What is RAS? Recirculating aquaculture explained

How the loop works

Water leaving the tanks carries four problems: solids, ammonia, carbon dioxide and a shortage of oxygen. Each treatment stage removes one of them.

• Solids removal. A drum filter screens out faeces and uneaten feed. Settlers and protein skimmers take heavier and finer fractions.
• Biological filtration. Bacteria in a biofilter turn toxic ammonia into nitrate.
• Degassing. A degassing tower strips CO₂ from the water.
• Oxygenation. Oxygen cones or aeration put oxygen back in.
• Disinfection. UV or ozone treats the water before it returns to the tanks.

A small share of water is replaced each day to control nitrate and make up for losses.

RAS, ponds and flow-through farms

When RAS makes sense

• Water is scarce or expensive to pump and discharge.
• Land is limited or close to market.
• The climate is wrong for the species, so temperature must be controlled.
• Biosecurity matters and you want to control what enters the farm.
• You want year-round production at a steady output.

Numbers from a working farm

A commercial indoor RAS in Sweden held tilapia and catfish water around 30 °C all year. Energy use was about 3.1 kWh per kg of tilapia and 0.8 kWh per kg of catfish, varying only about ±15% between seasons (Bergman et al. 2020). Power is the running cost that a pond does not have.

What RAS asks of you

A RAS depends on power, pumps and oxygen running all day. It needs trained staff, a backup plan for failures, and a design sized on the real feed load. Read RAS design basics next, or see our delivered farms.