Reclaimed Water Disifection Alternatives to Avoid NDMA and THM Formation
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.
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), in the Los Angeles County. Approximately 65 MGD of reclaimed effluent is reused for a variety of applications including groundwater replenishment, landscape and agricultural irrigation, wildlife habitat maintenance, and industrial process water supply. Among these various applications, groundwater replenishment is most significant and represents approximately half of the total reuse amount. Reclaimed water used for groundwater replenishment and applications described above is required to meet the drinking water standards set by the California Department of Health Services (DHS) in the California Code of Regulations Title 22 (California Title 22).
Typical processes employed at the Districts’ tertiary WRPs include primary sedimentation, activated sludge with biological nitrogen removal, secondary clarification, media filtration, disinfection with chloramination, and dechlorination (see Figure 1). Sludge produced from the WRP is usually transported to a regional plant for treatment and disposal. A Mannich type cationic polymer is often added to the return activated sludge or to the mixed liquor entering the secondary clarifiers to enhance sludge settling and for foam control. Mannich polymer has a polyacrylamide/methyl amine formulation with dimethylamine (DMA) and formaldehyde added to activate the “Mannich” process and to increase the chain length. The polymer is delivered as a solution flocculent and is diluted with chlorinated final effluent before use. Depending on needs, the polymer dose may vary from approximately 0.4 to 2 mg/L.