Towards reducing DBP formation potential of drinking water by favouring direct ozone over hydroxyl radical reactions during ozonation

Abstract

When ozonation is employed in advanced water treatment plants to produce drinking water, dissolved organic matter reacts with ozone (O3) and/or hydroxyl radicals (OH) affecting disinfection byproduct (DBP) formation with subsequently used chlorine-based disinfectants. This study presents the effects of varying exposures of O3 and •OH on DBP concentrations and their associated toxicity generated after subsequent chlorination. DBP formation potential tests and in vitro bioassays were conducted after batch ozonation experiments of coagulated surface water with and without addition of tertiary butanol (t-BuOH, 10 mM) and hydrogen peroxide (H2O2, 1 mg/mg O3), and at different pH (6-8) and transferred ozone doses (0-1 mg/mg TOC). Although ozonation led to a 24-37% decrease in formation of total trihalomethanes, haloacetic acids, haloacetonitriles, and trihaloacetamides, an increase in formation of total trihalonitromethanes, chloral hydrate, and haloketones was observed. This effect however was less pronounced for samples ozonated at conditions favoring molecular ozone (e.g., pH 6 and in the presence of t-BuOH) over •OH reactions (e.g., pH 8 and in the presence of H2O2). Compared to ozonation only, addition of H2O2 consistently enhanced formation of all DBP groups (20-61%) except trihalonitromethanes. This proves that •OH-transformed organic matter is more susceptible to halogen incorporation. Analogously, adsorbable organic halogen (AOX) concentrations increased under conditions that favor •OH reactions. The ratio of unknown to known AOX, however, was greater at conditions that promote direct O3 reactions. Although significant correlation was found between AOX and genotoxicity with the p53 bioassay, toxicity tests using 4 in vitro bioassays showed relatively low absolute differences between various ozonation conditions.

Overview of the Main Disinfection Processes for Wastewater and Drinking Water Treatment Plants

“…It is believed that the degradation of the residual chlorine dioxide and other final reaction products is faster than that observed in the case of chlorine and therefore these compounds should not represent a hazard to the aquatic environment [45]. During the disinfection with ozone, the formation of organic (e.g., aldehydes, carboxylic acids, and ketones) and inorganic (e.g., bromate) disinfection by-products has been well documented [46][47][48][49][50][51][52][53]. Ozone does not cause the formation of chlorinated by-products such as trihalomethane, but recent studies indicate that ozone induce the formation of NDMA [54][55][56].…”

Novel chemical and biological pre-treatments to improve water quality in drinking water treatment

“…Overall, 5-75% THM and HAA precursors reduction could be achieved by ozonation (von Gunten, 2003b). However, the increase of DBP FP after ozonation was also reported by researchers, including THMs (Mao et al, 2014), HAAs (Mao et al, 2014;Speitel Jr et al, 1993), HANs (Molnar et al, 2012), CH, HKs and HNMs (De Vera et al, 2015). It is hypothesized that the increased DBP FPs may result from the formation of intermediate species by partial oxidation processes (e.g., the formation of phenolic compounds from the oxidation of aromatic compounds by OH radicals, formation of methyl ketones from the oxidation of olefines with ozone) (von Gunten, 2003b).…”

“…Overall, within 6 m of filtration, the DBP FP reduction efficiencies of reactor A for THM4 (sum of TCM, BDCM, DBCM and TBM), CH, HK2 (sum of 11DCP and 111TCP), HAN4 (sum of TCAN, DCAN, BCAN and DBAN) and HNM2 (sum of TCNM and TBNM), were 77±8%, 90±1%, 75±5%, 51±3% and 37±11% respectively, and the final DBP FP for these DBPs are equal or even lower than the values of coagulated water from the same WTP (De Vera et al, 2015). In general, the reduction efficiency of DBP FP for C-DBPs (including THM4, CH, and HK2) was higher or similar than DOC removal (i.e., 71±2%).…”

“… Engineered short residence time RBF (about 6 days HRT) could achieve substantial DOC (60-70%) and UV 254 (70-80%) removal efficiency, which is even better than the performance of conventional coagulation used in the WTP (De Vera et al, 2015). For different regulated and emerging DBP families, at least 50% DBP FP reduction could be achieved.…”

“…De Vera et al (2015) suggested that, compared with OH radicals dominated processes, O 3 dominated processes removed more THM, HAA, and HAN precursors. Overall, 5-75% THM and HAA precursors reduction could be achieved by ozonation (von Gunten, 2003b).…”