RAS System

Overview

Recirculating aquaculture systems (RAS) are closed-loop fish farms that treat and reuse water continuously, enabling high-density farming in land-based facilities far from coast or rivers. Unlike traditional pond or net-pen farming, RAS circulates water through mechanical filters, biofilters, and aeration units, removing waste while maintaining dissolved oxygen and nitrifying ammonia. This control makes RAS suitable for premium species (Atlantic salmon, sea bass, sea bream) and reduces disease pressure and regulatory risk compared to open-water systems.

A typical RAS comprises rearing tanks (50–200 m³ each), a drum filter for solids removal, a moving-bed or fixed-bed biofilter for ammonia conversion, oxygen injection, and an automated control system monitoring dissolved oxygen, pH, temperature, and ammonia in real time. The entire water column is recirculated 3–5 times daily, ensuring no stagnant zones.

How it works

Fish in rearing tanks release ammonia (NH₃) and suspended solids as waste. The Circulation Pump pulls water from the tank into the Drum Filter, where a rotating microscreen (100–200 µm) traps food waste, feces, and dead matter. The drum rotates continuously (8–12 rpm) and undergoes timed backwash cycles to eject collected solids and maintain low pressure drop.

Filtered water flows to the Biofilter, a vessel packed with plastic media or ceramic rings. Here, nitrifying bacteria (Nitrosomonas and Nitrobacter) oxidize ammonia to nitrite, then to harmless nitrate. This biological nitrification requires oxygen; the Air Diffuser System diffuser system provides 0.5–1.5 m³ air per m³ biofilter volume. Additional Oxygen Injection System oxygen (pure O₂ at 0.5–2 bar) is injected directly into the main water circulation or tank inlet to maintain dissolved oxygen above 7 mg/L.

The Degasser Unit strips excess CO₂ and volatile compounds, stabilizing pH. Water returns to the Rearing Tanks via the Supply Line, completing one recirculation cycle every 4–8 hours.

The Control & Monitoring System monitors four critical parameters:

• Dissolved oxygen (DO) via optical or electrochemical probe, target 7–9 mg/L
• pH via glass electrode, maintained 6.8–7.4 by degassing and alkalinity dosing
• Temperature via RTD or thermistor, held ±1°C by chiller or heater
• Ammonia via ion-selective electrode (ISE) or spectrophotometry, target <1 mg/L

The PLC adjusts oxygen injection rate, biofilter blower speed, and solenoid backwash timing based on real-time feedback, minimizing manual intervention and preventing fish stress or mortality from poor water quality.

Design considerations

System scale and redundancy. A single 100 m³ tank with full treatment equipment costs 1–1.5 million EUR. Larger facilities (500–1000 m³) achieve economies of scale on equipment and labor, but add single-point-of-failure risk if the drum filter or biofilter fails. Site-critical systems install backup pumps, oxygen supplies, and generators.

Species and stocking density. Salmon require higher oxygen (8–9 mg/L) and cooler water (12–16°C) than tilapia (6–7 mg/L, 26–28°C). Stocking density ranges 50–150 kg/m³; denser stocks demand larger biofilters and higher aeration, increasing operational cost.

Biofilter surface area. Nitrification capacity scales with bacterial attachment surface (200–400 m²/m³ for K3/K5 media). Undersized biofilters allow ammonia accumulation; oversized ones increase cost and oxygen demand. A rule of thumb: 10–15 m³ biofilter per 100 m³ rearing capacity.

Energy footprint. Circulation pump (15–30 kW), biofilter blower (3–7 kW), aeration/oxygen (2–5 kW), and chiller (if needed) consume 25–50 kWh per metric ton of fish produced. Cold-water species (salmon) require chiller energy; warm-water species (tilapia) may save energy in tropical climates.

Discharge and waste. RAS produces concentrated sludge (50–80 kg/day in a 100 m³ system), containing uneaten feed and feces. This sludge can be dewatered and composted, used as biodigestion feedstock, or applied to land as fertilizer. Nutrient-dense discharge also attracts aquaponics integration, growing vegetables in the ammonia-rich water.

Standards and regulation

RAS facilities typically comply with:

• ISO 14644 cleanroom classifications for hatchery systems (controlled microbial environments)
• IUCN Best Practice guidelines on species introduction and escapement prevention
• Local environmental permits for discharge quality (typically <2 mg/L total nitrogen in treated effluent)
• Food safety (EU Reg. 853/2004 for farmed fish intended for human consumption)