Abstract: A waste water treatment system including an oxidation vessel which receives waste water and has at least one wall and is adapted to introduce oxygen into the waste water contained therein. A clarifier is mounted on the wall at the outlet from the vessel and is narrow in comparison to the vessel. The clarifier includes generally vertical end walls and an imperforate elongated generally vertical sidewall defining upper and lower ends of the clarifier. The lower end of the clarifier is opened to provide for admission of waste water into the clarifier and an apertured plate extends along the length of the clarifier at the upper end and is hydraulically interposed between the clarifier and the vessel outlet.