Recirculating aquaculture systems (RAS) are used in home aquaria and for fish production where water exchange is limited and the use of biofiltration is required to reduce ammonia toxicity. Other types of filtration and environmental control are often also necessary to maintain clean water and provide a suitable habitat for fish. The main benefit of RAS is the ability to reduce the need for fresh, clean water while still maintaining a healthy environment for fish. To be operated economically commercial RAS must have high fish stocking densities, and many researchers are currently conducting studies to determine if RAS is a viable form of intensive aquaculture.
RAS water treatment processes
A series of treatment processes is utilized to maintain water quality in intensive fish farming operations. These steps are often done in order or sometimes in tandem. After leaving the vessel holding fish the water is first treated for solids before entering a biofilter to convert ammonia, next degassing and oxygenation occur, often followed by heating/cooling and sterilization. Each of these processes can be completed by using a variety of different methods and equipment, but regardless all must take place to ensure a healthy environment that maximizes fish growth and health.
Biofiltration
All RAS relies on biofiltration to convert ammonia (NH4+ and NH3) excreted by the fish into nitrate. Ammonia is a waste product of fish metabolism and high concentrations (>.02 mg/L) are toxic to most finfish. Nitrifying bacteria are chemoautotrophs that convert ammonia into nitrite (NO2−) then nitrate (NO3−). These include bacteria of the genera Nitrobacter, Nitrococcus, Nitrospira, and Nitrospina. Although nitrite is usually converted to nitrate as quickly as it is produced, lack of biological oxidation of the nitrite will result in elevated nitrite levels that can be toxic to the fish. High levels of nitrite are also indicative of biofilter impending failure. Nitrate is the end-product of nitrification, and is the least toxic of the nitrogen compounds, with 96-hour exposure LC50 values in freshwater in excess of 1,000 mg/L. A biofilter provides a substrate for the bacterial community, which results in thick biofilm growing within the filter. Water is pumped through the filter, and ammonia is utilized by the bacteria for energy. In recirculating systems, daily water exchanges are commonly used to control nitrogen levels. Stable environmental conditions and regular maintenance are required to ensure the biofilter is operating efficiently.
Solids removal In addition to treating the liquid waste excreted by fish the solid waste must also be treated, this is done by concentrating and flushing the solids out of the system. Removing solids reduces bacteria growth, oxygen demand, and the proliferation of disease. The simplest method for removing solids is the creation of settling basin where the relative velocity of the water is slow and particles can settle at the bottom of the tank where they are either flushed out or vacuumed out manually using a siphon. However, this method is not viable for RAS operations where a small footprint is desired. Typical RAS solids removal involves a sand filter or particle filter where solids become lodged and can be periodically backflushed out of the filter. Another common method is the use of a mechanical drum filter where water is run over a rotating drum screen that is periodically cleaned by pressurized spray nozzles, and the resulting slurry is treated or sent down the drain. In order to remove extremely fine particles or colloidal solids a protein fractionator may be used with or without the addition of ozone (O3).
Oxygenation Reoxygenating the system water is a crucial part to obtaining high production densities. Fish require oxygen to metabolize food and grow, as do bacteria communities in the biofilter. Dissolved oxygen levels can be increased through two methods, aeration and oxygenation. In aeration air is pumped through an air stone or similar device that creates small bubbles in the water column, this results in a high surface area where oxygen can dissolve into the water. In general due to slow gas dissolution rates and the high air pressure needed to create small bubbles this method is considered inefficient and the water is instead oxygenated by pumping in pure oxygen. Various methods are used to ensure that during oxygenation all of the oxygen dissolves into the water column. Careful calculation and consideration must be given to the oxygen demand of a given system, and that demand must be met with either oxygenation or aeration equipment.
pH control In all RAS pH must be carefully monitored and controlled. The first step of nitrification in the biofilter consumes alkalinity and lowers the pH of the system. Keeping the pH in a suitable range (5.0-9.0 for freshwater systems) is crucial to maintain the health of both the fish and biofilter. pH is typically controlled by the addition of alkalinity in the form of lime (CaCO3) or sodium hydroxide (NaOH). A low pH will lead to high levels of dissolved carbon dioxide (CO2), which can prove toxic to fish. pH can also be controlled by degassing CO2 in a packed column or with an aerator, this is necessary in intensive systems especially where oxygenation instead of aeration is used in tanks to maintain O2 levels.
Temperature control All fish species have a preferred temperature above and below which that fish will experience negative health effects and eventually death. Warm water species such as tilapia and barramundi prefer 24 °C water or warmer, where as cold water species such as trout and salmon prefer water temperature below 16 °C. Temperature also plays an important role in dissolved oxygen (DO) concentrations, with higher water temperatures having lower values for DO saturation. Temperature is controlled through the use of submerged heaters, heat pumps, chillers, and heat exchangers. All four may be used to keep a system operating at the optimal temperature for maximizing fish production.
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