Effectiveness of AOC removal by advanced water treatment systems: a case study

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 µg 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 µg 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.

The following conclusions can be drawn from the present investigation. In this study, AOC removal efficiency of the advanced water treatment processes of the CCL was assessed. Moreover, effects of two different filling materials on the efficiency of biofiltration process were evaluated in the column study. Results of the laboratory and field investigation show that a significant AOC removal efficiency was observed by the applied BAC system in the WTP. Conclusions of this study include the following: (1) Significant AOC removal efficiency was achieved in the WTPCCL, and the AOC concentrations in the effluent could meet the established standard. (2) The increased AOC concentrations after the treatment of preozonation and chlorination might be caused by the oxidation of organic matters to more biodegradable and assimilable of organic compounds. (3) The removal of AOC had a correlation with the decrease in concentrations of other drinking water indicators (e.g., coliform, TPC, TDS, and particle counts). (4) The addition of sodium thiosulfate in water samples could enhance the performance of the AOC analysis. (5) GAC is a more appropriate filling material than anthracite in the biofiltration system for the removal of AOC. (6) BAC filtration played an important role in the removal of the trace AOC. Thus, the application of BAC for AOC removal is feasible and should be included as a required treatment unit in the advanced WTP. From this reason, anthracite could not apply instead of GAC. (7) GAC filtration proved to be most effective at removing AOC with a high removal efficiency of more than 80%. (8) Treatment processes (like ozonation) which increased the amount of organics in carbonyl group would likely lead to a product water with poor biological stability. Ozonation therefore, should be combined with the GAC or biological processes to minimize the AOC formation potential.