In spite of all public enthusiasm for sorting and recycling waste, a considerable amount of unsorted, often evil-smelling household rubbish always remains. This is left to rot on waste tips, where it represents a burden on the environment through pollution of ground water and emissions of gas. Methods of reducing the volume of such waste include combustion and biological degrading; alongside composting plant, digestion processes have greatly gained in favour. They significantly reduce the volume of waste, and the bio-gases produced by digestion can be used to generate power. Researchers at the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB in Stuttgart and the Schwarting-Uhde company in Flensburg have now improved the technology involved. They have succeeded in reducing the dry organic matter which makes up around 60 percent of such residual waste by almost 90 percent, simultaneously doubling the yield of biogas.

The trick used by the Fraunhofer researchers was to refine the normal two-stage process, adding micro-filters to the second phase of processing under hermetically sealed conditions. Sludge liquor passes through these filters while the residual mass continues to rot, and shrink – to less than half the volume in comparison with earlier processes. The scientists then bring air into play, and treat what remains with special fungi in order to reduce the amount of lignin in the biomass, which is not easily broken down through fermentation. This substance is contained in almost all plants, fruits and vegetables, and lignifies their fibres, or makes them “woody“. Finally, the mass is digested one more time, further reducing the residual volume. The odour-free residual solids can then be disposed of at landfill sites.

In the words of Professor Dr. Walter Trösch of the Fraunhofer IGB: “There is no problem involved in retrofitting existing fermentation plant with our system, except of course for the question of cost.“ However, the new system represents a worthwhile investment in the future; the results achieved already fulfil the latest technical requirements for disposal of domestic waste in Germany, due for enforcement as of the year 2005. And the process is not only suitable for treatment of household waste; tangible benefits are also gained through the reprocessing of other organic forms of waste such as biowaste, slurry, sewage sludge or natural materials used by industry.