The Sanitation Districts of Los Angeles County (Districts) operate seven tertiary water reclamation plants (WRPs) with a combined treatment capacity of approximately 200 million gallons per day (MGD). Chloramination is used at these WRPs for effluent disinfection. It was recently discovered
that chloramination results in formation of N-nitrosodimethylamine (NDMA), a compound with high carcinogenic potency. NDMA can be formed when chloramines react with dimethylamine (DMA) in water. DMA is present in raw sewage and is a component of Mannich polymer that is commonly used as a settling
aid in the activated sludge process. To minimize NDMA formation from chloramination, the Districts developed a disinfection alternative, called “sequential chlorination”. In this chlorination scheme, free chlorine is added to the fully nitrified secondary effluent, and chloramines
are then added to the filtered secondary effluent. Both laboratory and full-scale tests were conducted to evaluate the effect of sequential chlorination on disinfection efficacy and formation of disinfection byproducts, especially NDMA and trihalomethanes (THMs). Results from plant-scale
testing at four WRPs indicated that sequential chlorination provided effective disinfection and significantly reduced NDMA formation. The effluent total THM concentrations were moderately higher than the levels when chloramination was practiced, but they were well below the drinking water
standard for this parameter. In addition, sequential chlorination did not cause aquatic toxicity and had insignificant cyanide and cyanogen chloride formation.
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