This paper proposes a new configuration of a stirred anaerobic bioreactor to improve the biomass retention in sequencing batch processes for treating low strength wastewater. In the proposed configuration, the biomass was immobilized on a polyurethane foam matrix, thus resulting in the elimination of or reduction in settling time. The reactor was operated ata temperature of 30 degrees C and an agitation rate of 500 rpm supplied by means of a stirring bar at the bottom of the reactor. An 8-h cycle was used to treat 0.51 of a synthetic substrate containing a chemical oxygen demand (COD) of 485 mg/l. The overall COD removal efficiency attained 86% in the first 3 h of the cycle, as shown by the concentration profile.