Recirculating aquaculture systems ( RAS ) are used in home aquaria and for fish products Where water exchange is limited and the use of bio-filtration is required to Reduce ammonia toxicity.  Other types of filtration and environmental control are also required to maintain a suitable habitat for fish. The main benefit of RAS is the ability to reduce the need for fresh water. To be exploited economically commercial RAS must have high fish stocking densities, and many researchers are currently conducting studies to determine if it is a viable form of intensive aquaculture . 
RAS water treatment processes
A series of treatment processes is used to maintain water quality in intensive fish farming operations. These steps are often done in order or sometimes in tandem. After the introduction of a biofilter to the ammonia, the next degassing and oxygenation occur, often followed by heating / cooling and sterilization. Each of these processes can be completed by a variety of different methods and equipment, but regardless of the size of the product.
All RAS related to biofiltration to convert ammonia (NH 4 + and NH 3 ) 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 to nitrite then nitrate. 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 used by the bacteria for energy. Nitrate is less toxic than ammonia (> 100 mg / L), and can be removed by a denitrifying biofilter or by water replacement. Stable environmental conditions and regular maintenance are required to ensure the biofilter is operating efficiently.
In addition to treating the liquid waste, it is necessary to treat the problem by concentrating and flushing the solids out of the system.  Removing solids reduces growth bacteria, oxygen demand, and the proliferation of disease . The simplest method for the removal of solids is the creation of a settling basin where the flow of water is slow and able to move to 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 sand filtration or particle filtering. Another common method is the use of a mechanical drum filter, which is a rotating drum screen that is periodically cleaned by pressurized spray nozzles, and the resulting slurry is treated to the drain. Incomplete solids or colloidal solvents can be used with or without the addition of ozone (O 3 ).
Reoxygenating the system is a crucial part of 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 of aeration and oxygenation . In aeration air is pumped through an air stone or similar device that creates small bubbles in the water column, which results in a high surface area where oxygen can dissolve into the water. In general, this method is considered to be inefficient and the solution is to be used in a controlled manner. Various methods are used to ensure that during oxygenation all the oxygen dissolves into the water column. Careful computation and consideration must be given to the application of the system and to the use of oxygenation or aeration equipment. 
In all RAS pH must be carefully monitored and controlled. The first step of nitrification in the alkaline biofilter and lowers the pH of the system.  Keeping the pH in a suitable range (5.0-9.0 for freshwater systems) is crucial to maintaining the health of both the fish and the biofilter. pH is typically controlled by the addition of alkalinity in the form of lime (CaCO 3 ) or sodium hydroxide (NaOH). A low pH will lead to high levels of dissolved carbon dioxide (CO 2 ), which can prove toxic to fish.  pH can also be controlled by degassing CO 2In 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 O 2 levels. 
All fish species have a preferred temperature above and beyond which Warm water species Such As tilapia and barramundi prefer 24 ° C water or warmer, Where have 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 of these can be used to keep the system operating at optimal temperature for maximizing fish production.
Disease outbreaks occur more frequently when dealing with high density fish stocks in intensive RAS. Outbreaks can be reduced by operating multiple independent systems with the same building and isolating water to water contact between systems and personnel that move between systems.  Also the use of Ultra Violet (UV) or ozone water treatment system reduces the number of free floating viruses and bacteria in the water system. These treatment systems reduce the risk of an outbreak of disease.
- Reduced water requirements as compared to raceway or pond aquaculture systems. 
- Reduced land needs due to high stocking density 
- Site selection flexibility and independence from a large, clean water source. 
- Reduction in wastewater effluent volume. 
- Increased biosecurity and ease in treating disease outbreaks. 
- Ability to closely monitor and control environmental conditions to maximize production efficiency. Similarly, from climate and variable environmental conditions. 
- High upfront investment in materials and infrastructure. 
- High operating costs mostly due to electricity, and system maintenance. 
- A need for highly trained staff to monitor and operate the system. 
Special types of RAS
Combining plants and fish in a RAS is referred to as aquaponics. In this type of system ammonia produced by the fish is not only converted to nitrate but is also removed by the plants from the water.  In an aquaponics system fishes effectively fertilize the plants, this creates a closed loop system where very little waste is generated and inputs are minimized. Aquaponics provides the advantage of being able to harvest and sell multiple crops.
Home Aquaria and Inland Commercial Aquariums are a form of RAS where the water quality is very carefully controlled and the stocking of fish is relatively low. In these systems the goal is to display the fish rather than producing food. However, biofilters and other forms of water treatment are still used to reduce the need to exchange water and maintain water clarity.  RAS water must be removed periodically to prevent nitrate and other toxic chemicals from building up in the system. Coastal aquariums often have high rates of water exchange and are typically not operated as a result of their proximity to a large body of clean water.
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