The stability and efficiency of an anaerobic reactor containing biomass immobilized on polyurethane foam were assessed. The reactor with mechanical stirring of 500 rpm and maintained at 30 +/- 1 degrees C-treated synthetic wastewater with a concentration of approx 500 mg of chemical oxygen demand/L and was fed with different influent volumes and cycle times maintaining organic load. Operation was in batch mode with renewal of only part of the volume of wastewater to be treated; that is reactor discharge was not complete, but partial. The main operational characteristic investigated was the ratio of the volume of wastewater fed per cycle (V-A) to the volume of wastewater in the reactor (V-U) maintaining the same volumetric organic load. This way, operating flexibility could be verified in relation to the volume of treated wastewater at each cycle and the cycle time for the same organic load. The results indicated that the reactor was able to operate with different V-A/V-U ratios with no significant loss in performance, thus allowing increased operational flexibility. For conditions in which V-A was >= 50% of V-U, removal efficiencies of filtered and nonfiltered organic matter were about 84 and 79%, respectively, whereas at conditions of higher initial influent dilution, these efficiencies were slightly lower, about 80 and 74%, respectively. At higher initial influent dilutions, it became difficult to maintain a constant reactor medium volume, owing to a high formation rate of viscous polymer-like material, likely of microbiologic origin.