RAS are used in home aquarium  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 and commercially, RAS must have high fish stocking densities.



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  • Ability to closely monitor and control environmental conditions to maximize production efficiency. Similarly, independence from weather and variable environmental conditions.
  • Reduced water requirements as compared to raceway or pond aquaculture systems.
  • Site selection flexibility and independence from a large, clean water source.
  • Reduced land needs due to the high stocking density.


  • High operating costs mostly due to electricity and system maintenance.
  • High upfront investment in materials and infrastructure.

Recirculating Aquaculture System (RAS)

RAS water treatment processes

A series of treatment processes are utilized to maintain water quality in intensive fish farming operations. These steps are often done in series or sometimes in parallel. After leaving the vessel holding fish, 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, regardless all must take place to ensure a healthy environment that maximizes fish growth and health. Key systems of an RAS are:


What are Biological filters?

Biological filters are devices to culture microorganisms that will perform a given task for us. Different types of organisms will perform different tasks. Part of the art of designing and using bio-filters is to create an environment that will promote the growth of the organisms that are needed.

Why do we need biological filters for aquaculture?

We use bio-filters to help maintain water quality in recirculating or closed loop systems. Bio-filters are also used to improve water quality before water is discharged from a facility. There are many different methods of maintaining good water quality and bio-filtration is only one component of the total picture. It is however, a very important and essential component especially for recirculating aquaculture or aquarium systems.

How will bio-filters help us?

Depending on design and application, bio-filters have the ability to accomplish the following functions. The first three functions are performed by biological means and the last four are done by physical processes that do not depend on living organisms.

  • Remove ammonia
  • Remove nitrites
  • Remove dissolved organic solids
  • Add oxygen
  • Remove carbon dioxide
  • Remove excess nitrogen and other dissolved gasses
  • Remove suspended solids


Disease outbreaks occur more readily when dealing with high fish stocking densities typically found in intensive RAS. Outbreaks can be reduced by operating multiple independent systems within the same building and isolating water to water contact between systems via thorough equipment and personnel cleaning. Additional Ultra Violet (UV) and/or ozone treatment reduces free floating viruses and bacteria in the water system. These treatment methods reduce disease loadings and thus reduces the chance of an outbreak.


Reoxygenating the water is crucial to achieving high fish 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, resulting 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 maintainO2 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.

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 back-flushed 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 disposed. In order to remove extremely fine particles or colloidal solids, a protein fractionator may be used with or without the addition of ozone (O3.)

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 fish effectively fertilize the plants, this creates a closed looped 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 density of fish is relatively low. In these systems the goal is to display the fish rather than producing food. However, bio filters and other forms of water treatment are still used to reduce the need to exchange water and to maintain water clarity.Just like in traditional 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 RAS due to their proximity to a large body of clean water.