water flowing through the treatment basin is determined by several factors, the most important being the pathogenic microorganisms, the intensity of UV radiation, the linear dimensions of the treatment unit and the turbidity of the water. Aspects about the influence of UV radiation intensity and water turbidity on the germicidal effect of UV lamps have been discussed previously. Irradiation parameters, namely radiation intensity and exposure time, shown in Tables 2÷6 for different species of microorganisms, should be adjusted according to the turbidity of the water. The maximum permissible flow rate of a suspended disinfection system is usually determined experimentally. Under conditions of radiation irradiation required by the species of microorganisms (radiation intensity and time of exposure) and maximum expected turbidity, the experiment consists in changing the system flow and monitoring the presence of pathogenic microorganisms in the effluent of the treatment unit.
Submerged systems
Submerged UV radiation systems use a sunken or water-surrounded lamp. The constructive solution and operating principle of a submersible system are shown in fig. 32.
Figure 32 .Submerged UV treatment system (Wheaton, 1985)
A tube made of quartz or glass protects the lamp from direct contact with water. The material from which the tube is made allows the transmission of UV radiation and its absorption into the water loaded with pathogenic microorganisms. Although the glass tube could be removed, its presence provides several advantages. In the case of a protective tube, the water is kept at a considerable distance from the lamp. Therefore, the water temperature has a limited influence on the emission of the lamp. Changing the lamp is also greatly facilitated in this case, keeping the lamp tight. Due to the high resistance of the material from which the tube is made, this system can be placed directly into a high pressure water stream.
The manual tube cleaning device (fig. 32) is an indispensable component of the system. Its role is to effectively remove the solids deposited on the outer surface of the protective tube in order not to block the emission of the UV lamp radiation. In case of large disinfection units, lamp cleaning is done automatically by a device that slides along the tube in both directions, driven by the water flow.
The main functional feature of submerged systems is that they use all the radiation produced by the lamp, with the exception of transmission and reflection losses. Also, the removal of the reflector and the possibility of direct installation in the water stream are important arguments for water disinfection with a submerged system. Particularly, suspended systems cannot be easily adapted to pressurized water flow, require special construction reflectors, and do not allow automatic lamp cleaning. Therefore, submerged UV radiation treatment plants are used on an industrial scale, while suspended systems have a more limited use in experimental research.
4.6.5. Ozone
The qualities of ozone, as a disinfectant, have been recognized for a long time. The first information on water sterilization with ozone dates back to 1893, and the first commercial use of ozone for water disinfection in an urban power system was reported in 1906 in France.
Ozone, the triatomic oxygen molecule, is formed when the oxygen molecules are sufficiently excited to break down into atomic oxygen. Collisions of these atomic oxygen atoms cause ozone formation. Achieving the degree of excitation of oxygen molecules required for ozone formation is achieved by passing oxygen through a high voltage corona discharge or exposing oxygen to ultraviolet radiation with wavelengths between 1000 and 2000 Ǻ (Klein et al., 1975; Koller, 1965 quoted by Wheaton, F., 1985). Most commercial ozone generators use the high voltage corona discharge system.
Several types of ozone generators are available for aquaculture recirculating systems. As a constructive solution, all types of generators consist of two parallel plates spaced at a certain distance between them. The operating principle consists in connecting the plate system to a high voltage source, resulting in the formation of an electric current between the two surfaces. Various dielectric materials can be used to provide uniform electrical current across the entire surface of the plates. Gas, ordinary air or pure oxygen, is passed through the two surfaces where the oxygen molecules are sufficiently excited to form ozone. Ozone production in these units is dependent on several variables. The distance between the plates must be large enough to allow relatively free flow of gas and ensure a uniform current. The upper limit of the plate spacing must not exceed a certain value so that the required tension is excessively high. The gas pressure must be high enough to ensure the desired flow, taking into account the recorded pressure losses. As the gas pressure increases, the electrical resistance of the gas changes, a phenomenon that affects the optimum operating voltage of the ozonator. The rate of ozone decomposition (the return to molecular oxygen) increases with the increase in temperature. Because approx. 90% of the energy with which the ozonator is fed is lost in the form of heat, ensuring an optimal operation of the ozonator requires its cooling (Klein et al., 1975 cited by Wheaton, F., 1985). Depending on the design of the ozone generator, cooling can be done with water or air. The dielectric material placed between the surfaces of the electrodes should have the highest electrical resistance and also a high thermal conductivity, two properties that rarely meet together. Therefore, the compromise solution is usually the use of a high resistivity dielectric material disposed in a thin layer which solves the problem of thermal conductivity.
The installed power of an ozone generator depends on its requirements regarding the electrical characteristics of the current formed between the electrodes (voltage, frequency, intensity). It is known that the efficiency of an ozonator is directly proportional to the frequency of the electric current. The increase in frequency is accompanied, however, by electrical losses which, in certain situations, can become economically prohibitive, which is why the choice of dielectric material for the design of an ozonator is an important problem.
The gas used in the ozonator is air or oxygen. The energy yield of oxygen generators is very different for the two gases. Thus, the same ozonator will consume less than half of the amount of energy consumed when operating with air when using oxygen. Therefore, even if oxygen is expensive, it is recommended to use it for large units. It is known that ozone is a good oxidant and has a strong corrosive effect that is potentiated in a wet environment. Therefore, regardless of the nature of the gas, it must be dried before use.
Gas flow is also an important parameter that determines the concentration of ozone measured at the generator output. Thus, in the case of lower flows, ozone generation is higher. The concentrations of ozone produced by most ozonators vary between 0.5÷10 percent, expressed massic.
Effectiveness of ozone disinfecting action
The use of ozone as a disinfectant is dependent on contact time and dosage. Ozone reacts very quickly compared to other compounds, such as chlorine, for several reasons. Thus, chlorine has to dissociate in water before it reaches the toxicity stage, while ozone is toxic by simple contact. Ozone efficiency is less determined by pH and temperature than chlorine. However, it seems that ozone is more efficient at a pH equal to 6 than at a pH equal to 8. Ozone does not react significantly with ammonia, as is the case with chlorine (Nebel et al., 1975). This phenomenon makes it possible for ozone to react much faster than chlorine. Also, as an oxidizing action, ozone is twice as powerful as chlorine.
In the secondary ozone treatment of urban effluent, Nebel et al. (1975) found that between the number of microorganisms in water and the ozone concentration there is a correlation which, graphically expressed in a logarithmic coordinate system, it is in the form of a straight line; the dependence is linear for concentrations of ozone of 0.5÷10 mg/L. The same study indicates that BOD, ammonia, nitrites, suspended solids, turbidity and color decrease as ozone concentration increases and nitrate concentration increases with increasing ozone levels.
Pavoni et al., 1975, monitored the effect of ozone in a concentration of 15 ppm on viruses and bacteria in distilled water. The results show that the survival of microorganisms over time is described by a sigmoid curve. The disinfection was complete after about 15 seconds under the initial bacterial count of about 2 x 106 bacteria/mL and that of viruses of 109 units/mL.
Pavoni (1975), Nebel (1975) have shown that both bacteria and viruses are destroyed very quickly in the case of ozone treatment, a phenomenon confirmed in time by several researchers.
The results of some experiments on the effect of ozone on microorganisms (E. coli, other coliforms, polioviruses, T-2 and F-2 bacteriophages, Entamoeba histolytica, Schistosoma manioni) were synthesized by Kelly (1974, quoted by Wheaton, F., 1985) showing that, in conditions of residual ozone concentrations of 0.05÷1.8 mg/L and contact times of 1÷22 minutes, almost 99% inactivation was achieved and the minimum percentage of 95% was quite rare.
Organic matter also exhibits a certain affinity for ozone. This phenomenon is the basis for treating wastewater in order to eliminate its color, odor and turbidity. Therefore, purifying a high-loading organic material requires higher ozone concentrations to achieve a degree of disinfection similar to that obtained in a water with a lower organic matter content. Ozone reacts most rapidly with organic compounds by ozonolysis of carbon-carbon double bonds (Bailey, 1975). Ozone has an oxidizing effect on the following chemical structures: multiple carbon-carbon linkages of olefinic and acetylene type; aromatic, carbocyclic and heterocyclic molecules; carbon-nitrogen bonds and other similar unsaturated groups; nucleophilic molecules such as amines, sulfides, sulfoxides, phosphites and phosphorus hydrogen; carbon-hydrogen bonds from alcohols, ethers, aldehydes, amines and hydrocarbons; silicon-carbon, silicon-silicon and silicon-hydrogen bonds; other types of carbon-metal bonds (Bailey, 1975.) Inorganic substances in water can also exhibit ozone affinity. It is well known, for example, that iron and manganese can be oxidized by ozone to forms of insoluble oxides. By treating a freshwater with ozone, the iron concentration was reduced from 9.54 mg/L to 0.07 mg/L and manganese from 1.21 mg/L to 0.05 mg/L (Kjos and others, 1975 cited by Wheaton, F., 1985). By using oxygen, instead of ozone, for the treatment of the same water, the iron concentration dropped to only 3.99 mg/L while the manganese concentration was reduced to only 0.71 mg/L, much higher than the ozone value. Knowing the ozone oxidation effect on inorganic compounds, its use in water disinfection in closed-loop marine culture systems implies rigorous monitoring because some salts, whose presence in water is indispensable, can be oxidized to insoluble forms, which, in fact, are thus removed from the system.
Ozone toxicity
Ozone has a pronounced toxic effect for both humans and most aquatic organisms. The measure of the effect of ozone toxicity on living organisms is given by the volume of the administered dose, expressed in volume, and the duration of exposure.
For humans, the ozone is lethal in the case of an exposure of 0.1 minutes at a dose of 11000 ppm or, for a longer exposure of 1oo minutes, at a much lower dose of about 20 ppm. The ozone toxicity in humans is felt starting at a dose of 3000 ppm associated with an exposure time of 0.1 min, a dose of 4 ppm and a duration of exposure of 1000 min (Nebel et al., 1975).
Data from the literature on ozone toxicity on aquatic organisms are insufficient and less precise except for punctual and dispersed information that cannot be linked to conclude on this issue. However, it seems that any amount of ozone, whether small, present in a culture system is harmful and should be avoided. For example, Arthur and Mount (1975 cited by Wheaton, F., 1985) found that an ozone concentration of 0.2÷0.3 ppm is fatal for certain species of fish. Fortunately, ozone is very unstable, decomposing rapidly into oxygen. Under these circumstances, the few minutes elapsed between water treatment and ozone introduction into the culture system are sufficient to avoid toxicity problems. With regard to other aquatic organisms, for example oyster larvae, the literature (De Manche et al., 1975 cited by Wheaton, F., 1985) signals that they are extremely sensitive even at very low ozone concentrations. In this situation, it is necessary that, before being introduced into the crop system, the ozone water, especially the marine, is passed through an activated carbon filter at which the water quality parameters are rebalanced.
Ozone water disinfection systems
Regardless of the constructive solution adopted, an ozone water treatment system must ensure intimate contact between target organisms and ozone. Therefore, ensuring an effective mixing of water with the gas is a primary requirement for the application of this disinfection system. Depending on how this is achieved, ozone-water contactors can be grouped into four groups, namely: spraying columns; columns with filling; columns with trays or screens; gas bubble dispersing systems (fig.33).
The operating principle of spraying columns consists of injecting water as fine drops into the ozone atmosphere of the column. In this way, an appropriate contact is made between the two phases, liquid and gaseous, resulting in the transfer of ozone into water. The ozone transfer efficiency is directly proportional to the ratio of the gas phase to the liquid phase (G/L) flow rate. Therefore, the need to achieve higher G/L ratios, to the extent of the proposed efficiency, plus ozone instability, is two main arguments explaining the limited use of spray columns for water ozone.
Figure 33. Ozone treatment system schematic
for disinfecting 400–2400 L/min ofsurface water (Timmons et al., 2002)
Filled columns consist of a compact bed of granular material through which water and ozone pass. The granular medium provides a sufficiently large contact surface at which the gas transfer occurs. "Trickling" filters, currently used for wastewater treatment, are a good example of a water-ozone contactor that falls into the category of filler columns. On the height of the column, gas and liquid can move in the same direction or counter-current. Styrofoam-type contactors have many advantages that justify their use in ozonizing water, namely: small size, which means costs or reduced technological spaces; appreciable capability of operation over a wide range in terms of the ratio between the gas and liquid phase flow (G/L); low pressure losses on the passage of gas and water through the granular medium; proper operation also for liquids producing a large amount of foam; possibility of realization in constructive variants that allow operation in pressure lines. The use of filling columns for ozonizing water for disinfection involves the knowledge of functional disadvantages, among which: unequal distribution of liquid and gas inside the column; the possibility of clogging the filter medium with suspended solids transported by water.
Water-ozone contactors such as tile or screen columns are widely used in large systems. Ozonators of this type are made up of one or more ozone screens/plates. The water flows over/through these trays/screens and contacts the gas, thereby making the gas transfer. The functional characteristics of the columns of screens or trays are similar to those of the columns with filler. The difference between the two types consists in the fact that for columns with screens or plates, both phases, liquid and gaseous, are evenly distributed over the height of the column. Also, trays/screens columns can be operated in a larger scale of G/L ratios than in columns with fillings. The possibility of easier removal of sedimentary solids is also another major advantage of tile/screen columns compared to those with filler (Stahl, 1975, cited by Wheaton, F., 1985).
The dispersion systems are most commonly used for the transfer of ozone into water. The dispersion of a gas consists, in its simplest form, in the introduction of the gas in the form of bubbles into a column of liquid. It is known that mass transfer is potentiated by the increase of the gas-liquid interface area, which is why small bubbles are preferred. Hence, gas is usually introduced at the bottom of the column or basin using specific distribution elements capable of producing bubbles as fine as possible. The distribution elements are usually represented by microporous structures made of different materials, such as: stone, ceramic products, silicon carbide, plastics, etc. Making a contact surface requires, in some situations, vigorous mixing of water with special devices, most often using the Venturi vacuum cleaner.
The choice of one of the four types of contactors depends on several factors, among which the most important are: the target micro-organisms, the water quality, the required technological flow and the technical and functional characteristics of the contactors. However, the main objective to be satisfied by the chosen system is to ensure as much as possible the best possible contact between ozone and the target micro-organisms. This implicitly implies a larger transfer area between the two gaseous and liquid phases. Ensuring the necessary contact time, appropriate to the factors presented above, is an important criteria for assessing the performance of the chosen contactor. The possibility of recycling oxygen produced by the contactor is also an important objective, especially when the ozonator operates with pure oxygen.
The residual ozone concentration along with time and contact surface are the main parameters determining the effectiveness of disinfection. The value of these parameters varies with target organisms and water quality. Thus, it appears that a concentration of 0.5 mg/ozone in the effluent of a contactor and a contact time of 5÷10 minutes ensures, in the case of urban sewerage, a degree of destruction of the microorganisms in water higher than 95%. In most aquaculture systems, a similar percentage of disinfection of wastewater is usually obtained at lower values of residual ozone concentration and contact time due to lower water loading in microorganisms, organic compounds and inorganic compounds. In the literature it is estimated that contact times of 1÷5 minutes and concentrations of ozone in the contactor of 0.56÷1 mg/L represent reasonable parameters for water treatment in the aquaculture of the recirculating systems (Sproul and Majumdar, 1955 quoted by Wheaton, F., 1985).
NOTE: Regarding the scoring system: Space requirements (where is large and is small); Operational management (Ease of use – where is hard and is easy); Operational performance (where is low and is high); Acquisition and implementation cost (where is expensive and is cheap); Maintenance and operation costs (where is expensive and is cheap). All scoring was done by comparing a technology/equipment with another (within the same category).
7. Aquaponics
7.1 Introduction
An aquaponic system integrates hydroponic techniques within a recirculating aquaculture system (RAS). Integrated aquaponic systems have become more and more popular in the last years, presenting sustainable new ways to produce food. The main advantage that an aquaponic integrated system brings to the RAS is the extra biofiltration provided by the plants. As a consequence, the make-up water daily exchange rate in reduced.
The main components of an integrated aquaponic system are: the rearing tank, the solid removal units (sump and mechanical filter), the biofiltration unit, the aeration unit, the degassing unit, the pumps and the hydroponic culture module (fig. 34).
Figure 34. Schematic overview of an integrated aquaponic system (Thorarinsdottir et al., 2015)
Rakocy et al., 2006, considers that there is an optimum arrangement of these components, as can be seen in figure 35, thus the solid removal unit and the bio filtration unit must precede the hydroponic culture module.
Figure 35.Optimum arrangement of aquaponic system components (not to scale) (Rakocy et al., 2006)
In this way, the effluent from the rearing unit is treated first by removing its suspended and settable solids, then it is biofiltered by removing as much ammonia and nitrite as possible, finally reaching the hydroponic culture module, where nutrients (nitrate and other micro and macro elements) are absorbed by the plants and additional ammonia and nitrite are removed by the bacteria growing on the surfaces of the hydroponic module and/or the grow media. After passing through the hydroponic culture module, the water is collected into a sump from where it is returned to the rearing unit.
There are different types of aquaponic techniques, such as: media grow beds, deep water culture (DWC), and nutrient film technique (NFT), each presenting advantages and disadvantages over each other. The design and choosing of the best aquaponic system might seem challenging, but in fact it can be simple when practicing a proper fish stocking density so as to provide a good nutrient level for plant production.
4.7.2. Media grow beds
A media grow beds uses a tank or container that is filled with a solid medium suitable for plant cultivation such as: light expanded clay aggregate (LECA) (fig. 36A), pumice (fig.36B), Growstone (fig.36C), gravel (fig.36D), perlite (fig.36E), expanded shale (fig.36F), peat, coconut, sand, mineral wool, slag, ceramic balls, bricks and polystyrene or marble.
Figure 36 – Different types of growing media
This cultivation method uses a hydroponic substrate and includes a drain hole that is 1.5 cm above the bottom surface. This allows the existence of a thin water mass at the bottom of the substrate, which serves as a permanent water store (Bradley and Tabares, 2000).
In order to choose the best material for plant cultivation by this method, must have the following characteristics (Abou Hadid and El-Behairy, 1999):
provide solid to support plants;
be inert "do not contain any chemical elements";
retain enough water;
retain enough oxygen at the same time as water retention;
contain no chemical that could be toxic to plants;
be free of diseases.
These media provide the plants with a good root fixation substrate, also because of the porosity (high specific surface area) and a good water and air retention rate, the media provides extra biofiltration/nitrification and mineralization.
In small scale aquaponic systems that practice a low fish stocking density, the biofilter can be completely excluded, the nitrification process being realized by the grow bed media. Also red earthworms (Eisenia fetida) can be added in the grow bed.
These annelids help break down solid waste and excess roots, they suppress plant diseases and provide extra nutrients to the plants through their excrements – vermicompost. Vermicompost has been shown to enhance plant growth, crop yield, and improve root structure and development (Pant et al., 2009).
The media grow bed system can be implemented by choosing one of the two flood regimes: constant (continuous) flow or reciprocating flow (“ebb and flow”) (fig,37).
Figure 37 – Bell siphon schematic (A) and examples of bell siphon (B) (Fox et al., 2010)
Water flows into the grow bed and it fills it up to a specific level, then it drains out almost completely only to start the process once more. This effect is achieved by narrowing the outlet pipe, thus creating a suction effect once the water reaches a certain (desired) level.
In the reciprocating flow variant the media and the plant roots get better aeration (Rakocy et al., 2006), unlike the constant flow variant in which the same media and the roots are always under the water level. When cultivating a plant with high nutritional requirements, a constant flow system is recommended, but the reciprocating flow system has a better biofiltration and solid removal.
This hydroponic module is quite easy to build and depending on the media type that is used, the cost vary, making it not the cheapest, but also not the most expensive solution.
Maintenance can be difficult if large amounts of solids get through the mechanical filtration unit and end up in the media, over time clogging the media, the roots and even the piping.
The media grow bed is better suited for plants that bear fruit since the media provides better fixation for the roots and support for the plant.
The crop yield of the media grow bed system is better than that of the DWC system, which in turn is better than the NFT system (Lennard and Leonard, 2006).
4.7.3. Deep water culture (DWC)
Also known as the floating raft system, the deep water culture system is optimal for both small and large scale production systems.
The design is simple, the hydroponic tanks are large, not too deep, and usually a minimum of 30cm water depth level is maintained.
The plants grow on polystyrene or plastic sheets that float on the surface of the water, usually fixed in small plastic net pots (fig. 38).
Figure 38. UVI deep water culture aquaponic production system (Rakocy, J.E.)
Just like the media grow bed troughs, the deep water culture tanks, in this case, have a simple design; rectangular shape, made of plastic, metal, wood or concrete covered with impermeable lining, etc. Water flows in the hydroponic module through an inlet at one end and it flows out through an overflow outlet at the other end of the unit. This way a constant water level is always maintained and in case of a pump malfunction or power failure, the plants do not die.
The rectangular basins have two distinct advantages: the nutrient basins are in the form of conveyor belts for the rafts in which they are planted and for those on which they are harvested, and the plants are disposed in a single horizontal plane so that the interception of the sunlight on each plant is (Jensen, 1991).
This is a highly productive method, but requires intensive biological and mechanical filtration to maintain clean water.
The roots of the plant grow directly into the oxygenated water flowing from the fish tanks with a volumetric exchange rate of approximately 30% per hour. (Thorarinsdottir et al., 2015). If the solids filtration and removal is not done properly, sludge accumulating at the bottom of the hydroponic tank and on the roots of the plant blocking oxygen and nutrients uptake.
In aquaponics it is often encountered the use of these systems as decanting basins. Due to the fact that these channels on which the plants are grown are long and narrow and contain a large amount of water, they act as decanters (sumps). This is a good thing as long as solids are removed periodically and do not accumulate. However, once covered with floating plates is difficult, it is consumed for a long time and it is laborious to remove solids from the bottom of the channels. It is much better for the solids to settle in a different place from the plant cultivation for a mild and regular removal (Lennard, 2012).
Floating bed hydroponics also provides sufficient nitrification space if the solids are removed from the effluent before they reach the hydroponic component (Rakocy et al., 2006).
Crop management involves transplanting the seedlings into the system and harvesting the plants once they achieve marketable size. This process can be improved by applying a conveyor movement of the rafts similar to the conveyor production system (CPS) used in the NFT systems. The seedlings are introduced on floating trays into the system at inlet end of the hydroponic module and moved along the length of the tank as it grows, reaching the outlet end of the tank when it’s ready to be harvested.
A great improvement to the DWS is the implementing a conveyor production system (CPS) (fig. 39) within the hydroponic modules.
Figure 39. Schematic of a conveyor production system (CPS) (Adler et al., 2003)
The DWC system is easiest to build and the least expensive of the three types of aquaponic systems.
4.7.4. Nutrient film technique (NFT)
The nutrient film technique system uses long and narrow plastic tubes with a thin layer (film) of water continuously flowing through them.
An absolute requirement in the case of the NFT systems is a good preliminary mechanical filtering (Thorarinsdottir et al., 2015). Solids accumulating on the roots must be avoided, not only because it can inhibit plant growth, but also because it can lead to clogging of the system.
The design of the NFT systems might be more complex than the other two aquaponic systems, but it certainly is the easiest to manage as long as clogging is avoided. The system uses long tubes with rectangular or circular section, 10-15 cm in width or diameter. These tubes have holes cut into the top part where plants (optionally in plastic pots) are set (fig.40).
The NFT tubes are placed at a 1% angle, the water entering at the raised end, flowing through the tube and being gravitationally evacuated at the lowered end. Water is introduced into the NFT hydroponic modules with a low flow pump, aiming for a 1-2 L/min flow regime (Thorarinsdottir et al., 2015). Also these pumps require less energy to recirculate the water into the hydroponic units, reducing the operational cost.
Figure 40. Nutrient film technique aquaponic system with rectangle troughs (A) and tubes (B)
An absolute advantage of the NFT system over the other two systems is the possibility to design and build the system even vertically, making excellent use of the available growing area.
In order to avoid the oxygen depletion be the plant roots due to the small volume of water flowing through, the tube must not be longer than 10 m.
NFT systems also have some drawbacks, like the risk of root clogging and the decrease of the water’s nutrients towards the end of the channel.
4.7.5. Lighting
One important component of the aquaponic system, that must not be overlooked, is the lighting equipment needed for assuring a fast and proper plant development. In most cases natural lighting just won’t be sufficient enough for an optimum system yield.
There are several options to choose from, such as: fluorescent bulbs, metal halide bulbs, and LED lamps. The lights come in various wave lengths, they cover a certain area and need to be mounted at a certain height above the plants, depending on plant species. When choosing the lighting solution for an aquaponic system, a very important aspect also needs to be the acquisition to operating cost ratio. Basically the more expensive the lighting system acquisition is the less expensive the operation cost over a longer period of time is.
The lighting operation period during a day can vary depending on the crop that is grown and the time of year, but usually around 10 hours of light : 14 hours of dark (Lennard and Leonard, 2004).
NOTE: Regarding the scoring system: Space requirements (where is large and is small); Operational management (Ease of use – where is hard and is easy); Operational performance (where is low and is high); Acquisition and implementation cost (where is expensive and is cheap); Maintenance and operation costs (where is expensive and is cheap). All scoring was done by comparing a technology/equipment with another (within the same category).
WATER QUALITY IN RAS SYSTEMS
Due to the environmental restrictions which appeared in many countries (land restrictions, water limitations and environment pollution) recirculating aquaculture systems (RAS) are developed rapidly in many areas of the fish farming sector.
RAS offers the advantage of growing fish in a controlled environment with a big reduction of water consumption due to waste-management and nutrient recycling, being environmentally friendly (Timmons and Ebeling, 2013; Jacob Bregnballe, 2015; Martins et al., 2010). These kinds of systems can control the water quality, the water temperature, are not dependent on the weather and it has low water and land requirements (Ebeling et al., 1995).
Mainly, the technology uses mechanical and biological filters for water treatment but some systems contains more treatment units such as: unit for the administration of ozone, for the wastewater disinfection and organic waste removal; degassing unit, for carbon dioxide removing; monitoring and control systems (Michaud, 2007; Jacob Bregnballe, 2015).
Water has an important impact at the fish growth and the final quality of the fish (tissue quality, texture and taste). Thus, maintaining the physico-chemical parameters of the water in the recirculating system, in the optimal range for the raised species, ensures a higher growth rate of the fish biomass, while poor water quality affects the physiology of the fish, the rate of growth, causes pathological changes and disorders of the functioning of internal organs, in some cases even leading to high mortality. Also, water quality has a decisive influence on the growth of biological material by direct and/or indirect action of organic or inorganic solvates in water, which, in optimal quantities corresponding to biological necessities, ensures a normal development. Any deviation of certain parameters from the optimal interval may be detrimental to the fish.
Mainly, water quality in the growing units depends on two things (fig. 41), the quality of the influent water and changes in the water quality inside the rearing units which is influenced by feed input and biological activity (Colt et al., 2009).
Figure 41. Processes that affect water quality in a recirculating aquaculture system
(adapted after Colt et al., 2009).
The primary water quality parameters include temperature, dissolved oxygen (DO), pH, ammonia, nitrites, nitrates, suspended solids, but it is also important to control aspects like salinity, alkalinity, biochemical oxygen demand (BOD) (Table 8). So, in order to be able to reuse the water in a RAS system it is important to monitor and control variables, since they directly affect animal health, feed utilization, growth rates and carrying capacities.
Table 1. Water criteria quality for a recirculation system (Jacob Bregnballe, 2015)
Temperature. In a RAS system, water temperature is the most important parameters to consider when assessing water quality. Because fish are poikilothermic or cold-blooded, their body temperature is approximately the same as their environment and each species has an optimum temperature range that maximizes growth and an upper and lower limit beyond which they cannot survive (table 9).
Knowing the thermal optimum for each species (table 10) have a great importance in the intensive growing and exploitation technologies since temperature directly affects the physiological processes, such as respiration rate, efficiency of feeding and assimilation, growth, behavior, reproduction (Timmons and Ebeling, 2013).
Table 9. Optimum Temperature Ranges (°C) for some fish species raised in a RAS system
Above the optimum temperature, the increased energy requirements for food conversion and other metabolic processes ensure that the law of diminishing returns applies. Also, at higher than optimum temperatures, the fish food conversion ratios are lower. Further temperature increases beyond optimum are of no benefit, and may in fact approach lethal levels (Timmons and Ebeling, 2013).
Changes of the environmental temperature affect the fishes' rate of biochemical reactions, which leads to different metabolic and oxygen consumption rates. At the lower ranges of the species tolerable temperature range, these rates decrease. As water temperatures increase, fish become more active and consume more dissolved oxygen, while simultaneously producing more carbon dioxide and other excretory products, such as ammonia. These increasing rates of consumption of necessary elements and production of detrimental elements can have a direct effect on overall fish health and survival if these parameters are allowed to exceed nominal values. If not corrected, the fish will become stressed to some degree. Even low levels of stress can have adverse long-term consequences in the form of reduced growth rates or mortality due to opportunistic organisms that take advantage of the stressed fish. (Timmons and Ebeling, 2013).
Besides that, temperature affects several other parameters and can influence the physical and chemical properties of water, such us (Figure 41):
Salinity and conductivity
Metabolic rates and photosynthesis production
Compound toxicity
Dissolved oxygen and other dissolved gas concentrations
Oxidation reduction potential
pH
In a RAS system, the temperature can be regulated with different heaters and coolers (Losordo et al., 1999) and water exchanges (fig. 42).
Figure 42. Solution for controlling temperature in a RAS system
Table 10. Criteria and Effects of Temperature on some Fish Species
Dissolved oxygen (DO). After temperature, dissolved oxygen is the most important and most critical parameter, which requires continuous monitoring in intensive production systems. Low oxygen concentrations in water lead to serious adverse effects such as respiratory stress, hypoxia, lethargy, including fish death.
In aquaculture systems there are many competitors for dissolved oxygen.The competition for oxygen dissolved in water is given on the one hand by the fish biomass and on the other hand by the chemoautotrophic nitrifying bacteria (which carries out the oxidation of ammonia) and the chemoheterotrophic bacteria that consume the organic carbon.
The required dissolved oxygen level is directly proportional to the density of the fish from the RAS, so a higher stocking density involves a high biological oxygen demand. Minimum values of dissolved oxygen as also highly dependent on species and growing conditions (table 11, fig.43).
Table 11. Criteria and Effects of Dissolved Oxygen on the Health of Fish (SOUTH AFRICAN WATER QUALITY GUIDELINES, 1996)
Chronically low DO levels cause stress, resulting in reduced appetite, poor growth and production and an increase in susceptibility to infectious diseases. Symptoms include gulping of air at the water surface, stress coloration and an increase in swimming activity
Also, high DO concentrations (>20 mg/L) are toxic to fish and can cause physiological dysfunctions (including gas bubble disease) and developmental abnormalities in fertilized eggs and larvae (Department of Water Affairs and Forestry, 1996).
Figure 43. Solution for controlling DO in a RAS system
pH. In a RAS system, the pH tends to decrease when nitrifying bacteria produce acidity, consuming alkalinity as well as carbon dioxide resulting from the breathing process of fish and micro-organisms. Carbon dioxide reacts with water to form carbonic acid that lowers pH. At a pH of less than 6 pH units, the nitrifying bacteria are inhibited and no longer converts ammoniacal nitrogen.
The optimum pH value for growth and health of most aquatic animals is 6,5-9 (Timmons et al., 2007). If the fish are exposed to extreme values of pH, growth rate will be significantly reduced with stressful or lethal effects on the fish biomass.
Table 12. Criteria and Effects of pH on the Health of Fish (SOUTH AFRICAN WATER QUALITY GUIDELINES, 1996)
Generally, in recirculating systems, the optimal pH range is maintained by the addition of alkaline buffer solutions. The most commonly used buffer solutions are sodium bicarbonate and calcium carbonate. The pH value needs to be constantly monitored and controlled (fig. 44) to keep it at optimum levels in the rearing tank and the biofilter. If nothing is done the environment will eventually become toxic (Ebeling et al., 1995; Lekang, 2007; Losordo et al., 1999).
Figure 44. Technical solution for controlling pH in a RAS system
Nitrogen compounds (Ammonia, Nitrites, Nitrates).
Ammonia is the main source of nitrogen removed from fish and is excreted especially in the gills in the form of ammoniacal gas. Ammonia is the product of catabolism resulting from protein digestion and is extremely toxic.
In the aquatic environment, ammonia is present in two forms: non-toxic and non-toxic (NH4+) dissociated or ionized (NH4+), which is toxic to most fish at a constant value exceeding 0,03 mg/L. Ammonia in undissociated form easily penetrates the tissue barriers, penetrating through the branch epithelium in the blood of the fish, acting harmful to the nervous system. The rate of conversion from NH4 + to NH3 depends both on temperature and pH. Also, low concentrations of dissolved oxygen (DO) increase the toxicity of NH3 (table 13)
Table 13. Criteria and Effects of ammonia on the Health of Fish (SOUTH AFRICAN WATER QUALITY GUIDELINES, 1996)
For most recirculating aquaculture systems, ammonia removal usually takes place in specialized filtration compartments (biological filters, chemical filters) located outside the growing basins. Biofilters are formed from active bacterial cultures attached to specific surfaces.
High concentrations of internal ammonia affect intracellular and blood pH, and osmoregulation. This may result in reduced internal ion concentrations, increased urine flow (>12% of the lethal threshold concentration) and plasma renin activity (in rainbow trout) and acidemia, which adversely affects the oxygen-binding capacity of hemoglobin.
Also, at long-term exposure of fish to reversed water-to-blood ammonia concentrations leads to various manifestations of chronic toxicity:
Increases in the primary stress indicators in the blood, i.e. increased concentrations of cortisol and catecholamines;
Capillary congestion and dilation; renal blood vessel damage; connective tissue inflammation and hyperplasia;
Tissue damage of the gills (epithelial cell hyperplasia and hypertrophy; separation of epithelial cells from pillar cells which results in inflammation of the gill tissue;
cellular degeneration; lamellar detachment; congestion; hemorrhage; aneurysms; karyolysis and karyorrhexis) (Department of Water Affairs and Forestry, 1996).
Nitrites. Generally, nitrites occur in aquatic environments as a product of the activity of gram-negative bacteria, chemoautotrophs, being intermediates in the nitrification process, which is the two-step oxidation of ammoniacal nitrogen to nitrates. This nitrification is dependent on the degree of aeration of the water in the system (Cristea et. al.,2002).
Although nitrite is converted to nitrate relatively quickly by ozone and the nitrifying bacteria in a properly balanced biofilter, it is a problem in recirculating systems because it is being created on a constant basis, so the fish are continually exposed to certain concentration (Timmons and Ebeling, 2013). Nitrites are very toxic to aquatic organisms because they enter the body of fish through chlorine cells in the branched epithelium by a mechanism in which chlorine molecules play a role as mediator.
The immediate effect of nitrites on fish is observed in the blood. Plasma can accumulate nitrites, acting as a means of transport to spread them into tissues. In red blood cells, nitrites oxidize Fe2 + hemoglobin into Fe3+ producing methemoglobin, unable to transport oxygen. Exposure of fish to high nitrite concentrations causes gills to deteriorate (hypertrophy, hyperplasia, epithelial separation) and thymus (hemorrhage and necrotic lesions) (Wedemeyer, 1996).
For most recirculating aquaculture systems, ammonia removal usually takes place in specialized filtration compartments (biological filters, chemical filters) located outside the growing basins. The toxicity of nitrite can be reduced or stopped by chloride ions. Usually between 6 ÷ 10 parts of chloride protect the 1part nitride fish. In this situation, pH, alkalinity and dissolved oxygen values must be checked, reduced feed intensity, or system water may be replaced, and if the nitrite concentration increases, NaCl is added (table14).
Table 14. Criteria and Effects of Nitrite on the Health of Fish (SOUTH AFRICAN WATER QUALITY GUIDELINES, 1996)
Nitrates are the final product of nitrification, they are relatively non-toxic, except for very high values. Generally, nitrates do not go up to critical levels if there is a change of water in the system with fresh water in a proportion of 5 ÷ 10% daily. At the same time, in many recirculating systems, it appears that denitrification occurs within the system, which maintains nitrate concentrations below the toxic level. In systems with low water exchange or high hydraulic retention times, denitrification has become increasingly important (Timmons and Ebeling, 2013) (table 15).
Table 15. Criteria and Effects of Nitrate on the Health of Fish (SOUTH AFRICAN WATER QUALITY GUIDELINES, 1996)
For the RAS systems a non-specific method of removing nitrate can be: passing the water stream through an ion exchange column with a selective affinity for nitrates. Unfortunately, the is expensive because other anions will also be removed, depending on the nature of the resin used. On a commercial scale the process described requires competent operation, control and maintenance and is generally not viable under culture conditions (fig. 45).
Figure 45. Technical solution for controlling N in a RAS system
Alkalinity. Alkalinity is a measure of the pH-buffering capacity or the acid-neutralizing capacity of water. Alkalinity is defined as the total amount of titratable bases in water expressed as mg/L equivalent calcium carbonate (CaCO3) (Timmons and Ebeling, 2013).
At pH values less than 8.3 the hydrogen carbonate ion concentration is the predominant form whereas at pH values greater than 8.3 and 9.6 the concentrations of the carbonate and hydroxide ions, respectively, are of consequence. Other ions which may also contribute to the alkalinity of water are borates, silicates, phosphates and organic bases.
For intensive fish growth, the recommended alkalinity range is 100 ÷ 150 mg /L (table 16)
Table 16. Criteria and Effects of alkalinity on the Health of Fish (SOUTH AFRICAN WATER QUALITY GUIDELINES, 1996)
Water quality problems associated with alkalinity are usually the result of inadequate alkalinity and therefore poor buffering capacity. Treatment normally entails increasing the alkalinity of the water by the addition of lime. In a RAS system alkalinity can be adjusted through the addition of sodium bicarbonate (NaHCO3), common baking soda. Other materials can be used, but sodium bicarbonate safe, inexpensive, and easy to be used (fig. 46)
Figure 46. Technical solution for controlling alkalinity in a RAS system
Hardness- represents a measure of the amount of calcium (Ca2 +) and magnesium (Mg2 +), salts that are present in water. In freshwater farming systems, fish are hypertonic in their environment. This means that water tends to balance the differences in osmosis; for this reason, fish must dispose of large amounts of urine to maintain their internal physiological balance.
Waters have traditionally been classified as soft (0–75 mg/L), moderately hard (75–150 mg/L), hard (150–300 mg/L), or very hard (> 300 mg/L) (Timmons and Ebeling, 2013). Hard waters usually exhibit fairly high alkalinities, although during acidification, alkalinity can be rapidly reduced while the concentrations of calcium and magnesium (i.e. hardness) remains the same (fig.47)
Figure 47. Technical solution for controlling hardness in a RAS system
Table 17. Criteria and Effects of hardness on the Health of Fish (SOUTH AFRICAN WATER QUALITY GUIDELINES, 1996)
Calcium and magnesium interact with metals, decreasing the toxicity of metals such as zinc and copper. Similarly, increased calcium concentrations decrease the toxicity of ammonia. Water is commonly softened either by the addition of lime followed by recarbonation, or by using ion exchange, sometimes preceded by a precipitation if the feed water is particularly hard.
Adequate concentrations of calcium and magnesium are necessary to ensure growth and survival of fish. Calcium is necessary for bone formation, blood clotting and other biological processes. Calcium in culture water reduces the loss of other salts from fish. A loss of salts causes a reduction in growth, as the fish must use energy supplied through the diet to re-absorb lost salts. Low levels of calcium also result in reduced disease resistance in fry.
Magnesium and calcium have an important role in muscle contraction and the transmission of nervous impulses in animals. In a hard water (> 250 mg / L alkalinity) will lose less metabolic energy for osmoregulation than fish in poor water (<100 mg / L alkalinity) so they have to consume more metabolic energy for growth.
REFFERENCES
Timmons B. Michael and Ebeling M. James, 2013, recirculating Aquaculture 3rd Edition, Ithaca Publishing Company LLC, ISBN 978-0-9712646-5-6.
Jacob Bregnballe, 2015, A Guide to Recirculation Aquaculture – An introduction to the new environmentally friendly and highly productive closed fi sh farming systems, Published by the Food and Agriculture Organization of the United Nations (FAO) and EUROFISH International Organisation.
Martins C.I.M., E.H. Eding, M.C.J. Verdegem, L.T.N. Heinsbroek, O. Schneider, J.P. Blancheton, E. Roque d’Orbcastel and J.A.J. Verreth, New developments in recirculating aquaculture systems in Europe: A perspective on environmental sustainability, Aquacultural Engineering, November 2010, Volume 43, Issue 3, Pages 83-93.
Michaud L. (2007) – Microbial communities of recirculating aquaculture facilities: interaction between heterotrophic and autotrophic bacteria and the system itself – PhD Dissertation in “Scienze Ambientali: Ambiente Marino e Risorse” (XVIII CICLO), University of Messina.
Colt, J., Watten, B., & Rust, M. (2009). Modeling carbon dioxide, pH, and un-ionized ammonia relationships in serial reuse systems. Aquacultural Engineering, 40(1), 28-44. doi:10.1016/j.aquaeng.2008.10.004.
Losordo, T. M., Masser, M. P., & Rakocy, J. (1999). Recirculating aquaculture tank production systems. A Review of Component Options. Retrieved from http://www.lssu.edu/faculty/gsteinhart/GBS-LSSU/BIOL372-Fish
Wedemeyer, G.A. (1996).Physiology of fish in intensive culture systems, ISBN 0-412-07801-5, Chapman and Hall, New York.
Department of Water Affairs and Forestry, SOUTH AFRICAN WATER QUALITY GUIDELINES Volume 6: Agricultural Water Use: Aquaculture Second Edition, 1996.
Piper, R.E., McElwain, I.B., Orme, L.E., McCraren, J.P., Fowler, L.G Leonard, J.R., 1982. Fish Hatchery Management. U.S. Fish and Wildlife Service, Washington, DC.
Tucker, C.S., Robinson, E.H., 1990. Channel Catfish Farming Handbook. New York, Van Nostrand Reinhold.
Kaiser, G.E. and F.W. Wheaton. 1983. Nitrification filters for aquatic culture systems: State of the art. Journal of the World Mariculture Society 14: 302-324
Libey, G.S. and G.E. Miller. 1985. Evaluation of three biological filters suitable for aquacultural applications. Journal of the World Mariculture Society 16: 158- 168.
Losordo, T.M. 1991. Engineering considerations in closed recirculating systems. Pp. 58-69 in Aquaculture systems engineering. American Society of Agricultural Engineers, St. Joseph, Michigan
Losordo, T.M. Michael. Masser and James E. Rakocy,1999, Recirculating Aquaculture Tank Production Systems A Review of Component Options, SRAC Publication No. 453
Malone, R.F., B.S. Chitta, and D.G. Drennan. 1993. Optimizing nitrification in bead filters for warmwater recirculating aquaculture. Pp. 315-325 in J.K. Wang, editor. Techniques for modern aquaculture. American Society of Agricultural Engineers, St. Joseph, Michigan.
Spotte, S. 1979. Fish and invertebrate culture: Water management in closed systems. John Wiley & Sons, New York, NY.
Bailey, Philip , 1975, Reactivity of ozone with various organic functional groups important to water purification in the First International Symposion on Ozone for Water and Wasterwater Treatment. International Ozone Institute Waterbury Conn.
Kelly Cornelius B. 1974 The Toxicity of Chlorinated Waste Effluents to fish and Considerations of Alternative Processes for the Disinfections of Waste Effluents. Virginia State Water Control Board.
Pavoni, J.L. Tittlebaum M.E. Spenser H.T. and Fleishman M.R. 1975, Ozonation as a Viral Disinfection Technique in Wastewater Treatment Systems, in the First International Symposium on Ozone for Water and Wasterwater Treatment. International Ozone Institute Waterbury Conn.
Nebel C. Unangst P.C, and Gottschling R.D. 1975. Ozone Disinfection of secondary Effluent:Laboratory Studies. on Ozone for Water and Wasterwater Treatment. International Ozone Institute Waterbury Conn.
Timmons, M. ,B., Losordo T. M., 1994. Aquaculture water reuse system: engineering, design and management. Elsevier.
Timmons, M.B., Ebeling, J.M., Wheaton, F.W., Summerfelt, S.T., Vinci, B.J., 2002. Recirculating Aquaculture Systems. NRAC Publication, NY, USA (2002).
Wedemeyer, G. A. (1996). Physiology of Fish in Intensive Culture Systems. New York: Chapman & Hall.
Wheaton, F.W., 1985. – Aquacultural Engineering. Robert E. Krieger Publishing Company, Malabar, Florida,
Grace și Piedrahita, 1991
Bisogni J.J and Timmons M.B., 1991, Control of pH in closed cycle aquaculture Engineering Aspects of Intensive Aquaculture. Northeast Regional Agricultural Engineering Service Publication No. 49. Cooperative Extension, Ithaca, NY.
General Electric Company, 1953, Bulletin LD 14, General Division, Lamp, Departament, Cleveland
Rakocy, J. E., Michael P. Masser and Thomas Losordo 2006. Recirculating aquaculture tank production systems: aquaponics – integrating fish and plant culture. Southern Regional Aquaculture Center. SRAC Publication No. 454
Ragnheidur Thorarinsdottir, 2015, Technical Report ·DOI: 10.13140/RG.2.1.4975.6880, ISBN: 978-9935-9283-1-3, Affiliation: University of Iceland
Lennard, W.A. and Leonard, B.V. 2004. A comparison of reciprocating flow versus constant flow in an integrated, gravel bed, aquaponic test system. Aquaculture International 12
Lennard, W.A. 2012. Aquaponic System Design Parameters: Basic System Water Chemistry. Melbourne: Aquaponic Solutions
Pant A.P., Radovich T.J.K., Hue N.V., Talcott S.T., Krenek K.A., 2009. Vermicompost extracts influence growth, mineral nutrients, phytonutrients and antioxidant activity in pak-choi (Brassica Rapa cv. Bonsai, Chinensis group) grown under vermicompost and chemical fertiliser. Journal of Science Food Agriculture (89), 2383-2392.
Bradley P., and Tabares C.H.M, 2000, Building by design hydroponics in developing countries, The Growing Edge
Copyright Notice
© Licențiada.org respectă drepturile de proprietate intelectuală și așteaptă ca toți utilizatorii să facă același lucru. Dacă consideri că un conținut de pe site încalcă drepturile tale de autor, te rugăm să trimiți o notificare DMCA.
Acest articol: General considerations regarding atmospheric and biological models for aquaponic production systems Atmospheric and water data model for aquaponics… [307339] (ID: 307339)
Dacă considerați că acest conținut vă încalcă drepturile de autor, vă rugăm să depuneți o cerere pe pagina noastră Copyright Takedown.