Background Information on the effects of initial biomass concentration on bioregeneration rate and efficiency is lacking as many of the bioregeneration studies were conducted at constant initial biomass concentration. The objectives of this study are: (i) to develop kinetic models incorporating adsorption, desorption, biodegradation and the irreversibility of adsorption in describing the bioregeneration of granular activated carbon (GAC) loaded with 4-chlorophenol (4-CP); and (ii) to investigate the effect of initial biomass concentration on the rate and efficiency of bioregeneration of 4-CP-loaded GAC in simultaneous adsorption and biodegradation processes. Results The bioregeneration model with Haldane biodegradation kinetics gave a better fit (R2 > 0.956) than the model with first-order biodegradation kinetics (R2 > 0.805) to the experimental data. Values of the rate constant of desorption generated from the models showed that there was no observed effect of varying biomass concentration on the bioregeneration rate. The efficiency study revealed no observed effect of varying biomass concentration on the bioregeneration efficiency provided there was no inhibition effect due to 4-CP. Conclusions Change in the initial biomass concentration had no observed effect on the bioregeneration rate and efficiency, if there was no inhibitory effect due to the substrate in the latter case. (c) 2012 Society of Chemical Industry.