Disinfection By-Products in Drinking Water - ACS Publications

Chapter 21

Downloaded by KTH ROYAL INST OF TECHNOLOGY on August 24, 2015 | http://pubs.acs.org Publication Date: August 5, 2008 | doi: 10.1021/bk-2008-0995.ch021

Formation of Nitrosamines in Effluent-Impacted Drinking Waters Stuart W. Krasner, Michael J. Sclimenti, Chih Fen Tiffany Lee, and Jessica Schramm Water Quality Standards Branch, Metropolitan Water District of Southern California, 700 Moreno Avenue, La Verne, CA 91750-3303

Treated wastewater discharges are a source of precursors for nitrosamines (e.g., N-nitrosodimethylamine [NDMA]). Drinking water treatment plants on effluent-impacted rivers or lakes may form NDMA if they chloraminate their water. The amount of treated wastewater in the drinking water supply can affect the level of NDMA that can form. However, prechlorination with a sufficient free-chlorine contact time can destroy or transform NDMA precursors, resulting in less NDMA upon chloramination.

Introduction In recent years, greater portions of treated wastewater are contributing more toward the drinking water supplies through reclamation, recycling, and reuse (intentional and incidental) processes. Many rivers, lakes, and groundwaters that supply water to drinking water treatment plants (DWTPs) contain discharges from upstream wastewater treatment plants (WWTPs). It is not uncommon to have a significant portion of the source water for these DWTPs originally derived from the upstream wastewater contribution. Effluent organic matter (EfOM) from WWTPs is a source of disinfection by-products (DBPs), if chlorine disinfection is practiced, and DBP precursors (1). Nitrosamines (e.g, N-nitrosodimethylamine [NDMA]) are found in treated wastewater and drinking water (2). NDMA is a by-product of the disinfection of 304

© 2008 American Chemical Society In Disinfection By-Products in Drinking Water; Karanfil, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.


305 some natural waters and wastewaters with combined chlorine (3-4). Although there is no U.S. regulation for any of the nitrosamines, the California Department of Health Services has notification levels (at 10 ng/L each) for the following species: NDMA, N-nitrosodiethylamine (NDEA), and N-nitrosodipropylamine (NDPA). In addition to the presence of NDMA in chloraminated wastewaters, EfOM is also a source of NDMA precursors. On a central tendency basis, achieving some nitrification at WWTPs resulted in reducing the level of NDMA precursors (/). Nitrification reduced the level of hydrophobic and transphilic natural organic matter (NOM) to some extent, but really reduced the level of hydrophilic NOM considerably (7). These results suggest that the NDMA precursors in EfOM may have been primarily in the hydrophilic NOM fraction. When a DWTP uses chloramines to control the formation of regulated DBPs (e.g., trihalomethanes [THMs]), there may be a trade-off issue with the formation of NDMA. Thus, the use of chloramines might be precluded in waters that are high in NDMA precursors—e.g., in source waters heavily impacted by EfOM—because of the formation of too high of a level of NDMA. Because only a percentage of NDMA precursors will be converted to NDMA during fullscale chloramination practices, it is unknown under what conditions too much NDMA (e.g., >10 ng/L) might be formed in effluent-impacted rivers. Therefore, research was conducted with a range of EfOM/river mixtures under drinking water chloramination conditions to access vulnerability to NDMA formation.

Experimental Methods Eight nitrosamines were measured: NDMA, N-nitrosomethylethylamine (NMEA), NDEA, N-nitrosomorpholine (NMOR), N-nitrosopyrrolidine (NPYR), NDPA, N-nitrosopiperidine (NPIP), and N-nitrosodibutylamine (NDBA). Nitrosamine samples were concentrated using solid-phase extraction with Ambersorb (5) and were analyzed using chemical ionization gas chromatography (GC)/mass spectrometry (6). Selected samples were also analyzed for THMs. The THMs were measured using a salted liquid/liquid extraction and GC/electron-capture detection method (7). Two EfOM samples were collected, one that was well nitrified and one that was poorly nitrified. Nitrification transforms ammonia and organic-nitrogen to nitrate. In addition, a river sample upstream of any WWTPs was collected for the "background" water quality. Mixtures of these samples were prepared to simulate a range of effluent-impacted waters (e.g., 25, 50, and 75 percent). Each EfOM/river mixture was chloraminated under DWTP chloramination conditions. Typically, most DWTPs that use chloramines will first use free chlorine for primary disinfection. In the case in which there is a significant

In Disinfection By-Products in Drinking Water; Karanfil, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

306 amount of ammonia in the plant influent, many DWTPs use breakpoint chlorination to achieve afree-chlorineresidual. In this testing, the samples were prechlorinated to break out all of the ammonia and have afree-chlorineresidual (e.g., -2-5 mg/L) for the formation of chloramines during post-ammonia addition. In addition, the impact offree-chlorinecontact time (e.g., 5, 20, and 60 min) was evaluated. Chloramines, with a chlorine/ammonia-nitrogen (C1 /NH -N) ratio of 3:1 or 4:1 (on a weight basis), were set up. In parallel, selected samples were chloraminated with no prechlorination step (where ammonia was added first). The final chloraminated samples were held for 2 or 3 days (at a pH - 8 and at 25°C).


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Results Table I shows the water quality (i.e., total organic carbon [TOC], ultraviolet absorbance [UVA], ammonia-nitrogen [NH -N]) of EfOM and river samples. For some experiments with the poorly nitrified EfOM, there was insufficient river water remaining. For those tests, surface water collected at the Weymouth DWTP influent (La Verne, Calif.) was used. That water was a mixture of California State Project water and Colorado River water. Subsequently, fresh samples of river water and EfOM were collected and used (Table II). 3

Table I. Water Quality of Samples Initially Used Water Source

UVA (cm )

TOC (mg/L)

NH N (mg/L) r

1

River 3.0 Well nitrified EfOM 9.3 Poorly nitrified EfOM 12.2 SOURCE: Reproduced with permissionfromreference Research Foundation.

0.057

0.03

0.160

0.08

0.181

-17

1.

Copyright 2008 Awwa

Table II. Water Quality of Samples Subsequently Used Water Source

TOC (mg/L)

UVA (cm')

NHj-N (mg/L)

Bromide (mg/L)

Weymouth DWTP influent ~3 ~0 River 3.1 0.052 0.12 0.08 Well nitrified EfOM 9.8 0.146 0.04 0.20 Poorly nitrified EfOM 14.4 0.177 14 0.22 SOURCE: Reproduced with permission from reference 1. Copyright 2008 Awwa Research Foundation.


307 Impact of Free-Chlorine Contact Time Figure 1 shows the impact offree-chlorinecontact time (before ammonia was added to form chloramines) on NDMA formation during postchloramination in a mixture of 75-percent river water and 25-percent well nitrified EfOM (water from Table I). The well nitrified EfOM had a background level of NDMA (i.e., 12.9 ng/L), which corresponded to 3.2 ng/L when the EfOM was diluted fourfold. As part of the post-chloramination, the chlorine dose ranged from 2 to 7 mg/L. In these tests, a Cl /N (weight) ratio of 4:1 was used and the samples were held for 2 days. The following observations can be made:


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Three tests with 1-hr prechlorination gave the same NDMA formation (7.08.6 ng/L). Of this, -4-5 ng/L of NDMA was formed over the background amount. One test with 20-min prechlorination gave the same NDMA formation (7.5 ng/L) as the 1-hr prechlorination tests. Three tests with 5-min prechlorination gave highly variable, but high NDMA formation (14-86 ng/L). Three tests with pre-ammoniation gave highly variable and even higher NDMA formation (52-252 ng/L).

According to Schreiber and Mitch (5), the order of chlorine and ammonia addition to form chloramines can highly affect NDMA formation. According to their research, in which they conducted experiments with (1) dimethylamine in distilled water or (2) secondary municipal wastewater effluent, dichloramine forms much more NDMA than monochloramine. When chlorine is added first and ammonia is added second, one should get monochloramine and some NDMA formation (which is what happened in the 20-min and 1-hr prechlorination tests). When ammonia is added first and chlorine is added second, one can get a localized region in which dichloramine forms, and more NDMA is produced. Not only did the latter happen in the pre-ammoniation tests, it appeared to have happened in some of the 5-min prechlorination tests (perhaps due to inadequate mixing). This explains some of the variability in the latter tests. According to Charrois and Hrudey (9) or Chen and Valentine (70), prechlorination can significantly reduce NDMA formation during subsequent chloramination. This may be due to the destruction or transformation of NDMA precursors by the free chlorine. This explains (in part) why NDMA formation was relatively low, compared to its pre-ammoniation, in the tests with a sufficient amount of free-chlorine contact time. However, with longer freechlorine contact times, more THM formation can occur. For example, in 1-hr


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NOTE: Well nitrified EfOM had background amount of NDMA (3.2 ng/L in 4x diluted sample)

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Free-Chlorine Contact Time (min) Figure 1. Impact offree-chlorine contact time (before ammonia was added to form chloramines) on NDMA formation during post-chloramination in mixture of 75-percent river water and 25-percent well nitrified EfOM. (Reproduced with permissionfromreference 1. Copyright 2008 Awwa Research Foundation.)

prechlorination/post-chloramination tests, THM formations for the 75/25 blend and a 50/50 mixture of river water and well nitrified EfOM were 78 and 99 μΐξ/L, respectively. In order to reliably comply (e.g., 20-percent safety factor) with the U.S. THM maximumum contaminant level (MCL) of 80 μ&Τ_,, a DWTP would want to form

Disinfection By-Products in Drinking Water - ACS Publications

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