PROBLEM TO BE SOLVED: To provide a wastewater treatment method which effectively uses a waste alkali solution and a waste acid solution generated from a washing step of a reverse osmosis membrane when treating high COD wastewater (COD concentration: 30-70 mg/L) discharged from a waste incineration plant.SOLUTION: A wastewater treatment method for wastewater discharged from an incineration plant includes an alkali washing step of washing a separation membrane with alkali washing water and an acid washing step of washing a reverse osmosis membrane with acid washing water, and also includes (A) a waste alkali solution treatment step of supplying a waste alkali solution, discharged from the alkali washing process, to a temperature lowering device to bring the waste alkali solution into contact with a heat-recovered flue gas and/or (B) a waste acid solution treatment step of mixing a waste acid solution discharged from the acid washing process with fly ash discharged from a dust collector.
