To provide an effective treating method for a filtrate obtained by washing ash such as incinerator ash and fly ash, bypass dust recovered from a chlorine bypass system of cement kiln, dust from a melting furnace with water.

This method comprises 2 processes. Namely, the filtrate from ash washing is reacted with potassium sulfate to form gypsum and potassium chloride by a double decomposition reaction in the first process, and the resulting potassium chloride is reacted with sulfuric acid to form potassium sulfate and hydrochloric acid by a double decomposition reaction in the second process, where potassium sulfate is recycled between the two processes and gypsum and hydrochloric acid are recovered from the processes.