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

Abstract

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:1. Maintain a minimum FCRCT of 9 mg Cl 2 -min/L; 2. Maintain a minimum FCR concentration of 1.0 mg Cl 2 /L; and 3. Maintain 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. WEFTEC 2013

Free Chlorine Disinfection of Full‐Scale MBR Effluent to Achieve 5‐Log Virus Inactivation

“…This indicates that the MS2 inactivation was very rapid in the CTF MBR effluent and did not require a high NaOCl dose to meet the 5-log inactivation goal. This observation is consistent with the previous reports on bench-and pilot-scale free chlorine disinfection studies with MBR effluents (Hirani et al, 2014;Mansell et al, 2008;Mansell, et al, 2009), as well as those granular media-filtered tertiary effluents (Adelman et al, 2016;Huitric et al, 2013).…”

“…This also suggests that proper membrane cleaning to maintain the integrity of the MBR membranes is very important because impaired membranes would deteriorate the water quality and cause turbidity spikes, which could lead to Ct requirement to increase as demonstrated by the earlier works (Hirani et al, 2014;Mansell et al, 2008;Mansell et al, 2009). The minimum Ct value determined in this study was also comparable to the reported values for the free chlorine disinfection of granular media-filtered tertiary effluent in pilotand full-scale studies conducted to aim at the conditional approval of free chlorine disinfection at full-scale water reclamation facilities in California (Adelman et al, 2016;Huitric et al, 2013). The CTF became the first MBR-based water reclamation facility that received a conditional approval of free chlorine disinfection from the State Water Resources Control Board based on the result of this study.…”

“…It was realized that the CTF used a fully nitrified MBR that produced high quality, membrane-filtered effluent with very low turbidity (around 0.1 NTU) and little to no ammonia-N. A preliminary study confirmed that free chlorine was the primary chlorine species in the CCB. It was anticipated that a larger flow of recycled water could be disinfected without building additional facilities if a free chlorine Ct value similar to those reported in the previous works (Adelman et al, 2016;Huitric et al, 2013) Chlorine Contact Basin and Chlorine Injection Facilities. The existing CCB was constructed during the first MBR construction in 2004.…”

“…A number of research papers have been published in the past 10 years on the free chlorine disinfection of tertiary effluent to revisit the Ct value determination for effective disinfection of nitrified recycled water in California (Adelman et al, 2016;Hirani et al, 2014;Huitric et al, 2013;Mansell et al, 2008;Mansell et al, 2009). The researchers in the Sanitation Districts of Los Angeles County (LACSD) have conducted a series of pioneering work at their Whittier Narrows Water Reclamation Plant (Mansell et al, 2008;Mansell et al, 2009) and San Jose Creek East Water Reclamation Plant (Huitric et al, 2013). Mansell et al (2008) reported that Ct values above 30 and 40 mgÁmin/L were sufficient to achieve 5-log MS2 inactivation and less than 2.2 MPN/100 mL total coliform goal, respectively, in a series of bench-and pilot-scale experiments of free chlorine disinfection of membrane bioreactor (MBR) effluent spiked with mixed liquor from the aeration tank to simulate the effluent from damaged hollow fiber membranes.…”

“…They also reported that poliovirus was more susceptible to chloramine than MS2 in the bench-scale experiment. Huitric et al (2013) performed a similar pilot-scale experiment with anthracite and sand-filtered tertiary effluent and found that the Ct values as low as 2 to 3 mgÁmin/L was sufficient for the 5.7-log MS2 inactivation, which led to the first conditional approval of free chlorinebased disinfection as an accepted alternative disinfection technology for the production of disinfected tertiary recycled water by California Department of Public Health and State Water Resources Control Board (Division of Drinking Water, 2014). Hirani et al (2014) performed a comprehensive bench-scale disinfection study on microorganism inactivation by free chlorine in MBR effluent samples from several full-scale water reclamation facilities and studied the effect of membrane cleaning on the passage and inactivation of various viruses and coliform bacteria.…”