Recirculating Aquaculture Systems (RAS): A Guide
How recirculating aquaculture systems work: closed-loop water reuse, filtration and biofiltration, the nitrogen cycle, oxygenation, and the trade-offs of land-based fish farming.
Overview
A recirculating aquaculture system (RAS) is a land-based, closed-loop method in which fish are raised in tanks whose water is continuously treated and reused rather than discharged. NOAA describes RAS as closed-loop production systems that continuously filter and recycle water and waste, with typically more than 99 percent of the flow recycled after treatment. Because water exchange is limited, biofiltration is required to keep ammonia from reaching toxic levels.
Why RAS is used
- Water reuse: the bulk of the water is treated and returned, sharply reducing freshwater intake.
- Site flexibility: production can be located away from natural water bodies, including near markets.
- Biosecurity: a closed, controlled environment limits pathogen entry and escapes.
- Environmental control: temperature, oxygen and water chemistry are managed independent of weather.
Core components
Water leaves the culture tank carrying solids and dissolved wastes and passes through a treatment train before returning. The main stages are mechanical filtration, biological filtration, gas management and disinfection, driven by pumps and a sump.
- Mechanical filtration: drum, disc or sand filters and settling chambers remove uneaten feed and faeces.
- Biofilter: provides surface area where nitrifying bacteria convert ammonia to nitrate.
- Oxygenation: pure oxygen is commonly added rather than air, as it is more efficient at high density.
- Degassing / CO2 stripping: packed columns or aerators remove carbon dioxide and help stabilize pH.
- Disinfection: UV and/or ozone reduce free-floating bacteria and viruses.
- Sumps and pumps: circulate water through the treatment stages and back to the tanks.
The nitrogen cycle in RAS
Fish excrete ammonia as the main nitrogenous waste, which is toxic at low concentrations. In the biofilter, ammonia-oxidizing bacteria convert ammonia to nitrite, and nitrite-oxidizing bacteria such as Nitrospira and Nitrobacter convert nitrite to nitrate, the least toxic of the three. Nitrate accumulates and is controlled by limited water exchange or, in some systems, by anaerobic denitrification that converts nitrate to nitrogen gas.
Advantages and disadvantages
| Advantages | Disadvantages |
|---|---|
| High water reuse and low intake | High capital cost to build |
| Flexible siting near markets | High energy use for pumping and oxygenation |
| Strong biosecurity and control | Requires trained operators and reliable power |
| Year-round, weather-independent production | System failure can cause rapid losses |
Species suited to RAS
RAS is used for warm-water species such as tilapia (Oreochromis niloticus) and barramundi, which prefer warmer water, and for cold-water salmonids such as rainbow trout (Oncorhynchus mykiss) and Atlantic salmon (Salmo salar). A widespread use is producing salmon smolts on land in freshwater RAS before transfer to sea cages, and full grow-out of species like trout and tilapia in closed systems.