Abstract: A sewage treatment plant in which sewage is introduced into a tank or other suitable container, to form a mixed biomass and sewage liquid for treatment within the tank, by means of both fixed and suspended biomass. The fixed biomass attaches itself to a plurality of free draining media surfaces that are alternately exposed and submerged by the raising and lowering of liquid levels within the tank. The raising and lowering of the liquid surfaces is achieved by displacing liquid with pressurized air from an inverted box that also contains a plurality of media surfaces. The treated sewage overflows into a final zone which contains random media surfaces. Surplus sludge collecting in the final zone is fed back close to the inlet by gravity through a non-return valve.