Case
AAU researchers turn sewage sludge into a sustainable resource
Published online: 09.11.2022
Case
AAU researchers turn sewage sludge into a sustainable resource
Published online: 09.11.2022
The toxic sludge that remains after sewage water has been treated at a sewage plant is either burnt or spread over fields to utilize the nitrogen and phosphorous in the sludge as fertilizer. But as the sludge often contains a large number of toxic residues, it is unfit to fertilize fields for crops or animals.
However, sewage sludge may soon shift from being a hazardous waste product and turn into a valuable resource. Researchers at Aalborg University have helped develop a method that turns the carbon in the sludge into biofuels. The approach is called Hydrothermal Liquefaction (HTL). In plain terms, this constitutes heating up moist sludge under pressure. This enables a series of chemical processes that isolate the carbon in the sludge and turns it into a biogenic crude oil, explains Associate Professor Thomas Helmer Pedersen from AAU Energy.
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We have used sewage sludge to produce jet fuel, which is the most refined type of carbon-based fuel. There is definitely a large potential in utilizing sludge-based fuel for ships and airplanes that are much harder to electrify than cars and trains.
The oil from the sludge has to be treated at a refinery in the same way as fossil oil. In order for that to be possible, it has to pass through a catalyst. After this, it is every bit as useful as any fossil fuel.
Huge environmental benefit
Besides producing sustainable jet fuel, utilizing the sewage sludge entails a number of other environmental benefits. Thomas Helmer Pedersen and his colleagues have succeeded in extracting all the nitrogen and phosphorous from the sludge, turning it into an effective and safe fertilizer.
Often, the sludge also contains medicinal residues and a growing amount of micro plastic that is also preferable to keep out of the biological cycle. When the bio-oil from the sludge passes through Thomas Helmer Pedersen and his colleagues’ thermal process, microplastics, medicine and other waste materials are subjected to temperatures high enough to make them decompose.
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There is a positive circular effect in manufacturing bio-oil and extracting nitrogen that can be re-used. At the same time, we see a huge environmental benefit in removing micro plastics and medicinal residue from the natural cycle
Ready within years
The government’s goal of making all domestic flights in Denmark fly on sustainable fuels within three years cannot be achieved by Thomas Helmer Pedersen alone, but the timespan is not much longer. The method has been proved in the lab, and with the right amount of investment, a fully functional commercial facility could be ready within three to five years, he estimates.
