Abstract: A membrane separator assembly for separating solids from water includes opposing enclosure wall structures, a membrane module unit, and a gas diffuser disposed below the membrane module unit. The membrane module unit includes a plurality of membrane modules respectively having at least one separating membrane extending between at least two membrane fixing members. At least one of the membrane fixing members has a main passage which is in fluid communication with a membrane passageway. The membrane passageway is accessible to water via permeation through the separating membranes and is inaccessible to solids which are impermeable relative to the separating membranes. The membrane separator assembly is preferably used with a detergent containing percarbonate, preferably sodium percarbonate. The assembly also includes a gas diffuser with two opposing main pipes and branch pipes extending therebetween to avoid undesired pressure differentials when cleaning the assembly.