in a system for treatment of biochemical waste such as activated sludges , the waste is subjected to anaerobic stabilization processes , which include two phases of an acid fermentation and a methane fermentation effected in separate reactors . the waste after the acid fermentation is then separated into a liquid effluent for the methane fermentation and a waste sludge . the waste sludge which may cause secondary environmental pollution is re - treated within the anaerobic stabilization processes without withdrawing from the system .