A process for operating filtering membranes submerged in a tank involves, in one aspect, periodically deconcentrating the tank by partially emptying and refilling the tank with fresh water. The emptying and refilling may be performed generally simultaneously or sequentially. In another embodiment, the membrane modules are arranged in a series of filtration zones between a feed water inlet and a retentate outlet of a tank and the zone adjacent the outlet is emptied and refilled.