Maintaining high concentrations of microorganisms is crucial for the anaerobic treatment of organics. The granulation of sludge is a good method but it takes more than 6 months to form sludge granules spontaneously. In this study, a large amount of granular sludge was prepared within 5 min by adding organic-inorganic hybrid polymers to the sewage digester sludge. The size of the granular sludge ranged from I to 5 mm and its falling velocity was higher than that of the granular sludge from the actual wastewater treatment facility. Since the granule was stable throughout the operation of a UASB, the removal efficiency was as high as 90% at high organic loading rate up to 18 kg-COD/m(3) day. (C) 2003 Elsevier Ltd. All rights reserved.