A filtering element has ultrafiltration or microfiltration hollow fiber membranes extending horizontally between a pair of opposed horizontally spaced, vertically extending headers. Side plates extending between the pair of vertically extending headers define a vertical flow channel through the element. Modules are created by placing the elements in place without obstructing the vertical flow channels. Each element may be released from the frame, however, and removed by sliding it in a direction substantially normal to its headers without disassembling the remainder of the module. The elements have associated releasable water tight fittings between the elements and a permeate collector, the releasable water tight fittings being releasable by removing an element from the frame. An aerator below the module has a plurality of air holes located to provide a line of air holes below each element or below a side plate between each pair of elements. In a preferred process, permeate flux is less than 50 L/m /h and preferably less than 35 L/m /h, aeration to scrub the membranes is provided during periods of time when permeation is periodically stopped and accumulated solids are removed by periodically emptying and refilling a tank containing the elements. A bubble point test is applied to each row of the module in turn, the presence of bubbles in a particular flow channel indicating which, if any, elements of a row are suitable.