Chapter 6: Manage re-circulatory aquaculture

Recirculating Aquaculture Systems (RAS) have revolutionized fish farming practices in Kenya by enabling sustainable, high-density fish production with minimal water usage. This technology is particularly vital in regions facing water scarcity or land limitations, such as urban and peri-urban areas around Nairobi and Kisumu. For animal production professionals, designing an effective RAS is crucial to optimize fish health, growth rates, and operational efficiency while controlling environmental impacts. Understanding the design principles of RAS ensures that fish farmers can maintain stable water quality, reduce disease risks, and achieve economic viability.

6.1 Design recirculating aquaculture system (RAS)

A recirculating aquaculture system, or RAS, is a closed-loop fish production system in which the water leaving the fish tanks is cleaned and returned instead of being discharged. Only a small amount of new water is added to replace what is lost through evaporation, cleaning and the removal of sludge. This makes RAS suited to places where water is scarce, and it allows the farmer to control temperature, oxygen and cleanliness closely. The cost is complexity: if one part fails, the water quality can collapse within hours. Design therefore starts with a clear picture of the loop that the water follows, and every component is sized to match the amount of fish and feed the system will carry.

The loop works as follows. Water leaves the fish culture unit, passes through a mechanical filter that removes solids, flows to a biofilter where bacteria change ammonia into nitrate, is aerated or oxygenated, and is pumped back to the fish. A water reservoir holds the system's reserve, and a drainage system removes sludge and spent water.

Principles of RAS design

• Water reuse: water is filtered and reused continuously, so only small volumes of fresh water are needed.
• Water quality control: oxygen, pH, temperature and nitrogen wastes are kept in the safe range, with sensors where the farm can afford them.
• Biofilter capacity: the biofilter must handle the ammonia that the fish produce at the heaviest feeding rate.
• Reliability: pumps, aeration and power each have a stand-by, because the fish depend on circulation and oxygen all the time.
• Modularity: units are built so that the farm can add tanks and filters as production grows.

Bio-filters

The biofilter is the part of the loop that makes water safe to reuse. Fish waste releases ammonia, which is toxic. Nitrifying bacteria living on the surface of the filter medium change ammonia to nitrite and then to nitrate, which is far less harmful. A biofilter therefore needs a medium with a large surface area, such as plastic media, gravel or special beads, a steady flow of water, and plenty of oxygen so that the bacteria stay alive. Common designs are the trickling filter, the moving-bed filter in which media float in aerated water, and the submerged fixed-bed filter. The size of the biofilter is chosen from the amount of feed given each day, because ammonia production follows the feed. A new biofilter takes several weeks to build up its bacteria before the system can carry a full stock of fish.

Mechanical filters

A mechanical filter takes out solid particles, mainly uneaten feed and faeces, before they decay and increase ammonia and oxygen demand. Common types are settling tanks or clarifiers, in which solids settle by gravity, screens and drum filters, which sieve the water, and sand or bead filters. The filter must be placed before the biofilter so that the bacteria are not smothered with solids. It must be easy to clean, since it needs cleaning daily or even more often in a busy system, and it needs a drain or valve through which the removed solids are discharged.

Fish culture unit(s)

The culture unit is the tank or raceway in which the fish are kept. Circular tanks with a centre drain are widely used because the water circulates evenly, solids move to the centre and can be removed, and there are few dead corners. Tanks are made of a smooth, non-toxic material such as fibreglass, food-grade plastic or lined concrete, and have a tight cover or net to stop fish jumping out. Inlets are placed to create a swirling flow, and each tank has its own valve so that it can be isolated for cleaning. The volume of each tank is chosen from the planned stocking density and the number of fish per batch.

Water reservoir

A reservoir, sump or storage tank holds the water that returns from the culture units and acts as the system's buffer. When the pumps are switched off the water in pipes and filters drains back, and the reservoir must be large enough to receive it without overflowing. It also holds the reserve of water for top-ups, dilutes sudden changes in water quality and provides a place for heaters, coolers and dosing equipment. The reservoir has a lid to keep out light and dirt, a float or sensor that shows the level and an overflow pipe leading to the drain.

Water drainage system

The drainage system carries sludge and spent water out of the system. It consists of bottom drains in the tanks, a sludge outlet from the mechanical filter, a waste line and a sump or settling tank for the waste water. Pipes are laid with a fall so that water and solids flow without settling, and they are wide enough to avoid blockage and easy to clean. Waste is treated and used on crops, for example by drying the sludge or by spreading the nutrient-rich waste water on a crop field, and it is not discharged to rivers or drains.

Water aeration system

Fish and the nitrifying bacteria both need oxygen. Aeration is by air blowers with diffusers or air stones in the tanks, the biofilter and the reservoir, and by oxygen cones or pure-oxygen injection in heavily stocked systems. The blower is sized for the oxygen demand at the highest stocking and feeding level, and it has a back-up unit or an emergency source of power, because a power cut stops both circulation and aeration. Air stones and diffusers are placed so that bubbles rise through the water evenly.

Other components and design considerations

Pumps move water around the loop and are chosen for reliability and low energy use, with pipes of a size that reduces friction. Some farms add an ultraviolet steriliser to reduce the number of disease organisms in the water, and a heater, cooler or shade where temperature needs control. The water source matters: borehole, surface and mains water need different treatment, and chlorinated water must be dechlorinated before use. Power availability and cost are major design factors in rural areas, so standby generators or solar power are considered. Effluent from the system must be managed responsibly.

Example: a 4 cubic metre fish tank is to have its water pass through the filters once every 30 minutes.

Tank volume = 4 m³ = 4 000 litres

Flow rate = 4 000 litres ÷ 30 minutes = 133.3 litres per minute, or about 8 m³ per hour

The pump is chosen to deliver at least this flow, allowing for losses in the pipes and filters.

Steps to design a RAS

1. Define the production aims: the species, the number of fish, the size at harvest and the target yield.
2. Assess the site and the water: test the water, check space, power supply and distance to the market.
3. Select the components: choose tanks, mechanical and biological filters, aeration equipment, pumps and controls to suit the aims.
4. Plan the water flow and layout: draw the pipes and the water path so that water flows through every part and nothing stagnates.
5. Add monitoring and control: fit sensors or test points for dissolved oxygen, pH, temperature and ammonia, and alarms for pump and power failure.
6. Plan waste treatment and disposal: provide settling tanks, a sludge handling routine and a way to use or treat the effluent.

Chapter Summary

This chapter covered the essential aspects of maintaining a recirculatory aquaculture system, starting with the design principles that ensure efficient water flow, filtration, and oxygenation tailored to the specific fish species being farmed. It then addressed the installation process, highlighting the importance of proper setup of tanks, pumps, filters, and plumbing to achieve a fully functional system. Bio safety measures were emphasized to prevent disease outbreaks and maintain water quality, including protocols for quarantine, disinfection, and monitoring of fish health. The chapter concluded with management activities necessary for the ongoing operation of RAS, such as regular maintenance of equipment, water quality testing, feeding schedules, and waste removal. Together, these elements form a comprehensive framework for successful fish production in a controlled environment. Proper integration of design, installation, bio safety, and management ensures sustainability and productivity in recirculatory aquaculture.