Trihalomethanes and haloacetamides as disinfection by-products in different types of drinking water of Wuxi

Background The concentrations of disinfection by-products (DBPs) are varied by different water sources, disinfectants, or treatment processes in Wuxi, and the associated health risks are also different.

Objective To understand the levels of trihalomethanes (THMs) and haloacetamides (HAcAms) in drinking water in Wuxi, and their variations by water sources, seasons, disinfectants or treatment processes, aiming to provide technical support for ensuring the safety of drinking water.

Methods In dry period (December 2019) and wet period (July 2020), the finished water and tap water (from the beginning, middle, and end of the drinking water distribution network) from 12 centralized water treatment plants in Wuxi were collected to detect the concentrations of THMs and HAcAms in water samples. A purge and trap-gas chromatography-mass spectrometry method was applied to detect trichloromethane (TCM), bromodichloromethane (BDCM), dibromochloromethane (DBCM), and tribromomethane (TBM), and a solid-phase extraction-gas chromatography-mass spectrometry method to detect dichloroacetamide (DCAcAm), trichloroacetamide (TCAcAm), bromochloroacetamide (BCAcAm), dibromoacetamide (DBAcAm), bromodichloroacetamide (BDCAcAm), dibromochloroacetamide (DBCAcAm), and tribromoacetamide (TBAcAm). Analyses and comparisons were made on the concentrations of THMs and HAcAms in drinking water by water sources (the Yangtze River/the Taihu Lake/reservoir), wet/dry seasons, disinfection methods (liquid chlorine/sodium hypochlorite), and treatment processes (conventional treatment/conventional+advanced treatment).

Results A total of 96 drinking water samples were collected in Wuxi. THMs were positive in all the water samples (100%), with concentration ranging from 1.027 to 40.225 μg·L⁻¹ and the M (P₂₅, P₇₅) concentration being 24.782 (17.784, 30.932) μg·L⁻¹. None of the 4 THMs exceeded the standard limit of the Standards for drinking water quality (GB 5749-2022 ), and the order of the 4 THMs concentrations from high to low was TCM > BDCM > DBCM > TBM. Five of the 7 HAcAms were detected, the total concentration ranged from 0.137 to 3.288 μg·L⁻¹, and the M (P₂₅, P₇₅) was 0.808 (0.482, 1.704) μg·L⁻¹. The DCAcAm concentration was the highest (2.448 μg·L⁻¹), followed by BCAcAm, while TCAcAm and DBCAcAm were not detected. The M (P₂₅, P₇₅) of the total concentration of THMs in the drinking water from the Taihu Lake was 33.353 (26.649, 36.217) μg·L⁻¹, that of the Yangtze River was 27.448 (24.312, 31.393) μg·L⁻¹, and both were higher than the level of the reservoir 16.359 (2.305, 21.553) μg·L⁻¹ (P<0.05), while the M (P₂₅, P₇₅) of the total concentration of HAcAms in the drinking water from the Taihu Lake was 0.616 (0.363, 0.718) μg·L⁻¹, which was lower than those of the Yangtze River 0.967 (0.355, 2.283) μg·L⁻¹ and the reservoir 1.071 (0.686, 1.828) μg·L⁻¹ (P<0.05). There were no statistically significant differences in the total concentrations of THMs and HAcAms between wet season and dry season, or between different disinfection methods (P>0.05). The M (P₂₅, P₇₅) concentrations of THMs and HAcAms in drinking water after advanced treatment process involving ozone, activated carbon, and membrane were 20.565 (3.316, 27.185) μg·L⁻¹ and 0.623 (0.452, 1.286) μg·L⁻¹ respectively, and were lower than the corresponding values after conventional treatment process, 28.740 (23.431, 35.085) μg·L⁻¹ and 0.934 (0.490, 2.116) μg·L⁻¹ respectively (P＜0.05).

Conclusion The concentrations of THMs and HAcAms in drinking water in Wuxi are generally at a low level. The levels of controlled THMs meet the requirements of national standards, and the levels of uncontrolled HAcAms as new DBPs are up to μg·L⁻¹. The concentrations of the two kinds of DBPs in drinking water vary by water sources. The concentrations of THMs and HAcAms produced by the advanced treatment process are lower than that by the conventional treatment process.