Synergistic effects of chlorine and ozone on ultraviolet disinfection of different microorganisms in secondary wastewater effluent.

This study investigated the synergistic effects of chlorine or ozone application to secondary effluent prior to ultraviolet (UV) disinfection on the inactivation of E. coli, total coliforms, Clostridium perfringens, Giardia spp. cysts, and Cryptosporidium spp. oocysts. The physicochemical parameters remained statistically similar in the chlorine assays. In contrast, ozonation reduced the COD, solids, turbidity, and absorbance at 254 nm. The order of microorganism resistance was as follows: E. coli = total coliforms < C. perfringens across all treatments (both individual and sequential). The ozone dosage was more strongly correlated with microbial inactivation than was the applied CT (concentration × contact time), indicating greater efficacy with greater ozone consumption. Chick's kinetic model provided the best fit for UV radiation, whereas the Hom model was more suitable for chlorination. Standalone ozone treatment notably reduced Giardia cyst concentrations, and standard fluorescence reduction after sequential treatments suggested oxidative damage to cyst walls. The high viability of Cryptosporidium oocysts after disinfection raises significant public health concerns. Synergistic inactivation varied by treatment: ozone-UV (0.02 to 1.28 log) and chlorine-UV (0.07 to 0.82 log), depending on the target organism. These findings indicate that lower CT values for primary disinfectants can effectively reduce pathogen levels, offering a more sustainable approach to wastewater treatment.