Article pubs.acs.org/est
Impact of Nitrification on the Formation of N‑Nitrosamines and Halogenated Disinfection Byproducts within Distribution System Storage Facilities Teng Zeng†,‡,§ and William A. Mitch*,†,‡ †
Department of Civil and Environmental Engineering, Stanford University, 473 Via Ortega, Stanford, California 94305, United States National Science Foundation Engineering Research Center for Re-Inventing the Nation’s Urban Water Infrastructure (ReNUWIt), 473 Via Ortega, Stanford, California 94305, United States § Department of Civil and Environmental Engineering, Syracuse University, 151 Link Hall, Syracuse, New York 13244, United States ‡
S Supporting Information *
ABSTRACT: Distribution system storage facilities are a critical, yet often overlooked, component of the urban water infrastructure. This study showed elevated concentrations of N-nitrosodimethylamine (NDMA), total N-nitrosamines (TONO), regulated trihalomethanes (THMs) and haloacetic acids (HAAs), 1,1-dichloropropanone (1,1-DCP), trichloroacetaldehyde (TCAL), haloacetonitriles (HANs), and haloacetamides (HAMs) in waters with ongoing nitrification as compared to non-nitrifying waters in storage facilities within five different chloraminated drinking water distribution systems. The concentrations of NDMA, TONO, HANs, and HAMs in the nitrifying waters further increased upon application of simulated distribution system chloramination. The addition of a nitrifying biofilm sample collected from a nitrifying facility to its non-nitrifying influent water led to increases in Nnitrosamine and halogenated DBP formation, suggesting the release of precursors from nitrifying biofilms. Periodic treatment of two nitrifying facilities with breakpoint chlorination (BPC) temporarily suppressed nitrification and reduced precursor levels for N-nitrosamines, HANs, and HAMs, as reflected by lower concentrations of these DBPs measured after re-establishment of a chloramine residual within the facilities than prior to the BPC treatment. However, BPC promoted the formation of halogenated DBPs while a free chlorine residual was maintained. Strategies that minimize application of free chlorine while preventing nitrification are needed to control DBP precursor release in storage facilities.
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formation of N-nitrosamines.10 The U.S. Environmental Protection Agency’s (USEPA’s) Integrated Risk Information System database indicates that even low nanogram-per-liter concentrations of N-nitrosamines in drinking water are associated with a 10−6 lifetime excess cancer risk,11 and the USEPA is considering future regulation of N-nitrosamines as a contaminant group.12 In California, a 10 ng/L notification level has been established for three N-nitrosamines,13 including Nnitrosodimethylamine (NDMA), by far the most commonly detected N-nitrosamine species in distribution systems.14,15 According to the USEPA’s second Unregulated Contaminant Monitoring Rule (UCMR2) database, the mean NDMA concentrations in chloraminated distribution system waters with maximum residence times were higher than levels in drinking water treatment plant effluents, regardless of the type of source waters (i.e., groundwater, surface water, and mixed
INTRODUCTION Nitrification, a microbial process by which ammonia is converted sequentially to nitrite and nitrate by nitrifying bacteria, impacts a significant portion of U.S. water utilities practicing chloramination for secondary disinfection.1,2 Chloramination may favor the proliferation of nitrifying bacteria within distribution systems because of ammonia overfed during chloramine formation or released from chloramine demand and decay reactions.3 The occurrence of nitrification episodes leads to accelerated loss of disinfectant residuals, excessive accumulation of nitrite or nitrate, increased release of pipe corrosion products, and decreasing pH and dissolved oxygen concentrations.4−7 Thus, nitrification control is of practical importance as utilities move forward with optimizing distribution system water quality for regulatory compliance. Periodic switching from chloramines to free chlorine, referred to as breakpoint chlorination, is a common measure to control nitrification within chloraminated distribution systems but may not provide long-term prevention of nitrification recurrence.8,9 While effectively reducing the formation of regulated disinfectant byproducts (DBPs), chloramination promotes the © 2016 American Chemical Society
Received: Revised: Accepted: Published: 2964
November 23, 2015 January 14, 2016 February 9, 2016 February 9, 2016 DOI: 10.1021/acs.est.5b05668 Environ. Sci. Technol. 2016, 50, 2964−2973
Article
Environmental Science & Technology ground and surface water).16,17 However, the nitrification status of distribution systems was not included as part of the UCMR2 survey, thereby preventing the possibility to explore associations between nitrification and NDMA formation. Although N-nitrosamine precursors are known to originate from wastewater-impacted source waters or certain polymers used for water treatment (e.g., amine-based cationic coagulants),18,19 only limited information is available regarding the origin and significance of N-nitrosamine precursors within distribution systems. Two recent studies have shown that certain distribution system synthetic materials can serve as a source of NDMA and other N-nitrosamines.19 Morran et al. demonstrated that rubber pipeline sealing rings leached NDMA and precursors capable of forming additional NDMA upon chloramination, but no clear relationship was found between the amount of NDMA released and the types or ages of rubber materials.20 Teefy et al. also reported that newly installed rubber gaskets in a temporary storage tank leached NDMA, N-nitrosodi-n-butylamine (NDBA), and N-nitrosopiperidine (NPIP).21 Nevertheless, the possible impact of nitrification on Nnitrosamine formation within distribution systems remains largely unknown. Singer et al. showed that NDMA concentrations were close to, or slightly above, 10 ng/L in two North Carolina distribution systems exhibiting historical evidence of nitrification,22 but no comparison was made to the treatment plant effluent or non-nitrifying systems. Krasner et al. suggested that NDMA concentrations in distribution system sections undergoing nitrification were generally similar to those in sections without nitrification,23 but in several cases, the data were confounded by interference from additional groundwater supplies merging within the distribution system. For four of the six distribution system sections without groundwater input, NDMA concentrations were higher in sections where nitrification was indicated by low total chlorine residuals and measurable nitrite. For the other two sections, NDMA concentrations were equal to or higher than in the non-nitrifying sections of the distribution system. Though yielding important information, these two earlier investigations mainly focused on NDMA and did not present conclusive evidence as to whether nitrification within distribution systems is a potential contributor to the formation of NDMA or other N-nitrosamines. Similarly, despite one previous study that showed temporary increases in trihalomethanes and haloacetic acids during breakpoint chlorination of a Florida distribution system,24 the role that nitrification plays in the formation of regulated and unregulated halogenated DBPs has not been well defined. The goal of this work was to examine the impact of nitrification on the levels of N-nitrosamines and halogenated DBPs and their precursors in full-scale storage facilities (e.g., elevated storage tanks and open-cut reservoirs). Storage facilities are particularly prone to nitrification as a result of poor mixing and prolonged stagnation.25−27 Herein, we compared the concentrations of specific N-nitrosamines, total N-nitrosamines, and an array of regulated and unregulated halogenated DBPs in the nitrifying and non-nitrifying waters collected from storage facilities within five different chloraminated drinking water distribution systems. We further treated nitrifying and non-nitrifying waters with preformed monochloramine under simulated distribution system conditions to examine the amount of chloramine-reactive DBP precursors remaining in the nitrifying waters with low chloramine residuals. For one nitrifying storage facility, we spiked the non-nitrifying influent water with a nitrifying biofilm sample collected from the same facility to directly evaluate the
contribution of nitrifying biofilms to DBP formation. For two other nitrifying storage facilities, we analyzed waters sampled across a time sequence encompassing nitrification, breakpoint chlorination treatment, and the re-establishment of chloramines to evaluate whether breakpoint chlorination effectively reduced DBP formation associated with nitrification. Results from this work may contribute to an improved understanding of the interplay between nitrification and DBP formation within distribution systems, which is critical for utilities to balance corresponding control strategies.
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MATERIALS AND METHODS Chemicals and reagents used are described in the Supporting Information (Text S1). Water samples were collected between September 2014 and October 2015 from 15 storage facilities (design capacities ranging from 0.2 to 22.3 million gallons) within five chloraminated drinking water distribution systems fed by different treatment plants treating different source waters. These storage facilities were selected based on consultation with water utility personnel regarding their nitrification status, as determined by recent measurements of nitrite and chloramine residuals. Three of the five distribution systems were operated by the same water utility. Storage facilities within all five distribution systems received exclusively chloraminated water from an individual treatment plant. When practiced, breakpoint chlorination (BPC) at Systems A, B, and C involved pumping sodium hypochlorite from a truck-mounted tank directly into a single storage facility (i.e., instead of treating the entire distribution system) until a ∼3 mg Cl2/L free chlorine residual was achieved. Chloramine residuals were re-established via inflow of chloraminated water from the distribution system displacing the chlorinated facility water. For the purpose of this study, storage facility waters were classified as “nitrifying” or “non-nitrifying” based on the levels of nitrite and chloramine residuals. Nitrifying waters were operationally defined as waters containing >0.2 mg N/L nitrite and
