Recirculating Aquaculture System: How RAS Works

A recirculating aquaculture system (RAS) is a fish farm in tanks where the same water is cleaned and reused again and again. Pumps move water from the fish tanks through a solids filter, a biofilter that turns toxic ammonia into nitrate, and gas and disinfection units, then back to the fish. The USDA Census of Aquaculture defines recirculating systems as tanks, usually above ground and under cover, where 90 percent or more of the system water is recycled. That is the core idea: grow fish anywhere, close to buyers, with tight control of water, waste and disease.

This guide explains how a recirculating aquaculture system works, what each part does, where RAS stands in the United States and the world, and what it costs to run. It also shows the trade-offs: RAS saves water and land but uses much more electricity than ponds or cages. A calculator lets you test a small tilapia unit with your own numbers.

What is a recirculating aquaculture system (RAS)?

In a pond or a sea cage, nature does most of the water treatment. The pond’s algae and bacteria break down waste, and the sea carries it away. In a recirculating aquaculture system, machines do this job. Fish live in round or rectangular tanks indoors or in a covered building. Their water flows out of the tank, is treated, and returns.

NOAA Fisheries describes recirculating systems as “closed-loop” production that continuously filters and recycles water and waste. Only a small share of water is replaced each day, to make up for losses from cleaning the filters and from evaporation. The Southern Regional Aquaculture Center (SRAC) notes that aquaponic systems, which are recirculating systems with plants, run with very little daily water exchange, less than 2 percent.

Because the farmer controls water temperature, oxygen and light, RAS can grow fish all year, in cold or dry places, and near city markets. Common species are tilapia, trout, Atlantic salmon smolts, barramundi, hybrid striped bass, shrimp and ornamental fish.

RAS compared with other ways to farm fish

Meeting mounting global seafood demand sustainably

World demand for fish keeps rising. FAO’s State of World Fisheries and Aquaculture 2024 reports that global fisheries and aquaculture production reached a record 223.2 million tonnes in 2022, including 185.4 million tonnes of aquatic animals. For the first time, aquaculture produced more aquatic animals than capture fisheries: 94.4 million tonnes, or 51 percent of the total. FAO projects that aquatic animal output will reach 205 million tonnes by 2032, with 111 million tonnes from aquaculture.

Wild catches have been fairly flat for decades, so most extra supply has to come from farms. The question is how to grow more fish without more pollution, disease or habitat loss. RAS is one answer, because it keeps waste inside the building and lets farms sit inland, away from sensitive coasts.

The United States shows why this matters. NOAA Fisheries states that the country imports 70 to 85 percent of its seafood, and that about half of the imports are farmed abroad. The US ranks 18th in world aquaculture output. Land-based recirculating farms near big cities are one way to shorten that supply chain.

Chart

World output of aquatic animals, 2022

Million tonnes, live weight; algae excluded

Technological components and integration in RAS

Every RAS has the same basic parts. Size and brand differ, but the treatment steps stay the same.

1. Culture tanks

Round tanks are common because a circular flow carries solid waste to a centre drain. Tank size and the number of tanks depend on how often you want to harvest. Many farms keep several tanks with fish of different ages so they can sell every week.

2. Solids removal

Uneaten feed and fish faeces must leave the water fast. Settling tanks, swirl separators, drum filters and bead filters remove them. If solids stay, they rot, use up oxygen and feed unwanted bacteria.

3. Biofilter

Fish release ammonia through their gills. Ammonia is toxic even at low levels. In the biofilter, nitrifying bacteria living on plastic media with a large surface area turn ammonia into nitrite and then into nitrate, which is far less harmful. FAO’s manual on small-scale aquaponics notes that these bacteria grow best at 17 to 34 °C, and that action is needed when ammonia or nitrite rises above 1 mg per litre. A new biofilter needs several weeks to “cycle” before it can carry a full load of fish.

4. Aeration, oxygen and degassing

Dense fish stocks need a steady supply of dissolved oxygen. Farms use air blowers, diffusers or pure oxygen with cones or contactors. Degassing towers strip out the carbon dioxide that builds up in reused water.

5. Disinfection

Ultraviolet (UV) lamps or ozone kill bacteria and parasites in the returning water. This helps keep disease pressure down.

6. Monitoring and backup

Sensors track oxygen, temperature, pH, ammonia and water level. Alarms and backup generators are essential. In RAS, a pump failure can kill a whole tank of fish within hours, so power backup is part of the design, not an extra.

How dense can fish be stocked?

Stocking density is one of the key design choices. SRAC’s fact sheet on tank culture of tilapia states that highly aerated systems with fish fed for good growth reach a maximum biomass density of about 0.25 pound per gallon (30 kg/m³), and that some operators report 0.5 to 0.6 pound per gallon (60 to 72 kg/m³). The same fact sheet says the daily feed rate for market-size fish is usually 1.0 to 1.5 percent of body weight. Higher density means more output from each tank, but also more oxygen demand, more waste and more risk if anything fails.

Chart

Tilapia tank densities reported by SRAC

Maximum fish biomass, kg per cubic metre of tank water

Recirculating hydroponic systems and aquaponics: maximizing resource efficiency

Aquaponics joins RAS with hydroponics. Water rich in fish waste flows to plant beds. Plants take up the nitrate and other nutrients, and cleaner water goes back to the fish. FAO’s guide to small-scale aquaponic food production describes it as the integration of recirculating aquaculture and hydroponics in one production system.

The best-known commercial design comes from the University of the Virgin Islands (UVI). SRAC’s aquaponics fact sheet describes the UVI system: four fish tanks, solids removal, degassing and six raft beds for lettuce or basil, on about one-eighth of an acre. In the published example, annual output was about 4.16 metric tons of Nile tilapia. The fact sheet reports a break-even price of US$1.47 per pound for tilapia against a sale price of US$2.50 per pound, and US$6.15 per case of lettuce against a sale price of US$20.00 per case. Those are figures from a university budget for the Virgin Islands, not current market prices, and SRAC notes they do not show all capital, operating and marketing costs.

Aquaponics suits leafy greens and herbs best. It also works as a learning tool for schools and small farms. For larger fish output, many growers keep a separate RAS and use only part of the waste water for plants.

Comparative benefits table: RAS, conventional aquaculture, and hydroponics

Environmental stewardship and the blue-green economy

RAS has clear environmental strengths. It uses little water and land, keeps fish away from wild stocks so escapes are rare, and collects waste instead of releasing it into rivers or bays. The collected sludge can go to compost, biogas or fields as a fertilizer, under local rules.

The main weakness is energy. A review of energy use in recirculating aquaculture systems by Badiola and colleagues (Aquacultural Engineering, 2018) explains that RAS cuts water use and land use, but its high energy demand can raise the impacts linked to fossil fuels. The authors looked at published studies and a survey of commercial farms, and saw room for renewable energy.

A meta-analysis of life cycle studies by Clark and Tilman in Environmental Research Letters (2017) found that recirculating aquaculture emitted several times more greenhouse gases per unit of food than non-recirculating aquaculture, because of its high energy needs. So the climate footprint of a RAS farm depends heavily on its power source. Farms that run on low-carbon grids, solar or heat recovery narrow the gap.

Other points to weigh:

• Feed: feed is often the largest operating cost and a large part of the footprint. Feed conversion ratio (FCR), the kilograms of feed per kilogram of fish gained, matters as much as the hardware. The SRAC aquaponics example uses an FCR of 1.7 for tilapia.
• Medicines: closed water and UV disinfection lower disease risk, though outbreaks can still spread fast inside one building.
• Waste water: in the US, discharge from larger farms can need a permit under the National Pollutant Discharge Elimination System (NPDES). Check the rules with your state agency.

Farms that track their emissions can use tools similar to those for carbon footprinting on crop land, and buyers increasingly ask for product traceability from farm to plate.

▶ How satellite vegetation maps show crop stress on fields, such as those that receive fish sludge. · 4:09

Regional and international aquaculture adoption

RAS is used in many countries, mostly where water is scarce, winters are cold or coastal sites are limited.

• United States: The 2023 Census of Aquaculture by USDA’s National Agricultural Statistics Service counted 467 farms with sales that used recirculating systems, up from 370 in 2018. All US aquaculture sales were about US$1.91 billion in 2023, from 3,453 farms. In the census table on production methods, Florida had the most farms using recirculating systems (160).
• Norway and the North Atlantic: salmon farms raise young salmon (smolts) in land-based recirculating hatcheries before moving them to sea cages, and some companies grow salmon to market size on land. FAO’s species fact sheet for Atlantic salmon describes the hatchery and freshwater stage that these systems serve.
• Europe: countries such as Denmark and the Netherlands use RAS for species such as trout and eel, and water rules push farms to treat effluent.
• Canada, Australia and New Zealand: farms grow salmon, trout and barramundi in tanks, often to supply city markets with fresh fish.

In the United States, recirculating farms still make up a modest share of all farms. Ponds remain the most common method, mainly for catfish, crawfish and baitfish.

Chart

US aquaculture farms by production method

Number of farms with sales using each method; one farm can use several methods

Key challenges and future prospects

RAS can work well, but many projects have struggled. The main challenges are:

1. High capital cost. Tanks, filters, pumps, buildings and backup power cost far more per kilogram of capacity than ponds. SRAC’s aquaponics fact sheet notes that these systems need a large capital investment.
2. Energy bills. Pumping, aeration, oxygen and heating or cooling run day and night.
3. Skills. Staff must understand water chemistry, fish health and machinery. Small mistakes can cause large losses.
4. Biological risk. Off-flavour from bacteria, gas imbalance and disease can affect a whole building at once.
5. Market price. RAS fish must sell at a price that covers higher costs. Many farms aim at fresh, local, premium markets.
6. Finance and insurance. Lenders may see RAS as high risk. Good records of output and water quality help, as with crop loan and insurance checks in field farming.

The outlook is still positive. Sensors, automation, better biofilters, renewable power and heat recovery are lowering risk and cost. Data platforms and APIs let farms combine water quality logs with weather and site data, and developers can pull weather through the satellite weather API documentation.

▶ Six plain steps to start precision farming with digital tools. · 4:18

Calculator: a simple RAS fish budget

Use this calculator to test a small RAS on your own numbers. All default values are examples only, set near break-even, and not benchmarks. Use today’s feed and fish prices from your suppliers and buyers. For a real project, get advice from an extension service, a fish health vet and an accountant.

Try the numbers for your own farm below.

How satellite and farm data support aquaculture management

RAS runs indoors, so satellites do not see the fish. But many farms combine RAS with ponds, crop land or plant beds, and site data still matters. Satellite images help with choosing a site, checking nearby land use and water bodies, tracking weather, and watching the fields that receive fish sludge as fertilizer. Water index maps such as NDWI show changes in surface water and moisture in ponds and fields.

Farmonaut Satellite + AI Farm Monitoring System gives crop and land monitoring from Sentinel-2 and other satellites on web and mobile apps. Teams that manage many sites can use the agro admin dashboard, and developers can bring satellite data into their own aquaculture software through the agriculture API.

Related pages on our site: web and mobile apps.

▶ How satellites and the NDWI water index help track water on farms. · 7:57

5 bullet points: the future of RAS in sustainable aquaculture

• Aquaculture now supplies more aquatic animals than wild catch, at 51 percent of the world total in 2022 (FAO).
• Recirculating farms in the US rose from 370 to 467 between 2018 and 2023 (USDA census).
• RAS reuses 90 percent or more of its water, but its energy demand drives its climate footprint.
• Low-carbon power, heat recovery and better feed conversion are the main levers for cleaner RAS fish.
• Aquaponics turns fish waste into plant nutrients and works best for leafy greens and herbs.

FAQ: recirculating aquaculture system

What is a recirculating aquaculture system in simple words?

It is a fish farm in tanks where water is filtered and reused instead of being thrown away. Machines remove waste, turn ammonia into nitrate, add oxygen and kill germs, then send the water back to the fish.

How much water does RAS save?

The USDA defines recirculating systems as those that recycle 90 percent or more of system water. Aquaponic recirculating systems can run with less than 2 percent daily water exchange, according to SRAC.

Which fish grow well in a recirculating aquaculture system?

Tilapia, rainbow trout, Atlantic salmon (especially young smolts), barramundi, hybrid striped bass, sturgeon, shrimp and ornamental fish. The right choice depends on water temperature, market price and local rules.

Is RAS profitable?

It can be, but only with good management and a strong market. Costs for buildings, equipment and energy are high, so the sale price must cover them. Test your own numbers with the calculator above and get professional advice before investing.

What is the difference between RAS and aquaponics?

Aquaponics is a recirculating system that also grows plants. The plants use the fish waste as fertilizer, which helps clean the water.

Is a recirculating aquaculture system better for the environment?

It uses far less water and land and controls waste and escapes. But it uses more energy, and a 2017 meta-analysis found higher greenhouse gas emissions per unit of food than non-recirculating systems. Clean power makes a big difference.

Conclusion: charting a sustainable course for global aquaculture

A recirculating aquaculture system lets farmers grow fish almost anywhere, with tight control of water, waste and fish health. Aquaculture already provides more than half of the world’s aquatic animal output, and that share is set to grow. RAS will be part of that growth, especially near cities and in places short of water or coastline. Its success depends on lower energy use, cleaner power, skilled staff and careful budgets. Plan carefully, start with proven designs, and check local rules before you build.