The rice husk based activated carbon (RHAC), used in this study was precarbonized and activated using phosphoric acid. The activated carbon was characterized with the help of N2-adsorption/desorption, FT-IR and scanning electron microscope to determine surface area, functional groups and surface morphology respectively. Bacillus sp. isolated from tannery wastewater was immobilized in this rice husk based activated carbon and packed in a reactor (named CAACO reactor). The wastewater having the following characteristics - chemical oxygen demand (COD): 1716 ± 392 mg/l; biological oxygen demand (BOD): 393 ± 114 mg/l, sulphate: 940 ± 217 mg/l and sulphide: 193 ± 56 mg/l - was applied at a hydraulic loading rate of 0.7376 m 3/m3/day and surface loading rate of 0.2438 m 3/m2/day to the CAACO reactor. The temperature was maintained at 30°C and the pressure at one atmosphere. It was found that the reduction in COD, BOD, sulphide and sulphate were respectively 87%, 96%, 100% and 40% under an air oxidation of 2 h. © 2004 Elsevier Ltd. All rights reserved.