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This paper focuses on the mechanisms of the biological activated carbon (BAC) process. It is found that the bioregeneration hypothesis of desorption by exoenzymatic reactions is the common basis for some previous mechanisms. But by considering properties of the enzymes involved in the biodegradation pathway of phenols and aromatic hydrocarbons and distribution of exoenzymes in carbon pores, it is suggested that the hypothesis is not valid in most circumstances. A series of tests has been made in which phenol was used as the testing pollutant. The BAC column test was performed to investigate the changes in carbon adsorption capacity during operation. Properties of biological activity in BAC were measured by a Warburg respirometer. A comparative test between the BAC bed reactor and biological aerated filters was also made. All test results were contradictory to the mechanisms based on the bioregeneration hypothesis and supported the authors' suggestion that the mechanism of the BAC process is basically a simple combination of biodegradation and carbon adsorption. It is suggested that some specially designed biological aerated filter (BAF), instead of a BAC bed reactor, may be used in advanced wastewater treatment.