Turning fish waste into biogas

Sludge discharge from aquaculture is a growing problem, particularly in some fjords where seawater circulation is limited. Working with the technology companies Lift Up and Aquapro, aquaculture company Lerøy Sjøtroll has developed a new sludge collection technology to remove natural waste, such as feed residues and faeces, from fish pens.

‘The natural ecosystem on the seabed benefits from some organic matter being released from the fish pens, as it acts as fertiliser. The problem arises when the amount of sludge exceeds the environment’s natural capacity, which can happen in fjords where oxygen levels in the bottom waters are low,’ says Nina Møgster, CEO of Lerøy Sjøtroll.

The aquaculture company currently operates 40 facilities in Hordaland, farming either salmon or trout. Three of these facilities are located in Sørfjorden near Osterøy. This is a sill fjord with a narrow inlet from the sea, which creates challenges due to limited water exchange and low oxygen levels near the seabed. The first pilot project was installed here in August 2020.

The fish sludge is collected in a fine-meshed net installed at the bottom of the cage. It is then pumped through a filtration system into a tank on the surface. The waste is transported away from the facilities by boat.

'It is too early to draw conclusions about its impact on the seabed environment, but we can see that we are able to collect large quantities of sludge and that the fish do not appear to be adversely affected by the new technology,' says Møgster.

We supported the project with around NOK 6.5 million through our Environmental Technology Scheme. Møgster emphasises that this essential risk relief enabled the aquaculture company to launch the project.

'We were awarded the funding in March 2020, just as the pandemic was creating greater uncertainty in both the market and the economy. If we had not already had the capital and funding for the project at that point, the whole project might have been postponed. The funding also validates the project itself, which has been motivating for everyone involved,' she says.

Tore Alfeim is a financial adviser with us and has been closely involved in the project. He believes it is important that we support companies developing new technology that makes the aquaculture industry more sustainable.

‘In this case, several companies collectively had the technology and solutions, but they could not move forward until someone invested in the project. It is exciting to see them devote time and resources to testing value chains for reusing waste materials. Experience from other sectors suggests that higher volumes can generate greater demand. In the long term, this could become a profitable project for them,’ he says.

Sludge consists entirely of organic matter and can therefore be converted into other biological products. Through agreements with transport vessels and biogas companies, the sludge from Lerøy Sjøtroll is shipped to Denmark, where it is converted into biogas.

‘Our goal has always been to develop the project into a viable circular economy solution. For now, sludge has a negative value, which means we pay to have it collected and delivered. However, we are very optimistic about the opportunities this could create in the future,’ says Møgster.

The company has also allocated funding to research the use of sludge as a soil improver in agriculture.