Abstract: Biological oxidation and flotation apparatus and method are disclosed for use in the treatment of sewage or other aqueous waste material by the activated sludge process. A treatment and flotation chamber is provided in which both biological oxidation and flotation take place. In some forms of the method and apparatus, a separate biological oxidation tank is also used prior to the treatment and flotation chamber.The treatment and flotation chamber includes a quiescent zone at the top of the chamber that extends horizontally through at least the downstream part of the chamber, and extends downward at least about three inches blow the surface of the contents of the chamber. The contents of the chamber below the quiescent zone are continuously recirculated, for example by pumps whose inlets lie below the quiescent zone and which transfer a part of the aqueous material under treatment to the bottom of the chamber to be employed as the shearing liquid in a bubble shearing apparatus.