Abstract: The basin includes a standpipe having axially extending slots spaced vertically from its bottom. A baffle extends coaxially within the pipe around the slots to separate oil and grease from the runoff. The accumulated water is held in the standpipe thereby allowing the larger particles to settle out into the lower portion of the standpipe before the water flows through the slots into an outer chamber. The outer chamber contains a cylindrical filter element through which the water must flow. The filter element removes suspended lighter solids and any remaining grease or oil not removed in the standpipe. The cleansed runoff finally is discharged through an outflow pipe extending from the outer chamber. The standpipe can be periodically cleansed by removal of a drop inlet grate which covers the pipe. Furthermore, the filter element can be removed for cleaning.