RAS Design Principles for Commercial Fish Farms

A recirculating aquaculture system is not an equipment list — it is a mass balance. Every component is sized from one number: peak daily feed at peak standing biomass. Design in that order and the quotations you receive become comparable; design around a tank catalogue and they never will. The capital consequences are set out in the RAS system cost breakdown.

1 — Start from feed load, not tank volume

Peak feed load drives oxygen consumption, total ammonia nitrogen production, carbon dioxide production and solids load. As planning rules of thumb, each kilogram of feed consumes roughly 0.4–0.5 kg of oxygen, produces about 25–35 g of TAN, releases 1.2–1.4 kg of CO₂ and generates 0.25–0.3 kg of dry solids. Fix tonnes per year, harvest weight, cycle length, survival and FCR first with the biomass calculator and the fish growth calculator.

2 — Set density, then derive culture volume

Density is a species and welfare decision, not a cost lever. Trout and salmon post-smolt typically run 40–70 kg/m³, tilapia and barramundi 60–90 kg/m³, sensitive marine species lower. Peak standing biomass divided by the design density gives culture volume — and only then tank count and diameter. Round tanks with a central drain and a swirl separator outperform rectangular tanks on solids removal at almost every commercial scale.

3 — Flow is set by oxygen and CO₂, not by turnovers

Turnover rate is an output, not an input. Required recirculation flow is the flow that keeps dissolved oxygen above the species minimum at the tank outlet and CO₂ below roughly 15 mg/L. In practice commercial grow-out lands at 0.7–1.5 tank turnovers per hour, but you should arrive there through the RAS water turnover calculator and the oxygen requirement calculator — never by copying another farm's number.

4 — Remove solids early and gently

Solids that break up in a pump become dissolved organics the biofilter must handle and the fish must tolerate. Sequence matters: central drain, swirl separator, drum filter at 40–60 µm, then biofilter. Size drum filters for 100–150% of design flow so one unit can be taken offline for maintenance without stopping production. For marine and brackish systems a protein skimmer removes fine solids the drum filter cannot.

5 — Size the biofilter on TAN load, with margin

Moving bed (MBBR) media typically converts 400–800 g TAN per m³ of media per day at commercial temperatures; fixed-bed and trickling filters differ. Size on peak TAN, not average, and add capacity for the nursery stage, which produces disproportionate loading per kilogram. Nitrification is also the slowest system to start: budget 4–8 weeks of biofilter maturation before the first significant stocking.

6 — Oxygenation and degassing are separate problems

Oxygen cones or low-head oxygenators fed by a PSA generator with liquid oxygen backup deliver oxygen efficiently, but they do not strip CO₂. Cascade columns, packed degassers or forced ventilation handle that separately, and in warm high-density systems CO₂ — not oxygen — usually becomes the limit that caps biomass. Check both with the oxygen demand calculator.

7 — Design redundancy around failure modes, not budget

In a RAS, life-support failure is measured in minutes. Non-negotiable: standby power covering the full critical load, duty/standby pumps, emergency oxygen diffusion independent of electricity, and alarms that reach a human phone. Size backup on critical load with the generator sizing calculator and the power requirement calculator.

8 — Biosecurity is architecture, not procedure

Separate nursery and grow-out water loops, all-in/all-out batch flow, UV or ozone on makeup and recirculation, one-way personnel flow and quarantine for incoming juveniles all have to be drawn into the building layout. Retrofitting biosecurity into a finished building costs several times what designing it in does.

9 — Building envelope and energy

Commercial RAS typically consumes 3.5–5.5 kWh per kilogram produced, dominated by pumping, oxygenation and temperature control. Insulation, heat recovery on discharge water and heat pumps are capital items that pay back through the OPEX line, so they belong in the first budget — see the aquaculture OPEX guide and the energy cost calculator.

10 — Turn the design into one specification

A comparable RFQ states species, tonnes, harvest weight, design density, peak feed load, water temperature and source, target DO and CO₂, makeup water percentage, redundancy requirements, control scope and the civil/mechanical scope boundary. That is the difference between three quotes you can compare and three quotes for three different farms. See the aquaculture RFQ guide and the RAS planning hub.

FishMatch Group is an independent aquaculture project sourcing and RFQ platform. We are not a fish farm, feed producer, equipment manufacturer, EPC contractor, lender, broker-dealer, investment advisor or financial advisor. Supplier and project partner outreach happens only after a commercial project brief is reviewed. Start with a human-reviewed RFQ.