Full-Scale Implementation of Sequential Chlorination to Produce California Title 22 Recycled Water

Sequential chlorination is a two-step disinfection process developed by the Sanitation Districts of Los Angeles County (Districts). This process uses free chlorine followed by chloramines to produce disinfected tertiary recycled water meeting California Title 22 regulations. 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. The chloramines used in the second step of sequential chlorination further reduce total coliform levels and help to stop trihalomethane formation. Thus, the combination of free chlorine and chloramines in sequential chlorination provides effective disinfection of viruses and bacteria while balancing the formation of disinfection byproducts. In April 2013, the California State Water Resources Control Board, Division of Drinking Water (DDW) approved the use of sequential chlorination at the Districts’ San Jose Creek East Water Reclamation Plant (SJCEWRP). In April 2015, DDW approved an Operations Plan proposed by the Districts for the implementation of sequential chlorination at SJCEWRP. This paper discusses practical issues encountered and resolved during full-scale implementation. Specific issues include managing ammonia residuals in secondary effluent, controlling free chlorine dosing and residual, calculating CT values, and compliance with other standards such as total coliform and trihalomethanes.