What is RAS?

Recirculating aquaculture systems (RAS) are a type of land-based aquaculture production platform utilized to raise a variety of aquaculture species, including Atlantic salmon, in a highly controlled environment.

Features include:

  • Water reuse to minimize reliance on water resources
  • Enhanced biosecurity—protecting fish from pathogens or predators
  • Preventing escapees
  • Protecting wild fish populations and surrounding environment (low to no impact)
  • Local production—lower transportation costs/carbon footprint
  • Control and collection of fish waste to reduce pollution discharge and recover nutrients
  • Optimized environment for fish growth—control of temperature, water quality, feeds, and other environmental parameters

Why RAS?

“RAS technology offers the ability to effectively capture and repurpose wastes—reducing environmental impact—to enhance biosecurity to prevent fish escapement, minimize pathogen entry or release to the surrounding environment, and highly control the rearing environment for increased fish performance and welfare. It also increases flexibility in site selection for proximity to markets. Farming fish locally to the market improves sales, reduces transportation costs, requires a smaller carbon footprint, improves traceability and product freshness, and supports the economy in local communities.”

–Steve Summerfelt, Chief Science Officer, Superior Fresh, LLC

For the first time, RAS is making it possible for Atlantic salmon to be a local, “farm to table” option on menus and store shelves that are far from ocean sources.

How does it work?

RAS requires specialized equipment and technologies designed to provide the fish with optimal water quality and control. A simple diagram by The Conservation Fund Freshwater Institute highlights general steps of RAS technology.

Step-by-step guide for RAS technology


Fish are grown and housed in tanks with special considerations accounting for their biological needs. The size, shape and construction of the tanks will differ based on species, biological needs, stocking rates, life stage, water supply, and the capacity of the entire RAS setup (size of pumps, biological filters, etc). Generally, round tanks which provide a double drain system for quick solids removal are recommended.

Solids removal
A mechanical filter, such as a microscreen drum filter or radial flow settler, is used to physically remove suspended solid waste (primarily fecal matter and some uneaten food), just like a colander. Water is physically cleaner after mechanical filtration but still must be treated through biofiltration processes to remove dissolved wastes before it is ready for reuse and fish use (see below). The solid waste is collected to be removed without hurting the environment. A promising approach is the use of marine microorganisms that very efficiently convert the solid waste to fuel-grade biogas. Solid waste is thus biologically treated without being discharged to the environment and bioenergy is generated in the process that can be used to offset the energy cost of the RAS farm.
Pumping/pump sump
Most RAS systems contain a pump sump and various pumps for collection and movement of water throughout the system. Some water which may be lost through evaporation, cleaning, etc. is replaced by fresh water into the sump.
Nitrogen Removal

One waste product from fish is ammonia, which is dissolved in the water. Ammonia can become toxic and rapidly accumulate if not removed from water using biological filtration. The process of biological filtration cleans water at the molecular level using healthy, beneficial bacteria to transform toxic ammonia into other nitrogen-based compounds that are safer for fish. Nitrogen removal is the biological process by which bacteria convert the toxic ammonia to nitrate. Nitrate can also accumulate to toxic levels unless being removed via water replacement which is not desired in fully contained RAS farms. In such cases, nitrate is biologically removed by beneficial bacteria in another biological filter that converts nitrate into non-toxic nitrogen.

Temperature control

Each type of fish has a preferred temperature range based on its unique biology. Atlantic salmon, for example, are a cold-water species and can be sensitive to temperature changes outside of their normal ranges. Temperature is controlled using heating and cooling systems to guarantee that fish are held at their most optimal range.

Degassing and oxygenation
As fish swim and feed they consume dissolved oxygen related to their activity level. When fish consume dissolved oxygen (O2), they also release carbon dioxide (CO2) back into the water. CO2-saturated water is toxic to fish therefore it must be removed before water can be reused. For salmon, ideal levels would be no more than 10 mg of C02 per liter of water. A degasser actively removes CO2 from the water andO2 is then diffused to match ideal levels needed by fish.
Disinfection systems, usually UV or ozone, can be used to provide biosecurity control. Disinfection occurs last because water is the cleanest and clearest it can be, and this maximizes treatment of water.
Monitoring and control
Many RAS systems are carefully monitored for proper water quality on a regular basis to guarantee that they provide the optimum water quality for the fish species being reared. This monitoring is accomplished through use of both electronic and hands on monitoring by qualified aquaculturists. Many facilities utilize SCADA systems to effectively monitor and control various parameters in the systems to guarantee proper conditions for fish in the system.

Photo credit for header photo at top: Narayan Mahon, taken at the University of Wisconsin-Stevens Point Northern Wisconsin Demonstration Facility.