NOVELTY - Dephosphorizing and denitrifying urban sewage comprises: feeding pre-processed urban sewage, and returning sludge and digestion sludge returned from precipitating tank to anaerobic reactor; feeding anaerobic reactor outlet water and liquid mixture to anoxic reactor; feeding discharged water to aerobic reactor; feeding water discharged liquid mixture to precipitating tank for sludge-water separation; digesting concentrated active sludge by anaerobic sludge digesting unit; returning partial digested sludge to anaerobic, anoxic or aerobic reactor; and dewatering remaining digested sludge.    USE - The method is useful for treating urban sewage by dephosphorizing and denitrifying.    ADVANTAGE - The method: has zero energy consumption and low sludge yield; improves methane yield and reduces the discharging amount of digested sludge and recycles the methane energy to provide energy for sewage and sludge processing energy (electric energy and heat) by combining anaerobic digestion of sludge, biological dephosphorization and denitrification of sludge, and aerobic biological processing technologies.    DETAILED DESCRIPTION - Dephosphorizing and denitrifying urban sewage, comprises: (a) feeding the pre-processed urban sewage and the returning sludge returned from a secondary precipitating tank into an anaerobic reactor, feeding partial or all of digested sludge returned from the anaerobic sludge digesting unit into the anaerobic reactor, performing aeration and stirring the sewage and sludge in the anaerobic reactor to form liquid mixture A; (b) feeding the liquid mixture A into an anoxic reactor from the anaerobic reactor, feeding partial or all of digested sludge returned from the anaerobic sludge digesting unit into the anoxic reactor, returning the liquid mixture B into the anoxic reactor from the aerobic reactor, performing aeration and stirring the sewage and sludge in the anoxic reactor to form liquid mixture C; (c) feeding the liquid mixture C into an aerobic reactor from the anoxic reactor, feeding partial or all of digested sludge returned from the anaerobic sludge digesting unit into the aerobic reactor, returning partial liquid mixture B discharged from the aerobic reactor into the anoxic reactor and feeding the remaining liquid mixture D into the secondary precipitating tank; (d) after the liquid mixture D is fed into the secondary precipitating tank, performing ternary sewage processing or discharging the sewage out of the system as an outlet water, discharging the sludge from the bottom of the secondary precipitating tank, in which most sludge is fed into the anaerobic reactor as the returning sludge, and the remaining sludge is discharged into the sludge concentrating unit; (e) discharging the concentrated sludge from the sludge concentrating unit to an anaerobic sludge digesting unit for digestion, returning the sludge-removed water (supernatant) for sewage pre-processing; and (f) digesting the sludge at anaerobic condition by the anaerobic sludge digesting unit to produce methane mainly containing methyl hydride and digested sludge, returning partially digested sludge of the sludge discharged by the sludge digesting unit to the anaerobic reactor, anoxic reactor or the aerobic reactor; and dewatering the remaining digested sludge for sludge processing.    DESCRIPTION OF DRAWING(S) - The drawing shows a process for treating urban sewage by dephosphorizing and denitrifying (Drawing includes non-English language text).