Recirculating Aquaculture System

Advantages of Recirculating Aquaculture System:

  • Relative small area is needed
  • Relative low water consumption
  • Controlled environment
  • Easy harvesting
  • Can be put in place in almost every area, even in cold climates
  • Intensive production levels
  • Security is easy to manage

Disadvantages of Recirculating Aquaculture System:

  • High and constant demand of power 24/7
  • Highly technical installation
  • Maintenance is relatively high
  • Technical high skilled people needed
Recirculating Aquaculture System for African Catfish

Recirculating Aquaculture System for African Catfish

Introduction to the recirculating aquaculture system

In a recirculating aquaculture system the culture water is purified and reused continuously. A recirculating aquaculture system is an almost completely closed circuit. The produced waste products; non soluble waste, ammonium and CO2, are either removed or converted into non-toxic products by the system components. The purified water is subsequently, saturated with oxygen, and returned to the fish tanks. By recirculating the culture water, the water and energy requirements are limited to an absolute minimum. It is however not possible to design a fully closed recirculating system. The non-degradable waste products must be removed and evaporated water must be replaced. Still our recirculating aquaculture systems are capable of reusing 90% or more of the culture water. To ensure good water purification, recirculating aquaculture systems consist of a number of components with specific functions. The fish culture systems must be able to:

  • Remove solid particles (waste, spilled feed)
  • Remove dissolved organic matter
  • Convert ammonium into the less harmful nitrate
  • Remove CO2 and add O2

The basic recirculating aquaculture system consists of the following components:

  1. Fish tanks
  2. Sedimentation tank
  3. Pump tank
  4. Biotower
  5. Reception tank

schematic overview of a recirculating aquaculture system

The following figure shows the different processes within a recirculating aquaculture system:
Processes within a recirculating aquaculture system -640

Mechanical filtration used in a in a recirculating aquaculture system

In a RAS system the mechanical filter is used to remove the suspended solids from the system water flow. These solids, primarily faecal matter, need to be removed in order to maintain water quality. There are multiple types of mechanical filters, including drumfilter and sedimentation filter.

Sedimentation filter

The water flowing from the fish tank(s) is led through the sedimentation tank. In this tank the non-soluble, solid particles are separated from the system water using gravity. The sedimentation tank is filled with a specially adapted polypropylene filter pack. The solids are allowed to settle on these filters. The settled particles form a layer of silt on the bottom of the sedimentation tank. This silt is biologically very active: up to 60% of the nitrates produced by the biofilter are denitrified by the bacteria in the silt into nitrogen gas.
Depending on the feed load on the system, the sedimentation tank must be cleaned regularly. The silt and water for cleaning is drained to the waste water sewage system in the farm. After this mechanical treatment, the system water flows to the pump tank.

Filtermaterial to be used inside a sedimentation filter

Filtermaterial to be used inside a sedimentation filter


The system water coming from the fish tanks can also be mechanically cleaned using a drumfilter. A drum filter is an active mechanical filter; suspended solids are continuously and automatically removed from the system water and therefore requires no daily maintenance. The system water flows from the fish tanks in the drum filter. In the drumfilter the system water passes the filter screen (blue arrow). In this process, the particles in the system water are blocked by the filter screen. Due to the clogging of the filter screen the water level inside the drum will rise. As a result, the flushing mechanism is put into operation. The drum starts to rotate, and from the outside of the drum water is sprayed under high pressure from nozzles through the filter cloth, hereby flushing the waste particles from the screen. Together with the waste particles the water is discarded in the gutter.

UV-C device

After mechanical filtration, the system water is treated with UV-C light. This light is electromagnetic radiation just below the visible light spectrum. The wavelength of UV-C light is 200-280 nanometers.

UV-C light is highly germicidal and has an antiseptic effect. With the use of irradiation of UV-C light to a contaminated surface, or in running water, it is possible to control excessive growth of micro-organisms including bacteria. UV disinfection is a proven method to kill off bacteria, viruses and fungi, but first and foremost to keep the system water clear and free of small particles even at low dosage.

Biological filtration used in a recirculating aquaculture system

The nitrifying bacteria inside the biological filter convert the for fish harmful ammonium in to nitrate using oxygen. Nitrate is only toxic for fish at very high concentrations. Nitrification is an acidifying process. It is therefore very important to check, among other water quality parameters, the pH regularly.


From the pump tank the system water is first pumped to the biotower. A biotower is on average 3 to 4 meters high and consists of polypropylene net filter blocks suited for bio-filtration. On top of the biotower the water is dispersed over the filter using special sprayers. The water flows through the filter packs to the bio tower reception tank or directly back into the pump tank. On the surface of the filter packs in the biotower a biofilm of nitrifying bacteria is formed. A biotower is also added to the system to remove dissolved gasses (i.e. CO2) from the water and saturation of the system water with oxygen.

Biotowers on a tilapia spawning system

Biotowers on a tilapia spawning system

Upflow filter (or movingbed filter)

Another type of biological filter is the upflow filter. These filters have a high capacity with relatively low volume. From the pump tank the culture water is pumped into the upflow filter. The upflow filter consists of a large circular tank with aeration device and filled with bed PP particles (see picture at the right). A biofilm of nitrifying bacteria is present on the surface of the PP particles.

PP particles inside upflow filter

PP particles inside upflow filter

Contact Fleuren & Nooijen for prices and possibilities of recirculating aquaculture systems to be used for hatcheries and grow out.