Patent ID: 8153007

Claim:
A method of treating a wastewater, comprising the steps of: feeding a wastewater into an activated sludge tank; conducting biological treatment of the wastewater in the activated sludge tank; feeding a biologically-treated wastewater containing activated sludge from the activated sludge tank to a membrane module installed outside of the activated sludge tank; filtering the biologically-treated wastewater containing activated sludge in the membrane module to produce a treated wastewater; disconnecting the membrane module from the activated sludge tank with a valve installed between the membrane module and the activated sludge tank; chemically-cleaning the membrane module with a membrane-cleaning agent comprising chlorine or an alkali agent while the membrane module is disconnected from the activated sludge tank by the valve, and water-flushing off a substance contained in the wastewater that reacts with the membrane-cleaning agent to form a hazardous substance or an operation-inhibiting substance, or the membrane-cleaning agent remaining in the membrane module, with water, while the membrane module and the activated sludge tank are disconnected from each other by the valve, wherein the membrane-cleaning agent is prevented from contacting the substance contained in the wastewater that reacts with the membrane-cleaning agent to form hazardous substances or operation-inhibiting substances and also influence in-tank microbes and the wastewater contains an organic matter which generates a toxic chlorine-based organic matter in a reaction with chlorine or contains a metal which generates a metal compound which precipitates on the surface of the membrane or in the membrane from a reaction with chlorine or an alkali agent.