Making biogas from fish sludge

Discharges in the form of sludge from fish farms are a growing problem, especially in fjords that have an infrequent exchange of basin water. In collaboration with the technology companies Lift Up and Aquapro, the aquaculture company Lerøy Sjøtroll has developed a completely new sludge collection technology to remove natural waste substances such as uneaten fish feed and faeces from net-pens.

“The natural ecosystem on the seabed benefits from some discharges of organic materials from the net-pens, it acts as a fertiliser. The problem arises when the amount of sludge exceeds nature’s ability to tolerate such high levels, which can happen in fjords that have low levels of oxygen in the bottom water”, says Nina Møgster, Managing Director of Lerøy Sjøtroll.

Pilot project in Western Norway

The aquaculture company currently has 40 facilities in Hordaland, containing either salmon or trout. Three of these facilities are located in Sørfjorden near Osterøy. This is a threshold fjord where the inlet from the sea is narrow. This presents challenges related to infrequent exchange of basin water and low concentrations of oxygen in the depths of the fjord. The first pilot project was installed in Sørfjorden as early as August 2020.

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

“It is too early to draw conclusions about the effect it has on the seabed environment, but we see that we are able to collect large amounts of sludge, and that the fish do not appear to be adversely affected by the new technology”, says Møgster.

Important support from Innovation Norway

The project was supported by Innovation Norway through the environmental technology scheme which granted NOK 6.5 million. Møgster emphasises that risk mitigation was necessary in order for the aquaculture company to start the project.

“We were allocated the funding in March 2020, just as the pandemic made both the market and the economy more uncertain. Had we not had the capital and funding for the project at that point, the whole thing might have been postponed. In addition, the support also provides recognition of the project itself, which has been motivating for those involved”, she says.

Making a profit from sludge

Tore Alfeim is a finance adviser at Innovation Norway and has been very much involved with the project. He believes it is important that Innovation Norway supports actors who develop new technology that ensures a more sustainable aquaculture industry.

“In this situation, there were several companies that had developed the technology and solutions together, but they were not able to move forward until someone invested in the project. The fact that they spend time and resources testing value chains for the reuse of waste materials is very exciting to follow. Experience from other areas suggests that increased volume may create greater demand. In the long term, it can become a profitable project for them”, he says.

Sending sludge to Denmark

Sludge consists exclusively of organic materials, which means it can be converted into other biological products. Through agreements with transport boats and biogas companies, the sludge is sent from Lerøy Sjøtroll to Denmark where it is converted into biogas.

“We have always had the goal of developing the project so that it is feasible in terms of a circular economy. At the moment, sludge has a negative value, which means that we pay for it to be picked up and delivered, but we are very positive about the opportunities this can provide for the future”, says Møgster.

The company has also set aside funds to research using sludge for soil improvement within the field of agriculture.