Abstract: An aerobic sewage treatment method and apparatus for use with a conventional septic system collection tanks having an inlet conduit (baffle) and an outlet conduit (baffle). A separate treatment vessel is mounted horizontally above the septic tank, the vessel having tangential inlet and outlet at opposite ends; the inlet connected to the output of a pump immersed in the main tank with the outlet from the treatment vessel returning to the septic tank. The vessel contains baffle means to contact floc particles and means to inject an oxygen-containing gas into the liquid as the liquid flows through the vessel. Because of the configuration of the treatment vessel inlet and outlet and baffle means in the vessel, liquid in the vessel flows tangentially under conditions of hydraulic turbulence with large shear forces generated by the baffle means to reduce the floc particle size.