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
• Control and biologically treat fish waste to prevent discharge to the environment
• Optimized environment for best fish growth and performance—control of temperature, salinity, water quality, and other parameters
• Enhanced biosecurity—protecting fish from environmental pathogens or pollutants
• Preventing fish escapes from the production systems.
• Protecting wild fish populations and the surrounding environment (low to no impact)
• Local production—lower transportation costs/carbon footprint
Why RAS?
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 in non-coastal regions like the Midwest.
“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
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 shows the general steps of RAS technology. You can access more information about each step by expanding the orange tabs at the bottom of the page.
Step-by-step guide for RAS technology
Farmers grow and house fish in specialized tanks that account for their biological needs. The species, biological requirements, stocking rates, life stage, water supply, and overall RAS setup capacity (including pump and biological filter sizes) determine the tanks’ size, shape, and construction. Experts generally recommend round tanks with a double drain system for efficient solids removal.
Mechanical filters, such as microscreen drum filters or radial flow settlers, physically remove suspended solid waste (primarily fecal matter and some uneaten food), functioning much like a colander. While this filtration physically cleans the water, biofiltration processes must still treat it to remove dissolved wastes before fish can reuse it (see below). The solid waste is collected for removal without environmental harm. A promising approach uses marine microorganisms to efficiently convert solid waste into fuel-grade biogas. This method biologically treats solid waste without environmental discharge and generates bioenergy, which can offset the RAS farm’s energy costs.
Most RAS systems incorporate a pump sump and various pumps to collect and move water throughout the system. Freshwater replenishes the sump to replace any water lost through evaporation, cleaning, or other processes.
Fish produce ammonia as a waste product, which dissolves in the water. If not removed through biological filtration, ammonia can rapidly accumulate to toxic levels. Biological filtration cleans water at the molecular level, using healthy, beneficial bacteria to transform toxic ammonia into safer nitrogen-based compounds. In this nitrogen removal process, bacteria convert toxic ammonia to nitrate. However, nitrate can also accumulate to toxic levels unless removed. While water replacement can remove nitrate, fully contained RAS farms prefer to avoid this method. Instead, these farms employ another biological filter where beneficial bacteria convert nitrate into non-toxic nitrogen gas.
Each fish species thrives within a specific temperature range based on its unique biology. Atlantic salmon, a cold-water species, can react sensitively to temperature fluctuations outside their normal range. Heating and cooling systems maintain optimal temperatures, ensuring fish remain within their ideal thermal conditions.
Fish absorb dissolved oxygen through their gills as they swim and feed, with absorption levels correlating to their activity. As fish use oxygen (O2), they release carbon dioxide (CO2) into the water. CO2-saturated water can harm fish, so it must be removed before water recirculation. For salmon, CO2 levels should not exceed 10 milligrams per liter of water – equivalent to less than one teaspoon of CO2 in a typical 12-ounce can of soda.
Disinfection systems, typically UV or ozone, provide biosecurity control. The process occurs last because the water reaches its cleanest and clearest state at this point, maximizing treatment effectiveness.
Many RAS systems undergo careful and regular monitoring to guarantee optimum water quality for the fish species they rear. This monitoring combines electronic devices and hands-on techniques. Many facilities employ SCADA (Supervisory Control and Data Acquisition) systems to effectively monitor and control various parameters, ensuring proper conditions for fish in the system. SCADA systems are computer-based tools that collect real-time data on water quality factors like temperature, oxygen levels, and pH, allowing for precise control of the aquatic environment.
Photo credit for header photo at top: Narayan Mahon, taken at the University of Wisconsin-Stevens Point Northern Wisconsin Demonstration Facility.