Abstract: An improved microporous, modular filter structure assembled from replaceable filter cartridges (25) housed within a filter cartridge assemby (10) connected at opposed ends by means of symmetrical headers (41). The headers include a tube within a tube structure whereby they are adapted to conduct first and second fluids independently to both adjacent, like interconnected headers and into and out of filter cartridges (25) to which each header (41) is connected at both ends of each cartridge. Manifolds (100, 101) include a similar tube within a tube structure which allows them to be connected to the ends of rows of interconnected headers (41) whereby a modular filter bank can be constructed of indefinite dimension and capacity.