waste liquid is cleaned or purified primarily by removing organic contaminants therefrom by combination of a floating filter layer and an anaerobic biological process . the floating filter layer consists of irregularly shaped small pieces of lipophilic substances such as polypropylene and other plastics , and the organic contaminants are on the one hand filtered by this floating filter layer , and on the other hand decomposed on the surface of the small pieces forming the floating filter layer through anaerobic biological process . the system of the present invention is economical to build , and can be used as a part of sewage and waste liquid processing installations either for industrial or for household use .