A circular economy approach to aquaculture could produce energy to power farms, as well as fertilisers for growing vegetables according to latest research.

Aquaponics is a food production system that couples aquaculture - raising aquatic animals such as fish in tanks, with hydroponics - cultivating plants in water, free from soil. In such a system, the aquaculture water, which is rich in nutrients excreted from fish, is fed to the plants.

This food production model imitates the fertilisation that occurs naturally around river and lakes. Now, a doctoral student at the University of Gothenburg has shown how the waste can also be used to produce a biogas that can help meet the energy needs of farms.

Marine biologist Victor Lobanov says, “By breaking down fish faecal matter in an anaerobic environment – known as digestion – we can obtain a concentrated gas mixture of 70% methane that can be used as fuel. This can make aquaponics a source of energy."

Lobanov's study also shows that the nutrients released during the anaerobic digestion of fish waste are easier for plants to take up, compared to synthetic nutrition solutions.

“Fish waste contains a lot of nutrients," he says. "These should also be usable in aquaponics to enable even more sustainable food production than today.”

The digestion process has only been tested in a lab environment so far, but a pilot project in a commercial aquaponics facility gets underway this summer. It will give researchers insights into how well the method can handle disruptions to the system, and what more needs to be done to create more robust digestion processes.

Lobanov’s goal is to create modular digestion systems that can be integrated into existing aquaculture and aquaponic facilities. He says there is significant interest from the aquaculture industry, and the technology could also be used in other farming applications such as piggeries.

Even the sludge left over after digestion is still extremely rich in nutrients and, because it has been processed, can be used to fertilise crop fields. This reduces the risk of eutrophication of rivers and lakes - where algal blooms harmful to aquatic life form due to excess nutrients from unprocessed biological waste being discharged to the environment.

“In many countries, the quantity of fertiliser produced in livestock farming is a problem," Lobanov explains. "It can only be spread on fields during certain times of the year and removing wastes from the farm is associated with extra costs during pumping and transportation.

"Digestion of the fish solids reduces the quantity of waste produced by farms while additionally producing energy and a great fertiliser for hydroponics.”