Biodegradation of haloacetic acids in water treatment processes

Formation and degradation of haloacetic acids (HAAs) in Tai Lake Water Treatment Plant (WTP) in Kin-Men County, Taiwan, were evaluated in this study. The results showed that formation of HAAs after chlorination is a fast process. Owing to the presence of fairly high organic precursors in the raw water, a large amount of HAAs (up to 80 μg/L in summer) was formed after addition of the pre-chlorine, and only a small portion of the HAAs was removed during the coagulation, flotation, and rapid filtration units. However, more than 80% of HAAs were removed in slow sand filtration (SSF) unit. Laboratory batch filtration tests showed that the HAAs can not be effectively removed by conventional coagulation and filtration treatments. However, the HAAs in water was effectively removed by biodegradation in batch biodegradation tests using filter sands taken from the top of the SSF unit in Tai Lake WTP. For comparison with the results obtained in batch experiments, simulated SSF systems were also installed in laboratory to evaluate the effects of biodegradation for HAAs removal in filter columns. Results of parallel laboratory SSF column tests showed that HAAs was quickly degraded when the simulated SSFs have been operated for a suitable time to allow the microbial growth on the sand surface. In both batch and simulated SSF biodegradation treatments, the biodegradation rates for HAAs decreased as the number of halogen atoms increased. The results in this study demonstrated that biological degradation is the major mechanism responsible for HAAs removal in the SSF units.