The growing Norwegian aquaculture industry faces the challenge of sustainably managing its waste. A recent scientific study published by researchers from NORCE, Wageningen University and Research, the Technical University of Darmstadt, and the University of Bergen explores how fish farming sludge and effluent water from recirculating aquaculture systems (RAS) can cease to be a problem and instead become a source of nutrients and energy, promoting a circular economy model.
The research, titled “Utilization of fish sludge and aquaculture effluent water from Norway for nutrient and energy recovery,” published in the journal Resources, Conservation & Recycling Advances, evaluates the environmental impacts of different waste treatment scenarios, comparing current practices with innovative alternatives that include anaerobic digestion and microalgae cultivation.
The challenge of waste in modern aquaculture
Recirculating aquaculture systems (RAS) allow for greater control of the production environment and the collection of fish feces and uneaten feed, known as aquaculture sludge. This sludge, along with the effluent water, contains nitrogen and phosphorus that, if discharged directly, can contaminate aquatic ecosystems. Current management, which often involves sending part of this sludge to other countries for treatment or releasing it into the environment, raises questions about its sustainability.
This study focused on evaluating alternatives for an average RAS facility in Norway that produces 5,000 tons of salmon per year, seeking more localized and efficient treatment options.
How was the study approached?
The researchers used a Life Cycle Assessment (LCA) methodology to compare the potential environmental impact of four different scenarios for treating fish sludge and effluent water. LCA is a tool that allows for the evaluation of the environmental “footprints” of a product or process throughout all its stages.
The four scenarios analyzed were:
- Scenario 1 (Baseline): Reflects the current situation where 50% of the fish sludge is dried and sent to Denmark for biogas production, while the remaining 50% and the effluent water are discharged into the local environment.
- Scenario 2: Similar to the first, but assumes that 100% of the fish sludge is dried and sent to Denmark for anaerobic digestion.
- Scenario 3: Proposes local treatment in Norway. Nutrients from the reject water (water remaining after dewatering the sludge) are recovered through microalgae cultivation, and the dewatered fish sludge is used as the sole substrate for an anaerobic digestion process.
- Scenario 4: Also a local treatment in Norway. Similar to scenario 3, but the dewatered fish sludge is added as a co-substrate (along with municipal sewage sludge) in an anaerobic digestion plant.
To compare the scenarios fairly, a “system expansion” was applied, adjusting for differences in the final products of each scenario (such as microalgae biomass, energy, and organic fertilizer) and considering the location of the treatments.
What did the research reveal?
The LCA results showed significant differences between the scenarios:
- Improving the current situation: Treating 100% of the RAS sludge in Denmark (Scenario 2) proved to be environmentally more sustainable in most impact categories than the current practice of treating only 50% and releasing the rest (Scenario 1). This is mainly due to the reduction of direct nutrient discharge into the environment.
- Promising local treatment: Scenarios proposing treatment in Norway using microalgae cultivation and anaerobic digestion (Scenarios 3 and 4) showed a significantly lower impact than the baseline (Scenario 1) in categories such as stratospheric ozone depletion, fossil resource scarcity, and, very notably, marine eutrophication. Nitrogen recovery through microalgae was a key factor in reducing eutrophication.
- Advantageous co-digestion: Co-digestion of fish sludge with other substrates in Norway (Scenario 4) resulted in a potentially lower environmental impact than using fish sludge as the sole substrate for anaerobic digestion (Scenario 3) in almost all impact categories. This is partly due to higher energy production that can be used, for example, in microalgae cultivation.
- The energy factor: A crucial finding was the significant impact of the national electricity grid composition (hydropower in Norway vs. a mix with more fossil fuels in Denmark, at the time of the study) on environmental sustainability. Microalgae cultivation in Nordic latitudes, which requires artificial lighting, is highly energy-demanding.
- Operational costs: A preliminary economic analysis indicated that all proposed scenarios could have similar or even lower operational costs compared to the baseline, thanks to nutrient recycling and energy production.
Implications for the salmon industry
This study underscores the potential to transform aquaculture waste from an environmental problem into a valuable resource. For an aquaculture producer, implementing treatment systems like those described could mean:
- Reduced environmental impact: Decreasing the ecological footprint of the operation by minimizing nutrient discharge and recovering resources.
- New sources of income or savings: The production of biogas (energy) and fertilizers from sludge, and microalgae biomass with various potential uses, could generate additional income or reduce operational costs (e.g., for fertilizers or energy).
- Improved public image: Adopting circular economy practices can enhance the perception of the company’s sustainability.
The study also identifies areas where more research and development are needed, such as optimizing anaerobic digestion of saline sludge, more energy-efficient sludge drying technologies, and the regulatory framework for utilizing microalgae grown in wastewater and the biofertilizer obtained.
The research suggests that local sludge treatment in Norway (Scenario 4, co-digestion) may be the most environmentally favorable option in many categories, provided factors such as the availability of biogas plants and logistics are considered.
Conclusion: Towards a circular economy
The Norwegian study offers valuable insight into how aquaculture can advance towards greater sustainability by implementing circular economy strategies. Anaerobic digestion of fish sludge, especially in co-digestion, and microalgae cultivation in effluents, are presented as promising technologies for recovering nutrients and energy.
While challenges exist, such as the high energy demand of some processes and the need to optimize technologies and the regulatory framework, the potential environmental and economic benefits are significant, opening doors for a cleaner and more efficient aquaculture industry.
Reference (open access)
Böpple, H., Brussino, G., Engel, A., Breuhaus, P., Dopffel, N., An-Stepec, B. A., Kleinegris, D. M., & Slegers, P. M. (2025). Utilization of fish sludge and aquaculture effluent water from Norway for nutrient and energy recovery. Resources, Conservation & Recycling Advances, 200256. https://doi.org/10.1016/j.rcradv.2025.200256
Editor at the digital magazine AquaHoy. He holds a degree in Aquaculture Biology from the National University of Santa (UNS) and a Master’s degree in Science and Innovation Management from the Polytechnic University of Valencia, with postgraduate diplomas in Business Innovation and Innovation Management. He possesses extensive experience in the aquaculture and fisheries sector, having led the Fisheries Innovation Unit of the National Program for Innovation in Fisheries and Aquaculture (PNIPA). He has served as a senior consultant in technology watch, an innovation project formulator and advisor, and a lecturer at UNS. He is a member of the Peruvian College of Biologists and was recognized by the World Aquaculture Society (WAS) in 2016 for his contribution to aquaculture.
