The aim of this study was to determine the effects of the volume occupied by the filter bed zone on the behavior of an anaerobic hybrid UASFB reactor. This configuration contains a sludge bed in the lower part of the reactor and a filter bed in the upper part. Three such reactors, each with a working volume of 9.8-L, were studied; these differed in proportion of packing medium occupying 50, 35 and 20% of the working volume of the reactors, respectively. All the reactors were fed with wine distillery vinasse and were operated for a period of 232 days at 33+/-1 degrees C with increasing OLRs. No significant differences were observed between the reactors for the range of OLRs studied. Indeed, relatively high OLRs could be reached for all the three reactors with an average OLR of about 18 g COD/L d towards the end of the experiment, while maintaining a constant HRT of 26 h. COD(t) and COD(s) removal efficiencies were always more than 80 and 86%, respectively, even for the highest OLR. High quantities of VS were accumulated in the reactors with an average value of 438+/-17 g per reactor. Most of the biomass was in the sludge bed zone suggesting that this part was playing a major role in COD removal. The internal packing medium on the top of the reactor had a key role in retaining the small flocculated particles inside the reactor, while minimizing the escape of solids in the effluent. This study suggests that low packing volume ratios can be used in the design of UASFB reactors, which allows a reduction in the costs of packing medium required as the supports are expensive.