Sludge is human waste (like faeces) in waste water left during sewage treatment. Since it is organic waste, it is used to produce bio gas and manure. Sludge is collected by a scrapper. It is transferred to a separate tank where it is decomposed by the anaerobic bacteria. The biogas produced in the process can be used as fuel. After the aerator tank, microbes and human waste settle down and form activated sludge. The activated sludge is about 97% water. The water is removed by sand drying beds or machines. Dried sludge is used as manure, returning organic matter and nutrients to the soil.
