Bioregeneration is defined as the renewal of the adsorptive capacity of activated carbon by microorganisms for further adsorption. Bioregeneration of activated carbon increases the service-life of activated carbons. It is traditionally believed that bioregeneration occurs as a result of a concentration gradient between the carbon and the bulk liquid. Therefore, bioregeneration can only occur for compounds that readily desorb. Some researchers suggest that also exo-enzymes act in bioregeneration. Bioregeneration has been demontrated in offline systems, which involve desorption and biological removal of adsorbed organic matter in a closed-loop recirculating batch system. It has also been shown that bioregeneration of carbon occurs during the time course of wastewater treatment processes that are based on biological activated carbons (BACs). However, most of the studies aiming at quantifying bioregeneration were performed using offline systems because of difficulties encountered in quantifying bioregeneration during BAC treatments of wastewater. Bioregeneration is dependent on several factors including biodegradability, adsorbability and desorbability of sorbate, characteristics of activated carbon and process configuration, and it can be optimized by varying the operational conditions. In this review, we are addressing the enhancement of biological treatment by activated carbon, the definition and mechanisms of biological regeneration, the relationship between adsorption reversibility and bioregeneration, the factors affecting bioregeneration, the methods for determination and quantification of bioregeneration and the mathematical models of bioregeneration. Future research is still required to determine the optimum conditions for an increased bioregeneration. Particularly, factors such as the activated carbon type, nature of the microbial community and optimum process configuration need further investigation. The validity and efficiency of exo-enzymatic activities on bioregeneration should still be investigated. (c) 2007 Published by Elsevier Ltd.