Removal of disinfection residual bacteria in UV222, UV222/H2O2 and UV222/peroxymonosulfate systems: what is the safe usage for wastewater reclamation

Abstract

Disinfection residual bacteria (DRB) are widely present in the reclaimed treatment effluents and can regrow during the downstream distribution and storage, posing a threat to the biosafety of reuse applications. Recently, far ultraviolet (UV₂₂₂) have garnered augmented attention due to the highly efficient and energy-intensive oxidation, making them a potential approach for the deep inactivation of DRB. However, there remains a lack of quantitative analyses on how to monitor the disinfection intensity to mitigate the health risks associated with DRB. In this study, we used the UV₂₂₂, UV₂₂₂/H₂O₂ and UV₂₂₂/peroxymonosulfate (PMS) systems to treat model DRB including Escherichia coli, Pseudomonas aeruginosa, and Bacillus subtilis, and developed a multiparameter model to accurately present the dose-culturability relationship. On this basis, we conducted the simulated disinfection, and detected the viability status and regrowth potential of DRB during the post-disinfection processes. It turned out that UV₂₂₂ alone exhibited the superiority over UV₂₅₄, especially for treating Pseudomonas aeruginosa. UV₂₂₂/H₂O₂ and UV₂₂₂/PMS systems further improved the inactivation rates. The practical UV doses for full-scale reclaimed disinfection (10–200 mJ/cm²) were sufficient for the UV₂₂₂-based systems to inactivate DRB (initial 10⁷ CFU/mL) to the safe level in effluent measured by culture methods. But substantial DRB still persisted in VBNC state, which necessitated higher doses of 200–450 mJ/cm² to further inhibit the regrowth under accidental contamination and prolonged transport/storage culture. Fortunately, H₂O₂ provided residual disinfection for Bacillus subtilis, and PMS performed promising sustained disinfection for all the three DRB. This study provided valuable insights for the expanded application of UV₂₂₂ disinfection and future updates of pathogen standards in reclaimed water treatment.