Oxygenation in RAS: degassing, PSA oxygen and cones

Where oxygen goes

Fish use oxygen to breathe and grow. The biofilter bacteria use it too, and so does any organic matter left in the water. Demand rises with feed and with temperature, while warm water holds less oxygen.

Aeration or pure oxygen

Blowers and diffusers add air. They are enough at lower stocking densities and are always needed for MBBR biofilters and degassing. At high density, air alone cannot keep up and pure oxygen is added.

Remove CO₂ first

CO₂ harms fish even when oxygen is fine. In a pikeperch RAS trial, feed conversion worsened from 1.09 at 5 mg/L CO₂ to 1.82 at 30 mg/L (Frontiers, pikeperch trial). Cold-water species are often given a tolerance of about 10–20 mg/L and warm-water species about 20–40 mg/L (FloNergia). A degassing tower strips CO₂ as water falls through packing against a flow of air.

Making and dissolving oxygen

• PSA generators make oxygen on site from air, so the farm does not rely on deliveries.
• Oxygen cones dissolve it under pressure into the return flow, so little is lost.
• Standby supply covers a generator stop or power failure.

See PSA oxygen generators and oxygen cones.

Size on the peak

A common rule of thumb is about 350 g of oxygen per kg of feed for the fish (Novair). Average oxygen demand is still the wrong number. Size on the demand after the largest feeding of the day, at the warmest water temperature you expect, and add a standby. The first layout we send shows this oxygen balance.

Watch it

Dissolved oxygen falls fast when a pump, blower or oxygen supply stops. A sensor and alarm on dissolved oxygen is one of the most useful things a farm can fit. One farm manager’s fish survived a 20-minute outage only because backup oxygen opened by itself (Global Seafood Alliance). See pumps, drains and controls.