Abstract: A waste water treatment system including an oxidation vessel for receipt of waste water and including structure defining an elongated, closed loop path for the flow of waste water within the vessel. One or more rotors (a) direct waste water along the path and (b) introduce oygen into the waste water within the vessel. An elongated grit chamber is disposed within the vessel and has an inlet and an outlet. The inlet and the outlet are spaced from each other along the flow path and there is provided a further inlet for introducing raw waste water into the grit chamber near the first mentioned grit chamber inlet.