A bulk mass transfer coefficient (BMTC) equation was derived from the mechanism of mass transfer in surface liquid membrane in this study, which was based on the analysis of biosorption process, conservation of mass in sludge granule and the unification of the dimension. A biosorption experiment was carried out in which anoxic sludge from an anoxic baffled reactor for printing and dyeing wastewater treatment was used to adsorb Acid Red GR dye. The results showed that there was a linear regression curve between ln[qe/(qe-q)] (qe and q were the amount adsorbed at equilibrium and at time t, respectively.) and time t. There was also a good agreement between the adsorbate amount measured and that predicted by the equation of BMTC. The BMTC of Acid Red GR dye adsorbed by anoxic sludge was 6.816 kgm(-3)min(-1). Experimental results indicated that the BMTC determined by a simple adsorptive experiment using this equation was credible. It could be a feasible and effective way to determine BMTC of activated sludge for biosorption performance.