Aquaculture sludge can be recycled
Sludge from recirculating aquaculture systems (RAS) can be utilised in biogas plants and end up as fuel, or be converted to fertiliser, a new report shows.
Recirculation means that the water in land-based hatcheries is reused, as opposed to traditional flow-through (FT) systems where the water is used only once.
This will lead to the saving of large quantities of water in land-based aquaculture, such as production of Atlantic salmon smolts. RAS-based production also offers exceptionally good control of the water environment, sensible use of water resources and enhanced protection against infection.
Another advantage of RAS-based production is that it is easier to separate the nutrients from the waste water than in traditional FT systems, and the sludge is separated before the water is reused.
According to a report by scientists from Nofima and AVS Chile, up to 1600 tonnes of dry matter from sludge could be produced on an annual basis by RAS-based production in Norway and Chile in 2015.
The scientists have evaluated possible uses for this waste as it is not only water than can be reused in such systems.
The waste originates mostly from the metabolism and excrement of the fish as well as from waste feed and, as such, is rich in carbon, nitrogen and phosphorous.
According to the scientists, the most profitable use of the sludge is as a source for biogas or as fertilizer for agriculture. By treating the sludge in biogas plants, it may be used to produce fuel which can be used in cars that are adapted to run on biofuel.
“Fish farm sludge can be sent to regional biogas plants and in this way make a positive contribution to the fish farmer’s environmental accounts,” says Scientist Harald Takle from Nofima. “However, insufficient sludge will be produced in Norwegian salmon hatcheries to make it economically viable for fish farmers to build their own biogas plants.”
In order to be able to utilise the waste, the report recommends additional R & D focussing on the economic potential, the chemical composition of the sludge, experiments involving different types of sludge as fertilisers and the building of a pilot-scale biogas reactor system.
There is increasing interest in RAS-based production. In 2006, less than 2 % of Atlantic salmon smolt production in Norway was in such systems. The report states that the capacity of RAS in Norway will reach 40 million smolts by the end of 2011 and this number could rise to 85 million per year in 2015.
Later this year, Nofima will open a new, large-scale Centre for Recirculation in Aquaculture at Sunndalsøra in Møre og Romsdal.
