The Fraunhofer Institute for Interfacial Engineering and Biotechnology has been testing its new digestion process in Heidelberg’s municipal sewage plant for the past year. Engineers are delighted by the speed and efficiency of the new procedure, with biogas being produced in five days compared with typical processing times of 20 to 30 days. The digestor also operates at a much smaller volume of four litres compared with the usual 60 litres.
“We have combined an excellent biological process with excellent biochemical engineering,” Professor Walter Trösch from the Fraunhofer Institute told edie. “Of the electricity generated from the biogas produced, only a third is used in the process, leaving two-thirds to be sold to the grid or diverted to other areas of the plant.” Thus the city of Heidelberg runs off its own sewage.
The new method has the added advantage of reducing the organic content of sewage by 50 to 60% compared with an average 40% in normal decomposition, resulting in fewer residues to dispose of. But although some processing plants could be updated to incorporate the new procedure, for older plants in Germany the cost of conversion is too high. “Old-fashioned plants would need to be re-built, so their conversion is not yet possible,” says Professor Trösch.