NOVELTY - Preparation of flocculating agent composite fertilizer and methane, comprises adding algae-containing sludge to hydrothermal oxidation reactor, adding hydrogen peroxide and standing the reactant; drying solid sludge and grinding, passing into a 100-mesh sieve, adding hydrochloric acid (HCl), reacting, curing and drying polymeric aluminum ferric chloride to obtain solid product, and regulating pH of organic residual wastewater.    USE - Method for preparing flocculating agent composite fertilizer and methane (claimed).    ADVANTAGE - The method is simple and low cost, and has high economical benefits, and is capable of being applied to zero-discharging and clean production for processing sewage of lake polluted by algae and the algae-containing sludge polluted by sewage plant.    DETAILED DESCRIPTION - Preparation of flocculating agent composite fertilizer and methane, comprises:    (A) adding algae-containing sludge with water content of 96-99% from sewage plant to a hydrothermal oxidation reactor; adding 30 mass% hydrogen peroxide at adding quantity of 50 mg/L; reacting for 1.5-2.5 hours at 150-180 degrees C and 0.8-1 MPa; standing the reactant for 30 minutes to obtain brown organic residual wastewater with chemical oxygen demand (COD) concentration of 10000-20000 mg/L and solid sludge containing ferro-aluminum inorganic substance;    (B) drying solid sludge at 120 degrees C and grinding; passing into a 100-mesh sieve, in which substance passing the sieve is sludge powder less than 100 meshes; then adding HCl to substance passing the sieve based on mass ratio of sludge powder to HCl of 1: (3-5); reacting for 1-2 hours at 90-100 degrees C; standing the reactant for 30 minutes and obtaining supernatant liquid; using alkali to regulate pH of supernatant liquid to 2.5-4; curing the product for 12 hours at 50-60 degrees C to obtain polymeric aluminum ferric chloride and drying the polymeric aluminum ferric chloride to obtain the solid product of polymeric aluminum ferric chloride; drying the liquid product at 90-100 degrees C to obtain the solid product of polymeric aluminum ferric chloride, in which the content of aluminum oxide in solid product after detection is greater than 30%, the alkalinity is more than 45%, the iron content is 3%, the content of the insoluble substance is less than 0.5%, and the other indexes meet national standard of polymeric aluminum chloride flocculating agent, and the molar concentration of hydrochloric acid is 5 mol/L; and    (C) regulating pH of organic residual wastewater obtained in step (A) to 8-9; using a measuring pump to pump the wastewater to an up-flow anaerobic sludge bed reactor based on flow rate of 0.002 m3/day, controlling the temperature of reactor to 35 degrees C and performing anaerobic reaction for 24 hours to obtain granular organic fertilizer, in which flammable gas with 65% methane is generated during reaction; and the granular organic fertilizer is composite fertilizer comprising 46% nitrogen and 5% phosphorus.    DESCRIPTION OF DRAWING(S) - The drawing is a block diagram of a method for preparing flocculating agent composite fertilizer and methane (Drawing includes non-English language text).