Abstract: A modular membrane filtration system (5) includes a number of banks (6) of membrane modules (7). Each module (7) includes upper and lower modular manifolds (8) and (9), respectively which are connected to one another to form upper and lower common manifolds (10) and (11) for each bank (6). The upper and lower manifolds (8) and (9) are used for delivery of feed and removal of filtrate and waste liquid from the modules (7). A gas feed manifold (12) runs along the top of the module banks (6) and feeds gas to lower gas manifold (13) through drop lines (14). A further manifold (15) runs in parallel with the common upper and lower manifolds (10) and (11) and is connected thereto at connections points (16) between the manifold banks (6).