Effects of ozonation on halogenated acetaldehydes and trihalomethanes formation: Strategy of process control for a full-scale plant

Ozonation provides several advantages during the drinking water treatment process, including controlling the formation of regulated disinfection by-products (DBPs) such as trihalomethanes (THMs) and haloacetic acids (HAAs). However, ozonation can lead to the formation of bromate and unregulated halogenated acetaldehydes (HALs). In order to better understand the effects of ozonation on the formation and speciation of different DBPs and on the quality of drinking water, high-frequency water quality monitoring was conducted in a Canadian full-scale drinking water treatment plant (DWTP). We collected samples during a period when the DWTP underwent changes to their ozonation strategies: applying only post-ozonation (Post-O-3) and only pre-ozonation (Pre-O-3), and a transition period comprising two-stage (pre- and post-) ozonation and no-ozonation. In this case study, four different phases of disinfection were examined chronologically: Phase 1 (P1), Post-O-3 with Post-Cl-2; Phase 2 (P2), Pre-O-3 and Post-O-3 with Post-Cl-2; Phase 3 (P3), Post-Cl-2 only; Phase 4 (P4), Pre-O-3 with Post-Cl-2. In finished water, we observed that ozonation significantly effected the removal of total nitrogen (TN) and aromatic content (UV-254, SUVA) only, but not the removal of TOC. Total ozone dosage (Pre-O-3 + Post-O-3) appeared to be a contributing factor for HAL formation. Compared to non-ozonated water (chlorination only), concentrations of chloral hydrate (CH, the most prevalent HAL) were double in ozonated water. Dibromoacetaldehyde (DBAL) formed only in the presence of pre-ozonation. Ozonation effectively reduced the formation of total THMs, whereas more brominated THMs were formed with higher ozone dosages.