A comparative analysis of finished water quality disinfected by liquid chlorine or sodium hypochlorite in Yancheng, China, 2018—2022

Objective  To investigate the main pollution of disinfection by-products on the quality of finished water in Yancheng, China, 2018—2022, and analyze the influence of liquid chlorine and sodium hypochlorite disinfection on the quality of finished water.

Methods  From 2018 to 2022, a total of 40 samples of finished water and source water were collected from waterworks A(disinfected by liquid chlorine) and B(disinfected by sodium hypochlorite), respectively, to detect the levels of disinfection by-products, disinfectants and microbial indicators.

Results  There was no significant difference between the waterworks A and B in the contents of trichloromethane (Z=-0.38, P>0.05), bromodichloromethane (Z=-1.07, P>0.05), dibromochloromethane (Z=-0.61, P>0.05), and tribromomethane (Z=-1.37, P>0.05) in source water. The disinfection by-products of finished water in both waterworks was qualified. The contents of trichloromethane (Z=-5.41), bromo-dichloromethane (Z=-3.47) and bromo-dichloromethane (Z=-2.34) in source water were lower than those in finished water (P < 0.05). The contents of chlorodibromomethane (Z=-2.19) and tribromomethane (Z=-2.04) in water from waterworks A were higher than those from waterworks B (P < 0.05). The contents of trichloromethane, dichloroacetic acid, and trichloroacetic acid in finished water with a free residual chlorine concentration of>0.8 mg/L were significantly higher than those in finished water with a free residual chlorine concentration of ≤0.8 mg/L (Z=-2.36, -2.79, and -2.16, P < 0.05). The contents of trichloromethane, dichloroacetic acid, and trichloroacetic acid in finished water in wet season were significantly higher than those in dry season (Z=-3.48, -2.85, and -3.16, P < 0.01).The finished water disinfected by liquid chlorine showed a significantly higher content of residual chlorine than that disinfected by sodium hypochlorite (Z=-2.41, P < 0.05). Disinfection with liquid chlorine or sodium hypochlorite had no significant effect on the microbial indicators of finished water.

Conclusion  Both disinfection method with liquid chlorine and sodium hypochlorite have good disinfection effect, and some disinfection by-product indicators are affected by the differences in water sample type, disinfection method, water seasons, and residual chlorine level.