AWWA WQTC71467

The overall objective of this research was to characterize the impact of ClO₂-only and combined ClO₂/Cl₂ pre-oxidation processes on total trihalomethane (TTHM) and haloacetic acid (HAA-5) formation in simulated distribution systems for a wide range of raw water quality. Specific objectives for the combined process were to determine the impact of raw water TOC concentration on TTHM/HAA-5 formation, and to determine the optimum ClO₂:Cl₂ mass ratio. Approximately 50 water utilities across the U.S. participated in this 3-year project, providing raw water samples and full-scale treatment information. The raw water TOC concentrations ranged from 3.09 to 7.80 mg/L. In general, an attempt was made to conduct each set of experiments under similar conditions, particularly during the simulated distribution system (SDS) phase. However, the full-scale operating conditions of each utility did play a role in determining the experimental approach in each study. For example, although a vast majority of the SDS post-chlorine doses were in the range of 2.5 - 3.5 mg/L, they were not all exactly the same, given the wide range of settled water TOC levels. Likewise, although a majority of the SDS experiments were conducted at pH = 8.0, the overall range was 7.8 ± 0.4. Bench-scale SDS experiments were conducted with each sample in order to determine the impact of various preoxidation scenarios on TTHM/HAA-5 formation. The pre-oxidation scenarios generally included a minimum of: control (no pre-oxidant); ClO₂ only; ClO₂/Cl₂ (dose 1); and, ClO₂/Cl₂ (dose 2). The same coagulation/flocculation/sedimentation processes, based on full-scale information, were then applied to all samples, followed by pH adjustment, chlorination, and a 5- day holding period, in the dark at 20ºC, before TTHM/HAA-5 samples were collected. Includes 6 references, figures.