Two lab-scale UASB reactors were used to treat black liquor. In both the reactors, granulation was achieved. Granular sludge could be formed with or without supplementation of biodegradable carbohydrates in the treatment of black liquor, although the supplementation could accelerate the rate of granulation and lead to a quick enhancement of COD removal. The UASB reactor without a gas-solid separator (GSS) was more suitable for sludge granulation, due to accelerated wash-out of the dispersed sludge. Low influent concentration was also essential for granulation. At a loading of 10 kg COD/m(3) d, 40-46% of influent COD and 56-65% of BOD were removed. The specific yield of biogas was 510 l/kg COD removed.