September 10, 2020


Bacteria have more roles in successful RAS farming, says Finnish researcher


Recirculating aquaculture systems (RAS) technology that recycles and saves water, has expanded in recent years. The technology has not yet achieved economic viability, mainly due to high investment and operating costs. In addition, the operation and management of bioreactors has been one of the biggest issues, which microbiological processes were studied by M.Sc. Jani Pulkkinen in his dissertation.

Bioreactors utilise bacteria in the water purification process. In bioreactors, toxic ammonia excreted by fish is microbially converted to more harmless nitrate in the nitrification process.

"Although bioreactors are designed for the decomposition of nitrogen compounds, the main functions of bacteria in bioreactors were the decomposition of carbohydrates, amino acids and fats. The impact of bioreactors as a whole on water quality is thus much more diverse than previously thought. A diverse and stable bacterial community can maintain good water quality, not only in terms of nitrogen compounds, but also in organic matter," Pulkkinen says.

Different types of bioreactors can trap solids from water or affect the gas balance, but different bioreactors also have different nitrification efficiencies, i.e. how fast ammonia can be converted to nitrate.

"The sizing and selection of bioreactor type should be done taking into account the characteristics of the entire water treatment system," says Pulkkinen.

Principle of a recirculating aquaculture system and the main factors affecting water quality and system operation.

The biological and mechanical solids removal capacity of bioreactors can compensate the properties of the rest of the water treatment system. By optimising the entire water treatment system, the best possible water quality can be maintained for the well-being and growth of the fish, which enables cost-effective and environmentally friendly aquaculture.

All studies in the dissertation were conducted in the experimental recirculating aquaculture facilities of the Natural Resources Institute Finland (Luke) Laukaa fish farm.The dissertation has been funded by Luke, the European Union and the Ministry of Agriculture and Forestry from the European Maritime and Fisheries Fund.

The dissertation is available at:

Pulkkinen defends his dissertation in English on September 11, 2020 at 12pm in a Zoom webinar: