Abstract: A plant for the treatment of waste water, such as municipal sewage and industrial wastes containing organic matter which must be decomposed, comprises a mechanical clarifier for the removal of non-degradable and coarse contaminants, an activation vessel for a first digestion stage operating with an oxygen content close to 0 mg/l with facultative anaerobic operation, an intermediate clarifier, an aeration stage formed by a trickling filter for oxidative decomposition, an afterclarifier and an arrangement for removal and preferably the treatment of surplus sludge. An unaerated mixture (facultative anaerobic) basin is provided upstream of the afterclarifier in which water from the trickling filter is contacted with surplus sludge from the activation basin.