Far-UVC 222 nm Treatment: Effects of Nitrate/Nitrite on Disinfection Byproduct Formation Potential

Irradiation at far ultraviolet C (far-UVC) 222 nm by krypton chloride (KrCl*) excilamps can enhance microbial disinfection and micropollutant photolysis/oxidation. However, nitrate/nitrite, which absorbs strongly at 222 nm, may affect the formation of disinfection byproducts (DBPs). Herein, we evaluated model organic matter and real water samples and observed a substantial increase in the formation potential for trichloronitromethane (chloropicrin) (TCNM-FP), a nitrogenous DBP, by nitrate or nitrite after irradiation at 222 nm. At a disinfection dose of 100 mJ·cm^-2, TCNM-FP of humic acids and fulvic acids increased from ∼0.4 to 25 and 43 μg·L^-1, respectively, by the presence of 10 mg-N·L^-1 nitrate. For the effect of nitrate concentration, the TCNM-FP peak was observed at 5-10 mg-N·L^-1. Stronger fluence caused a greater increase of TCNM-FP. Similarly, the increase of TCNM-FP was also observed for wastewater and drinking water samples containing nitrate. Pretreatment using ozonation and coagulation, flocculation, and filtration or the addition of H₂O₂ can effectively control TCNM-FP. The formation potential of other DBPs was minorly affected by irradiation at 222 nm regardless of whether nitrate/nitrite was present. Overall, far-UVC 222 nm treatment poses the risk of increasing TCNM-FP of waters containing nitrate or nitrite at environmentally relevant concentrations and the mitigation strategies merit further research.