Evaluation of synergy and bacterial regrowth in photocatalytic ozonation disinfection of municipal wastewater

The use of solar and ultraviolet titanium dioxide photocatalytic ozonation processes to inactivate waterborne pathogens (Escherichia coli, Salmonella species, Shigella species and Vibrio cholerae) in synthetic water and secondary municipal wastewater effluent is presented. The performance indicators were bacterial inactivation efficiency, post-disinfection regrowth and synergy effects (collaboration) between ozonation and photocatalysis (photocatalytic ozonation). Photocatalytic ozonation effectively inactivated the target bacteria and positive synergistic interactions were observed, leading to synergy indices (SI) of up to 1.86 indicating a performance much higher than that of ozonation and photocatalysis individually (SI ≤ 1, no synergy; SI > 1 shows synergy between the two processes). Furthermore, there was a substantial reduction in contact time required for complete bacterial inactivation by 50–75% compared to the individual unit processes of ozonation and photocatalysis. Moreover, no post-treatment bacterial regrowth after 24 and 48 h in the dark was observed. Therefore, the combined processes overcame the limitations of the individual unit processes in terms of the suppression of bacterial reactivation and regrowth owing to the fact that bacterial cells were irreparably damaged. The treated wastewater satisfied the bacteriological requirements in treated wastewater for South Africa.