Fish sludge study reveals circular business opportunities for Vestland

The project “Does Salt Prevent Circular Value Chains in Aquaculture?” has been led by the cluster NCE Seafood and is based on collaboration between stakeholders across Vestland’s seafood cluster, including the aquaculture industry, technology developers, and the horticultural sector. The project has investigated how fish sludge from sea-based aquaculture can be converted into biochar and become part of new circular value chains, and points towards significant business potential in areas such as carbon storage, nutrient recovery, renewable energy and soil improvement, creating opportunities for companies seeking to develop new products and services linked to aquaculture by-products.

- We find the results very interesting and a good paltform for workeing with future value chains. Circularity isn't about managing waste. It's about rebuilding the resource base that makes sustainable growth possible, says Investment manager Charlotte Hartvigsen Lem in Invest in Vestland, and adds:

- The findings align closely with the ambitions of Grøn region Vestland, which works to accelerate the green transition through industrial symbiosis and new circular value chains. By demonstrating both the potential and the challenges of converting fish sludge into biochar, the project identifies new opportunities for value creation across the aquaculture, technology and agricultural sectors while helping to unlock more resource-efficient industries in Vestland.

Turning a waste stream into a resource

The Norwegian aquaculture industry generates substantial volumes of fish sludge containing values like carbon, phosphorus and other nutrients. Much of these resources are currently underutilised.

The project explored whether dried fish sludge could be processed through pyrolysis, a technology that converts organic material into biochar, gas and energy. Tests confirmed that fish sludge is an energy-rich feedstock with significantly higher energy content than expected. The process also reduced the material volume dramatically, making transport and handling considerably more efficient.

These findings demonstrate that fish sludge has the potential to become a valuable raw material in future circular value chains rather than being treated as a residual waste stream.

Three types of biochar were used in the cultivation trial: biochar produced from woodchips, biochar produced from saline fish sludge, and biochar produced from highly saline fish sludge. Photo: NCE Seafood. 

Strong potential for carbon and nutrient recovery

Analysis of both the fish sludge and the resulting biochar revealed high levels of carbon, nitrogen and phosphorus. Importantly, much of the carbon and nutrients were retained in the biochar after processing.

The resulting biochar showed characteristics that make it attractive for several applications, including carbon storage and soil improvement. High phosphorus and calcium content further enhances its potential value, particularly in sectors seeking sustainable alternatives to conventional inputs.

If fish-sludge biochar can be successfully utilised in agriculture and other industries, it could create a pathway for returning nutrients and carbon from aquaculture back to productive use on land, strengthening links between the blue and green economies.

Salt remains the main challenge

While the project confirmed the resource potential of fish sludge, it also identified the biggest obstacle to circular utilisation: salt.

Researchers found that sodium and chloride largely remain in the material throughout the pyrolysis process. As a result, significant salt concentrations were detected in the final biochar product.

To understand the implications, cultivation trials were carried out in collaboration with the local commercial grower Toppe Gartneri, using a range of crops including barley, ryegrass, lettuce, kale and swede.

The results were clear: Biochar produced from woodchips delivered growth comparable to the control group. Biochar produced from highly saline fish sludge resulted in very limited germination, while biochar with moderate salt levels reduced both germination and plant growth.

However, the trials also provided an encouraging indication that the challenge may be manageable. When irrigation methods were changed, plant growth improved significantly, suggesting that salt can potentially be washed out and its negative effects reduced.

Aquaculture is an important industry in Vestland. A lot of sludge is generated and should be recycled in a sustainable way. Photo: Espen Bierud.

Building the foundation for new circular industries

Although the project does not provide a final answer to whether salt prevents circular value chains in aquaculture, it has delivered important new knowledge about both the opportunities and the technical barriers.

The next step will be to develop methods for reducing salt content, map variations between different fish-sludge sources and identify the most suitable applications for biochar derived from sea-based aquaculture.

The study was initiated and carried out through collaboration between actors in Vestland’s seafood value chain, reflecting the region’s ambition to develop more resource-efficient and circular solutions for the aquaculture industry.

- Circularity isn't about managing waste. It's about rebuilding the resource base that makes sustainable growth possible. For Vestland, the findings point towards new business opportunities at the intersection of aquaculture, agriculture, climate technology and circular industry. With further innovation and commercial development, fish sludge may become the foundation for entirely new value chains that capture more value from the region’s growing seafood sector, says Charlotte Hartvigsen Lem.

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