NOVELTY - The process comprises treating low strength wastewater with a primary treatment to produce a primary effluent and a primary sludge, treating the primary sludge by anaerobic digestion, treating the primary effluent by a secondary treatment to produce an effluent and a secondary sludge, treating the primary sludge and a thickened portion of the secondary sludge in an anaerobic digester, treating a feed stream in an anoxic tank, treating effluent from the anoxic tank in a nitritation tank, and recirculating a portion of effluent from the nitritation tank to the anoxic tank.    USE - The process is useful for treating low strength wastewater such as municipal sewage to produce biogas, which is useful as a fuel for onsite combined heat and power generation unit.    ADVANTAGE - The process is capable of energy-efficiently treating the low strength wastewater thus providing high quality water without contaminants and enhancing the biogas generation.    DETAILED DESCRIPTION - The process comprises treating low strength wastewater with a primary treatment to produce a primary effluent and a primary sludge, treating the primary sludge by anaerobic digestion, treating the primary effluent by a secondary treatment to produce an effluent and a secondary sludge, treating the primary sludge and a thickened portion of the secondary sludge in an anaerobic digester, treating a feed stream in an anoxic tank, treating effluent from the anoxic tank in a nitritation tank, and recirculating a portion of effluent from the nitritation tank to the anoxic tank. The primary treatment comprises process of solids contact aeration of the wastewater or a process of addition of a coagulant and/or a flocculant to the wastewater prior to passing the wastewater through a clarifier or micro-screen such that the primary sludge contains 50% of biological oxygen demand (BOD) in the wastewater. The anaerobic digestion is performed at a solids content of greater than or equal to 4% dry solids produced or maintained by influent thickening or recuperative thickening or by addition of a co-digestion waste. The secondary treatment comprises a process of nitritation and denitritation, a process of nitrogen removal by anammox bacteria, or a process of passing wastewater through a moving bed bioreactor at a hydraulic retention time of less than or equal to 90 minutes or at a loading rate of 6 kg BOD per cubic meter of tank per day or more. A portion of the digestate is treated with a membrane filtration unit such as air scrubbed filtration unit.    DESCRIPTION OF DRAWING(S) - The diagram shows a flow diagram of a wastewater treatment plant.    Preliminary treatment area (1)    Enhanced primary treatment area (2)    Secondary or biological treatment area (3)    Anaerobic digestion area (4)    Wastewater treatment plant. (10)