A system for treating wastewater comprises a source of activated sludge, a compressor for supplying pressurized air, at least one anaerobic reactor for anaerobically treating wastewater with the activated sludge, at least one aerobic reactor for aerobically treating wastewater with the activated sludge and pressurized air, a pressure control system for regulating pressure in the system, a discharge system for removing byproducts of the system, and a discharge system for removing treated wastewater and activated sludge from an anaerobic and an aerobic reactor, and a surge tank for holding removed wastewater and activated sludge and thereby further treating the wastewater.