A column packed with clean sheets of reticulated polyurethane foam was fed with either raw sewage, synthetic sewage or skimmed milk for a short time to evaluate the mechanism of physical entrapment and filtration of particles. The results revealed that clean media were effective in the removal of suspended chemical oxygen demand (COD) (>75%) even with hydraulic retention time (HRT) as low as 0.5 h or upflow velocity as high as 10 m/h. However, the removal of colloidal COD remained poor under all experimental conditions. The treatment of raw sewage and paper-filtered sewage in a fed-batch recirculation system under anaerobic conditions resulted in average removal efficiencies for colloidal COD of 63% and 81%, respectively at a temperature of 24 +/- 1 degrees C. Therefore, the presence of an anaerobic biofilm and the removal of suspended solids (SS) from the influent increased the removal efficiency of colloidal COD in the treatment of domestic sewage in an anaerobic polyurethane-foam filter.