Occurrence and control of nitrogenous disinfection by-products in drinking water – A review

w a t e r r e s e a r c h 4 5 ( 2 0 1 1 ) 4 3 4 1 e4 3 5 4

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Review

Occurrence and control of nitrogenous disinfection by-products in drinking water e A review Tom Bond*, Jin Huang, Michael R. Templeton, Nigel Graham Department of Civil and Environmental Engineering, Imperial College London, London SW7 2AZ, United Kingdom

article info

abstract

Article history:

The presence of nitrogenous disinfection by-products (N-DBPs), including nitrosamines,

Received 10 November 2010

cyanogen halides, haloacetonitriles, haloacetamides and halonitromethanes, in drinking

Received in revised form

water is of concern due to their high genotoxicity and cytotoxicity compared with regu-

29 May 2011

lated DBPs. Occurrence of N-DBPs is likely to increase if water sources become impacted by

Accepted 30 May 2011

wastewater and algae. Moreover, a shift from chlorination to chloramination, an option for

Available online 7 June 2011

water providers wanting to reduce regulated DBPs such as trihalomethanes (THMs) and haloacetic acids (HAAs), can also increase certain N-DBPs. This paper provides a critical

Keywords:

review of the occurrence and control of N-DBPs. Data collated from surveys undertaken in

NDMA

the United States and Scotland were used to calculate that the sum of analysed haloni-

Nitrosamines

tromethanes represented 3e4% of the mass of THMs on a median basis; with Pearson

Haloacetonitriles

product moment correlation coefficients of 0.78 and 0.83 between formation of dihaloa-

Cyanogen halides

cetonitriles and that of THMs and HAAs respectively. The impact of water treatment

Halonitromethanes

processes on N-DBP formation is complex and variable. While coagulation and filtration are

Haloacetamides

of moderate efficacy for the removal of N-DBP precursors, such as amino acids and amines, biofiltration, if used prior to disinfection, is particularly successful at removing cyanogen halide precursors. Oxidation before final disinfection can increase halonitromethane formation and decrease N-nitrosodimethylamine, and chloramination is likely to increase cyanogen halides and NDMA relative to chlorination. ª 2011 Elsevier Ltd. All rights reserved.

Contents 1. 2.

Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Occurrence of N-DBPs in drinking water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Interpreting N-DBP occurrence data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Haloacetonitriles (HANs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Haloacetamides (HAcAms) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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* Corresponding author. Present address: Pollution Research Group, School of Chemical Engineering, University of KwaZulu-Natal, Durban 4041, South Africa. Tel.: þ27 31 260 3131; fax: þ27 31 260 3241. E-mail address: [email protected] (T. Bond). 0043-1354/$ e see front matter ª 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2011.05.034

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3. 4.

5.

1.

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2.4. Cyanogen halides (CNX) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Halonitromethanes (HNMs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6. Nitrosamines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7. Other N-DBPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Precursor sources and formation pathways . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Key factors in the formation of N-DBPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Water quality parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1. The impact of pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.2. The impact of bromide and iodide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. The impact of treatment and disinfection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1. The impact of treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2. The impact of pre-oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.3. The impact of chlorination and chloramination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions: minimising N-DBPs in water treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Background

Research into disinfection by-products (DBPs), an unintentional result of water treatment, stems from the mid-1970s, when the formation of trihalomethanes (THMs) was linked to reactions between chlorine and natural organic matter (NOM) in Dutch drinking water (Rook, 1974). By the turn of that decade the THMs were regulated in the USA at 100 mg L1 due to a cancer risk, and a second group of DBPs, the haloacetic acids (HAAs), had been identified in drinking water at comparable levels to THMs. Soon after, the haloacetonitriles (HANs), an important group of N-DBPs, were detected in chlorinated natural waters (Oliver, 1983; Trehy and Bieber, 1981). An enhanced risk of bladder cancer has been associated with exposure to DBPs (Villanueva et al., 2007), although the contribution of various DBPs to this association remains uncertain, given that no identified chlorination DBPs are believed to be plausible bladder carcinogens (Hrudey, 2009). Over 600 DBPs have been reported in drinking water or simulated laboratory disinfection tests, resulting from the use of chlorine and other disinfectants, notably chloramines, ozone and chlorine dioxide (Krasner et al., 2006). However, data for many nitrogenous DBPs (N-DBPs) remains relatively limited. In the United States (US) selected N-DBPs were analysed in water treatment plant (WTP) surveys undertaken in 1988e1989 (Krasner et al., 1989), 1997e98 (McGuire et al., 2002), 2000e2002 (Krasner et al., 2006; Weinberg et al., 2002) and 2006e2007 (Krasner et al., 2007; Mitch et al., 2009). The 2000e2002 study encompassed over 70 emerging DBPs and among the analysed N-DBPs were HANs, halonitromethanes (HNMs) and haloacetamides (HAcAms). There is less information available on the occurrence of N-DBPs in other countries, although relevant surveys have been carried out in Canada (Williams et al., 1995, 1997), Australia (Simpson and Hayes, 1998) and Scotland (Goslan et al., 2009). Another important N-DBP is N-nitrosodimethylamine (NDMA), formerly used in the production of rocket fuel and other industrial processes. Initially detected in Canadian drinking water in the 1980s (Jobb et al., 1994) NDMA has since been reported as a DBP produced from reactions

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between monochloramine (NH2Cl) and dimethylamine (DMA) (Choi and Valentine, 2002). Several factors have seen a particular recent focus on NDBPs. Firstly, many N-DBPs are of greater perceived health risk than regulated DBP species. Comparison of data from in vitro mammalian cell tests demonstrated the HANs, HNMs and HAcAms are all far more cytotoxic and genotoxic than the non-nitrogenous THMs and HAAs, although the haloacetaldehydes also exhibit very high cytotoxicity and genotoxicity (Plewa and Wagner, 2009). Moreover, the nitrosamines may play a significant role in human carcinogenesis (Loeppky and Michejda, 1994) and the United States Office of Environmental Health Hazard Assessment (OEHHA) have issued a public health goal of 3 ng L1 for NDMA (OEHHA, 2006). At present, however, no N-DBPs are formally regulated by large governmental bodies anywhere in the world (Box 1). Secondly, water utilities are increasingly switching from chlorination to alternative disinfectants, particularly chloramines, in order to limit the formation of regulated THMs and HAAs (Seidel et al., 2005). Disinfectant type is a key factor in NDBP formation since, depending on the compound and reaction conditions, the nitrogen can derive either from the organic precursors, i.e. dissolved organic nitrogen (DON), or in the case of chloramination, from the disinfectant. Finally, the impact of human activity upon drinking water sources is increasingly being felt in the form of wastewater effluent and algal activity (Mitch et al., 2009). Since these are both enriched in DON, their presence is likely to lead to raised concentrations of many N-DBPs. Of the components of DON, amino acids are known to act as precursors of HANs, HAcAms and cyanogen halides (CNX) (Hirose et al., 1988; Ram, 1985; Reckhow et al., 2001; Trehy et al., 1986), while amine precursors of NDMA are believed to be largely anthropogenic in origin (Sacher et al., 2008; Schreiber and Mitch, 2006b), in contrast to the THMs and HAAs, where NOM, typically of terrestrial origin, is the main precursor pool. Hence, understanding and controlling the incidence of NDBPs is a contemporary challenge to the water industry. The objectives of this review are to highlight typical concentrations of identified N-DBPs drinking water, investigate

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Box 1 Acronyms used. AOM CNBr CNCl CNMs CNX CNX DBPs BCAN DBAN DHAN DCAA DBAcAm DCAN DEA DHAN DHNMFP DHNMs DMA DON DPA EEM EfOM FP HAA9 HAAs HAcAms HAN4

Algal organic matter Cyanogen bromide Cyanogen chloride Chloronitromethanes Cyanogen halides CNX formation potential Disinfection by-products Bromochloroacetonitrile Dibromoacetonitrile Dihaloacetonitrile Dichloroacetic acid Dibromoacetamide Dichloroacetonitrile Diethylamine Dihaloacetonitrile Dihalonitromethane formation potential Dihalonitromethanes Dimethylamine Dissolved organic nitrogen Dipropylamine Excitation-emission matrix Effluent organic matter Formation potential Sum of nine surveyed HAAs Haloacetic acids Haloacetamides Sum of four HANs (DCAN, BCAN, DBAN and TCAN)

potential relationships between N-DBPs and other DBPs based on published data and to examine strategies for their mitigation. (Table 1).

2.

Occurrence of N-DBPs in drinking water

2.1.

Interpreting N-DBP occurrence data

A number of caveats should be kept in mind by those reviewing and comparing available data on N-DBPs. Firstly, important differences in disinfection practice between countries impact upon exposure of precursor material to disinfectants. Many US WTPs operate pre-chlorination or pre-chloramination (i.e. before or intermediate to other treatment) as well as post-disinfection, whereas in Europe post-chlorination/chloramination alone is typical. Furthermore, in the US 2000e2002 survey many selected WTPs had high bromide (median level 120 mg L1) and total organic carbon (median 5.8 mg L1) levels (Krasner et al., 2006) and were thus thought likely to generate relatively high DBP loads. A confounding factor when comparing relative effects of disinfectants on N-DBP formation is that it is often WTPs treating high bromide and/or organic carbon waters which switch from chlorination to chloramination in an attempt to lower formation of regulated DBPs. In the 2006e2007 US NDBP survey most WTP intake waters were impacted by upstream algal blooms and/or the discharge of treated wastewater (Mitch et al., 2009), where it can be expected that

HANs HNMs ICR MOR ND NDBA NDMA N-DBPs NDPA NEMA NMOR NOM NPOC NPYR NR OEHHA PYR TCAN TCNM THM4 THMs TMA TOC TOX UDMH US WTP WWTP

Haloacetonitriles Halonitromethanes Information collection rule Morpholine Not detected N-nitrosodibutylamine N-nitrosodimethylamine Nitrogenous disinfection by-products N-nitrosdipropylamine N-nitrosoethylmethylamine N-nitrosomorpholine Natural organic matter Non-purgable organic carbon N-nitrosopyrrolidine Not reported Office of Environmental Health Hazard Assessment Pyrrolidine Trichloroacetonitrile Trichloronitromethane (chloropicrin) Sum of four regulated THMs Trihalomethanes Trimethylamine Total organic carbon Total organic halogen Unsymmetrical 1,1-dimethylhydrazine United States Water treatment plant Wastewater treatment plant

N-DBP formation is above that found in more pristine water sources. Moreover, some N-DBPs have been identified in atypical waters: in the study where 2,3,5-tribromopyrrole was first detected (along with 3-bromopropanenitrile) bromide concentrations were 2 mg L1 (Richardson et al., 2003) and/or using sample concentration techniques followed by qualitative gas chromatography (GC) methods without commercially-available standards (Richardson et al., 1999, 2003). Meanwhile, disparate laboratory disinfection protocols are used for measuring DBPs formed from model compounds or isolates of NOM. To illustrate, in two studies testing the formation potential of NOM isolates the pH, chloramine dose and contact time were respectively 7.0, 45 mg of Cl2 per mg DOC (pre-formed monochloramine added) and 10 days (Lee et al., 2007) and 8.0, 3.0 mg Cl2 per mg DOC (ammonia added before chlorine at 1:3 weight ratio) and 3 days (Dotson et al., 2009).

2.2.

Haloacetonitriles (HANs)

Of the three major N-DBP groups captured by existing analytical methodologies e HANs, HAcAms and HNMs - in the 2000e2002 US survey, HANs occurred at the highest concentrations, with median and maximum levels of 3 and 14 mg L-1 respectively, and dichloroacetonitrile (DCAN) was the most prevalent species (Table 2) (Krasner et al., 2006; Weinberg et al., 2002). In the 2006e2007 US survey, median values for the sum of DCAN, bromochloroacetonitrile (BCAN), dibromoacetonitrile (DBAN) and trichloroacetonitrile (TCAN)

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Table 1 e Important nitrogenous disinfection by-products (N-DBPs). Group/formula

Structure

Important species

R Haloacetonitriles (HANs) R3CCN

R

N R

Cyanogen halides (CNX) RCN

R NH2

R

R

N R

Halonitromethanes (HNMs) R3CNO2

N+

R

Dichloroacetamide (DCAcAm) (right), dibromoacetamide (DBAcAm), trichloroacetamide (TCAcAm)

Cyanogen chloride (CNCl) (right), cyanogen bromide (CNBr)

N Cl

N

Cl

O

Cl

NH2

H

Cl

O

N

O

N Cl

Trichloronitromethane (chloropicrin) (right), tribromonitromethane (bromopicrin), bromodichloronitromethane, dibromochloronitromethane

R

R

H

O-

R

Nitrosamines R2NNO

Cl

O

R Haloacetamides (HAcAms) R3CCONH2

Dichloroacetonitrile (DCAN) (right), bromochloroacetonitrile (BCAN), dibromoacetonitrile (DBAN), trichloroacetonitrile (TCAN) and tribromoacetonitrile (TBAN)

Structure

N-nitrosodimethylamine (NDMA) (right), N-nitrosopyrrolidine (NPYR), N-nitrosomorpholine (NMOR), N-nitrosodiethylamine (NDEA)

ON+

Cl

O

Cl

H3 C N

N

O

H3 C

R is typically Cl, Br, I, H or an alkyl group, though it can also be a larger aliphatic or aromatic group.

(collectively HAN4) were slightly higher at 4.0 mg L1 (Krasner et al., 2007), presumably a reflection of the mainly algal and wastewater-impacted waters chosen. In Australia, HAN4 levels up to 36 mg L1 have been observed, something most likely related to high organic content and bromide, which also resulted in THM levels up to 191 mg L1 (Simpson and Hayes, 1998). In contrast, HANs were lower in Scotland than the US, with median and maximum HAN4 concentrations of 1 mg L1 and 4 mg L1 respectively (Goslan et al., 2009). Median non-purgeable organic carbon (NPOC) and bromide levels were 3.6 mg L1 and 55 mg L1 respectively, in the Scottish waters, versus equivalent figures of 5.8 mg L1 and 120 mg L1 in the US survey (Goslan et al., 2009; Krasner et al., 2006). It has been proposed that the mass of HANs typically represents around 10% of the THMs (Krasner et al., 1989; Oliver, 1983). To investigate such rules the extensive DBP data from the US 2000e2002 survey was collated with that from the 2006e2007 US survey and Scotland and relevant ratios and correlations between the DBP groups calculated (Tables 3 and 4). At the 12 WTPs in the US 2000e2002 survey HAN4 as a proportion of the four regulated THMs (THM4) varied from 2% to 14%, with a median value of 8%, while median ratios for HAN4 and DHAN (HAN4 without TCAN) were respectively 2% and 7% in the Scottish and US 2006e2007 surveys (Table 3, Scottish ratios computed from median

values across the whole of the survey). Thus, the 10% value is an approximate guide to HAN formation. A good positive correlation (r ¼ 0.90) has previously been observed between HAN and THM formation (Krasner et al., 1989). For this review correlations between DHAN and THM4 was calculated as 0.78, with a correlation of 0.83 between HAN4 and the nine surveyed HAAs (HAA9) (Table 4, n ¼ 15). This indicates HAAs may be at least as good a predictor of HAN formation as THMs. As DCAN hydrolyses to 2,2-dichloroacetamide (DCAcAm) and consequently dichloroacetic acid (DCAA) in the presence of free chlorine or at alkaline pH (Reckhow et al., 2001) this is perhaps unsurprising. In the US 2000e2002 survey and Scotland median values of HAN4/DHAN in finished water accounted for 7% of HAA9 formation (Table 3).

2.3.

Haloacetamides (HAcAms)

HAcAms were reported for the first time during the 2000e2002 US survey, DCAcAm being the most prominent species, with a median concentration of 1.3 mg L1 (Table 2). The median and maximum concentrations of the sum of HAcAms were 1.4 mg L1 and 7.4 mg L1 respectively, though note that not all the possible brominated and chlorinated HAcAms were quantified. HAcAms were frequently identified in finished water from three sites where chlorine dioxide was applied prior to chlorine/chloramine. Krasner and co-workers noted

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Table 2 e Summary of N-DBP occurrence in effluents of selected US WTPs (Krasner et al., 2006; Weinberg et al., 2002). Occurrence (mg L1)

Name

WTP of max occurrence

Median Maximum Haloacetonitriles Dichloroacetonitrile Bromochloroacetonitrile

Conditions of max occurrence TOC (mg L1)

Bromidea (mg L1)

pH

Disinfection

12.0 10 (opposite) 3.0e4.2 7.5e7.7 50e70 Cl2 - chloramines 3.0 2 (opposite), 1.6e3.0 7.9e8.7 120e400 Cl2 - chloramines 11 (opposite) and 12 (below) 2.9e4.2 7.4e7.5 160e210 ClO2eCl2 - chloramines Other HANs recorded: dibromoacetonitrile, trichloroacetonitrile, dibromochloroacetonitrile, chloroacetonitrile, bromoacetonitrile Sum of haloacetonitriles 3 14 10 (as above) Haloacetamides 2,2-dibromoacetamide 0.6 2.8 12 (opposite) 3.2e4.5 7.6e8.3 150e330 ClO2 - chloramines 2,2-dichloroacetamide 1.3 5.6 6 (opposite), 12 (as above) 3.5e4.5 5.8e7.0 39e80 ClO2eCl2 - chloramines Other HAcAms recorded: 2-bromoacetamide, 2,2,2-trichloroacetamide, 2-chloroacetamide Sum of haloacetamides 1.4 7.4 6 (as above) Halonitromethanes Chloropicrin 0.2 2.0 10 (as above) Bromodichloronitromethane 0.3 3.0 7 3.0e4.2 7.5e7.7 50e70 Cl2 - chloramines Dibromochloronitromethane ND 3.0 12 (as above) Bromopicrin ND 5.0 12 (as above) Other HNMs recorded: bromochloronitromethane, chloronitromethane, bromonitromethane, dichloronitromethane, dibromonitromethane Sum of halonitromethanes 1 10 12 (as above) 1 0.6

a ¼ Raw water bromide concentration.

that DCAcAm occurred at a similar level to DCAN (respective median values 1.3 mg L1 and 1.0 mg L1) and that HAcAm formation was w10% of HAAs, with dichloro representatives of the two groups found at higher levels than the trichloro species (Krasner et al., 2006).

2.4.

Cyanogen halides (CNX)

In the 1988e1989 US survey an association was noted between chloramination and CNCl formation, median values of CNCl in treatment works with free chlorine and chloramination were 0.4 mg L1 and 2.2 mg L1 respectively (Krasner et al., 1989). This finding has been re-confirmed by subsequent research (see Section 4.2.3). During the 2006e2007 NDBP survey CNX (i.e. CNCl plus CNBr) formation was generally

only observed at the plants with chloramination, in which the median and maximum formation was 2.6 mg L1 and 7.8 mg L1 respectively (Mitch et al., 2009). Nonetheless, CNX precursors were widely present, as shown by plant influent samples disinfected under laboratory conditions designed to maximise CNX (3 h pre-chlorination then chloramination for 21 h), which generated respective median and maximum levels of 12 and 34 mg L1 (Mitch et al., 2009). A low formation of CNX was found in plants where ozone was applied prior to biofiltration and chlorination/chloramination, suggesting biological treatment effectively removed formaldehyde and other CNX precursors resulting from ozonation (Krasner et al., 2007). In Australia CNCl has been recorded up to a level of 10 mg L1 from a WTP practicing monochloramination (Simpson and Hayes, 1998).

Table 3 e Ratios (mg/mg) between N-DBPs and other DBP groups in finished water samples from US and Scotland. Survey

US 2000e2002 Weinberg et al., 2002

US 2006e2007 a

Mitch et al., 2009

Ratios

Min

Median

Max

HAN4/THM4 DHAN/THM4 HAN4/HAA9 HAN4/DXAA TCNM/THM4 Sum of HNMs/THM4 Sum of HNMs/HAA9

0.02 0.02 0.02 0.02 0 0 0

0.08 0.08 0.07 0.13 0.00 0.03 0.03

0.14 0.14 0.12 0.2 0.01 0.23 0.19

Scotland b

Goslan et al., 2009 c

25th %ile

Median

90th %ile

0.07

0.07

0.16

Median 0.02 0.07

0 0

0.01 0.01

0.03 0.04

0.00

a ¼ Ratios calculated from mean values of 4e5 seasonal samples taken at each of 12 WTPs. Not reported taken as half the minimum reporting level; not detected taken as zero. Between four and nine HNMs quantified, depending on the sample (see Table 2). b ¼ Five HNMs quantified. c ¼ Ratios computed from median data across whole of survey. THM4 ¼ sum of four regulated THMs; HAA9 ¼ sum of nine surveyed HAAs. DHANs ¼ HAN4 e TCAN.

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Table 4 e Correlationsa between DBP groups in USb and Scottishb DBP surveys.

HAA9 (n ¼ 16) DXAA (n ¼ 12) TXAA (n ¼ 12) HAN4 (n ¼ 16) DHAN (n ¼ 15) Sum of HNMs (n ¼ 15) TCNM (n ¼ 19)

THM4 (n ¼ 19)

HAA9

DXAA

TXAA

HAN4

DHANs

HNMs

0.81 0.76 0.80 0.30 0.78 0.02 0.09

0.84 0.89 0.71 0.83 0.14 0.54

0.42 0.59 0.61 0.56 0.44

0.83 0.79 0.21 0.62

0.99 0.06 0.42

0.18 0.67

0.30

a Correlations ¼ Pearson product moment correlation coefficients (r). b Bulk of data (n ¼ 12 for all analytes) from US 2000e2002 survey (mean values of 4e5 seasonal samples taken at each of 12 WTPs. Between four and nine HNMs quantified). Remainder (n ¼ 3) from US 2006e2007 survey (THM4, DHAN, sum of (five) HNMs and TCNM) and (n ¼ 4) from Scottish survey (THM4, HAA9 and TCNM). DHANs ¼ HAN4 e TCAN. TCNM ¼ trichloronitromethane.

2.5.

Halonitromethanes (HNMs)

For the sum of HNMs, median and maximum levels of 1 and 10 mg L1 were recorded during the 2000e2002 US survey. Bromopicrin and dibromochloronitromethane maxima were 5.0 and 3.0 mg L1, respectively (Table 2) at a site characterised by high bromide (150e330 mg L1), pre-oxidation with chlorine dioxide and post-chloramination. The sum of analysed HNMs represented respectively 3% and 1% of THM4 on a median basis in the US in 2000e2002 and 2006e2007(Table 3), although a maximum 23% of THM4 formation was recorded at one location where THM4 was relatively low (mean ¼ 8.5 mg L1). As intimated by this, HNM formation does not appear related to the THMs or HAAs, with no meaningful correlations calculated between the sum of HNMs and the two regulated groups (Table 4, n ¼ 15). The formation of chloropicrin (trichloronitromethane (TCNM)) in the 2006e2007 N-DBP study was higher, with median and maximum values of 0.5 and 7.6 mg/L, respectively (Krasner et al., 2007), again highlighting the importance of wastewater and algae as precursor sources. Ozonation before chlorination can dramatically enhance HNM formation (Hoigne and Bader, 1988) (see Section 4.2.2).

2.6.

Nitrosamines

NDMA is typically observed in the low ng L1 range in drinking water, although concentrations equal to or above 1000 ng L1 have been recorded in chloraminated raw water (Sacher et al., 2008) and chlorinated or chloraminated wastewater effluent (Krasner et al., 2009a; Mitch et al., 2003). Analysis of a Canadian drinking water supply in 1986 detected NDMA at concentrations between 5 and 115 ng L1 (Jobb et al., 1994), which stimulated a survey of 145 WTPs in Ontario, Canada. Finished water was under 5 ng L1 in the majority of cases. These and other studies highlighted associations between elevated NDMA occurrence and municipal and industrial wastewater input, chloramination, cationic polymers and ion exchange resins (Najm and Trussell, 2001). Various synthetic chemicals containing a DMA moiety have subsequently been identified as NDMA precursors. These include the pharmaceutical ranitidine (Sacher et al., 2008) and diuron, a herbicide (Chen and Young, 2008). Charrois et al. (2004) developed an ammonia positive chemical ionisation method which enabled detection of two

additional nitrosamines in drinking water: N-nitrosopyrrolidine (NPYR) and N-nitrosomorpholine (NMOR), at 2e4 ng L1 and 1 ng L-1 respectively, in addition to NDMA at 2e180 ng L1. Also evident were increased levels of NDMA in the distribution system (180 ng L1) relative to treated effluent (67 ng L1) of a plant using chloramination and UV disinfection. A recent study compared the formation of eight nitrosamines in finished water samples from six utilities using various treatments with raw water samples from the same sources chloraminated under laboratory conditions (Sacher et al., 2008) (Table 5). In contrast to the treated water samples, where NDMA peaked at 4.9 ng L1 and no other nitrosamines were reported, the laboratory disinfected samples generated NDMA up to 110 ng L1 and NPYR and Nnitrosoethylmethylamine (NEMA) at maxima of 7.6 and 3.4 ng L1, respectively, indicating precursors of these species were present in raw waters. Chloramination of 81 river and lake samples revealed a similar pattern: NDMA was always the dominant species, with median and maximum levels of 45 and 1000 ng L1 respectively, while other nitrosamines were periodically present, albeit always at least an order of magnitude lower than NDMA. NPYR and N-nitrosodiethylamine (NDEA) were the second and third most frequently recorded nitrosamines and reached respective peaks of 35 and 23 ng L1 (Table 5). Another nitrosamine, N-nitrosodibutylamine (NDBA), has been detected in one UK water distribution system at 6.4 ng L1 (Templeton and Chen, 2010).

2.7.

Other N-DBPs

Intermediates in the reactions schemes portrayed (Figs. 1e3) are either known or presumed to occur in drinking water. In particular, hydrazine is carcinogenic and has been detected at 0.5e2.6 ng L1 in chloraminated drinking water, though was not detected in chlorinated samples (Davis and Li, 2008). Other organic hydrazines presumably form during the unsymmetrical hydrazine pathway of nitrosamine formation (displayed for NDMA in Fig. 3) (Choi and Valentine, 2002). Organic chloramines form from chlorination or chloramination of DON, for example, amino acid chlorination (Fig. 1) and can lead to a w10% overestimation of disinfection capacity in chloraminated water systems (Lee and Westerhoff, 2009). There are additional N-DBPs for which very limited occurrence data exists. Benzeneacetonitrile, heptanenitrile and cyanoformaldehyde have been detected as ozone

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Table 5 e Summary of nitrosamine levels in drinking water sources (Sacher et al., 2008) and of DBPs in wastewater (Krasner et al., 2009a and 2009b). Survey

Range of NDMA levels (ng/L)

Reference: Sacher et al., 2008 81 surface water samples (rivers and lakes) chloraminated under laboratory conditions (0.4 mM, 7 days). Eight nitrosamines. Original water samples from 6 water utilities. Samples from same 6 water utilities chloraminated under laboratory conditions (0.4 mM, 7 days). References: Krasner et al., 2009a and 2009b 23 US WWTPs at different seasons. Before chlorination/chloramination

Median (ng/L)

1e1000

45

<1e4.9

NR

8e110

NPYR: <1e7.6

ND e 34

2.7

8.5

ND e 8.2 ND e 3165

3 11

24

Range of HANs (mg/L)

Post-chlorination (Cl2/N > 10 mg/mg) Post-chloramination (Cl2/N < 10 mg/mg)

Median (mg/L)

0.9e30 ND e 12

COO-

R

NHCl

COO-

- CO2 + Cl-

NCl2

2 HOCl

-

HOCl

O

O

+

ND e 0.7 ND e 0.6

0.8

O

H 2O NH

R

R H Aldehyde

NCl

H

H H

NHCl - CO2 + Cl-

R C N

-HCl

NH

NH3

H2O

Nitrile

O NH3 H H Formaldehyde

NH3

O glycine

H R

- CO2 + Cl-

O -

Chloropicrin (mg/L)

case larger (and unidentified) nitriles and halonitroalkanes are perhaps expected to form during water treatment (Mitch et al., 2009). Since, on a median basis, halogenated DBPs quantified in the 2000e2002 US survey accounted for only around 30% of total organic halogen (TOX) formed after disinfection (Krasner et al., 2006), many unidentified DBPs occur in drinking water, some of which may contain nitrogen. The recent application of a total N-nitrosamine (TONO) assay to six recreational waters (swimming pool or hot tub) and their common tap water source produced a similar conclusion (Kulshrestha et al., 2010). This method established that NDMA accounted for only a mean of 13% (range 3e46%) of total nitrosamines present. Extrapolating the results of this study suggests that unidentified nitrosamines also occur in potable water.

COO-

R

NMOR: ND e 12700 (median ¼ 5.5; 75th %ile ¼ 17) NMOR: ND e w8 (median ¼ 5.6) NMOR: ND e 911. (median ¼ 3.3)

75th percentile (mg/L)

16 0.3

disinfection by-products (Richardson et al., 1999). However, cyanoformaldehyde was not detected in any of the treatment plants in the 2000e2002 US occurrence study. Other N-DBPs were recorded in Israeli water with very high bromide (2 mg L1) and chlorine dioxide disinfection: 2,3,5tribromopyrrole and 3-bromopropanenitrile (Richardson et al., 2003). The first of these compounds represented the first incidence of a halogenated pyrrole as a DBP. There are also various other nitrogenous compounds as yet unidentified in drinking water but which have been highlighted for future research on the basis of chemical and toxicological models (Bull et al., 2006). It has been suggested that degradation of larger compounds to one or two carbon amine precursors may be rate-limiting in the formation of many N-DBPs, in which

NH2 Amino acid

NDEA: <1e23

90

NEMA: <1e3.4

Other N-DBPs

R

Other nitrosamines (ng/L)

NPYR: <1e35

Post-chlorination (Cl2/N > 10 mg/mg) Post-chloramination (Cl2/N < 10 mg/mg)

HOCl

75th percentile (ng/L)

-

2 HOCl

O

H

NCl2 O

- CO2 + Cl-

H

HOCl NCl

H C N

Cl C N

-HCl Cyanogen chloride

Fig. 1 e Chlorination of amino acids (top), with specific reference to glycine (below). Based on Deborde and von Gunten (2008) and Joo and Mitch (2007).

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O

OH NH2Cl

H3 C

H

-HCl

-H2O

H3C

NHCl

H 3C

H3 C

2HOCl N

NCl

Cl2HC N

-H2O DCAN

Acetaldehyde O

OH NH2Cl

H

H

-HCl

-H2O H

NHCl

H

H N

NCl

Cl

HOCl -H2O

N

Cyanogen chloride

Formaldehyde

Fig. 2 e The aldehyde pathway, which converts aldehydes to the equivalent nitrile and subsequently halonitrile (Mitch et al., 2009).

3. Precursor sources and formation pathways DON species such as amino acids, proteins, amino sugars, amides, nitriles, pyrroles, purines and pyrimidines are widespread in surface water (Westerhoff and Mash, 2002). They typically comprise a small portion of NOM by weight (0.5e10%) and derive from the activity of microorganisms, leaching from soil, or the influence of wastewater discharge (Westerhoff and Mash, 2002). The hydrophilic neutral, hydrophilic base and colloidal fractions of NOM are especially rich in nitrogenous matter (Leenheer et al., 2007). Selected formation pathways which account for the formation of aldehydes and nitriles from amino acid chlorination; formaldehyde and CNCl from glycine chlorination (both Fig. 1 (Deborde and von Gunten, 2008; Joo and Mitch, 2007)); DCAN and CNCl from monochloramination of respectively acetaldehyde and formaldehyde (Fig. 2 (Mitch et al., 2009)) and NDMA from DMA (Fig. 3 (Choi and Valentine, 2002; Keefer and Roller, 1973; Schreiber and Mitch, 2006a)) are portrayed. Note that HANs and CNCl can result from either chlorination of nitrogenous precursors

H3 C NH H3 C

NH2Cl

Dimethylamine (DMA)

H3 C NH H3 C

NHCl2

N

H3 C

H N

Oxidation

H3 C

N

N Cl

Chlorinated UDMH

Dimethylamine (DMA)

N

O

H3 C N-nitrosdimethylamine

H3 C

NH H3 C

O

N-nitrosdimethylamine

Nitrosating agent, e.g. NO+, HNO2, N2O4

H3 C

N

H3 C

H3 C H3 C

H3 C

NH2

UDMH

-HCl

Dimethylamine (DMA)

Oxidation

H3 C N

-HCl

(i.e. amino acids) or monochloramination of non-nitrogenous precursors (i.e. aldehydes). Wastewater and algal activity are linked to increased DON levels in drinking water (Krasner et al., 2008). To illustrate this, the average DON level in 28 US WTPs was 186 mg-N L1 (Lee et al., 2006), whereas the equivalent value in 16 WTPs subject to algal or wastewater influence was 290 mg-N L1 (Dotson and Westerhoff, 2009). Meanwhile water sources affected by algal blooms can have DON levels around 1 mg L1 as N, with the median concentration in effluents of wastewater treatment plants (WWTPs) from 1 to 4 mg L-1 as N (Pocernich and Litke, 1997). Because the formation of N-DBPs, including HANs and HNMs, was highest from nitrogen-rich fractions of dissolved organic matter, increased effluent organic matter (EfOM) and algal organic matter (AOM) in drinking water supplies will generally increase N-DBP formation (Dotson et al., 2009). Several recent DBP studies have focused on EfOM as a source of both DBPs and their precursors. During a campaign sampling 23 WWTPs at different seasons, both NDMA and NMOR were present before disinfection, with NMOR quantified up to 12,700 ng L1. However, these data probably relate to

N

N

O

H3 C N-nitrosdimethylamine

Fig. 3 e NDMA formation pathways, involving UDMH (top) (Choi and Valentine, 2002); chlorinated UDMH (middle) (Schreiber and Mitch, 2006a,b) and nitrosation (bottom) (Keefer and Roller, 1973).

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contamination from an industrial or other anthropogenic process feeding the specific WWTPs. After postchloramination at the WWTPs NDMA median and maximum values were 11 and 3165 ng L1 respectively, and thus higher than equivalent concentrations post-chlorination (3 and 8.2 ng L1) (Krasner et al., 2009a, 2009b) (Table 5). In contrast, NMOR concentrations were generally lower postdisinfection. HANs were also higher in WWTP effluents with a chlorination stage than WTPs (e.g. Table 2), with respective median and maximum values 16 and 30 mg L1 postchlorination (Table 5). Conversely, chloropicrin only reached respective maxima of 0.7 and 0.6 mg L1 post-chlorination and post-chloramination (Table 5), lower than the maximum of 2 mg L1 in the nationwide WTP study (Table 2) and indicating treated wastewater was not a major source of chloropicrin precursors. Another factor impacting N-DBPs is the level of treatment in WWTPs: in those with nitrification lower amounts of NDMA and HAN precursors were present. Thus, median yields of HANs from samples classified as having treatment with no nitrification and good nitrification were 29 and 13 mg L1 respectively, and the corresponding data for NDMA were 878 and 440 ng L1 (Krasner et al., 2008, 2009b). These values were obtained via laboratory formation potential tests undertaken with chlorine for HANs and monochloramine for NDMA. Precursor concentrations are also liable to change after discharge into the environment. Sampling of an EfOM dominated Arizonan river fed by a WWTP featuring chlorination/dechlorination found the EfOM was biodegradable by up to 40% (over 38 km of river; estimated travel time 4.2 days) (Chen et al., 2009). Removal of HAN and nitrosamine precursors, as monitored by formation potential tests, correlated with that of EfOM. For chloropicrin precursors the situation was more complex, with peaks at intermediate sampling points tentatively related to variation in nitrite, a possible nitrogen source in chloropicrin (Choi and Richardson, 2004).

4.

Key factors in the formation of N-DBPs

4.1.

Water quality parameters

4.1.1.

The impact of pH

As a generalisation, there is a trade-off between operating disinfection at low pH to minimise THMs and using a higher

pH which disfavours other DBP groups (Stevens et al., 1989). As seen below, the TNMs are an exception to this rule (Joo and Mitch, 2007; Merlet et al., 1985) (Table 6), and DCAA is rather insensitive to pH change (Stevens et al., 1989). During a pilotplant investigation DCAN was stable only at pH 5, where its concentration increased over time. At pH 7 DCAN decreased over time and at pH 9.4 it barely formed at all (Stevens et al., 1989). Amongst the HANs, trihaloacetonitriles (THANs) had the highest rates of hydrolysis, followed by DHANs (Glezer et al., 1999; Oliver, 1983). Reaction rates with chlorine follow a similar trend, while all degradation rates increased with increasing pH (Reckhow et al., 2001). It has been noted that after chlorination of aspartic acid for 3 h at pH 5, 7 and 9, the DCAN yield was highest at pH 5 (Chu et al., 2010). However, DCAcAm formation was negligible at pH 5, 0.2% at pH 7 and 0.49% at pH 9 (Chu et al., 2010), hinting that DCAN hydrolysis may not be the only formation pathway. At the two sites of maximum DCAcAm, dibromoacetamide (DBAcAm) and total HAcAm occurrence (Table 2) in the US 2000e2002 survey, the pH ranged from 6.8 to 7.1 and 7.6e8.3 respectively. In chlorinated waters, decreased CNX presence with increasing pH values was attributed to its base-catalysed decomposition (Heller-Grossman et al., 1999), and the shift in equilibrium distribution of free chlorine is another factor (Xie and Reckhow, 1992). The hypochlorite ion (OCl), rather than hypochlorous acid (HOCl), is the reactive compound responsible for the decomposition of CNCl, and more OCl exists at pH 6 than at pH 5. This instability in the presence of free chlorine may explain more frequent appearance of CNCl in chloraminated waters. Fewer studies have examined pH-mediated effects on NDBPs in chloraminated waters. The net formation of DCAN and CNCl was only slightly reduced over a pH range of 7.5e9 (Pedersen et al., 1999). The highest formation of CNCl from formaldehyde was observed at pH 5 and the formation of HNMs upon chloramination was less influenced by pH as compared with chlorination (Joo and Mitch, 2007). Regarding NDMA, optimum conditions for its formation from UDMH occur at pH 7e8 (Mitch and Sedlak, 2002) (Table 6). Nitrosation of DMA by N2O3 is controlled by the formation of N2O3 and therefore nitrosation of DMA was most rapid at pH 3.4 and rather sluggish at neutral and basic pH (Mirvish, 1975). By a similar logic, free chlorine enhanced nitrosation was most rapid at neutral pH, when N2O4 formation was favoured (Choi and Valentine, 2003). Further, due to the postulated importance of dichloramine to NDMA formation, the pH

Table 6 e Effect of pH on occurrence of major N-DBP groups. Group Haloacetonitriles Haloacetamides Halonitromethanes Cyanogen halides Nitrosamines

pH effect

References

More stable at acidic pH, hydrolysed at alkaline pH. Uncertain but presumably hydrolysed at alkaline pH. See text. Chloropicrin formation increases with pH. Higher formation at acidic and neutral pH, unstable in presence of free chlorine. NDMA formation via UDMH peaks at pH 7e8. Chlorine enhanced nitrosation most rapid at neutral pH. Nitrosation itself increases with pH but normally limited by formation of nitrosating agent.

Glezer et al., 1999; Oliver, 1983; Stevens et al., 1989 Chu et al., 2010; Reckhow et al., 2001 Joo and Mitch, 2007; Merlet et al., 1985 Joo and Mitch, 2007; Mitch et al., 2009 Sacher et al., 2008; Mirvish, 1975; Choi and Valentine, 2003; Mitch and Sedlak, 2002

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dependence of chloramines speciation is another factor to consider. Alkaline conditions, especially above pH 8.5, limit dichloramine formation and this can lead to reduced NDMA formation (Schreiber and Mitch, 2006a).

4.1.2.

The impact of bromide and iodide

Most brominated and especially iodinated DBPs are more cytotoxic and genotoxic than their chlorinated analogues (Plewa and Wagner, 2009). Bromine species (HOBr/OBr-) are known to be more effective substitution agents than the equivalent chlorine species (Symons et al., 1993) and molar DHAN formation was found to increase up to 101% for a largely autochthonous water source and 73% for a heavily allochthonous water source after spiking each with bromide (up to 30 mM) (Hua and Reckhow, 2007). Increasing bromide concentrations shifted the distribution of DHANs from DCAN to BCAN and then to DBAN. Elevated levels of bromide can also raise the CNX yield on a molar basis, although HOBr can catalyse cyanogen bromide (CNBr) hydrolysis, analogous to chlorine enhancing CNCl degradation, resulting in a lower net concentration of CNBr (Heller-Grossman et al., 1999). Bromamines have been suggested to play an important role in DBP formation (Diehl et al., 2000), with monobromamine and dibromamine formed during ozonation found to outcompete chloramines in forming CNBr (Lei et al., 2006). There is scant information about the presence of iodinated NDBP compounds in drinking water, although at least several potentially occur. The compound, 2-2-bromoiodoacetamide has been detected in chloraminated drinking water (Plewa et al., 2008; Richardson et al., 2007). Since many nonnitrogenous iodinated DBPs have also been most frequently detected following chloramination it can be predicted that the same will be found to apply for iodinated N-DBPs.

4.2.

The impact of treatment and disinfection

4.2.1.

The impact of treatment

Nitrogen-rich colloidal, hydrophilic neutral and hydrophilic base fractions tend to dominate N-DBP formation (Dotson et al., 2009). As THM and N-DBP precursors have different physicochemical properties, treatments effective at removing the former may have less success with N-DBP precursors. In particular, the hydrophobic acid fraction is typically a major source for THM precursors and is susceptible to removal by coagulation. While coagulation is also effective for removing colloids (a fraction holding a significant portion of N-DBP precursors) the same does not apply to the similarly reactive

hydrophilic base and neutral fractions. Recently, DON removal by coagulation was reported to be 21% on a median basis by WTPs which had suspended chlorination and/or chloramination, compared with 37% for bulk DOC (Mitch et al., 2009) (Table 7). Moderate removals of N-DBP precursors have been observed by coagulation, ranging from 18% to 52% for DHAN precursors as assessed by chlorine and chloramine formation potential (FP) tests, respectively (Mitch et al., 2009) (Table 7). In the same study, the removal of precursors by filtration proved more variable, ranging from 3% for DHAN precursors (chlorine FP test) to 61% for chloropicrin precursors (chlorine FP test). Filtration did not significantly impact upon CNX or DHAN formation, except where biological filtration removed aldehydes identified as probable CNX precursors (Mitch et al., 2009). Where filters were believed to be biologically active formaldehyde was not detected and median acetaldehyde concentration was 0.6 mg L1, whereas equivalent median values in WTPs without biological filtration were 22 and 5 mg L1. However, the importance of free amino acids as HAN, HAcAm and CNX precursors, as repeatedly highlighted by model compound work, is not consistent with full-scale data. Mean values for total amino acids and amino sugars accounted for 15% (median 10%) of DON in algal and wastewater impacted plant influents, but only 5% (median 4%) of the DON in filter effluent samples (intermediate treatments included ozone, chlorine dioxide, chlorine, chloramines, lime softening and coagulation, depending on the plant) (Mitch et al., 2009). Thus amino acids showed preferential removal relative to that of DOC and DON. In both cases the proportion of free amino acids was low compared with total (combined) species. Hence, it is thought amino acids found in AOM and EfOM do not fully account for recorded N-DBP formation and consequently that unidentified precursors exist as part of DON (Dotson and Westerhoff, 2009). For comparison, although model compound studies have determined selected free amino acids, as well as the nitrosamine precursors dimethylamine, diethylamine, morpholine and piperidine, to be ineffectively removed by coagulation (Bond et al., 2010; Pietsch et al., 2001), aliphatic members of these groups are typically susceptible to biodegradation (Bond et al., 2009; Pietsch et al., 2001). Meanwhile, the 43% increase in NDMA after coagulation reported from WTPs which had suspended chlorination and/ or chloramination (Table 7) was thought to be linked to polymer use (Mitch et al., 2009). Overall, Table 7 shows the limited efficacy of the common water treatment processes (coagulation and filtration) in removing N-DBP precursors, therefore more advanced processes such as membranes and

Table 7 e Removal of N-DBP precursors and water quality parameters from WTPs that suspended use of chlorine and/or chloramines (Mitch et al., 2009). Median results reported for samples chlorinated and/or chloraminated before and after treatment under laboratory conditions in formation potential (FP) tests. Treatment

Coagulation Ozonation Filtration

DOC

37% 4% 12%

DON

21% 3% 18%

DHAN

DHAN

TCNM

TCNM

CNX

NDMA

(Cl2)

(NH2Cl)

(Cl2)

(NH2Cl)

(Cl2eNH2Cl)

(NH2Cl)

18% 20% 3%

52% 28% 24%

49% 226% 48%

44% 133% 61%

48% 4% 17%

43% 10% 52%

Negative value indicates increased formation. TCNM ¼ chloropicrin (trichloronitromethane).

w a t e r r e s e a r c h 4 5 ( 2 0 1 1 ) 4 3 4 1 e4 3 5 4

biodegradation are likely to be more appropriate for reducing concentrations of nitrogenous precursors.

4.2.2.

The impact of pre-oxidation

Ozone can have extreme effects on HNM formation, as demonstrated by an increased median chloropicrin occurrence of 226% and 133% when followed by chlorination and chloramination formation potential tests, respectively, (Table 7) (Mitch et al., 2009). Ozone can also increase CNX formation, probably by increasing levels of formaldehyde or related precursors. Formaldehyde is known to be produced during drinking water treatment by chlorination and ozonation at respective levels of 0.14 and 0.33 mM (Krasner et al., 1989; Richardson et al., 1999; Weinberg et al., 1993), which points to chlorination or ozonation followed by chloramination as a likely scenario in producing CNCl. Meanwhile, relatively high concentrations of HANs and CNX were formed when chlorine dioxide was used in combination with chlorine (Heller-Grossman et al., 1999; Richardson et al., 1994). UV irradiation can also influence subsequent N-DBP formation, depending on the lamp type. Chloropicrin increased from 0.6 mg L1 at 0 mJ cm2 to 1.8 mg L1 following 140 mJ cm2 of medium pressure irradiation, whereas low pressure UV had no impact (Reckhow et al., 2010), while its formation was nearly doubled when post-chlorination rather than post-chloramination was coupled with medium pressure UV (Shah et al., 2011). This enhancement has been linked to photolysis of nitrate below a wavelength of 250 nm, in which region low pressure UV does not emit significantly (Reckhow et al., 2010). It was postulated that nitrite radicals produced in this manner nitrated aromatic NOM structures. In addition, due to the photolability of NDMA, UV irradiation is an effective degradation method (Sharpless and Linden, 2003). Oxidation before chloramination has proved an effective means of controlling NDMA formation. Chlorine, ozone, permanganate and simulated sunlight were all capable of reducing NDMA formation relative to chloramination alone (Chen and Valentine, 2008), presumably due to alteration of precursor sites. For example, a 50% reduction was measured when chloramination of a concentrated river water sample was preceded by 10 min free chlorine contact time at 0.08 M. Ozonation was effective at degrading aliphatic and particularly alicyclic nitrosamine precursors, with half-lives of respectively 17, 58, 15 and 0.19 min (ozone dose 1.0 mg L1 at pH 7) for dimethylamine, diethylamine, piperidine and morpholine (Pietsch et al., 2001), although it only reduced NDMA formation by 10% on a median basis in surveyed WTPs (Table 7).

4.2.3.

The impact of chlorination and chloramination

Chloramination normally involves addition of both chlorine and ammonia under controlled conditions, with an average chlorine contact time of 26 min before ammonia addition noted in the US (AWWA, 2000). For the HANs, comparative formation potential tests with chlorine/chloramines on samples from surveyed WTPs showed a concentration ratio of 1.2/1.0 on a median basis (Mitch et al., 2009), using the method described for Dotson et al., 2009. This pattern is comparable to the Scottish survey, where median concentrations of HAN4 were 1.7 mg L1 in chlorinated waters and 1.3 mg L1 in chloraminated waters (Goslan et al., 2009).

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Dotson et al. (2009) isolated NOM fractions from nitrogenrich sources and tested their propensity to form N-DBPs. On a central tendency basis DCAN yields were approximately twice as high after chlorination than after chloramination, with highest yields from the most nitrogen-rich fraction (hydrophilic bases). In the study by Lee et al. (2007), DCAN formation was on average approximately five times higher after chloramination than chlorination of 17 fractions from various water sources. Since disinfection protocols varied widely between these studies (see Section 2.1) and HAN precursors can be either nitrogenous or non-nitrogenous (Figs. 1 and 2) contradictory literature results are perhaps not unexpected. Another relevant factor is that, in the presence of free chlorine, DCAN degrades to DCAA (Reckhow et al., 2001) while appearing to be more stable in the presence of monochloramine (Lee et al., 2007). Equal amounts of chloropicrin were found upon chlorination and chloramination (Richardson, 2008), while yields of chloropicrin were slightly higher from chlorination than chloramination of nitrogen-rich isolates (Dotson et al., 2009). Unlike free chlorine, chloramines do not catalyse CNX decay (Na and Olson, 2004), correlating with the noted trend for CNX to be more widespread in chloramination WTPs (see Section 2.4). By adding pre-formed chloramines to wastewater less NDMA resulted than from addition of chlorine and ammonia in situ, a result attributed to lower dichloramine levels (Schreiber and Mitch, 2005). Unequal distribution of chlorine and ammonia, especially around the point of chlorine addition, can lead to localised formation of dichloramine and this can also be alleviated by adding free chlorine before ammonia (Schreiber and Mitch, 2005). Enhanced dichloramine formation close to the breakpoint (at wCl:NH3 1.7) was thought to underlie NDMA formation an order of magnitude higher than during monochloramination (i.e. at lower Cl:NH3 ratios) (Schreiber and Mitch, 2007), while reactive nitrosating intermediates were thought to contribute to heightened NDMA formation when small amounts of free chlorine were present. Thus, to limit NDMA it was recommended that chloramination be operated away from the breakpoint and with no free chlorine residual (Schreiber and Mitch, 2007).

5. Conclusions: minimising N-DBPs in water treatment From the foregoing review of published research, several conclusions can be made regarding methods of minimising NDBPs in water treatment. Due to identified N-DBP groups having disparate precursors and formation pathways certain control parameters are group specific and may have the opposite effect on other N-DBPs. Thus no single method can be recommended for all N-DBPs, instead control strategies should be based upon a water specific assessment of precursor sources and DBP formation.  DHANs can be produced from both chlorination (e.g. of amino acids) and chloramination (e.g. of aldehydes). Thus, dependent on the availability of reactive precursors contradictory effects of chlorination versus chloramination

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on HAN formation are possible, with most literature showing higher levels following chlorination. Pre-oxidation with chlorine dioxide and post-disinfection at acidic pH have been observed to enhance HAN formation, thus avoiding these conditions may provide mitigation strategies. HAcAms are a recently reported group of DBPs whose formation is believed to be related to that of HAAs and HANs. Assuming this association is relevant to drinking water treatment, much of the above description of HAN formation and control may apply to the HAcAms, although further investigation is needed to confirm this prediction. No meaningful correlations were found between HNM formation and the THMs or HAAs using available N-DBP data, indicating disparate precursors and/or formation routes. UV irradiation or ozonation of drinking water before chlorination can increase chloropicrin formation by at least double and this is presumably related to the nitration of aromatic NOM. Thus, use of alternative treatments can be an effective method to control HNMs. Elevated levels of CNX in chloraminated drinking water are consistent with the pre-eminence of a formation route such as the monochloramination of formaldehyde. Further, the potential for formaldehyde release from ozonation and chlorination of NOM indicates either of these treatments preceding post-chloramination are combinations likely to promote CNX in potable water. Thus, CNX formation can be minimised by operating disinfection without secondary chloramination. Factors involved in the formation of NDMA in drinking water are chloramination, the presence of EfOM and the use of certain ion exchange resins or polymers. The majority of NMDA precursors are believed to be of anthropogenic origin. Oxidation prior to chloramination, for example by free chlorine, can limit NDMA formation. Other nitrosamines, notably NPYR, NMOR and NDEA, have been identified in the low ng L-1 range in waters with significant NDMA, indicating common precursor sources (e.g. wastewater) and that strategies reducing NDMA may also be effective for the other named nitrosamines. However, more work is needed to increase understanding of the formation and occurrence of nitrosamines other than NDMA. Common water treatment processes such as coagulation and filtration are typically rather ineffective for removing DON in general and N-DBP precursors in particular, therefore advanced processes such as membranes and biodegradation are likely to be more appropriate for reducing concentrations of these groups.

Acknowledgements The Department for Environment, Food and Rural Affairs (Defra) is gratefully acknowledged for funding a study by the authors entitled, ‘Review of the Current Toxicological and Occurrence Information Available on Nitrogen-Containing Disinfection ByProducts’, on which this paper is based. The authors wish to thank Stuart Krasner, Michael Plewa and Susan Richardson for their assistance with the original study.

references

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