A pressurized activated sludge reactor with a sand layer installed at the bottom of the reactor for filtration purposes was employed for treating synthetic organic wastewater. The intermittent high-pressure sequential bioreactor (IHPSB) was pressurized to facilitate efficient oxygen transfer under elevated biomass conditions with pressure released periodically, i.e. aeration, for mixing and exchanging air. The sand layer integrated in the bottom of reactor was employed to separate sludge from treated water during the effluent discharging period. The results show that the proposed system can achieve chemical oxygen demand (COD) removal of higher than 90% under COD loading ranging from 3.3 to 14.3 kg COD/m3/day. SS of the effluent is quite stable and is less than 30 mg/L when MLSS is less than 18,000 mg/L. Oxygen transfer in the IHPSB is quite effective. Dissolved oxygen (DO) ranging from 16 to 10 mg/L was achieved with aeration cycle varying from 3 to 15 min. Thus, IHPSB can be quite energy efficient compared with traditional aerobic activated biological systems and membrane biological reactor systems.