Water reclamation plants (WRPs) are facing the challenges of ensuring microbial safety and require efficient disinfection systems. Sequential ozone‑chlorine disinfection is supposed to be a favorable alternative for reclaimed water disinfection. This study compared the inactivation efficiency of E.coli by single ozone, single chlorine, and sequential ozone‑chlorine disinfection approaches. Notably, a single ozone or chlorine process could only achieve a log removal rate of up to 5 log, whereas the sequential ozone‑chlorine disinfection could completely inactivate microorganisms (7.3 log). For sequential ozone‑chlorine disinfection, the efficiency of chlorination was improved by 2.4%-18.5%. The synergistic effect mainly attributed to the elimination of chlorine consuming substances by ozone. Through the chlorine decay model (CRS) fitting and calculating the integral CT value, the enhancement ability of ozone to chlorine disinfection was quantified. By introducing an enhancement coefficient (β), a succinct and accurate model was established to estimate the inactivation rate of sequential ozone‑chlorine disinfection (mean absolute percentage error: 0.035). The results and methodology of this study are informative to optimize the disinfection units of WRPs.

Disinfection of secondary effluents is vital to provide a sustainable aquatic environment, minimize microbial risks and guarantee public and environmental safety. This study investigated the effectiveness of six treatment trains including single and combined disinfection processes (i.e., ozone alone, ultraviolet (UV) irradiation alone, chlorine alone, sequential ozone-UV, sequential ozone-chlorine and sequential ozone-UV-chlorine) on bacterial inactivation, as well as bulk water quality parameters such as color, turbidity, absorbance at 254 nm (UV254), dissolved organic carbon (DOC) and fluorescence based on samples collected from an actual water reclamation plant (WRP). For the single disinfection processes, when the ozone, UV and chlorine doses reached 5 mg/L, 15 mJ/cm2 and 4 mg/L, respectively, the log removal of Escherichia coli (E. coli) reached 5 log. A trailing phenomenon was observed with further increases in the disinfectant dosage. Under the combined treatment scenarios, ozone pretreatment resulted in substantial removal of color, turbidity, UV254, fluorescence excitation-emission matrix (FEEM) and chlorine consuming organics, thus enhancing the efficiency of subsequent UV irradiation or chlorine treatments. In the sequential ozone-UV-chlorine experiments, E. coli inactivation reached 7 log with ozone, UV and available chlorine of 3 mg/L, 5 or 10 mJ/cm2 and 2.5 mg/L, respectively. On the basis of the results from the actual WRP, the estimated operating cost per unit for the disinfection systems is 0.065 CNY/t, which is economical for long-term operation.