Oxygenation in RAS: Aeration, Pure Oxygen and Gas Supersaturation

How recirculating aquaculture systems meet the oxygen demand created by feed: air versus pure oxygen, contactor categories, gas bubble disease risk and oxygen monitoring.

Why oxygen is usually the first limit

According to SRAC Publication 451, the ability of a recirculating aquaculture system (RAS) to add dissolved oxygen (DO) is, in most cases, the first factor that limits how many fish it can carry. Oxygen has to be supplied at the same rate at which it is consumed, and total consumption is the sum of fish respiration, the biochemical oxygen demand (BOD) of bacteria breaking down wastes and uneaten feed, and the demand of nitrifying bacteria in the biofilter. The same fact sheet warns that in an intensively loaded system a failure of aeration or oxygenation can cost the whole crop in half an hour or less.

Oxygen demand created by feed

SRAC 451 links oxygen demand to the amount of feed and to how long solids stay in the loop. Where solids are removed quickly and the biofilter is not submerged, as little as 0.3 lb of oxygen is consumed per lb of feed; where solids are retained between backwashes in submerged biofilters, consumption can reach 0.75 lb per lb of feed. Because this is a mass ratio, the same figures read as 0.3–0.75 kg of oxygen per kg of feed (unit conversion, not a separate measurement).

SRAC 453 states that most warm-water RAS run at 5–6 mg/L DO, while cooler systems can operate above 8 mg/L, and that under high loads the aeration system must be able to replace all the oxygen in the system every 20–30 minutes at peak feeding. SRAC 452 recommends keeping DO above 60 percent of saturation, or above 5 mg/L, for most warm-water systems, and notes that nitrifying bacteria become inefficient below 2 mg/L.

Aeration with air

In the FAO guide, water leaving the fish tanks is typically at about 70 percent saturation and falls further in the biofilter; aeration with air usually brings it back to around 90 percent, and in some systems to 100 percent. SRAC 451 explains that aerating inside the culture tank is inefficient, because transfer efficiency drops as DO approaches saturation; a better point is the recycled flow just before it re-enters the tank, where DO is lowest.

Pure oxygen

When fish and bacteria consume oxygen faster than air-based equipment can supply it, pure oxygen is used. SRAC 451 notes that the saturation concentration of atmospheric oxygen rarely exceeds 8.75 mg/L in water above 20 °C, whereas with pure oxygen it rises nearly fivefold, to 43 mg/L at standard atmospheric pressure. Performance is judged by absorption efficiency, the mass of oxygen absorbed divided by the mass applied: properly designed devices exceed 90 percent, while air stones in tanks shallower than about 6 ft (1.8 m) reach no more than 40 percent. The FAO guide describes supersaturated side streams with 200–300 percent oxygen and supply either as delivered liquid oxygen or from an on-farm generator.

Contactor categories

Oxygen (Speece) cone: an inverted cone in which water enters at the top and oxygen bubbles are held in the widening section until they dissolve (SRAC 453). FAO gives a typical internal pressure of about 1.4 bar; only part of the flow is treated, and pumping against that pressure uses a lot of electricity.
Low-head oxygenators and oxygen platforms: enclosed units with cascading panels or a hooded agitator in an oxygen-rich atmosphere (SRAC 453). FAO gives a typical pressure of about 0.1 bar and use on the main recirculation flow.
Pressurized packed columns and spray towers: oxygen flows up as water falls through media or droplets; packed versions are prone to biofouling (SRAC 453).
In-tank diffusers: lower efficiency and higher cost (FAO), but widely fitted as emergency back-up.

SRAC 453 stresses that pure-oxygen systems add oxygen but do not remove carbon dioxide, so they must be matched by a CO2 stripper of similar size (see the CO2 degassing guide). It also notes that tank DO is generally kept below saturation to avoid losing costly oxygen to the air.

Supersaturation and gas bubble disease

SRAC 452 advises introducing supersaturated water near the tank bottom and mixing it quickly: if mixing is too slow, zones of supersaturation can cause gas bubble disease, in which gas comes out of solution inside the fish and forms bubbles in the blood; fry are particularly sensitive. SRAC 451 and 453 warn that nitrogen becomes supersaturated when air is entrained in pressurized flow, so air must never be fed to a U-tube or pressurized packed bed, and blown air should conservatively not be injected deeper than about 5 ft (1.5 m), with sites at higher altitude more sensitive. The FAO guide lists 70–100 percent as the normal oxygen saturation range and values below 40 or above 250 percent as unfavourable; for nitrogen, 80–100 percent saturation is normal and above 101 percent unfavourable.

A 2026 PLoS One study describes gas bubble disease as being caused by total dissolved gas (TDG) supersaturation, when total gas pressure in the water exceeds barometric pressure. It cites earlier experiments in which juvenile salmonids developed the disease at 102 percent TDG, with severity and mortality increasing above 110 percent. In its own trials, acute disease in salmon and trout parr appeared at 115–120 percent TDG, with gill emboli within 1–6 hours. These data come from river and hydropower research, not from RAS operating limits.

Monitoring and back-up

FAO recommends measuring after the oxygenation unit at atmospheric pressure, for example in a measurement chamber, because probes need regular wiping and calibration.
In circular tanks a probe near the outlet represents the tank well, but it must not sit near the oxygen injection point; in raceways it belongs in the lowest-oxygen zone near the outlet, where control lag can reach an hour (FAO).
Alarms should reach staff who can respond within 20 minutes (FAO, SRAC 453).
SRAC 452 calculates that after a power failure in an 84 °F system holding 1/4 lb of fish per gallon, DO falls from saturation to a stressful 3 mg/L in 16 minutes, and in under 6 minutes at 1 lb per gallon.
A simple back-up is a pure-oxygen tank with a solenoid valve that opens on power loss and feeds diffusers in every tank (SRAC 452, FAO).