Abstract: A flat-surfaced steep tank floor has a "U" shaped primary channel which extends diametrically across the floor, and a plurality of "U" shaped secondary channels which extend transversely to and intersect the primary channel. The channels have perforated covers which can be controllably removed to facilitate cleaning. The primary channel slopes downwardly from one side of tank to the opposite side. Each secondary channel slopes downwardly from one side of the tank to the primary channel. The secondary channels are perforated at regular intervals. The tank is aerated via aeration conduits which extend beneath the floor and communicate with the perforations.