A membrane filter module configured to treat a liquid contained in a tank at an ambient pressure. The module may have a header, with a bundle containing a plurality of substantially vertical hollow fiber membranes, wherein a lower end of each hollow fiber membrane is fixed in the header. The module may also have a gasification device adapted to periodically generate a gaseous bubble and configured to release the gaseous bubble within the bundle. The module may further have an enclosure that substantially surrounds the bundle that extends from a lower region to an upper region of the membrane bundle, wherein the enclosure is configured to retain the liquid introduced into the enclosure such that the liquid surrounds the membrane bundle. The gaseous bubble has a cross-sectional area that corresponds with a cross-sectional area of the enclosure, such that the cross-sectional area of the gaseous bubble occupies substantially the entire cross-sectional area of the enclosure as it flows along the bundle.