An aerobic bacteria sewage treatment apparatus using compressed air to aerate activated sludge, and to cause it to circulate about a cylindrical clarification compartment placed in the apparatus in a way to provide a zone of relative quiescence in the flowing sewage from which the clarification compartment withdraws liquid and further separates solid matter therefrom, finally delivering a very well clarified effluent, the effluent pipe being placed in the clarification compartment such that heavier particles tend to sink away from it and lighter particles tend to rise above it, whereby the effluent is substantially free of solid matter.