NOVELTY - The method comprises performing psychrophilic or mesophilic methanization of a liquid effluent in a methanizer (32) that produces biogas and methanized sludge and effluent, biologically treating the methanized effluent within a biological treatment zone, which produces biological sludge and a treated effluent, at a temperature of 5-15 degrees C, extracting the thicken biological sludge from the biological treatment zone, and performing mesophilic or thermophilic anaerobic digestion of a part of methanized sludge in a digester that produces biogas and digested sludge.    USE - The method is useful for treating urban or industrial liquid effluent.    ADVANTAGE - The method is capable of simply, energy-efficiently, economically and reliably treating the urban or industrial liquid effluent without polluting the environment thus providing highly purified liquid.    DETAILED DESCRIPTION - The method comprises performing psychrophilic or mesophilic methanization of a liquid effluent in a methanizer (32) that produces biogas and methanized sludge and effluent, biologically treating the methanized effluent within a biological treatment zone, which produces biological sludge and a treated effluent, at a temperature of 5-15 degrees C, extracting the thicken biological sludge from the biological treatment zone, performing mesophilic or thermophilic anaerobic digestion of a part of methanized sludge in a digester that produces biogas and digested sludge, recirculating a part of the digested sludge in the methanizer, extracting a part of digested sludge, recirculating a part of the thicken biological sludge in an upstream of a biological reactor (34) or a denitrification reactor, and controlling a flow of recirculation of the digested sludge in the methanizer, a flow of recirculation of the methanized sludge in the digester and a flow of extraction of the digested sludge. The biological treatment comprises aerobic biological treatment, and anoxia biological treatment. The flow of recirculation and the extraction is controlled to maintain a concentration (10-100 gMVS/L) of sludge in the methanizer and a concentration (30-100 gMVS/L) of sludge in the digester according to a temperature of effluent to be treated. An INDEPENDENT CLAIM is included for an installation for treating liquid effluent.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of an installation for treating liquid effluent.    Supply pipe (30)    Methanizer (32)    Outlet for methanized effluent (33)    Biological reactor (34)    Pipe. (35)