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How does sludge collection technology work?

September 17, 2025

AQUAPHOENIX is on a four-year mission to boost circularity in the aquaculture industry, reduce waste, and ensure no resources go astray.

The first step in this mission  is to test and develop novel technologies to collect, reduce and prepare aquaculture waste (known as sludge) for recycling. Let’s take a closer look at why this technology is needed, and how it works.

The context

Open pen aquaculture takes place in natural waterways, creating a direct interaction between industry and environment.

Illustration of an aquaculture cage with both the surface and underwater view. The cage is surrounded by nets and has small fish inside. Behind the cage is a small boat that fits 4 times the diameter of the round cage. The water is dark blue and shows fish sludge floating from the cage into the water column. Darker shapes visualise the bottom of the water body. Above the surface and in the back of the illustration is a relief landscape. The area representing the blue sky has a text in black fond that reads "Typically, the sludge from open pen aquaculture is partially dissolved and dispersed in the open water."
Illustration of an aquaculture cage with both the surface and underwater view. The cage is surrounded by nets and has small fish inside. Behind the cage is a small boat that fits 4 times the diameter of the round cage. The water is dark green to blue and shows fish sludge floating from the cage into the water column. The sludge spreads out and settles on the darker shapes visualising the bottom of the water. Above the surface and in the back of the illustration is a relief landscape. The area representing the blue sky has a text in black font that reads "if environmental monitoring shows unacceptable emissions, farmers must limit or cease production to reduce pollution."
A novel solution

Since 2012, LiftUp (a subsidiary of Framo) has been developing sludge collection technologies to collect waste from open pen fish farming.

Fine mesh nets are fitted with an intake and pumping unit, then placed at the bottom of the pen to capture faeces and unconsumed feed. This effectively prevents waste from entering the surrounding area.

Illustration of a round aquaculture pen showing both the surface and underwater view. The cage is surrounded by nets and small fish swim inside. At the bottom of the cafe is a cone collecting material floating inside the cage. From the cone a tube leads to the side of the cage, then to the surface of the water, and ultimately above the surface to the right of the cage. A small boat is shown behind the cage, with a size that fits approximately 4 times the diameter of the cage. In the back of the illustration are shapes showing a relief landscape.

Sludge is then pumped to the feeding barge into a storage tank, before being transported to shore by boat for further processing.

Wide illustration with an aquaculture cage on the left. Behind the cage is a small boat that fits 4 times the diameter of the round cage. On the bottom of the cage is a cone collecting material. This material is led to the surface through a tube. The tube leads to a platform floating on the surface of the water. The platform has a tank and inside-areas with windows. From there the tube is led to a smaller platform floating on the water, and then led to a larger boat with 2 tanks and a deck. From the second tank on the right a tube leads to one of the three tanks placed at a dock. On the dock is a large white building with few small windows and a roll-up door.
Testing the technology

Sludge collection has already been piloted at several Norwegian salmon farms to validate its efficiency, security and ease of use. Before fully introducing it to the market, further testing is needed to scale up performance. AQUAPHOENIX partners will test sludge collection technology across regions, nations and seas with varied environmental and production characteristics. For instance, the fjords of Norway require deeper pens than the shallow seas of the Baltic, where other AQUAPHOENIX testing sites are located in the Åland archipelago, Finland.

Sharing and scaling

Sludge collection technologies can be flexibly integrated into existing fish farms and used in multiple contexts. In the initial stages of the AQUAPHOENIX project, partners will continue to refine this technology using existing site infrastructure. Looking ahead, the technology can be incorporated into future designs and integrated solutions.

A circular action plan

Sludge collection technology is the first step in a circular action plan. After sludge is collected, treated and stored, AQUAPHOENIX partners will explore how this sludge can be reused in the manufacturing of sustainable products like novel fertilisers and feed ingredients and clean energy.

Meanwhile, scientific partners will observe the sites where sludge is being collected to quantify the environmental impact of this technology, which has great potential to contribute to ecosystem restoration and healthy seas. And partners in policy and governance will ensure there is a strong appetite and regulatory backbone for the widespread adoption of this technology.

Working for impact

From salvaging sludge to safeguard local ecosystems and support economic growth, to investigating its potential as a sustainable resource, the AQUAPHOENIX team is working to validate aquaculture waste as a circular resource and lead change on an international scale. Sludge collection technologies are one of the engines of this change: a key step towards reducing aquaculture emissions and realising a Zero Pollution Europe.

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