RAS Design: Unit Processes and Loop Layout

How a recirculating aquaculture loop is ordered, how water reuse is measured, where makeup water fits, and how every unit process is sized from the peak daily feed ration.

A recirculating aquaculture system (RAS) reuses culture water by treating it continuously. Fish consume feed and oxygen and release faecal solids, carbon dioxide and ammonia into the water; the treatment loop must remove these wastes and restore dissolved gases before the water returns to the tanks. This guide focuses on how the unit processes are ordered and sized rather than on the general concept of recirculation.

The five core processes

SRAC Publication 453 names five processes every tank-based RAS must address: circulation, clarification (solids removal), biofiltration, aeration and carbon dioxide stripping. The publication describes them as links in a chain: the weakest link sets the capacity of the whole system to hold or produce fish, and neglecting any one of them undermines a commercial operation. Secondary processes such as heating and cooling, UV or ozone disinfection, foam fractionation, pH dosing and denitrification are added once the core is covered.

Loop order and why it matters

The FAO guide to recirculation aquaculture describes the basic path: water leaves the fish tanks, passes a mechanical filter, then a biological filter, is aerated and stripped of carbon dioxide, and returns to the tanks. Oxygen enrichment with pure oxygen and UV disinfection can be added to this sequence as required.

1. Culture tank — tank outlets are built to remove waste particles quickly and are fitted with screens.
2. Solids removal — placed upstream of the biofilter so that organically rich suspended solids do not overload it; FAO notes that a microscreen keeps the biofilter from clogging and stabilises nitrification.
3. Biofiltration — nitrifying bacteria convert ammonia to nitrite and then nitrate, while heterotrophic bacteria break down dissolved organic matter.
4. Degassing — carbon dioxide from fish and biofilter respiration, and excess nitrogen gas, are stripped before water reaches the fish.
5. Oxygenation — SRAC 451 notes that aerating or oxygenating the return stream just before it re-enters the tank is more efficient than aerating inside the tank, because dissolved oxygen is lowest and carbon dioxide highest at that point.
6. Disinfection — UV works best when mechanical and biological filtration have already removed organic matter, because UV penetration depends on water clarity.

Pump position follows the same logic. FAO advises lifting the water only once and letting it return to the pump sump by gravity, and placing pumps after mechanical filtration so that solids from the tanks are not broken into finer particles. Total lift in most intensive systems is around 2–3 m according to FAO; SRAC 453 notes that older designs worked at about 8 m of recirculation pressure, whereas most modern designs target about 3 m.

Measuring water reuse and exchange

FAO expresses intensity as new water used per kilogram of fish produced per year. Indicative values are about 30 m³ for a traditional trout flow-through farm, around 3 m³ for a low-level outdoor RAS, about 1 m³ for an intensive RAS and as little as 0.3 m³ for a super-intensive indoor system. The same guide gives a formula for the degree of recirculation: internal recirculation flow divided by the sum of internal recirculation flow and new water intake, multiplied by 100. In its worked example of a 500-tonne farm, these intensities correspond to roughly 0%, 95.9%, 98.6% and 99.6% recirculation.

Exchange rate also controls nitrate. FAO states that nitrate above about 100 mg/L appears to impair growth and feed conversion, that new water above 300 litres per kilogram of feed is normally enough to dilute it, and that below this value a denitrification step becomes worth considering. SRAC 451 adds that nitrate is also flushed out during routine sludge removal and filter backwashing.

Makeup water

Makeup (intake) water replaces losses and is one of the main routes for pathogens. FAO recommends a disease-free source such as a borehole or well rather than a river, lake or the sea; where intake treatment is needed it typically consists of a sand filter followed by UV or ozone. SRAC 453 states that surface water that may carry disease should be disinfected, and that this is most common in marine systems without a suitable groundwater source. Intake water can also be used to regulate temperature, because heat from fish metabolism, biofilter activity and pump friction builds up in indoor systems.

Sizing from the feed load

SRAC 453 describes the design sequence: define the holding capacity of the system, calculate the peak daily feed ration, size every component for that peak feed load, and then apply a uniform safety factor (for example 1.5) to all major component calculations. All components must handle the same feed loading.

• Recirculation flow: typically 42–84 litres per minute per kilogram of daily feed (SRAC 453).
• Ammonia load: approximately 3% of feed becomes ammonia-nitrogen (SRAC 451). The required flow follows from a mass balance; if the treatment train removes 50% of ammonia per pass, the flow must be twice that needed with fresh water.
• Oxygen demand: about 0.3 kg of oxygen per kg of feed where solids are removed quickly and the biofilter is not submerged, up to about 0.75 kg where solids stay in the system between backwashes of submerged filters (SRAC 451).
• Aeration capacity: under high loads the aeration system must be able to replace all oxygen in the system every 20–30 minutes at peak feeding (SRAC 453).
• Stocking density: a widely accepted growout design figure is about 60 kg/m³; densities near 120 kg/m³ are achievable but often show unstable water quality (SRAC 453).