Abstract: A filter for treating liquids comprises an elongate housing having a liquid inlet port, an outlet port for concentrated liquid and a permeate discharge port. Extending through the housing is a parallel array of filter tubes, each having a permeable wall so that permeate passes into a common chamber defined by the housing. Manifold members, located at each end of the housing, are provided with channel means which communicate with the ends of the tubes. In use, the channel means direct the liquid along at least three passes through the tubes, the volume of liquid being reduced between the first and final passes so as to maintain the desired flow velocity. Each pass comprises a group of tubes, and the liquid may pass through seven or eleven groups of tubes.