Abstract: An insert for use in a drain basin, comprising an outer wall defining a periphery of the insert and having a top edge and a bottom edge. A perforated bottom surface extends across the lower periphery of the insert. A filter, resting on the perforated bottom surface, has a peripheral edge enclosing a central portion. A flange extends inwardly from the top edge of the outer wall past the peripheral edge of the filter. The flange directs drainage water past the periphery of the filter and to its more central portion, whereupon impurities within the drainage water are extracted and filtered water exits through the perforated bottom surface into the drain basin. In a preferred embodiment of the invention, the inward-extending flanges are angled downward toward the filter. In a second embodiment, the filter, bottom surface and inward-extending flanges are arranged in a cartridge, which is removably placed inside the periphery of the outer wall.