Pilot-Scale Demonstration of Virus Inactivation by the Sequential Chlorination Process

The sequential chlorination process was developed by the Sanitation Districts of Los Angeles County (Districts) for disinfection of fully nitrified secondary effluent produced at the Districts' water reclamation plants. The process applies free chlorine and chloramines sequentially. Plantscale testing has shown that the process effectively inactivates bacteria while minimizing the formation of N-nitrosodimethylamine (NDMA) and trihalomethanes (THMs). Bench-scale experiments previously conducted by the Districts indicated that sequential chlorination can achieve regulatory targets for virus disinfection at much lower CT values. In this study, the Districts conducted pilot-scale experiments to verify the effectiveness of sequential chlorination against the surrogate virus MS2 coliphage, which was seeded into nitrified secondary effluent from the San Jose Creek West Water Reclamation Plant, and disinfected in pilot-scale chlorine contact channels. A total of 10 free chlorine experiments were conducted. All test conditions achieved at least 4.5 logs virus inactivation (lowest free chlorine residual CT value tested was 0.8 mg Cl2-min/L); 30 out of 34 test conditions achieved >5 logs inactivation. Five sequential chlorination experiments were conducted; free chlorine residual CT values ranged from 0.41 to 1.10 Cl2-min/L, and total chlorine residual CT values ranged from 80 and 122 mg Cl2-min/L. These experiments all achieved at least 5 logs inactivation of MS2 coliphage.