Correlation between virus reduction and trihalomethanes risk after chlorine disinfection
Authors:   , Khantong Waraporn 
Published: June 2024

While wastewater disinfection is essential to mitigate the threats from waterborne pathogens, there arises a potential risk from disinfection byproducts. Striking a balance between achieving microbial reduction and managing the occurrence of disinfection byproducts is crucial for maintaining both microbial and chemical risks at acceptable levels. The cancer risk assessment was based on THMs through of oral ingestions, inhalation absorptions, and dermal absorptions exposure. This study aims to investigate the relationship between virus reduction and THMs formation, and to assess their respective cancer risk levels post-chlorination, and determined treatment process was used to decreased cancer risk post-chlorination. Effluent wastewater was collected from wastewater treatment plant employing cyclic activated sludge systems (CASS) in Chongnonsi wastewater treatment plant (CN-WWTP) and activated sludge type step feed and ultra-microfiltration in Bangsue wastewater treatment plant (BS-WWTP). From this study, the chlorine dose range of 4.8 to 14 mg/L is enough to inactivate 1–6 log of bacteriophage in CN-WWTP, while BS-WWTP used chlorine concentration range 1.5 to 5.0 mg/L, sufficient to according to the guidelines for virus reduction required by the World Health Organization (WHO). Results indicate that the dose of chlorine has a significant impact on the inactivation of bacteriophages and the level of log reduction. From chlorination process the chlorine concentration brought up the free chlorine residual after initial chlorine were founded range 0.147 to 2.613 mg/L, according to concentrations of residual chlorine limit in agriculture reused of WHO. Additionally, the concentration of chlorine appears to influence the formation of THMs. The concentrations of TCM, BDCM, DBCM, and TBM observed in our study ranged from 1.229 to 28.455 µg/L, 0.017 to 9.327 µg/L, 0.017 to 4.137 µg/L, and 0.028 to 1.205 µg/L, respectively. It was interesting that TCM dominated THMs occurrences, followed by BDCM, DBCM, and TBM, with averages of 79.72%, 15.87%, 3.55%, and 0.87%, respectively. These concentrations were detected during the inactivation of bacteriophages, which achieved a 1 to 6 log reduction. Importantly, the levels of THMs formed did not exceed the World Health Organization's (WHO) guidelines for THMs concentration in drinking-water quality.

The cancer risk associated with the oral ingestion, inhalation absorption, and dermal absorption of THMs resulting from chlorination. The result was founded cancer risk of TCM from two effluent wastewater were exceeded limit of USEPA in BS1 with 1.02 x 10-6 and BS3 with 1.12 x 10-6. Whereas the cancer risk of BDCM, DBCM, and TBM were shown acceptable risk. Specifically, the through oral ingestion was demonstrated unacceptable risk with 1.11 to 1.62 x 10-6 in CN-WWTP and in BS-WWTP of 1.00 to 1.91 x 10-6. The correlation between chlorine concentration and cancer risk, as well as log reduction and cancer risk, is significant in BS-WWTP. Furthermore, various treatment methods resulted in differences in effluent wastewater variables. However, the ultra-microfiltration method was deemed unnecessary to reduce cancer risk from the disinfection process by chlorine disinfectant to achieve a 1-6 log reduction of bacteriophage.