Garden waste such as grass and tree cuttings end up on the compost or in the green / brown bin. Whole truckloads of green stuff that is left over from pruning bushes and trees on roads and railways are processed in the composting plant. These are useful uses, but Nils Tippkötter, Professor of Bioprocess Engineering and Downstream Processing at the Aachen University of Applied Sciences in Jülich is much better used. It turns organic waste into hydrogen, which is one of the bearers of hope in an age that can do without fossil fuels.

Enzymes do the preparatory work

Tippkötter and his team shred the biological waste and bring it into contact with enzymes such as those used by biorefineries and breweries. These are manufactured industrially using biotechnological methods. It is a mix of cellulases and glucosides, as Tippkötter reveals. The solids are separated from the liquid fraction by centrifugation or a screw press.

Hot spring bacteria

In the end, what is left is a mixture of different types of sugar – mainly glucose, xylose and fructose. This can be used, for example, to produce alcohol through a fermentation process. But the Jülich researchers want more. In a bioreactor, they grow Thermotoga bacteria at a temperature of 77 degrees Celsius, which occur in hot springs on the sea floor. They feed on different substrates, including sugar. Arises as a metabolic product a gas that is 40 percent hydrogenthat needs to be separated. It also contains carbon dioxide, but only in the amount that the plants took from the atmosphere.

Competition with biogas plants

The yield is still too low as dhow it would be worthwhile to produce green hydrogen in this way. That is then with the attribute “green” proven if no carbon dioxide is released during its manufacture. So far, this has only been possible with electrolysis using electricity from renewable sources such as wind and sun. But Tippkötter is certain that the process can still be optimizedby introducing direct current, for example, which splits water and thus generate additional hydrogen. When it comes to excess green electricity, the bioreactor is used to store electricity.

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