UV-A based photocatalytic disinfection of secondary effluent from a wastewater treatment plant using Ag-ZnO: Effect of antibiotic presence, catalyst concentration, lamp and persulfate addition

In this study, applicability of UV-A based photocatalytic disinfection of secondary effluent from a sewage treatment plant (STP) was investigated. Three bacterial strains from the biological treatment effluent namely Stenotrophomonas nematodicola, Frediandcohnia salidurans and Shigella felxneri were isolated, cultured and plated for further experiments in the presence of a model antibiotic, metronidazole (MNZ). Silver doped zinc-oxide (Ag-ZnO) and titanium di-oxide (TiO ₂ ) were used as photocatalysts for the disinfection experiments where the former showed better disinfection kinetics under UV-A lamp. Out of the three microbial strains, Shigella flexneri strain showed maximum resistance towards photocatalytic disinfection. Increasing the catalyst dose to 3 g/L increased the disinfection kinetics. However, above 3 g/L, the rate of disinfection decreased due to agglomeration of nano-catalysts, and the increase in residual turbidity. Effect of shigella flexneri concentration showed that disinfection kinetics decreased with increase in bacteria concentration from 1x10 ⁴ to 1x10 ⁹ CFU/mL. In 2 and 3 h, complete removal of 1x10 ⁶ and 1x10 ⁸ CFU/mL was achieved under optimum conditions respectively. In visible region, the activity decreased slightly with a decrease in pseudo-first-order kinetics by ~1.9 times because of the polychromatic visible light. Further, the addition of persulfate in small amount of 200 mg/L increased the disinfection kinetics by ~1.5 times due to generation of persulfate radicals. The presence of MNZ with the shigella flexneri showed that there was a decrease in removal efficiency for both the antibiotic and bacteria with both photocatalysts. This was caused by the competitive antagonistic effect between the antibiotic and bacteria for the reactive oxygen species generated on the photocatalyst surface. Overall, Ag-ZnO showed good photocatalytic disinfection towards wastewater microbes under UV-A.