A filter-plate assembly has a support plate having an outwardly directed face and formed with an annular groove opening outward at the face and a chamber plate confronting the face of the support plate, forming therewith an inlet compartment, and formed with an inlet-port cutout opening into the compartment. A flexible filter plate adjacent the support plate has an outer peripheral frame set in the groove and a generally planar central panel within the frame and having a back face turned toward and forming a compartment with the face of the support plate and a front face turned away from the support plate and adapted to be overlain by a filter cloth. A rigid retaining bar overlying the frame extends parallel to it at the cutout. Bolts or the like secure the retaining bar relative to the support plate against the frame to hold the frame tightly in place in the groove of the support plate at the cutout.