Recirculating Aquaculture Systems


A functioning bioreactor enables the well-being and rapid growth of fish. The operations of recirculating aquaculture systems (RAS) are based on the fact that the water used for aquaculture can be recycled several times, when using biological filtration system. With this system it is possible to maintain a consistent temperature optimal for the growth of the fish around the year.

We offer fish farmers a turnkey solution, which includes both technology and equipment as well as fry raised from eggs.

Consistent production all year around

In a recirculating aquaculture system, the growth rate of the fish and the water quality can be optimized according to the species being cultured. For the economic efficiency of the system, the amount of fish biomass should be kept as consistent as possible around the year. This enables the maintaining of the end products delivery to the market at a consistent rate.

The system is measured according to the largest feeding, growth, and metabolic products of the fish. This requires careful production and equipment planning.

Food production close to the consumer

Recirculating aquaculture systems are particularly suited for areas where the amount or temperature of available water limits farming activities. Recirculating aquaculture system brings food production closer to the consumers and decreases the length and duration of the delivery chain.

Clewer’s bioreactors have been tested in a laboratory for coldwater fish (13—18°C) and warmwater fish (18—26°C). In each fish farming project, the water source and the cultured product are individually designed so that the fish farm concept can be optimised for the existing conditions. The design pays special attention to the well-being of the fish, the water quality, and the resulting cost-efficiency of the fish farm.

RAS treats nitrogen compounds

Biological filter constitutes a substantial part of the system. The recirculating aquaculture system accumulates ammonium due to the metabolism of the fish. Some of the nitrogen contained in the fish fodder is released from the gills to the recirculating water as ammonium. The released ammonia, whose share of the total ammonium depends on the pH value of the water, is dangerous to fish even in small quantities.

The nitrogen can be transformed into a less dangerous form with the help of biological treatment. This is carried out with nitrification (aerobic process) by transforming ammonium first into nitrite and then into nitrate with the bioreactor.

Water is changed in the system so that the amount of nitrate stays optimal, depending on the species of the fish. If the recycling ratio of the water is high and the amount of changed water is small compared to the total water mass, a denitrification reactor is installed in the system in order to remove nitrogen. The reactor reduces nitrate into nitrogen gas (anaerobic process) and releases it into the air.

Careful sizing ensures efficient water cleaning

The efficiency of the biological purification is influenced by a number of physical and chemical conditions. The efficiency of the biological filter depends on the amount and species of bacteria growing on the surfaces of the filter and their ability to utilise the ammonium entering the filter. To keep the optimal bacteria alive, the sizing of the biofilters must be done carefully according the demands of the production cycle and practices of each farm.


Wide experience and strong expertise in designing and constructing of recirculating aquaculture systems.

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We design and construct recirculating aquaculture systems as turnkey solutions.

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Customised solutions for commercial fish farms and research institutes across Europe

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