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Reclaimed Water Disinfection Alternatives to Avoid NDMA and THM Formation

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The Sanitation Districts of Los Angeles County (Districts) operate seven tertiary water reclamation plants (WRPs) with a combined treatment capacity over 200 million gallons per day (MGD). Chloramination is used at these WRPs for effluent disinfection. It was recently discovered that chloramination results in the formation of N-nitrosodimethylamine (NDMA), a compound with high carcinogenic potency. NDMA is formed when chloramines react with dimethylamine (DMA) in the water. Sources of DMA included polymer used as a settling aid in the activated sludge process.

To minimize NDMA formation from the chloramination disinfection process, the Districts evaluated two alternatives adapted from the existing practice. These alternatives were breakpoint chlorination while using the existing DMA containing polymer and chloramination while using emulsion polymers that do not contain DMA. Both laboratory and full-scale tests were conducted to evaluate disinfection efficacy and formation of NDMA and trihalomethanes (THM) with these alternatives.

Breakpoint chlorination effectively inactivated total coliform and generated insignificant amounts of NDMA. However, it generated higher levels of THM than chloramination. THM generation was directly related to chlorine residual and contact time. The results suggested that breakpoint chlorination could effectively inactivate total coliform while meeting the drinking water standard.

Chloramination with the use of emulsion polymers to enhance mixed liquor settling produced much lower NDMA levels than when Mannich polymer was used. However, emulsion polymers were less effective than the Mannich polymer as a settling aid. Depending on water reuse applications, these two approaches are viable options to manage effluent NDMA and THM concentrations.
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Document Type: Research Article

Publication date: 2006-01-01

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