NOVELTY - A wastewater is treated by adding sludge from waterworks to acclimate anaerobic reactor, and making suspended solid concentration in reactor reach 20-30 g/L; and using anaerobic reactor for treating sewage. The anaerobic reactor comprises water distributor (6), sludge expansion district, zero valent iron packed layer (4), suspended sludge district, biological filtering layer, 3-phase separation district and 3-phase separator (2) to process sewage, where refluxed sewage in suspended sludge district is delivered into water inlet tube on bottom of anaerobic reactor.    USE - The method can can decolour dye wastewater.    ADVANTAGE - The method can decolor dye wastewater with rate of 85-95%, removes more than 50% of chemical oxygen demand and increases biodegradability to at least 0.3.    DETAILED DESCRIPTION - The wastewater is treated by adding sludge from waterworks to acclimate anaerobic reactor, and making suspended solid concentration in reactor reach 20-30 g/L; and using anaerobic reactor for treating sewage. The use of anaerobic reactor for treating sewage involves:    (A) selecting iron sheet, iron slag or iron grain of 5-15 mm, and soaking into 0.1% sodium hydroxide before using 5% hydrochloric acid for washing; and using water for washing to remove grease contamination and rustiness on surface, drying obtained zero valent iron for use;    (B) mixing zero valent iron with active carbon grains whose diameter is 5-10 mm, where mass of zero valent iron is 1-3 times that of active carbon grains; adding mixture into zero valent iron packed layer, latching on wire-mesh screen, where height of mixture filled in zero valent iron packed layer is 2/3;    (C) placing 3 zero valent iron packed layers into acclimated anaerobic reactor, placing biological filtering layer and installing 3-phase separator, and covering upper cover of the anaerobic reactor;    (D) opening feed-water pump (9) to fill treating sewage whose pH is 8-9 into anaerobic reactor, determining flow according to concentration of treating sewage, and increasing flow load gradually;    (E) opening circulating pump, where mass of refluxed sewage is 16-20 times that of treating sewage;    (F) making treating sewage stay in anaerobic reactor for 18-24 hours, making clear water discharged and collected from effluent pipe (11), making biogas discharged and collected from gas header (10), and making sludge discharged and collected from suspended sludge district.    The anaerobic reactor comprises water distributor (6), sludge expansion district, zero valent iron packed layer (4), suspended sludge district, biological filtering layer, 3-phase separation district and 3-phase separator (2) to process sewage, where refluxed sewage in suspended sludge district is delivered into water inlet tube on bottom of anaerobic reactor through circulating pipe (12) and circulating pump to mix with inlet treating sewage and then flow into anaerobic reactor together.    DESCRIPTION OF DRAWING(S) - The drawing is a cross-sectional view showing an aerobic reactor.    Phase separator (2)    Zero valent iron packed layer (4)    Feed-water pump (9)    Gas header (10)    Effluent pipe (11)