Graded particle-size retention filter medium for cell-type filter unit

A filter unit comprising:

• an upper composite filter medium;
• a lower composite filter medium;
• a non-filtering separator element having a central void, operatively positioned between the upper composite filter medium and the lower composite filter medium, for facilitating liquid flow from the central void of the separator element and through the filter unit to allow liquid to move from an outer surface of the filter unit to a center bore and out through the center bore;
• and
• structure operatively connecting the lower composite filter medium and the upper composite filter medium at the outer periphery thereof, wherein the upper and the lower composite filter mediums each comprise:
• a first zone of filter material positioned distal to the separator element and comprising fibrous material and filter aids;
• and
• a cast polymeric symmetric or asymmetric microporous membrane positioned more proximal to the separator element than the first zone, the microporous membrane having an increased relative-PRC compared to the relative-PRC of the first zone.