Abstract: An intrachannel clarifier for producing clarified waste liquid within an oxidation ditch aerobic wastewater treatment system. The clarifier comprises a tank-like construction positioned in the upper portion of the oxidation ditch channel and having an opening in an upstream wall for admitting mixed liquor at a relatively slow rate. The region within the clarifier establishes a quiescent zone where solids may settle from the waste liquid by gravity to the tank bottom. A portion of the clarified liquid is removed by means of an overflow weir or launder. The bottom of the clarifier forms a perforated baffle made up of a plurality of spaced parallel slightly overlapping inverted V-shaped angle members extending between the walls of the oxidation ditch channel. The spaces between the angle members permit settled solids to return to the mixed liquor flowing beneath the clarifier, but prevent movement of excessive quantities of mixed liquor into the quiescent zone.