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

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.