Abstract: An apparatus for treating sewage is disclosed. The apparatus includes a modular treating tank which is divided by a perforated baffle or screen into primary and mixing zones. The mixing zone has a curved or semi-spherical end wall. A mixing and aeration system utilizing a high efficiency ejector withdraws liquid from the mixing chamber and directs the recirculated flow toward the primary chamber across the perforated baffle. The effluent from the treating tank is subsequently chemically disinfected by timed, periodic addition of a chemical such as a selected halogen. A recirculating generally oval or circular flow pattern in which liquid is withdrawn from the mixing zone, aerated and directed toward the primary chamber is established. Complete mixing with the incoming sewage occurs in the primary chamber to maintain a uniform concentration of oxygen, feed and purifying organisms throughout the tank volume.