To suppress bio-fouling in a reverse osmosis membrane module which is generated by microorganisms increased with organic components and inorganic components remaining in membrane filtered water as nutrients in a regenerated water manufacturing method treating which uses as the regenerated water the water obtained by treating with a reverse osmosis membrane the membrane filtered water obtained by treating dirty water such as sewage and industrial wastewater in a membrane separation and activated sludge method.

In a regenerated water manufacturing apparatus constituted of a biological treating vessel 1 treating with activated sludge water to be treated, a separation membrane module 2 comprising a flat membrane module immersed and installed in the biological treating vessel, and a reverse osmosis membrane module 13 treating with reverse osmosis the membrane filtered water obtained by membrane-filtering treated water by activated-sludge treated-water through the separation membrane module, the regenerated water manufacturing apparatus is cleaned by sterilizing the membrane filtered water flow passage of the separation membrane at a frequency of once an hour or once a week through feeding a chemical liquid in a direction opposite to the flow direction of the membrane filtered water in a membrane filtered water flow passage of the separation membrane module 2.

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