Sizing a RAS: From Feed Load to Oxygen Demand

Every reliable RAS design starts from feed. Feed drives biomass, biomass drives oxygen demand, oxygen consumption drives CO2 production, and protein metabolism drives ammonia excretion. Once daily feed load is fixed, the sizing of every major subsystem follows. This article walks through the calculation as it should appear in a bankable technical package.

Feed load and biomass

Start with the target production. For a 1,000 t/year facility with a design FCR of 1.1 and a 340-day operating year, the average daily feed load is approximately 3.2 t/day. Peak feed load — the number that sizes the equipment — is typically 1.4–1.6× the average, depending on cohort structure and grading schedule. A design that sizes to the average and not the peak will run out of oxygen or biofilter capacity at every peak.

Oxygen demand

Fish consume roughly 0.25–0.35 kg of oxygen per kg of feed, depending on species, temperature and activity. On top of that, nitrification in the biofilter consumes 4.57 kg O2 per kg of TAN oxidised. For a peak feed load of 5 t/day and a TAN generation rate of 30 g per kg of feed, total oxygen demand is approximately 1,750–2,000 kg O2/day. Oxygen supply must meet this demand with N+1 redundancy and hold dissolved oxygen above the species-specific minimum at the tank outlet, not the inlet.

TAN, CO2 and biofilter sizing

Total ammonia nitrogen (TAN) generation is roughly 30 g per kg of feed for high-protein diets. Biofilter media is sized by volumetric TAN removal rate at the design temperature — a moving bed biofilter typically achieves 0.3–0.9 g TAN/m² media/day at 15 °C, rising with temperature. Undersizing the biofilter is the single most common RAS design error and is invisible until stocking density approaches design.

CO2 production is approximately 1.375 kg CO2 per kg O2 consumed. Degassing capacity must hold CO2 below 15 mg/L for most species and below 10 mg/L for sensitive life stages. Degassing is frequently value-engineered out of first-generation designs and added back at 3–5× the original cost after commissioning.

Make-up water and effluent

Make-up water is set by nitrate accumulation, salinity drift and solids balance. A design targeting <100 mg/L nitrate-N without denitrification typically requires 300–500 L of make-up per kg of feed. Effluent treatment must meet the local discharge limit for total nitrogen, phosphorus and suspended solids — a constraint that is often discovered only during permitting, after equipment is already ordered.

From calculation to RFQ

The output of this sizing exercise is a one-page mass and energy balance that becomes the front page of every RFQ. Vendors quote against the balance rather than against a wish list. Lenders and technical due diligence reviewers expect to see it. Projects that cannot produce it on request are, by definition, not yet ready to procure.

Continue in the RAS planning cluster: RAS Design Fundamentals: What to Lock Before You Buy Equipment and Dissolved Oxygen Management for Bankable Aquaculture Farms. Pillar guide: Why Successful Aquaculture Projects Begin Long Before Equipment Is Purchased. See the full outline in the RAS Planning Topic Cluster.