Abstract: The present invention relates to a cleaning method for a submerged membrane element used in a membrane bioreactor treatment in which organic wastewater containing 1 mg/L or more of manganese is subjected a biological treatment in a bioreactor treatment tank and then subjected to solid-liquid separation by a membrane filtration device provided in the bioreactor treatment tank, in which after a first liquid containing an oxidizing agent is injected from a secondary side of the submerged membrane element to a primary side thereof and maintained for a certain period of time, the first liquid containing an oxidizing agent is suction-discharged from the secondary side to the outside of the system, and then a second liquid containing an acid is injected from the secondary side of the submerged membrane element to the primary side thereof and maintained for a certain period of time, followed by conducting suction filtration operation.

Abstract: Provided is a method for producing fresh water, having a plurality of desalination processes in parallel where raw water or pre-treated raw water is treated by a first semi-permeable membrane unit, and the resulting permeate of which a pH is raised is treated in the second semi-permeable membrane unit, comprising: temporally, in part of desalination process A; lowering a pH of raw water or pre-treated water to be supplied to a first semi-permeable membrane unit A1 and to make a pH of the resulting permeate of the first semi-permeable membrane unit A1 lower than that of feed water of a second semi-permeable membrane unit B2 in desalination process B to be supplied to a second semi-permeable membrane unit A2; and mixing the permeate obtained from the second semi-permeable membrane unit A2 with the permeate obtained from the second semi-permeable membrane unit B2 in said desalination process B.