Recently, the appearance of assimilable organic carbon (AOC) in the water treatment system and effluent of the treatment plant has brought more attention to the environmental engineers. In this study, AOC removal efficiency at the Cheng-Ching Lake water treatment plant (CCLWTP) was evaluated. The main objectives of this study were to: (1) evaluate the treatability of AOC by the advanced treatment system at the CCLWTP, (2) assess the relativity of AOC and the variations of other water quality indicators, (3) evaluate the effects of sodium thiosulfate on AOC analysis, and (4) evaluate the efficiency of biofiltration process using granular activated carbon (GAC) and anthracite as the fillers. Results show that the averaged influent and final effluent AOC concentrations at the CCLWTP were approximately 124 and 30 mu g acetate-C/L, respectively. Thus, the treatment plant had an AOC removal efficiency of about 76%, and the AOC concentrations in the final effluent met the criteria established by the CCLWTP (50 mu g acetate-C/L). Results indicate that the biofiltration process might contribute to the removal of the trace AOC in the GAC filtration process. Moreover, the removal of AOC had a correlation with the decrease in concentrations of other drinking water indicators. Results from a column test show that GAC was a more appropriate material than anthracite for the AOC removal. Results from this study provide us insight into the mechanisms of AOC removal by advanced water treatment processes. These findings would be helpful in designing a modified water treatment system for AOC removal and water quality improvement.