The azo dye acid red 151 (AR151) was aerobically biodegraded in a sequencing batch biofilter packed with a porous volcanic rock. AR151 was used as the sole source of carbon and energy for acclimated microorganisms. Acclimation was followed using the degradation time and the oxygen uptake rate. A maximal oxygen uptake rate of 0.5 mg O-2/(1 min) was obtained. Mineralization studies showed that 73% (as carbon) of the initial azo dye was transformed to CO2 by the consortia. A maximal substrate degradation rate of 247 mg AR151/(l(reactor)d) was obtained. Color removal was up to 99% using an initial concentration of 50 mg AR151/1. Anaerobic tests suggested that in the interior of the porous material, anaerobic biotransformations can occur, contributing from 14% to 16% of the decoloration of the azo dye. (C) 2003 Elsevier Ltd. All rights reserved.