Methylsiloxanes in Drinking Water Treatment Plants: Occurrence, Removal Efficiencies and Exposure Risks Across Multiple Stages of Treatment Process Development

Methylsiloxanes (MSs) are recognized as emerging pollutants and have been widely detected in surface waters that serve as drinking water sources. However, their removal efficiencies through various processes in drinking water treatment plants (DWTPs) remain poorly understood. The present study determined 17 MSs including 8 cyclic MSs (CMSs), 7 linear MSs (LMSs) and two hydride-terminated MSs (HDMSs) in 8 DWTPs which represented 4 typical treatment processes. In source water samples, 16 MSs were frequently detected, and the highest concentrations were observed in river network waters with the lowest ones in reservoir-derived waters. In the samples from DWTPs, the average total concentrations ranged from 34.2 to 147 ng/L for CMSs, 1.55 to 42.7 ng/L for LMSs and 14.1 to 133 ng/L for HDMSs, respectively. In each DWTP type, the total concentrations followed an order of CMSs > HDMSs > LMSs. The MSs can’t be completely removed by the DWTPs, with overall removal efficiencies ranging from 23.1%∼63.7% for CMSs, -58.9%∼27.4% for LMSs and 37.6%∼87.2% for HDMSs, respectively. In conventional DWTPs, disinfection functioned as the primary removal mechanism for MSs. In DWTPs with ozonation-biological activated carbon (O₃-BAC), both oxidation by O₃ and the disinfection contributed to the efficient removal. In DWTPs with pre-ozonation (or pre-chlorination), the O₃-BAC remained the key effective treatment step, while the disinfection showed negative contribution to the removal. The DWTPs with the artificial wetland or biological pretreatment with advanced processes including O₃-BAC, ultrafiltration (UF) and nanofiltration (NF) showed the highest removal efficiencies. The total average daily doses (ADDs) were below the corresponding chronic reference dose (RfD) values, suggesting low health risks. Further investigations on both toxicities and fates of these MSs are still necessary to strengthen health protection strategies.