The performance of an upflow anaerobic sludge blanket (UASB) reactor treating wastewater with volatile fatty acids (VFA) and alcohol generated during hydrogen fermentation of food waste was investigated. The chemical oxygen demand (COD) removal efficiency exceeded 96% up to the loading rates of 12.9 g COD/L d, corresponding to a food/microorganism ratio of 0.61 g COD/g VSS d. The methane production rate increased to 4.1 L/L d. Each gram of granule in the reactor had a daily maximum capacity of converting 0.55 g of COD to methane at the specific substrate utilization rate of 0.59 g COD/g VSS d. Of all the COD removed, 93% was converted to methane and the remaining presumably to biomass with a yield of 0.051 g VSS/g COD. At loading rates over 15.3 g COD/L d, the COD removal efficiency deteriorated due to sludge flotation and washout in the reactor, which resulted from short hydraulic retention time (HRT) of less than 5.8 h. The specific methanogenic activity (SMA) of granule was the highest for butyrate, and the lowest for propionate. Methanosaeta-like bamboo-shaped rods were present in abundance. (c) 2005 Elsevier Ltd. All rights reserved.