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Reverse Osmosis Shifts Chloramine Speciation Causing ReFormation of NDMA during Potable Reuse of Wastewater Daniel L. McCurry, Ken Ishida, Gregg Oelker, and William A. Mitch Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b01641 • Publication Date (Web): 03 Jul 2017 Downloaded from http://pubs.acs.org on July 6, 2017
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Environmental Science & Technology
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Reverse Osmosis Shifts Chloramine Speciation Causing Re-Formation of NDMA during Potable Reuse of Wastewater
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Daniel L. McCurry1,2, Kenneth P. Ishida3, Gregg L. Oelker4, William A. Mitch1,5* 1
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Present Address: Department of Civil and Environmental Engineering, University of Southern California, Los Angeles, CA 90089 3
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Department of Civil and Environmental Engineering, Stanford University, Stanford, CA 94305
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Orange County Water District, Fountain Valley, CA 92708
Suez, Edward C. Little Reclamation Facility, El Segundo, CA 90245
National Science Foundation Engineering Research Center for Re-Inventing the Nation’s Urban Water Infrastructure (ReNUWIt), 473 Via Ortega, Stanford, California 94305, United States
*Corresponding author phone: (650) 725-9298; fax: (650) 723-7058; e-mail:
[email protected] ACS Paragon Plus Environment
Environmental Science & Technology
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ABSTRACT UV-based advanced oxidation processes (AOPs) effectively degrade N-
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nitrosodimethylamine (NDMA) passing through reverse osmosis (RO) units within advanced
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treatment trains for the potable reuse of municipal wastewater. However, certain utilities have
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observed the re-formation of NDMA after the AOP from reactions between residual chloramines
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and NDMA precursors in the AOP product water. Using kinetic modeling and bench-scale RO
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experiments, we demonstrate that the low pH in the RO permeate (~5.5) coupled with the
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effective rejection of NH4+ promotes conversion of the residual monochloramine (NH2Cl) in the
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permeate to dichloramine (NHCl2) via the reaction: 2 NH2Cl + H+ ↔ NHCl2 + NH4+.
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Dichloramine is the chloramine species known to react with NDMA precursors to form NDMA.
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After UV/AOP, utilities generally use lime or other techniques to increase the pH of the finished
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water to prevent distribution system corrosion. Modeling indicated that, while the increase in pH
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halts dichloramine formation, it converts amine-based NDMA precursors to their more reactive,
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neutral forms. With modeling, and experiments at both bench-scale and field-scale, we
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demonstrate that reducing the time interval between RO treatment and final pH adjustment can
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significantly reduce NDMA re-formation by minimizing the amount of dichloramine formed
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prior to reaching the final target pH.
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INTRODUCTION
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purified municipal wastewater as a reliable source of potable water. Advanced treatment trains
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for potable reuse frequently employ Full Advanced Treatment (FAT), a combination of
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microfiltration (MF), reverse osmosis (RO) and advanced oxidation processes (AOPs; typically
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the UV/hydrogen peroxide AOP).1 A recent water reuse risk assessment identified disinfection
Droughts in the United States Southwest and California have increased interest in
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Environmental Science & Technology
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byproducts (DBPs) as important chemicals of potential human health concern, particularly N-
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nitrosodimethylamine (NDMA).2 NDMA is an extremely potent carcinogen; 0.7 ng/L in drinking
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water is associated with a 10-6 excess lifetime cancer risk, orders of magnitude lower than for
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many regulated DBPs.3 NDMA is the subject of a WHO standard (100 ng/L)4, California
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implemented a statewide drinking water notification level of 10 ng/L,5 and the compound is
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under consideration for federal regulation.6
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NDMA has long been associated with wastewater reuse. NDMA occurs in domestic
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sewage,7 but is also associated with industrial discharges.8 NDMA also forms during ozonation
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or chloramination of amine precursors9,10 and NDMA precursors are particularly associated with
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wastewater.9-11 Attempting to identify the source of NDMA precursors in wastewater is an active
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field of research. Certain pharmaceuticals that can occur in sewage form NDMA at high yield
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(e.g., methadone14 and ranitidine15,16). However, recent research has demonstrated that laundry
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effluent may be a dominant source of ozone- and chloramine-reactive NDMA precursors in
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domestic sewage; although specific precursors were not identified, they were not associated with
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detergents.7 NDMA forms by reaction of these precursors with the chloramines or ozone17,18
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applied upstream of MF in FAT treatment trains to control biofouling. This NDMA is only
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partially removed by RO.19 Indeed, the detection of NDMA in the RO permeate of Water
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Factory 21, one of the nation’s first potable reuse facilities, led to the suspension of operations in
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2000 pending the installation of a UV system to photolyze NDMA post-RO.20,21
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Hydrogen peroxide addition converts UV systems to AOPs to degrade other
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contaminants that may pass through RO membranes (e.g., 1,4-dioxane), but direct photolysis of
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NDMA remains a key criterion for selecting UV fluence.1 UV-based AOPs typically reduce
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NDMA concentrations to non-detectable levels (
