Manure and municipal solid waste fermentation in flanders: An appraisal

Abstract

In the last decade, some ten farm installations have been constructed in Flanders to digest piggery manure slurries. A variety of different technologies have been implemented. The conventional completely-mixed digester, provided with a mechanical mixing device, turned out as most reliable. The gas productions obtained, relative to the investment costs, permitted during the period of high energy prices an overall payback in ca 5–10 years. At current moderate oil prices and government tax levels on energy, the payback is of the order of 15–20 years. Hence, manure digestion on the farm is not economically attractive any more. However, increasing pressure against pollution, odour nuisance and disease (piggery pest) has had the result that manure digestion, particularly on a more industrial scale, remains of interest. In Gent, a conventional aerobic composting factory has been operating for more than 20 years. Yet the along maturing periods required for the compost have inspired attempts to try out an anaerobic process. In the so-called ‘Dranco’ process, the municipal solid waste (MSW) organics are subjected to solid-state fermentation at 55°C during 15–20 days. The pilot reactor (56 m 3) has been operating since 1984. Around 170 m 3 biogas is obtained per ton MSW-organics (56% total solids) subjected to digestion. The end product, labelled ‘Humotex’, is a stable product, free of volatile fatty acids and pathogens and capable, as such, to directly score as a plant-growth substrate. The overall costs of the Dranco process, operation on MSW-organics, are of the same order of magnitude as those of the conventional processes for treating municipal solid wastes. The Dranco process offers, also, potential to co-digest surplus sludge produced by conventional sewage-treatment plants.