What does RAS aquaculture mean in engineering terms?

A Recirculating Aquaculture System (RAS) is a closed-loop water treatment train that reuses 90–99% of system water per day. Water leaving the tanks passes through solids removal, biological nitrification, degassing, oxygenation and disinfection before returning to the culture tanks. Design is driven by feed load (kg/day), not by tank volume alone.

What is the correct treatment sequence in a RAS loop?

The standard sequence is: culture tank → dual-drain solids capture → drum filter (40–100 µm) → moving-bed or fixed-bed biofilter → CO₂ stripper / degasser → oxygenation (LHO or oxygen cone) → optional UV or ozone → back to tank. Solids must be removed before the biofilter; reversing that order overloads the media and destabilises nitrification.

How is biofilter capacity sized?

Biofilter media area is sized from daily feed load and the media's areal TAN removal rate (typically 0.3–0.9 g TAN/m²/day at 12–16 °C, higher in warm-water shrimp and tilapia systems). Rule of thumb: 1 kg of feed produces roughly 30 g TAN. Always size for peak biomass at the end of the production cycle, plus a safety margin of 20–30%.

How much oxygen does a land-based system need?

Budget approximately 0.4–0.5 kg of oxygen per kg of feed for salmonids, and 0.3–0.4 kg/kg for warm-water species. Oxygen transfer efficiency should be verified at the design DO setpoint (usually 90–110% saturation at tank inlet, never below 70% at outlet). Backup oxygen supply with independent alarms is a bankability requirement, not an option.

What CAPEX benchmarks apply to land-based projects?

Indicative, project-dependent ranges for a fully built facility (equipment, buildings and civil works, excluding land): land-based salmon grow-out USD 25,000–45,000 per annual tonne; smolt/post-smolt USD 18,000–30,000 per annual tonne; intensive indoor shrimp USD 12,000–25,000 per annual tonne. Equipment alone is typically 35–50% of total CAPEX. Use these only for early screening; a site-specific engineering estimate is required before financing.

How much makeup water does a RAS need?

Well-designed systems replace 2–10% of system volume per day depending on species, denitrification and TSS management. Lower exchange saves heating and pumping cost but raises nitrate, alkalinity and off-flavour management demands — most projects budget for alkalinity dosing and, above ~95% reuse, a denitrification stage.

Which risks most often break RAS business cases?

In order of frequency: undersized biofilter for final-stage biomass, energy cost assumptions (pumping plus heating/chilling can exceed 20% of OPEX), off-flavour and depuration capacity, mortality events from single-point oxygen or power failure, and understated commissioning periods. Financiers stress-test all five.

Engineering guide

RAS Aquaculture Design Guide: Sizing, Equipment Selection and CAPEX Benchmarks

A supplier-neutral primer on recirculating aquaculture system design for land-based salmon, tilapia and shrimp projects — how the treatment loop is sequenced, how each stage is sized, and what capital cost ranges to use for early project screening.

1. Start from feed load, not tank volume

Every RAS component is sized from peak daily feed. Feed determines TAN production, oxygen demand, CO₂ generation and solids load. Fix your target annual tonnage, biomass curve and feed conversion ratio first; only then size tanks, pumps and treatment units. Projects that begin with a tank layout almost always end up with an unbalanced loop.

2. Solids removal comes first

Dual-drain tanks capture settleable solids at the centre; drum or belt filters at 40–100 µm remove the remainder. Fine solids that pass the filter become biofilter fouling, oxygen demand and gill irritation. Specify filter capacity at peak flow with one unit out of service, and plan sludge handling and dewatering at design stage — permitting frequently hinges on it.

3. Nitrification: the core of the loop

Moving-bed (MBBR) and fixed-bed reactors both work; the decision is driven by footprint, energy, and operator skill. Size on areal TAN removal at design temperature and pH, and protect alkalinity — nitrification consumes roughly 7.05 g of alkalinity as CaCO₃ per gram of TAN oxidised. Provide a documented biofilter maturation plan for start-up; a 4–8 week ramp is normal.

4. Degassing and oxygenation

CO₂ accumulates fastest in high-reuse systems and limits stocking density long before ammonia does. Strippers should hold CO₂ below 10–15 mg/L for salmonids. Oxygen is added after degassing, using low-head oxygenators or cones with measured transfer efficiency. Redundant supply, independent DO probes and alarm-triggered emergency diffusion are standard in bankable designs.

5. Water exchange, nitrate and off-flavour

Reuse rate sets your operating cost and your risk profile. Above ~95% reuse, nitrate control usually requires denitrification, and geosmin/MIB management requires dedicated purge tanks with clean water and 3–10 days of depuration. Model heating, chilling and pumping energy with local tariffs — energy is the largest controllable OPEX line in most temperate projects.

6. CAPEX, OPEX and bankability

Split the budget into civil works, process equipment, buildings and services, and contingency (10–15% minimum). Lenders look for a mass-and-energy balance, an independent engineer's review, a commissioning schedule with biological ramp-up, and evidence of supplier capability and after-sales support. Equipment selection should be justified on process duty, not on brand preference.