Senior Researcher Jesper Ahrenfeldt and Ulrik Birk Henriksen from DTU Chemical Engineering with CEO Claus Thulstrup (right). Photo: Bax Lindhardt

Stinking sludge into energy and fertilizer

Thursday 15 Mar 18


Claus Thustrup, CEO at AquaGreen.
DTU start-up utilizes sludge from fish farming and wastewater to produce heat and fertilizer.

Found everywhere, sludge is difficult to dispose of and poses a global challenge. It is active, ferments, has a high water content—and can be hazardous to health. Now, the newly started cleantech business AquaGreen has found an innovative solution that eliminates all forms of noxious germs in the sludge, rendering it inert, odour-free, and at the same time removing the water, which is otherwise a costly and difficult process. Customers are already queuing up at AquaGreen.

AquaGreen’s solution consists of a processing plant which by means of steam drying and pyrolytic treatment (digestion of organic compounds by heating without oxygen) can extract natural fertilizers for agriculture and thermal energy for heating. All from foul-smelling mud.

Superheated steam drying (the green cycle).

Flue gas cycle with supply energy to pyrolysis and steam dryer (the brownish-red cycle). 

Pyrolysis chamber (box with zig zag auger).

Pyrolysis gas burner (box with flame).


Sludge de-watering and stabilizing is costly and energy-intensive, but often the calorific value of the sludge’s organic ingredients is sufficient to justify the process.

The start-up is currently focusing on the aquaculture industry—with wastewater and bio gasification targeted as the follow-up markets. In principle, the technology can be used everywhere where sludge is produced—e.g. cruise ships, festivals, refugee camps, and military bases.

That said, the entrepreneurs are also getting calls from interested parties wanting to know whether they can steam dry liquid manure, prawns, salmon louse, seaweed, mash from breweries—and apple cores.

“If you invent a technology that can transform a problem into a valuable product, you have a good business case. Today, the Norwegian aquaculture industry is our core business area. Here, we use thermal energy in land-based fish farming process heating, while the residual waste—namely biochar—can be used in agriculture due to a high fertilizer value in the form of plant-accessible phosphorus,” says Claus Thulstrup, AquaGreen CEO. 

Difficult to dispose of sludge
The start-up idea came about one night when Claus found himself talking to a good friend—the CEO of one of the world’s leading land-based fish farm companies. The friend told him that the biggest problem associated with the fish farms was disposing of the sludge created by fish faeces and fish feed residues. The two friends developed the idea of using the organic material in the sludge to drive a drying and combustion process based on pyrolysis.

Through a friend, Claus Thulstrup established contact with Senior Researcher Jesper Ahrenfeldt from DTU Chemical Engineering, who is conducting research into pyrolysis and biomass gasification. Together, they agreed that the idea was viable.

Since then—together with DTU—the company has secured grants from the Eurostars Programme, the Environmental Technology Development and Demonstration Programme (MUDP), and the InnoBooster Programme—among others. Concurrently, the company participated in the Danish Tech Challenge hardware competition at Scion DTU.

"If you invent a technology that can transform a problem into a valuable product, you have a good business case."
Claus Thustrup, CEO, AquaGreen

Photo: Bax Lindhardt

Sizeable Norwegian interest
“The exciting thing was that Claus came up with a business case and some immediately viable ideas, while my colleagues and I were able to draw on 30 years of biomass know-how. Suddenly, we began to realize that we could use some of the technologies we had been working with over the past 25 years. All we needed to do was connect a steam dryer to a pyrolysis unit and we had a simple, efficient plant which steam dried and pyrolyzed sludge instead of wood chips,” says Jesper Arenfeldt.

The process plant has generated particular interest in Norway, where fish farmers are gradually moving parts of their salmon farming from offshore to land-based production. At the same time, the Norwegian authorities have prohibited the direct emission of sludge from fish farms into the sea, as this creates huge environmental problems.

Another growing market is wastewater sludge, which is currently used as fertilizer directly on fields in commercial agriculture. In the Netherlands and Germany, however, legislation is underway banning the disposal of wastewater sludge on fields to avoid plastic residues, endocrine-disrupting substances, and heavy metals. That said, the new law will impose nutrient recirculation requirements.

Such legislation will most likely spread to the entire EU, thus creating an even bigger market for innovative solutions that break down the polluting substances and enable a secure recirculation of nutrients. AquaGreen also hopes to get their treated biochar products approved for organic farming.

Ulrik Birk Henriksen, Senior Researcher at DTU Chemical Engineering, chimes in:

“I really like the fact that our start-up is slightly back to front. Normally, you start by developing a technique, which you then try to sell to a company. We, however, began by going straight to the market. We know that there is a great need. The customers are queueing up. This gives you a completely different drive.” 

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