How to Size a RAS Project: Feed Load, Biomass, TAN and Flow Calculations

Reliable RAS sizing does not convert annual tonnage directly into an equipment model. It begins with a production schedule and peak simultaneous biomass, followed by separate mass balances for maximum daily feed, TAN, oxygen, solids and hydraulic flow.

Start with the production plan

A target of 100 tonnes per year does not mean that 100 tonnes of fish are present on any one day. Batch count, stocking weight, harvest weight, growth period, survival, grading and harvest intervals determine simultaneous biomass. The first sizing document should therefore be a production calendar.
For each month or week, the plan should show fish count, average weight, total biomass and feed input. The design point is not the annual average; it is the highest simultaneous biomass and maximum sustainable daily feed load the facility will experience.

1. Maximum biomass and tank volume

Core relationship: Required production-water volume (m³) = maximum biomass (kg) ÷ design stocking density (kg/m³).

Stocking density is not merely the amount of fish that physically fits in a tank. Species oxygen demand, swimming behaviour, feeding method, minimum outlet DO, CO₂, solids transport, grading frequency and operator experience must be assessed together. A single universal kg/m³ value should not be applied across projects.

2. Maximum daily feed load

Core relationship: Daily feed (kg/day) = system biomass (kg) × daily feeding rate (% biomass/day).

Feeding rate varies with species, live weight, water temperature, feed energy density and growth target. Treatment equipment must be checked against the highest sustainable daily feed load—not the average—because solids, TAN, oxygen demand and CO₂ production are all closely linked to feeding.

3. TAN load and biofilter capacity

Part of the nitrogen in consumed protein becomes growth, while part is excreted into the water as total ammonia nitrogen (TAN). Preliminary sizing combines daily feed with protein and nitrogen content, digestibility and the selected nitrogen-excretion coefficient. The coefficient depends on feed formulation and species, so one fixed factor should not be used for every project.

Biofilter selection must consider specific media area, validated nitrification rate, temperature, pH, alkalinity, dissolved oxygen, organic loading and start-up maturity—not only vessel volume. Atlas Aqua biofilter solutions can be configured around the project feed load.

4. System flow and tank turnover

Preliminary check: Recirculation flow (m³/h) = system or tank volume (m³) × target turnovers per hour.

Turnover is only a starting check. Final flow must be validated from the DO drop through the tank, CO₂ and TAN concentration difference, solids transport, filter surface loading and hydraulic elevations. The basis is measured site flow after pipe and valve losses—not the pump’s nameplate flow.

Mechanical-filter capacity is selected from actual flow, maximum solids load, screen opening, allowable head difference and backwash demand. Atlas Aqua drum filters are available in project-specific flow and micron configurations.

5. Oxygen and carbon-dioxide balance

Total oxygen demand includes fish respiration, the post-feeding metabolic peak, biofilter nitrification and microbial degradation of organic matter. A feed-based oxygen calculation is a strong primary method, but the result must also be checked against biomass, species, temperature and life stage.

Gas capacity alone does not define oxygen-transfer performance. Water flow, inlet DO, target outlet DO, pressure and actual transfer efficiency must be assessed together. Depending on the project, an oxygen cone or NANOB may be used. CO₂ requires a separate degasser capacity calculation.

6. Make-up water requirement

RAS achieves high water reuse, but make-up water is not zero. Demand depends on nitrate accumulation, salinity and mineral balance, drum-filter backwash, sludge purge, evaporation, fish-health policy and discharge strategy. Source water must also be analysed for temperature, pH, alkalinity, hardness, iron, manganese, ammonia, gas saturation and microbiology.

Sizing control table

Calculation stage	Primary design data
Production model	Batch count, stocking/harvest weight, growth period and survival
Tank system	Peak biomass, species, stocking density and tank geometry
Mechanical filtration	Actual flow, solids load, screen micron and backwash
Biofilter	Maximum feed, TAN load, temperature, pH and alkalinity
Oxygenation	Feed peak, biomass check, inlet/outlet DO and transfer efficiency
Degassing	CO₂ production, water flow, gas-transfer area and air ratio
Make-up water	Nitrate, purge/backwash, evaporation and source-water quality
Redundancy	N+1 pump/blower, emergency oxygen, generator and alarms

Why a blanket safety factor is not enough

Oversizing every component indiscriminately can increase capital and energy costs and create hydraulic imbalance. Safety margins should be applied to known failure modes. Pump N+1 redundancy, biofilter allowance for peak feed and maturity, and oxygen capacity for feeding peaks and emergency supply are separate engineering decisions.

Frequently asked questions

Can RAS capacity be selected from annual production tonnage?

Annual tonnage is an initial input. Equipment cannot be selected reliably without peak simultaneous biomass, daily feed, production cycle and harvest plan.

Should oxygen demand be calculated from feed or biomass?

Feed represents the main process load well, while a biomass-based check captures basal respiration, species, temperature and life stage. Use both approaches together.

Is tank-volume-based pump sizing sufficient?

No. Turnover is a preliminary check. Final flow must also satisfy oxygen, CO₂, TAN, solids transport and actual pipe-loss requirements.

Why does a biofilter not operate at full capacity on day one?

Nitrifying bacteria need time to colonise media and establish a stable biofilm. Feed must be increased gradually while TAN and nitrite are monitored.

FOR YOUR PROJECT: To convert your production schedule, peak feed load and site data into verified RAS equipment capacities, Contact Atlas Aqua.