Recirculating Aquaculture Systems

Recirculating Aquaculture Systems

Recirculating aquaculture systems (RASs) permit farmers to rear fish in an indoor facility and locations where water quality or quantity would otherwise preclude commercial culture.

Recirculating Aquaculture Systems

Recirculating aquaculture systems (RASs) permit farmers to rear fish in an indoor facility and

locations where water quality or quantity would otherwise preclude commercial culture,

permitting higher stocking densities and more complete environmental control. An outside

water source is necessary to replace water lost to evaporation and waste removal. Unlike

flow-through systems, RASs require filtration to remove particulate matter, nitrogen

compounds, and other solid and dissolved waste.

RASs are the most effective way to culture large quantities of fish in limited space. Unlike

other systems, RASs may be manipulated to manage temperature, water quality, and disease.

This is especially advantageous in the northeast where temperature fluctuates throughout the

year and the growing season is limited to a few months annually. In RASs, fish growth can

be maintained at an optimal rate year round and due to its flexibility, growers can rear cold

water, warm water or ornamental species.

RASs are more water efficient in terms of use of water per unit of production than other

systems, losing 5% or less of the culture system water volume. Lower water use also results

in reduced water discharge requirements. Collection of solids in RASs allows for higher

stocking densities. In RASs, 1/2 lb of fish of more can be grown in one gallon of water,

which is eight times more efficient than pond culture. However, increased production

comes with increased management and production costs. RASs are complex systems that

require considerable knowledge, commitment and resources to operate profitably.

Recirculating Aquaculture System (RAS) – System Design

Management Considerations

Because less space and land is required for RASs than other aquaculture systems, producers

have greater control and can produce more fish in less area. System size, species cultured,

economical feasibility and available land will determine system design. Growers should

research markets and system designs meticulously before committing to construction.

System components include culture tanks, particle filter (usually a screened drum filter) or

settling tank, bio-filter with chosen media (e.g. plastic, beads, or sand), heating or cooling

systems, pumps (including water circulation pump and sump pump), aeration/oxygenation

supply, appropriate lighting, CO2 stripper, waste sump and disinfection system (e.g. UV or

ozone, though ozone is not commonly used in South Africa).

Primary design considerations are water supply, culture tanks, filtration and waste discharge.

Water Supply

Water can be obtained from groundwater or municipal sources. Water from municipal

systems will have greater costs and may require additional permits. All water sources should

be investigated for possible contamination.

Culture Tanks

The size of the system, desired production goals and the cultured species determine the size

and shape of culture tanks. Any size or shape tank can be used but certain species may grow

more efficiently in specific tank designs. Generally, cylindrical tanks are preferable to other

shapes as they yield more uniform water quality and lend themselves to more efficient

aeration/oxygenation and waste removal than other geometric shapes. In recent times the use of mixed-cell raceways (MCR) has started to gain popularity.


Different types of waste are collected through different methods. Particulate matter,

composed largely of feces and uneaten food particles, is removed through such means as

settling tanks, mechanical (drum) filters or sand filters. Metabolic nitrogenous wastes

(ammonia and nitrite) are converted to non-toxic nitrate by bacteria contained within the

biofilter. Suspended solids are removed by bead filters, foam fractionation or other

flocculant-generating method. Dissolved solids are addressed through foam fractionation or


Waste Discharge

In some cases RASs may be allowed to discharge into municipal waste systems. This option

may be cost prohibitive in most cases.

One method of waste treatment for a RAS is the use of ponds or manure collection tanks.

Solids collected can be applied to land as an agricultural by-product. Treated waste water

should be discharged or reused in the system.

Constructed Wetlands (CWL) can be used to digest waste from a RAS. Solids are collected in

a lined bed where wetland plants are cultivated to digest solids and water is passed to a

secondary polishing bed to further filter the water. The size of a CWL will vary depending

upon the waste load.

Best Management Practices

• Full time maintenance with levels of backup is required for a RAS.

• Alarm systems should be installed for critical factors at appropriate locations such as

pump pressure, water flow, temperature, dissolved oxygen, and water level.

• Back-up generators and/or oxygen should be kept available to prevent loss in case of

power failure.

• Lighting should be provided when fish are raised species indoors.

• UV or ozone filtration is beneficial in some system designs.

• CO2 stripping may be required in a RAS due to high amounts fish waste and higher

stocking densities (> 0.5 lbs/gal).

Recirculating Aquaculture System (RAS) - Waste Management

Management Considerations

Waste management in RASs should be monitored in three parts: reducing waste in the

culture system, collection of solid waste outside the culture system, and treatment of

discharge water.

Waste Management Issues

o Feeding Rates

o Solids management

o Metabolite management

o Collection/ discharge of waste

The use of high quality feed is essential. Industry feed rates should be followed for a

particular species. Proper solids removal and maintenance of biological filters system will

help to eliminate undesirable organics from the culture system. Flushing or backwashing of

the biological filter may be required; frequency of flushes will depend on feeding rate and

filter design.

Solids removed from RASs should be collected and disposed appropriately. Disposal

methods are regulated by local, state and federal authorities. Wastes can sometimes be

discharged into municipal waste systems, but this is often expensive. Solids can be applied as

a soil amendment on local farms or used in composting operations. Waste water can be

reused in the system after proper treatment or discharged.

Best Management Practices

• Proper feed management should be employed to reduce generation of waste.

• Filtration systems (solids removal and biological filter) should be maintained and

monitored on a regular basis.

• Flow from the biofilter to the tanks should be adequate to maintain industry

standard water quality parameters for species cultured.

• Solids should be collected regularly and properly discharged.

Recirculating Aquaculture System (RAS) - Water Management

Management Considerations

A RAS recycles 90-99% of its water, thus, compared to FTS or pond culture very little

“make-up” water is needed. However, water lost to evaporation and solids removal must be

replaced. During nitrification, ammonia is released and CO2 is lost from the system,

diminishing buffering capacity of the water, leading to lower pH. Thus the grower must

monitor and manage water quality by the use of buffers and other additives.

Water Management Issues

o Species selection

o Water quality testing

o Alarm systems

Maintenance of water quality is essential: the same water is used continuously in the system.

Water quality in RASs primarily depends on the quality of recirculated water and the efficacy

of water treatment systems. Water chemistry can change on an hourly basis as fish feed,

metabolize and excrete: a number of water quality parameters should be monitored daily or

more frequently. These parameters include; ammonia, nitrite, nitrate, alkalinity, pH, dissolved

oxygen, turbidity and temperature. Suitable water quality parameters will depend on the fish

species cultured.

An alarm system should be set up to monitor and signal when temperature, dissolved oxygen

(DO) and pH are outside desired ranges. This allows for constant monitoring of critical

water parameters crucial to fish health. Optimally, corrective measures should be initiated

immediately when a critical parameter exceeds the desired range.

Best Management Practices

• An initial test of the water source should be conducted to determine it is free of

contaminants, including parasites, pathogens, disease or any other environmental


• Species reared should possess high tolerance to water quality fluctuations and the

ability to grow rapidly in high stocking densities.

• Water quality should be monitored daily for ammonia, nitrite, nitrate, alkalinity, pH,

dissolved oxygen, turbidity, temperature, using industry standards. Water quality data

should be recorded daily.

• Computer monitoring of basic water quality parameters should be employed in a

RAS. Alarm limits should be set just inside desired levels to insure that notification

and response before an emergency situation is present.

First published by the Massachusetts Center for Sustainable Aquaculture

Written by:  - Updated 7 Apr, 2020  
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