A batch process and apparatus for treating wastewater in a system employing anaerobic and aerobic zone reaction vessels under elevated pressure is disclosed in which an amount of processed activated sludge material is charged into an aerobic zone and wastewater to be processed from an inlet supply charge system is used to fill one or more aerobic zones. The charge system is pressurized by introducing high pressure air which is continuously infused and the material is circulated among anaerobic and aerobic zones as a multi-phase bubble flow while venting an amount of circulated air of lower oxygen content through a vapor release system. After treatment, the system pressure is reduced over a timed interval through the vapor release system, and the treated batch is discharged to an effluent storage system for separation of treated water and sludge.