NOVELTY - Urban sewage treatment with zero energy consumption and low sludge output comprises: e.g. introducing pretreated urban sewage into a primary precipitation tank; mixing effluent from the primary precipitation tank with active sludge and passing into an aeration tank; passing the mixed solution from the aeration tank into the secondary sedimentation tank for precipitation and discharging the sludge to a sludge concentrating unit; discharging concentrated sludge to the anaerobic sludge digestion unit; and utilizing the anaerobic sludge digestion unit for digesting the sludge to produce biogas.    USE - The method is useful for treating urban sewage.    ADVANTAGE - The method: has zero energy consumption and low sludge output; improves biogas output; reduces digested sludge emission; and recycles and utilizes biogas energy for providing the energy (electricity and heat) consumed by the sewage treatment and the sludge treatment.    DETAILED DESCRIPTION - Urban sewage treatment with zero energy consumption and low sludge output comprises: (i) introducing pretreated urban sewage into a primary precipitation tank and passing partial or overall digested sludge refluxed from an anaerobic sludge digestion unit flow into the primary precipitation tank; (ii) mixing effluent from the primary precipitation tank with active sludge refluxed from a secondary sedimentation tank and passing into an aeration tank to obtain a mixed solution and simultaneously allowing partial or overall digested sludge refluxed from the anaerobic sludge digestion unit flow into the aeration tank; (iii) passing the mixed solution from the aeration tank into the secondary sedimentation tank for precipitation and separation of active sludge suspended in the mixed solution and other solid substances from the water, passing the precipitated and clarified sewage flow into a third-stage sewage treatment or discharging the precipitated and clarified sewage as disposed water, discharging the sludge from the bottom of the secondary sedimentation tank, introducing a major portion of the sludge as refluxed sludge into the aeration tank and discharging the balance sludge to a sludge concentrating unit; (iv) in the sludge concentrating unit, discharging concentrated sludge to the anaerobic sludge digestion unit for digestion treatment and introducing the removed moisture (e.g. supernatant) of the sludge reflux to the sewage pretreatment or the primary precipitation tank or the aeration tank; (v) utilizing the anaerobic sludge digestion unit for digesting the sludge under an anaerobic condition to produce biogas mainly containing methane and to obtain digested sludge, returning one part of discharged sludge in the sludge digestion unit to the aeration tank or the primary precipitation tank or respectively returning to the aeration tank and the primary precipitation tank in a certain proportion and dehydrating and disposing the other part of the digested sludge according to a national standard.