Fate of THMs and HAAs in low TOC surface water

A total of 30 conventional surface water treatment plants (WTPs) implementing prechlorination and postchlorination simultaneously from different regions in Korea were investigated to assess formation and removal of THMs and HAA5. All water was low in total organic carbon (TOC) ranging from 0.74 to 6.20 mg/L with an average of 1.63 mg/L. The ranges of THMs and HAA5 levels were 4.5–84.3 μg/L and 1.5–90.8 μg/L, respectively. THMs concentration was more sensitive to water temperature than HAA5 and the ratio of THMs in summer over winter was 2.06. The sum of dichloroacetic acid (DCAA) and trichloroacetic acid (TCAA) was 97% of HAA5. The extent of formation and speciation of DBPs varied greatly by season and geography. The concentration of DCAA and TCAA of the finished water was comparable on a yearly base, but more TCAA was noticed in summer and the opposite trend was noticed in winter. This can be caused by different biodegradability in the sand filter between DCAA and TCAA that formed through prechlorination. Investigation on the removal of preformed DBPs in the GAC filter–adsorber (FA) revealed that breakthrough of THMs and HAA5 was noticed after 3 months of operation. However, gradual improvement (>90%) in HAA5 removal was observed again after breakthrough, which could be attributable to biodegradation. Heterotrophic plate counts confirmed active biological activity in the GAC FA.