Abstract: The invention describes a catch basin having an inlet zone and an outlet zone separated by a vertical baffle wherein:(i) the vertical baffle is positioned in such a manner that liquid is permitted to flow from the bottom of the inlet zone to the bottom of the outlet zone;(ii) the vertical baffle provides an inner wall for the outlet zone; and(iii) the outlet zone has an outer wall parallel to the vertical baffle, the outer wall having an outlet port at its upper end, the improvement comprising:locating a series of at least three alternating baffles in the outlet zone, these alternating baffles being parallel, and bearing a spaced relationship, to each other; the even numbered baffles in the series being attached at right angles to the vertical baffle; the odd numbered baffles in the series being attached at right angles to the outer wall; and each baffle in the series extending just beyond the midpoint between the vertical baffle and the outer wall, the first baffle in the series being attached to the bottom