The main objective of this work was to investigate the effect of volumetric loading rate (VLR), shock load, and alkalinity supplementation on the efficiency and stability of an Anaerobic Sequencing Batch Biofilm Reactor (AnSBBR) containing polyurethane foam cubes. Mixing in the reactor, which was kept at 30 +/- 1 degrees C, occurred by recirculating the liquid phase. The reactor treated 2.5 l cheese whey in 8-h cycles, at concentrations of 1, 2, and 4 g COD l-1, which corresponded to VLRs of 3, 6, and 12 g COD l-1 day-1, respectively. Application of single-cycle shock loads of 6, 12, and 24 g COD l-1 day-1 did not impair reactor performance. In addition, for VLRs of 3, 6, and 12 g COD l-1 day-1, alkalinity supplementation to the influent, at the end of each assay, could be reduced to 75, 50, and 50%, respectively, in relation to supplementation at the beginning of the assay. During reactor operation a viscous polymer-like material was formed between the polyurethane foam cubes, which increased at higher VLR. Finally, addition of salts to the influent improved reactor efficiency.