Wastewater treatment process produces sewage sludge, which is dewatered and dried either mechanically or thermally. In certain cases the thickened sludge is digested. During the digestion process the sludge volume decreases and drying features improve. During digestion the sludge decomposes in anaerobic conditions into methane, water, carbon dioxide and mineral salts. This so called biogas (appr. 60% methane and the rest carbon dioxide) is utilized in the biogas engine plant, and the remaining components are dried in centrifuges. The dried sludge is taken to a land fill to be composted, and the resulting compost is used in landscaping.