NOVELTY - Reduction of sludge comprises holding anoxic sludge in a container, in particular a sedimentation tank, where anoxic sludge comprises oligochaete-treated sludge.    USE - Reduction of sludge selected from primary sludge, secondary sludge, digested sludge, oligochaete-treated sludge, and/or sludge derived from wastewater treatment (claimed).    ADVANTAGE - The method is low-cost and labor-extensive. It reduces more than 50, preferably almost 70% of sludge, while conventional systems secondary sludge digested under mesophilic conditions degrades for 30%. The amount of sludge, which needs to be disposed of, is reduced, contributing to the economical benefit. Pretreatment of sludge by oligochaete worms releases an increased amount of previously inaccessible sludge, or material or compounds comprised by the sludge. Subsequent containing in and/or adding to a container of oligochaete-treated sludge allows prolonged oligochaete-induced sludge treatment in the tank and subsequent sludge reduction. The oligochaetes that grow on the digesting of the sludge, or parts of the oligochaetes, and the contents of their gastrointestinal track, decompose in the sedimentation tank, further contributing e.g. in a synergistic manner, to the reduction process, which takes place in the container. Fermentation takes place naturally but is facilitated by the increased availability of the organic fraction sludge facilitated by the oligochaetes. Pretreatment of sludge with oligochaete material, in combination with microbial activity originating from the sludge contained in the container, and the (bio)chemical and physical processes that occur, produces a synergistic effect, leading to improved or prolonged sludge reduction. Holding of oligochaete-treated sludge under anoxic conditions allows anaerobic digestion to efficiently proceed under unique conditions comprising high enzyme levels, including high lipase and protease levels in combination with small floc sizes. By the addition of oligochaete material to the container, enhanced reduction of sludge as obtained can be further supported, prolonged, or improved to allow more effective sludge reduction to take place. By adding or containing oligochaete-treated sludge, oligochaetes are first allowed to treat the sludge by passage of sludge through the digestive tract of these worms.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for sedimentation tank containing anoxic oligochaete-treated sludge.