The present invention is a filter assembly for vacuum filtration formed of a housing having an upper portion and lower portion, a filter separating the upper from the lower portion of the housing, an inlet formed in the upper housing, an outlet and a vacuum port formed in the lower portion, a gasket arranged at a lowermost surface of the lower portion and the upper portion is having a height sufficient to trap any air that might enter and keep it away from the membrane surface. Preferably, it is of a curved shape, preferably in the form of a dome. Optionally, the upper portion of the housing may also contain a vent, particularly a vent controlled by a valve, for the removal of air periodically from the upper portion, especially when a large incursion of air has occurred.