To provide a method for treatment of wastewater capable of decreasing running cost and installation cost by minimizing the quantity of generated excess sludge to zero or at least decreasing the quantity thereof to downsize a system, and simultaneously of recycling excess sludge as methane gas.

A method for wastewater treatment is characterized by comprising an aerobic biological treatment process which treat organic wastewater biologically under aerobic conditions, a solid-liquid separation process whereby sludge in mixture having been treated biologically is separated, a sludge feed- back process whereby at least a part of the separated sludge is returned to the aerobic biological treatment process as afeed back sludge, a solubilizing treatment process whereby the remaining sludge is made excess sludge and at least a part thereof is subjected to a solubilization treatment, an anaerobic digestion treatment process whereby remaining excess sludge is digested under anaerobic conditions, a digested sludge supply process whereby anaerobically digested sludge is supplied to the solubilization treatment process, and a solubilized sludge feed back process whereby the solubilized sludge is returned to the aerobic biological treatment process.