MEMBRANE FILTER AND ITS OPERATION

PROBLEM TO BE SOLVED: To enable washing the whole membrane surface and continuing to perform good operation for a long time. SOLUTION: This membrane filter has a treating tank 31, a membrane unit 50 immersed in the treating tank 31 and formed by laminating flat membrane- shaped membrane elements, a cylindrical skirt part 71 arranged in the bottom part of the membrane unit 50, an aerator 61 arranged below the skirt part 71 and for jetting gas, partitions 72, 73 arranged in the bottom part of the membrane unit 50 and for forming divided chambers 74 to 76 between the skirt part 71 and them in the skirt part 71, and a gas feeding means for feeding gas to the divided chambers 74 to 76. Since by this gas feeding means, gas is fed to each of the divided chambers 74 to 76, the quantity of bubbles flowing in both the side edge parts of the membrane unit 50 between membranes can be enlarged and the whole membrane surface 52 can be washed.