Biochar in the hand. Photo by Christian Ove Carlsson.

Sludge—from problem to resource

Thursday 01 Oct 20
by Morten Andersen


Jesper Ahrenfeldt
Senior Researcher
DTU Chemical Engineering
Sprung from DTU Chemical Engineering, start-up company AquaGreen sees sludge from waste water treatment as a resource rather than an environmental problem.

Waste water treatment, aquacultures, and production of biogas all have sludge as a waste fraction. In principle, the sludge can be used directly as fertilizer in agriculture, but complaints from neighbours over smell as well as content of substances which may be problematic for the environment and human health continue to raise concerns. Collaboration between the CHEC research centre at DTU Chemical Engineering and the young engineering company AquaGreen has developed an alternative: Turning the sludge into thermal energy and fertilizer.

The method builds on a combination of superheated steam-drying and pyrolysis. Pyrolysis is the cleaving of organic compounds by heating under anaerobic conditions.

The process is energy efficient, as the energy content of the sludge is utilized for both the drying and the pyrolytic process which converts the solid part of the sludge into a fertilizer. The end product, known as bio-char, has a high content of nutrients necessary for the growth of plants, especially phosphorous. Notably, the content of problematic substances such as pathogenic bacteria and residues from pharmaceutical products—both found in sludge from waste water treatment—has been eliminated in the up to 650° C pyrolytic heating.

Another major advantage is the significantly lower price of transportation.

“Whenever you transport sludge, you are really transporting more than 90 per cent water, which obviously does not contribute to fertilizing. As the bio-char produced in the new process has less than 10 per cent volume and weight in comparison to the sludge, you are no longer limited to finding application close to the source of the sludge,” explains Senior Researcher Jesper Ahrenfeldt, DTU Chemical Engineering.

The next challenge: Upscaling

AquaGreen was founded in 2014 based on findings in the group at DTU Chemical Engineering. As a first move, the company targeted the aquaculture sector in Norway. Here, environmental problems had forced local producers to move part of production onshore. This improved water quality in the fiords but also resulted in sludge being produced onshore, creating a market for AquaGreen’s technology.

After successful introduction in Norway, the company is now ready to conquer new markets. To this end, the collaboration with the CHEC group—which has been involved all along—will be extended further.

“While the fundamental technology is fully demonstrated and in commercial use, we still need to tackle various chemical engineering issues on the academic side. These are mainly related to upscaling of the processes. In a country like Denmark, waste water treatment is the most interesting application, but this will require building the plants somewhat larger to become economically feasible. As always, upscaling involves chemical engineering challenges. There are still some first-of-a-kind solutions waiting to be developed,” says Jesper Ahrenfeldt.

Legislation drives the transformation

Besides waste water treatment, production of biogas is another promising market for the technology. In Denmark, biogas is largely produced from manure and organic household waste. After the gas is produced, sludge remains, still having some 40-50 per cent of the original energy content. This fits in nicely with the AquaGreen technology.

While the technology is proven, several barriers still need to be overcome, Jesper Ahrenfeldt notes:

“The business case relies on farmers being willing to use bio-char. We see interest going up, but also know that agriculture is generally a conservative sector when it comes to introducing new technology and practises.”

Recent developments give rise to optimism:

“Germany has banned the direct disposal of sludge in agriculture, and we see similar legislation under way in other European countries. Denmark will surely follow at some point. This will obviously support the argument for using bio-chair instead of sludge, and thereby prompt a wider use of the new technology.”

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