Economic and practical considerations compel today's designers to investigate alternatives to conventional strategies for wastewater treatment systems. One such alternative is the Two-Zone process, a novel activated sludge process which combines the aerobic biological reactor and the clarifier in one tank for retrofit into existing plant tankage to increase capacity. A key feature of the Two-Zone process is the oxygenation of the recycled biomass with pure oxygen in an external transfer device. A 1,893-cu m/min (0.5-mgd) demonstration of the Two-Zone process was conducted at the Norristown Borough Municipal Wastewater Treatment plant in Pennsylvania. Objectives of the demonstration were to gain an insight into the functional requirements of the process and to test the system performance at higher loadings than attempted in previous studies. A basic conclusion of the demonstration was that the Two-Zone process functioned well by achieving up to 92 percent BOD5 removals at relatively low sludge ages (1 to 3 days) with average oxygen consumption of about 0.5 kg/kg BOD5 removed.