Disposing of organic waste is becoming more of a problem all the time. In some countries there is a separate "bio bin", but not everything that gets thrown in it is suitable for making compost. The number of fields on which sewage sludge can be spread is getting smaller every year. But where can we put such wastes when the number of land disposal sites is shrinking and there is every possibility that legislation may forbid tipping of organic wastes altogether in the long term? One possible alternative has been developed by researchers at the Fraunhofer-Institut für Grenzflächen- und Bioverfahrenstechnik IGB (Institute for Interfacial Engineering and Biotechnology) in Stuttgart. They have improved the familiar fermentation process to such an extent that organic waste and sewage sludge can be decomposed and converted to biogas. The volume of refuse is reduced, and at the same time the organic component can be used as an alternative source of energy.

The principle is extremely simple. As in nature, bacteria decompose organic material to produce methane. Two digesters are used in series: in the first, bacteria convert starch at a temperature of 37°C, while in the second reactor other micro-organisms convert more complex substances such as cellulose at 55°C. As the conditions in the two stages are optimized to the requirements of the different bacteria, the yield is very high - one kilogram (dry weight) of organic material produces about 1 cubic metre of biogas. The methane is then used as fuel in a cogenerator to produce electricity and heat. The heat can be used to keep the digesters warm, while the electricity is fed into the grid. On 14 October the results of a large-scale pilot plant in Leonberg will be presented at a colloquium in the Fraunhofer Institute's centre at Nobelstr. 12 in Stuttgart (in the foyer and lecture theatres A and B). Further topics will be new technologies for sewage plant; recycling technologies; and legislation governing waste disposal.