All along the long coastlines of the North Sea to the Northern Atlantic between Canada and Greenland, seafood is more than a staple — it’s a cultural mainstay and a booming industry.
But even sustainable salmon doesn’t swim away from its mess: every fish farm produces large volumes of waste in the form of fish sludge and nutrient-rich effluent water.
In a world where circularity is the new common sense, researchers from NORCE Norwegian Research Centre [60.1°N , 4.2°E] and the University of Bergen [60.4°N, 5.3°E] have found a way to turn this unwelcome by-product into valuable energy and nutrients. Their study compared different strategies for reusing aquaculture waste, from shipping dried fish sludge to Denmark for biogas production, to turning it into fertiliser and algae biomass right at home in Norway.
This means smarter, local treatment — especially when it includes anaerobic digestion and microalgae cultivation — could not only clean up fish farming’s act but also deliver real environmental and economic benefits.
Aquaculture’s Dirty Secret — and a Clean Solution
Modern fish farms, particularly recirculating aquaculture systems (RAS), keep their operations closed-loop. But that doesn’t mean they’re waste-free. Uneaten feed and fish faeces build up in the system, rich in phosphorus and nitrogen, and too saline and sludgy for traditional wastewater plants.
Currently, some of this sludge is shipped to Denmark for energy recovery via anaerobic digestion — a costly and carbon-heavy journey. The rest? Still often discharged into local fjords, with ecological consequences that don’t wash away easily.
The researchers examined four scenarios:
- Status quo — 50% of fish sludge shipped to Denmark, 50% discharged into nature.
- All-in export — 100% of fish sludge shipped to Denmark for biogas.
- Local solution — all waste treated in Norway, combining anaerobic digestion and microalgae cultivation.
- Co-digestion — Norwegian biogas plants treating fish sludge together with municipal waste, plus microalgae production.
Digest, Don’t Dump
The study found that scenario 4 — treating all waste within Norway using co-digestion and microalgae — offered the most balanced and circular solution. It avoided the environmental damage of dumping, eliminated the need for chemical fertilisers, and even produced heat and electricity through biogas.
Compared to sending sludge to Denmark, the local approach:
- Reduced marine eutrophication dramatically (nitrogen release down to 5% of the original impact).
- Avoided the emissions from long-distance transport.
- Made better use of Norway’s low-carbon energy grid to power microalgae cultivation.
Even more promising, the algae grown on this wastewater could become biofertiliser, bioplastics, or even cosmetics, offering a rare alchemy: waste turned into something truly useful.
A Model for Coastal Regions Worldwide
This research has clear implications where aquaculture is expanding and waste treatment often lags behind.
- Canada’s Atlantic provinces and Northern Europe’s fjord-lined coastlines share similar challenges: remote communities, sensitive marine ecosystems, and growing aquaculture footprints.
- The co-digestion model fits neatly into existing infrastructure, allowing fish sludge to be handled alongside sewage or agricultural waste.
- The concept of microalgae as a nutrient recovery system also holds promise for inland fish farms and regions with water treatment bottlenecks.
However, the study notes key hurdles remain: regulations currently limit the use of algae grown on wastewater in food or feed products, and energy-intensive processes like drying sludge still weigh heavily on both cost and emissions.
Turning Waste into Worth
Circularity isn’t just a buzzword here — it’s a genuine opportunity. Norway’s biorefinery-style model for aquaculture waste turns an environmental liability into multiple revenue streams: energy, fertiliser, biomass, and clean water. All from the back end of a salmon farm.
And while the infrastructure might be complex, the logic is simple: the closer we treat our waste, the less damage we export — and the more value we recover.
As Northern Europe and Canada push toward greener, more integrated food systems, this is the kind of solution that makes both ecological and economic sense. It’s time to stop flushing nutrients out to sea — and start farming them into something better.
Source
Hanna Böpple, et al, Utilization of fish sludge and aquaculture effluent water from Norway for nutrient and energy recovery, Resources, Conservation & Recycling Advances, 2025-05-13
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