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.
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
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 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.
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
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
• 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
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
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
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
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
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