Atlantic salmon
Salmo salar
The flagship RAS species. Several facilities exceed 5,000 t/year; smolt production in RAS is standard and grow-out is commercially proven. High value, about €6–9/kg.
12–16 °C · 0–35 ppt
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- Water Treatment
- Aquaculture (RAS)
Sustainable aquaculture technology
Recirculating aquaculture systems.
Fish and seafood grown on land in a closed loop, with water treatment keeping conditions right while recirculating up to 99% of the water. Year-round production, biosecurity, minimal discharge and full traceability, anywhere.
- Water recirculated and reused
- Up to 99%
- Days a year of production
- 365
- kg/m³ stocking density (ponds: 1–5)
- 40–120+
- Water per kg of fish
- 100–500 L
01 · The concept
A fish farm that keeps its water.
A RAS is a land-based fish farm where water is continuously cleaned and recycled through a series of treatment stages instead of being discharged. Fish are raised in tanks at high density, and treatment holds conditions steady around the clock.
The closed-loop principle
Typically 95 to 99% of the water is recirculated every day. Only 1 to 5% is replaced as make-up water, for evaporation, splashing and water removed with sludge. The footprint can be as low as 200 to 500 litres per kilogram of fish, against 5,000 to 50,000 L/kg in conventional aquaculture.
Location independence
Because RAS controls every parameter, temperature, salinity, oxygen and pH, a facility can be anywhere: deserts, cities, cold climates, landlocked countries. Production moves close to the market, food miles fall, and Norwegian salmon can be grown year-round in the middle of a continent.
02 · Water treatment train
Six stages, every litre, every lap.
Stage 1 of 6
Mechanical filtration
Faeces, uneaten feed and mucus must be caught before they decompose into ammonia. A rotating drum filter, usually 60 to 100 µm mesh, sieves solids out; swirl separators and bead filters handle coarse screening. Delay raises ammonia and oxygen demand.
- TSS, faeces, feed
- 40–120 µm screen
- Remove within minutes
Stage 2 of 6
Biofiltration: nitrification
Fish excrete ammonia through their gills, and it is toxic at very low levels. Nitrifying bacteria, mainly Nitrosomonas and Nitrospira, oxidise it to nitrite, then to far less toxic nitrate, on high-surface media (MBBR or fixed bed). The most critical and most sensitive component: colonies take weeks to establish.
- NH₃ → NO₂⁻ → NO₃⁻
- 200–1,000 m²/m³ media
- DO above 4 mg/L
Stage 3 of 6
CO₂ stripping & degassing
Respiration loads the water with CO₂. Above 15 to 20 mg/L for salmonids (30 to 35 for warm-water species) it causes hypercapnia and lowers pH. A degassing tower or cascade aerator drives it off, along with excess nitrogen that could cause gas bubble disease.
- Removes CO₂ and excess N₂
- Target below 15 mg/L (salmonids)
Stage 4 of 6
UV disinfection
In warm, nutrient-rich recirculating water, even a small pathogen load multiplies fast. UV inactivates bacteria, viruses and parasites before water returns to the fish, sized for at least 40 mJ/cm², often 80 to 120 in marine systems. Medium-pressure UV covers VHS and ISA viruses.
- 40–120 mJ/cm²
- LP or MP lamps
- No chemical residuals
Stage 5 of 6
Oxygenation
Fish and bacteria consume oxygen constantly, beyond what aeration can supply at RAS densities. Pure oxygen through pressurised cones, Speece cones or U-tubes holds dissolved oxygen at 90 to 110% saturation, from liquid oxygen or an on-site concentrator. The largest operating cost after energy.
- DO above 80% saturation
- LOX or PSA oxygen
Stage 6 of 6
pH & alkalinity control
Nitrification is acid-generating: each gram of ammonia nitrogen oxidised consumes about 7.1 g of alkalinity as CaCO₃. Sodium bicarbonate, calcium carbonate or sodium hydroxide dosing keeps pH at 7.0 to 8.0 and alkalinity above 100 to 150 mg/L, under automatic control.
- pH 7.0–8.0
- Alkalinity above 100 mg/L CaCO₃
03 · Biofiltration
The nitrogen cycle decides whether fish live.
Biological nitrification is the single most critical process in any RAS.
- NH₃ / NH₄⁺Ammonia from the gills. Acutely toxic.
- NO₂⁻Nitrite. Nitrosomonas step.
- NO₃⁻Nitrate. Nitrospira step. Far less toxic, but accumulates.
- N₂Removed by denitrification, or diluted by water exchange.
MBBR, moving bed biofilm reactor
The dominant biofilter in modern RAS. Plastic carriers of 25 to 50 mm tumble freely, driven by aeration or mixing, and nitrifying bacteria colonise their protected surfaces. The biofilm self-regulates: the outer layer sloughs off, preventing diffusion limits. Fill of 30 to 70% of reactor volume is typical.
Managing nitrate
Ammonia and nitrite are continuously converted; nitrate builds up. It is managed by daily water exchange of 1 to 5% of volume, or by denitrification reactors where bacteria turn NO₃⁻ into N₂ gas using methanol or acetic acid as a carbon source. Targets: below 100 to 200 mg/L for most species, below 50 for sensitive fish and shrimp.
04 · Water quality
Critical parameters, watched continuously.
Monitoring is continuous and automated, because deviations can kill fish within hours.
| Parameter | Target range | If out of range | Importance |
|---|---|---|---|
| Dissolved oxygen | >80% sat. / >8 mg/L | Hypoxia: lethargy, feed refusal, mortality | Critical |
| Total ammonia N (TAN) | <1.0 mg/L | Gill damage, immune suppression, neurotoxicity | Critical |
| Un-ionised ammonia (NH₃) | <0.05 mg/L | Acute toxicity threshold | Critical |
| Nitrite (NO₂⁻) | <0.1 mg/L | Methaemoglobinaemia: blood cannot carry oxygen | Critical |
| CO₂ (dissolved) | <15 mg/L salmonids, <35 warm-water | Hypercapnia: impaired O₂ uptake, lower blood pH | High |
| pH | 7.0–8.0 | Inhibits nitrification, shifts NH₃ fraction, gill damage | High |
| Temperature | Species-specific, e.g. 12–16 °C salmon | Poorer feed conversion, immune suppression, disease | High |
| Nitrate (NO₃⁻) | <100–200 mg/L | Chronic stress, reduced growth | Moderate |
| Alkalinity | >100 mg/L CaCO₃ | pH instability, nitrification collapse | Moderate |
| Suspended solids (TSS) | <15–25 mg/L | Gill irritation, biofilter clogging, pathogen shielding | Moderate |
05 · Technology comparison
RAS against ponds and flow-through.
When RAS makes economic and environmental sense, compared honestly with the alternatives.
| Criterion | Open pond / cage | Flow-through | RAS |
|---|---|---|---|
| Water use (L/kg fish) | 10,000–50,000 | 500–5,000 | 100–500 |
| Location | Site-dependent | Near a water source | Anywhere on land |
| Biosecurity | Open to the environment | Inlet risks | Closed loop |
| Environmental impact | Nutrient discharge, escapes | Significant effluent | Minimal discharge |
| Density | 1–5 kg/m³ | 10–40 kg/m³ | 40–120+ kg/m³ |
| Year-round control | Seasonal, weather-limited | Limited temperature control | Full climate independence |
| Capital cost | Low | Low to moderate | High (€2–10M+ per 1,000 t) |
| Operating cost | Low | Moderate | High: energy, oxygen, chemicals |
| Antibiotic use | Often high | Moderate | Very low to zero |
| Regulatory outlook | Increasing restrictions | Moderate scrutiny | Aligned with regulation |
When RAS pays
RAS becomes viable when land or water is scarce or expensive, when proximity to market earns a premium, when the species justifies the capital (salmon, eel, shrimp, seabass), when biosecurity is paramount, or when regulation prohibits open-water farming. Energy, at 3 to 10 kWh per kg of fish, is the critical variable: access to low-cost renewable energy changes the economics.
06 · Species & markets
What grows in a RAS.
From high-value marine fish to efficient freshwater species.
Temperature, salinity and chemistry are designed around each species' biology.
Yellowtail amberjack
Seriola lalandi / S. quinqueradiata
Premium marine species with strong growth in RAS. Major production in Japan, expanding globally; efficient feed conversion and disease resistance.
18–24 °C · 30–34 ppt
Nile tilapia
Oreochromis niloticus
The workhorse of tropical RAS: tolerant of fluctuations, fast-growing, efficient. Produced for fresh local markets across North America, Europe and Asia.
26–30 °C · Freshwater
Pacific white shrimp
Litopenaeus vannamei
A growing sector producing consistent, antibiotic-free fresh shrimp. Sensitive to ammonia and nitrite; local fresh shrimp earns a significant premium.
28–30 °C · 5–25 ppt
Seabass & seabream
Dicentrarchus labrax / Sparus aurata
Premium Mediterranean species increasingly produced in inland RAS across Europe, with strong demand for live and fresh product.
20–24 °C · 28–34 ppt
European eel
Anguilla anguilla
Wild eel is critically endangered; RAS is the only ethical route. Very high value, €10–30+/kg, with slow growth of two to four years.
22–26 °C · Freshwater
Rainbow trout
Oncorhynchus mykiss
Established in flow-through and RAS. Tolerates density, converts feed well, and RAS makes its cold-water needs possible in warm climates.
10–15 °C · Freshwater
Sturgeon & caviar
Acipenser spp.
The only sustainable caviar production, as wild sturgeon are endangered. Five to fifteen years to maturity, offset by extraordinary product value.
18–22 °C · Freshwater
Cobia & emerging species
Rachycentron canadum and others
Cobia reaches 4–5 kg in 12 months. With barramundi, snook and halibut, these are being pioneered in RAS as operators diversify.
24–30 °C · Marine
Choosing a species is choosing a water specification. We design the treatment train around it.
07 · Challenges & innovation
The engineering is the hard part.
RAS is complex and capital-intensive, and it is evolving fast as new technology lowers cost and raises reliability.
- 01
Energy
Typically 3 to 10 kWh per kg of fish: pumping 30%, oxygenation 25%, heating and cooling 20%, air handling 15%, lighting 10%. Answers include heat recovery, waste-heat use, variable-speed pumps and renewable co-location.
- 02
Capital cost & scale-up
€2–10M per 1,000 t of annual capacity. First-generation large projects saw significant overruns; standardised equipment, digital-twin validation and phased commissioning now reduce engineering risk.
- 03
Biosecurity & disease
A closed system has no dilution, so an outbreak can be catastrophic. Strict protocols, multi-barrier UV and ozone, microbiome management and early-warning monitoring, even real-time PCR on water samples, are critical.
- 04
Sludge & nutrients
5 to 15 kg of solids per tonne of fish must be dewatered and put to use: compost for fertiliser, anaerobic digestion for biogas, or aquaponics.
- 05
Digitalisation & AI
Dozens of sensors, feed systems and biomass cameras generate data continuously. Machine learning now warns of water-quality drift, predicts drum-filter and UV maintenance, optimises feed and estimates biomass, cutting reaction times from hours to minutes.
- 06
Aquaponics
Fish and hydroponic plants share one loop: fish nutrients fertilise leafy greens, herbs and tomatoes, which partly clean the water on its way back. Less waste and a second revenue stream.
An industry scaling fast
Landmark projects including Atlantic Sapphire, Salmon Evolution and Nordic Aquafarms are demonstrating industrial scale. More than 200 large-scale RAS projects are in planning, construction or early operation, and analysts project RAS to supply 10 to 15% of global salmon by 2030, up from about 2% in 2022.
RAS questions.
The questions we are asked most, answered briefly.
How much water does a RAS recirculate?
Typically 95 to 99% every day. Only 1 to 5% is replaced to make up for evaporation, splashing and water removed with sludge, which can mean as little as 200 to 500 litres per kilogram of fish.
What are the treatment stages in a RAS?
Mechanical filtration, biofiltration (nitrification), CO2 stripping and degassing, UV disinfection, oxygenation, and pH and alkalinity control, in a continuous loop back to the fish tanks.
When does RAS make economic sense?
When land or water is scarce or expensive, when being close to market earns a premium, when the species is valuable enough to justify the capital cost, when biosecurity is paramount, or when regulation prohibits open-water farming. Energy cost, typically 3 to 10 kWh per kg of fish, is the critical variable.
Can RAS be built in a hot or dry climate?
Yes. Because a RAS controls temperature, salinity, oxygen and pH, it can be located anywhere, including deserts, cities and landlocked countries, and cold-water species can be grown year-round.
Designed for your species, site and scale.
Tell us what you want to grow, and where. Book a meeting and we will confirm within minutes, or write to us on WhatsApp.
Denmark and Saudi Arabia. Serving the European Union and the Middle East, in English and Arabic.
- WhatsApp+966 55 455 4158
- Phone+966 55 455 4158
- Phone+966 53 968 6765
- Emailcontact@aggregatorsolutions.com
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Water quality parameters and performance figures are indicative and species-dependent. System design must be validated by qualified aquaculture engineers for each project. Fish welfare, veterinary and regulatory requirements vary by jurisdiction and species.