A screen decanter for screening solids from waste water in a waste water treatment system comprising a drain manifold and a plurality of cylindrical cartridges mounted to the drain manifold, each of the cylindrical cartridges having an outer screen defining a cavity. Each of the cartridges includes a drain standpipe disposed within the cavity in communication with the drain manifold. Each drain standpipe includes a plurality of patterned openings arranged in at least one row along the length and partial circumference thereof and preferably comprises three rows of patterned openings, each row being disposed at a 120° rotational angle from the adjacent row of patterned openings. Each drain standpipe is disposed at a 30° rotational angle from its neighbor drain standpipe such that no row of patterned openings in any of said drain standpipes faces directly at any other row of patterned openings in any other drain standpipe.