Determining Free Chlorine Residual CT Values to Meet California Title 22 Five-log Virus Inactivation Requirement

The Sanitation Districts of Los Angeles County (Districts) have developed a two-step disinfection process called sequential chlorination. The process applies free chlorine and chloramines in sequence and effectively inactivates viral and bacterial target organisms while minimizing the formation of N-nitrosodimethylamine (NDMA) and trihalomethanes (THMs). To meet California Title 22 requirements as an alternative disinfection technology for “disinfected tertiary recycled water”, the Districts embarked on this study and conducted experiments using a pilot-scale channel to determine the free chlorine residual CT values needed to achieve 5-log virus inactivation. Pulse tracer tests were first conducted to demonstrate that the pilot-scale results were applicable to the full-scale system. For the disinfection experiments, the target virus, MS2 coliphage, was seeded into filtered effluent produced from the San Jose Creek East Water Reclamation Plant (SJCEWRP). A total of 114 pilot-scale disinfection tests were conducted over a range of ammonia concentrations, turbidity levels, pH values, and water temperatures. Data from the 99 tests achieving breakpoint chlorination were analyzed and are presented in this paper. For the conditions tested (breakpoint chlorination, modal contact time ≥4 min), the lowest tested free chlorine residual CT (FCRCT) range of 2 to 3 mg-min/L achieved an average MS2 coliphage inactivation of 5.7-log and a lower 95% confidence level of 5.1-log. Both of these inactivation values exceed the California Title 22 requirement of 5-log virus inactivation for an alternative disinfection technology. Based on the results of this study, the Districts proposed to operate free chlorine disinfection as the first step of sequential chlorination, on filtered effluent at the SJCEWRP under the following conditions:Maintain a minimum FCRCT of 9 mg Cl2-min/L;Maintain a minimum FCR concentration of 1.0 mg Cl2/L; andMaintain a minimum free chlorine modal contact time of 4.0 min.Following a meeting with the California Department of Public Health (CDPH) staff, the Districts expect the CDPH to grant conditional acceptance of sequential chlorination as an alternative for producing disinfected tertiary recycled water at the SJCEWRP before the NPDES and reuse permit for the plant are renewed.