Sequential Chlorination for Recycled Water Disinfection: Advantages of Combining Free Chlorine and Chloramines

Although wastewater disinfection has historically been accomplished with a single method such as chloramination or UV, recent research has begun to evaluate the use of multiple disinfectants in a cost-effective way to maximize disinfection efficacy and minimize formation of byproducts. The Sanitation Districts of Los Angeles County have developed and patented a two-step process called “sequential chlorination,” which uses free chlorine followed by chloramines to produce disinfected tertiary recycled water meeting California Title 22 regulations, and have obtained conditional approval to use the process at one of their water reclamation plants. The free chlorine in the first step of sequential chlorination reliably achieves the ≥5-log MS2 coliphage inactivation required for disinfected tertiary recycled water at CT values (product of free chlorine residual and modal contact time) much lower than the prescriptive standard. Free chlorine also inactivates total coliform and does not form N-nitrosodimethylamine. However, free chlorine alone, at low doses and short contact times, does not consistently meet the total coliform standards for disinfected tertiary recycled water. The chloramines used in the second step of sequential chlorination reduce total coliform levels to meet the standards, and help to stop trihalomethanes formation. Thus, the combination of free chlorine and chloramines in sequential chlorination provides effective disinfection of MS2 coliphage and total coliform to meet regulatory requirements for disinfected tertiary recycled water, while balancing the formation of disinfection byproducts.