Inactivation of Bacteria by Peracetic Acid Combined with Ultraviolet Irradiation: Mechanism and Optimization

Peracetic acid (PAA) is an emerging disinfectant for municipal wastewater treatment owing to good biocidal effects and limited harmful by-product formation. This study investigated the inactivation of Gram-negative Escherichia coli (E. coli) and Gram-positive Enterococcus durans (E. durans) and Staphylococcus epidermidis (S. epidermidis) by PAA combined with UV concurrently (UV/PAA) or sequentially (PAA-UV/PAA) for enhanced disinfection. Under UV/PAA, the contributions of different mechanisms (UV, PAA, reactive radicals (mainly •OH and CH3C­(O)­OO•), and the synergistic effect of all mechanisms involved) to the overall inactivation were quantitatively assessed. Results revealed that radicals played a moderate role in the enhanced disinfection, while the synergistic effect presented a greater contribution, which could be partially linked to the diffusion of PAA into the cells as evidenced for the first time by a fluorescence microscopic method. Taking advantage of PAA diffusion into bacteria, pre-exposure of PAA followed by UV/PAA was demonstrated to yield the highest disinfection efficiency. Indeed, compared to UV/PAA, PAA-UV/PAA could achieve additional 4.7–5.4, 4.1–5.3, and 2.9–3.4 log inactivation of E. coli, E. durans, and S. epidermidis, respectively, in clean water and secondary/tertiary wastewater effluents when the same amounts of PAA and UV doses were applied in both approaches. Bacterial regrowth tests confirmed minimal regrowth potential after the disinfection.