N-nitrosodimethylamine (NDMA) formation during ozonation of wastewater and water treatment polymers

Chemosphere 144 (2016) 1618e1623

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N-nitrosodimethylamine (NDMA) formation during ozonation of wastewater and water treatment polymers Massimiliano Sgroi a, b, Paolo Roccaro a, b, Gregg Oelker c, Shane A. Snyder b, d, * a

Department of Civil Engineering and Architecture, University of Catania, Viale A. Doria 6, 95125 Catania, Italy Department of Chemical & Environmental Engineering, University of Arizona, 1133 E. James E. Rogers Way, Tucson, AZ 85721, USA c United Water, Edward C. Little Water Reclamation Facility, 1935 South Hughes Way, El Segundo, CA 90245, USA d National University of Singapore, NUS Environmental Research Institute (NERI), 5A Engineering Drive 1, T-Lab Building, #02-01, Singapore 117411, Singapore b

h i g h l i g h t s
NDMA forms at extremely high levels in some ozonated municipal wastewaters.
Water treatment polymers do not appear to impact NDMA levels.
UV and fluorescence surrogates are in excellent agreement with NDMA formation.
Potable water reuse using ozonation could face severe challenges due to NDMA formation.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 June 2015 Received in revised form 2 October 2015 Accepted 5 October 2015 Available online xxx

N-Nitrosodimethylamine (NDMA) formation by ozonation was investigated in the effluents of four different wastewater treatment plants destined for alternative reuse. Very high levels of NDMA formation were observed in wastewaters from treatment plants non operating with biological nitrogen removal. Selected experiments showed that hydroxyl radical did not have a significant role in NDMA formation during ozonation of wastewater. Furthermore, ozonation of three different polymers used for water treatment, including polyDADMAC, anionic polyacrylamide, and cationic polyacrylamide, spiked in wastewater did not increase the NDMA formation. Effluent organic matter (EfOM) likely reduced the availability of ozone in water able to react with polymers and quenched the produced $OH radicals which limited polymer degradation and subsequent NDMA production. Excellent correlations were observed between NDMA formation, UV absorbance at 254 nm, and total fluorescence reduction. These data provide evidence that UV and fluorescence surrogates could be used for monitoring and/or controlling NDMA formation during ozonation. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Nitrification Hydroxyl radical Polyacrylamide polyDADMAC Fluorescence UV absorbance

1. Introduction N-Nitrosamines constitute an emerging group of disinfection by-products (DBPs) (Richardson and Ternes, 2014) that exhibit unusually high carcinogenic risk [e.g., 106 cancer risk level at concentration as low as 0.7 ng/L for N-nitrosodimethylamine (NDMA)] (USEPA, 1993). Of all the nitrosamines, NDMA has been most commonly detected in drinking water and wastewater (USEPA, 2012) and USEPA placed it on the drinking water

* Corresponding author. Department of Chemical & Environmental Engineering, University of Arizona, 1133 E. James E. Rogers Way, Tucson, AZ 85721, USA. E-mail address: [email protected] (S.A. Snyder). http://dx.doi.org/10.1016/j.chemosphere.2015.10.023 0045-6535/© 2015 Elsevier Ltd. All rights reserved.

contaminant candidate list 3 (CCL3) (USEPA, 2009). Currently, the California Department of Public Health (CDPH) has set a notification level of 10 ng/L for NDMA in drinking water (CDPH, 2009). In addition, wastewater intended for indirect potable reuse (IPR) is also expected to comply with these drinking water standards (CDPH, 2009). NDMA can be generated in water and wastewater treatment system by chlorine-based disinfection (Schreiber and Mitch, 2006). More recently, researchers have reported that alternative disinfectants including chlorine dioxide (ClO2) and ozone (O3) are also able to produce nitrosamines (Najm and Trussel, 2001; Andrzejewski et al., 2007). A comprehensive review of the various mechanisms for NDMA formation in drinking water has been recently published (Krasner et al., 2013).

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Ozone is increasingly used as a disinfectant of choice in water treatment, since it is much more effective in inactivating various pathogenic microorganisms of public health concern, including Cryptosporidium parvum oocysts and Giardia lamblia cyst, compared to chlorine-based disinfectants (Gottschalk et al., 2000). In addition, its strong ability to degrade trace organic contaminants (Snyder et al., 2006) as well as its competitive capital and operation and maintenance costs suggested ozonation as a valuable oxidation process in modern IPR or multi-barrier systems (Roccaro et al., 2013). To date, there is a lack of knowledge about the nitrosamines precursor pool and formation mechanisms, particularly during ozonation. Typical secondary amines (such as dimethylamine (DMA)), some dyes, and related compounds have been shown to be NDMA precursors upon ozonation, but with very low NDMA yields (i.e., 0.02%) (Andrzejewski et al., 2007; Oya et al., 2008). On the contrary, a limited subset of precursors containing hydrazine (e.g., unsymmetrical dimethylhydrazine (UDMH) and semicarbazides) or sulfamide (e.g., N,N-dimethylsulfamide (DMS)) functional group have generated NDMA yields >50% (Schmidt and Brauch, 2008; Kosaka et al., 2009; Marti et al., 2015). Among potential NDMA precursors, cationic amine-based water treatment polymers containing DMA moieties have also been reported to form NDMA upon chloramination or ozonation and in both the cases NDMA formation has been related to polymer degradation and DMA release (Park et al., 2009; Padhye et al., 2011; Sgroi et al., 2014; Park et al., 2015). Particularly, Padhye et al. (2011) investigated NDMA formation from ozonation of several water treatment polymers, including poly(diallyldimethylammonium chloride) (polyDADMAC), polyamines and cationic polyacrylamides. In this cited study (Padhye et al., 2011), hydroxyl radicals generated from ozone were reported to play an important role in polymer degradation and DMA release, and polyDADMAC was the polymer that yielded the highest amount of NDMA (i.e., yield of 0.003%). However, the highest NDMA yield during ozonation of water treatment polymers was observed by Sgroi et al. (2014) during the ozonation of the Mannich polymer (i.e. yield of 0.011%), which is characterized by a very easily degradable structure. Most of the experiments reported in literature testing NDMA formation during ozonation of water treatment polymers were conducted in synthetic water and did not simulate the impact of a real water matrix (Padhye et al., 2011; Park et al., 2015). Recent studies have reported that molar conversion yields for some compounds, such as DMA and UDMH, can be higher in wastewater as compared to deionized water due to catalyzed reactions with constituents found in wastewater (e.g. ammonia and bromide) (Sgroi et al., 2014; Marti et al., 2015), on the contrary it can be decreased for some other compounds (e.g. DMS) likely owing to the high ozone demand created by effluent organic matter (EfOM) (Marti et al., 2015). Among water treatment polymers, NDMA formation in wastewater matrices upon ozonation has been verified only for the Mannich polymer (Sgroi et al., 2014), but this information is still missing for other polymers. PolyDADMAC and polyamines are used as coagulant in water treatment. On the contrary, polyacrylamide and Mannich polymers are used as flocculants in water treatment as well as conditioning agents in sludge dewatering and thickening processes. Typical dosages of polymers in water treatments are around 1 mg/L as active ingredient (Faust and Aly, 1983), whereas two order of magnitude higher dosage of polymer is typical for sludge dewatering and thickening operations (Sgroi et al., 2014). Nevertheless, to evaluate the final polymer concentration in the effluent of a coagulation process or the amount of polymer released from sludge treatments that goes back in the return line to the head of a WWTP is not of easy resolution

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and the impact on NDMA formation has to be verified by field studies. Overall, only few studies have investigated NDMA formation during ozonation of wastewaters (Kosaka et al., 2009; Sgroi et al., 2014; Gerrity et al., 2015; Sgroi et al., 2015). In Gerrity et al. (2015), ozone-induced NDMA formation of different secondary or tertiary wastewater effluents ranged from <10 to 143 ng/L, but the reasons for the variation in formation was not clarified by the authors. Very high levels of NDMA formation during ozonation were reported by Kosaka et al. (2009) in a wastewater effluent containing anti-yellowing agents (290 ng/L) and by Sgroi et al. (2014) in a wastewater secondary effluent impacted by Mannich polymer (538 ng/L). Finally, Sgroi et al. (2015) showed that the use of preozonation for membrane fouling control instead of prechloramination can be an inconvenient process for NDMA control in an IPR system due to the much faster reaction kinetics of ozone with some NDMA precursors than chloramines. The main objectives of the current study were: (i) to investigate NDMA formation in wastewater effluents of different characteristics and treated by different biological processes; (ii) to evaluate the role of hydroxyl radical in NDMA formation during ozonation of wastewater; (iii) to determine the production of NDMA from different water treatments polymers spiked in wastewater during ozonation. In addition, correlations between NDMA formation during ozonation and spectroscopic measurements were investigated in all the wastewaters tested. Indeed, to date few studies have reported relationships between NDMA formation and fluorescence or UV absorbance measurements and they are related to NDMA formation potential during chloramination (Chen and Valentine, 2007; Yang et al., 2015). 2. Materials and methods 2.1. Materials All purchased solvents, standards, and reagents were of high purity. The details concerning these materials are reported in the Supporting Data section (Text S1). Low molecular weight poly(diallyldimethylammonium chloride) (polyDADMAC) polymer (19e21% wt. solution, Clarifloc C308P), high charge anionic polyacrylamide emulsion (34e41% wt. solution, Clarifloc A-210P), very high charge cationic polyacrylamide emulsion (46.5e53.5% wt. solution, Clarifloc C-6288) were obtained by SNF Polydyne (Riceboro, GA). 2.2. Waters used Grab samples of wastewater effluents were collected at four treatment plants in the south-western United States producing water for different reuse applications and treated by different biological oxidation processes. The schemes of the investigated wastewater treatment plants (WWTPs) along with the applied reuse typology are presented in Table S1. All the samples were collected before any eventual chlorine addition at the WWTPs and kept refrigerated at 4  C until the ozonation tests. WWTP 1 serves about four million people and treats wastewater characterized by about 90% from domestic discharge. The plant scheme consists of preliminary treatments, primary sedimentation and secondary treatment where high purity oxygen is directly bubbled into the activated sludge reactor having a sludge retention time (SRT) of 1.5 days, and non able to achieve nitrification. The unchlorinated final effluent is in part sent to a downstream indirect potable reuse system. WWTP 2 serves approximately 500,000 people with domestic (70%), commercial (10%) and industrial contribution (20%). The facility's treatment process consists of

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headworks for initial screening of large materials and settling out of heavy sand and rocks, clarifiers to separate sludge and scum, trickling filters to remove suspended particles by biological treatment, and final chlorination. Effluent is discharged to a close river and in part reused for turf irrigation. The population served by WWTP 3 is approximately 220,000 with 90% contribution from domestic discharge. The treatment processes include extended biological oxidation by activated sludge process achieving nitrification. The dechlorinated effluent is discharged to a close river and reused for irrigation. Upgrading plans for WWTP 2 and WWTP 3 foresee possible ozonation of the final effluents in order to expand water reuse applications. Finally, WWTP 4 serves a considerably smaller population (approximately 17,000) with 73% of the population aged 65 or older (median age of 72 years old). The treatment train foresees preliminary treatments followed by a biological nutrient removal oxidation ditch, which operates on an extended aeration, nitrification, and denitrification process within the oxidation ditch by cycling the aeration on and off. The clarified effluent is then filtered and disinfected by chlorination before its utilization for aquifer recharge by soil aquifer treatment. Typical water quality parameters for the investigated wastewaters are reported in Table 1. 2.3. Ozonation

Table 2 NDMA formation during ozonation of different wastewater effluents. Wastewater effluent

TN (mg/L)

DOC (mg/L)

O3/DOC (mg/mg)

NDMA (ng/L)

WWTP 1

48.7

14.3

WWTP 2

39.5

10.5

WWTP 3

8.2

5.3

WWTP 4

2.4

5.4

0.0 0.3 0.6 0.8 1.1 0.0 0.1 0.3 0.5 0.0 0.2 0.6 0.9 0.0 0.3 0.6 1.1 1.7

30 161 284 419 537 17 26 35 46 5 8 9 10 1 1 1 4 4

TN, total dissolved nitrogen. DOC, dissolved organic carbon. O3, ozone.

Shimadzu total carbon analyzer TOC-LCSH (Kyoto, Japan) was used for DOC and TN quantification. Analysis of other inorganic water constituents was performed according to standard methods. Ultraviolet light absorbance was analyzed using Varian Cary 50 UV/Vis spectrophotometer. Fluorescence data was collected using Varian Cary Eclipse fluorescence spectrophotometer with the scanning range from excitation wavelength 240 nme450 nm at an interval of 5 nm and emission wavelength from 250 nm to 580 nm at the interval of 1 nm. Data processing in MATLAB (Natick, MA) included corrections for Raman scattering, blank response, the spectral sensitivity of the lamp, and the inner filter effect (i.e., the absorbance of the matrix) (MacDonald et al., 1997). Regional integration was performed according to published literature (Chen et al., 2003) with slight modifications (Anumol et al., 2015) to calculate the total fluorescence intensities (in Raman units) for each sample. Spectroscopic measurements have always been accomplished after 0.7 mm filtration.

Ozonation experiments were performed at pilot scale (WWTP effluent 4), in semi-batch mode (effluents of WWTP 1, WWTP 2 and WWTP 3) or by ozone stock solution injection (experiments in synthetic water). Semi-batch experiments were performed as described in Sgroi et al. (2014) and briefly illustrated in Supplementary Data (Text S2). The sand-filtered tertiary effluent of WWTP 4 was treated with a 40-L/min pilot-scale ozone/UV/H2O2 reactor (Wedeco/ITT, Herford, Germany), but it was operated only in the ozone mode for the purposes of this study. For polymers ozonation in synthetic water, ozone was applied by ozone solution spiking. Ozone stock solutions (40 mg/L) were produced by sparging O3 containing oxygen through deionized (DI) water cooled in an ice bath. Residual dissolved ozone concentration was measured by indigo method (Rakness et al., 2010).

3. Results and discussion

2.4. Analytical methods

3.1. NDMA formation in ozonated wastewaters

The NDMA extraction and analysis was performed according to the EPA Method 521. An Agilent 7000 Triple Quadrupole GC/ MSeMS (Santa Clara, California) equipped with Gerstel autosampler operating in the chemical ionization (CI) mode with ammonia as the reagent gas was used for analysis. Identification and quantitation were performed in the MSeMS mode with a resulting method reporting limit in ultrapure water of around 1 ng/L. For dissolved organic carbon (DOC) and total dissolved nitrogen (TN) concentrations analysis samples were filtered through 0.45 mm hydrophilic polypropylene filter (GHP Membrane Acrodisc, Pall Life Sciences) and acidified to pH < 3 with hydrochloric acid. A

NDMA formation by ozonation was investigated in the effluents of four different wastewater treatment plants destined for alternative reuse, including potable reuse. The selected wastewaters present different water quality characteristics and were treated by different biological oxidation processes. WWTP 1 and WWTP 2 show high TN and DOC concentrations in the final effluents. On the contrary, WWTP 3 and WWTP 4 effluents were treated for nitrogen removal and present low TN and DOC values. Particularly, WWTP 4 effluent was subjected to full nitrification and denitrification treatments. In Table 2 TN and DOC concentrations of the investigated wastewaters are compared to NDMA formation at different

Table 1 Water quality parameters for the investigated WWTPs. WWTP

pH

TN (mg/L)

Nitrate (mg/L as N)

Nitrite (mg/L as N)

DOC (mg/L)

1 2 3 4

7.7 7.4 7.5 8.0

48.7 39.5 8.2 2.4

1.91 0.09 6.68 na

2.15 0.66 BLQ na

14.3 10.5 5.3 5.4

BLQ, below limit of quantification. na, non available. TN, total dissolved nitrogen. N, nitrogen. DOC, dissolved organic carbon.

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ozone doses normalized to the DOC concentration. An ozone dose normalized to the DOC concentration has often been used as an operating parameter to compare waters with varying DOC concentration (Wert et al., 2009; Lee et al., 2013; Sgroi et al., 2014). In this study almost all the ozonation tests were carried out with an ozone to DOC ratio ranging from 0.0 to 1.0 mg/mg (exception was the WWTP 4 effluent, collected at the pilot-scale ozone/UV/H2O2 reactor). Indeed, ozone doses corresponding to O3/DOC ratios up to 1.0 mg/mg are generally usual for wastewater ozonation (Wert et al., 2009; Sgroi et al., 2014). Furthermore, in all the wastewater ozonation tests performed in semi-batch mode, the dissolved ozone residual measured immediately after ozone application was zero due to the high instantaneous ozone demand (IOD) common for wastewater (Wert et al., 2007), and the utilized reactor configuration and experimental set-up. Ozone residual in WWTP 4 effluent measured immediately after pilot scale ozonation ranged from 0 to 1.1 mg/L, but it was completely depleted after few minutes. Ozonation tests results showed that ozonation of wastewater often produced NDMA concentration not in compliance with California's potable reuse requirements (i.e. limit of 10 ng/L) (CDPH, 2009) and the NDMA formation was lower in effluents treated for nitrogen removal and by extended biological oxidation. WWTP 4 effluent was the only ozonated wastewater always in compliance with potable reuse requirements. Particularly, wastewater with lower TN concentration has always resulted in lower NDMA formation. This is in agreement with the consideration that the precursor for the amine moiety of nitrosamines should be a constituent of dissolved organic nitrogen (DON) (Lee W. et al., 2007; Shah and Mitch, 2012) and with the proved role of nitrogenous oxidants (e.g. hydroxylamine, brominated nitrogenous oxidants) generated during ammonia oxidation in NDMA formation (Sgroi et al., 2014). Hence, this result shows that a complete biological nitrification is an effective treatment method for reducing NDMA formation during ozonation of wastewaters, and it appears to be a strategic and essential treatment when wastewaters are destined for potable reuse.

3.2. Hydroxyl radical effect on NDMA formation WWTP 1 wastewater effluent presented a broadly larger NDMA formation than all the other wastewater. For this reason WWTP 1 wastewater was used for additional experiments in this study. In this WWTP, additional grab wastewater effluent samples were collected and supplementary ozonation experiments were performed adding 5 mg/L of H2O2 and 10 mM of TBA (a known $OH

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radicals scavenger) in order to investigate the role of hydroxyl radicals in NDMA formation. During the current experiments, 11 mg/L of ozone was applied to the investigated wastewater corresponding to a O3/DOC ratio of around 0.8 mg/mg. Fig. 1 shows NDMA formation after ozonation in absence of any manipulation, when TBA was added to quench the $OH radicals, and when hydrogen peroxide was added in order to have a higher production of $OH radicals. Results of all the performed tests suggest that $OH radicals did not contribute to any additional NDMA formation, and hydroxyl radical scavenging led to increased NDMA formation compared to the experiment with H2O2 addition, accordingly to what observed by Marti et al. (2015) during experiments with model compounds. The formation of $OH radicals may either produce reactions which form products other than NDMA or play a role of NDMA destruction. Indeed, it is noteworthy to observe that $OH radicals have higher value of rate constant (KNDMA 8 M1 s1) than O3 rate constant (KNDMA $OH ¼ 4.6  10 2 M1 s1) for NDMA (Lee C. et al., 2007). However, O3 ¼ 5.3  10 for some compounds such as the water treatment polymer polyDADMAC, hydroxyl radicals were shown to have an important role in NDMA formation (Padhye et al., 2011) as discussed in more detail in the next section.

3.3. Polymer coagulants tests Prior researches have shown that NDMA formation from polymers used for water treatment is strongly related to polymer degradation and dimethylamine (DMA) release during chloramination or ozonation (Park et al., 2009; Padhye et al., 2011). Particularly, in a study conducted by Padhye et al. (2011), polyDADMAC yielded the highest amount of NDMA during ozonation among several water treatment polymers, including polyamines and cationic polyacrylamides, and hydroxyl radicals generated from ozone played an important role in the degradation of polyDADMAC's quaternary ammonium ring groups and subsequent release of secondary amine. In this study, NDMA formation from three different water treatment polymers, including polyDADMAC, anionic polyacrylamide and cationic polyacrylamide, has been evaluated during ozonation in DI water (pH 7.6) and wastewater (pH 7.7) (Table 3). For each tested polymer, 10 mg/L of polymer emulsion was spiked in water. The corresponding active ingredient concentrations in water were: polyDADMAC 2.00 mg/L; anionic polyacrylamide 3.75 mg/L; cationic polyacrylamide 5.00 mg/L. Wastewater samples were ozonated in semi-batch mode (11 mg/L O3, or 0.8 mg/mg as O3/DOC ratio); in this case, the dissolved ozone residual measured

Fig. 1. NDMA formation by application of 11 mg/L of ozone (O3), 11 mg/L of ozone and 10 mM of tert-butyl alcohol (TBA), 11 mg/L of ozone and 5 mg/L of hydrogen peroxide (H2O2) in WWTP 1 effluent. Error bars indicate the minimum and maximum concentrations from experimental duplicate samples.

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Table 3 NDMA formation during ozonation of 2.00 mg/L of polyDADMAC, 3.75 mg/L of anionic polyacrylamide and 5.00 mg/L of cationic polyacrylamide as active ingredient. Polymer

No polymer addition polyDADMAC Anionic polyacrylamide Cationic polyacrylamide

DI water

WWTP 1 effluent

O3 (mg/L)

NDMA (ng/L)

O3 (mg/L)

NDMA (ng/L)

5.5 4.8 5.5 5.5

NDa 152b 1 3

11 11 11 11

386 376 371 374

Table 4 Max NDMA formation in different wastewaters and regression parameters of correlations between NDMA formation and UV absorbance at 254 nm (UV254) removal, and NDMA formation and total fluorescence (FT) removal. Wastewater effluent

NDMAmax (ng/L)

UV254 slope

FT

WWTP 1 WWTP 2 WWTP 3

537 46 10

13.13 0.96 0.09

7.98 0.64 0.07

R2UV

R2FT

0.93 0.99 0.97

0.92 0.99 0.97

slope

a

ND, non-detectable. Value reported in Padhye et al. (2011) and corresponding to a polyDADMAC concentration of 5 mg/L as active ingredient during similar ozonation conditions. b

immediately after ozone application was zero. Polymer solutions in DI water were ozonated by stock solution. In this case, ozone residual was not quenched to allow a complete polymer exposure to the oxidant. The polymers concentrations used in this study were higher than the optimum dosages of polymer required for coagulation (~1 mg/L as active ingredient) (Faust and Aly, 1983). However, no significant formation of NDMA due to polymers addition was observed in wastewater, even during ozonation of polyDADMAC, the polymer shown to have a significant yield of NDMA (Padhye et al., 2011). Likely, EfOM reduced the availability of ozone in water able to react with polymers and quenched the produced $OH radicals avoiding polymer degradation and DMA release. Anionic and cationic polyacrylamide produced low NDMA also when spiked in DI water. According to these results, the studied polymers seem to not be important NDMA precursors at the investigated concentrations during ozonation of wastewater. Nevertheless, it is noteworthy to observe that certain kinds of DMA-based polymers (i.e. Mannich polymer), characterized by a very high degradability, can be responsible for elevated NDMA formation during ozonation, even in wastewater matrices (126 ng/L was the additional NDMA formation observed during ozonation of a wastewater effluent containing 3 mg/L of Mannich polymer as active ingredient) (Sgroi et al., 2014).

3.4. Use of spectroscopic measurements to predict NDMA formation during ozonation Sgroi et al. (2014) reported that filtered wastewater samples (0.7 mm) produced more NDMA than unfiltered sample due to the different ozone demand created in filtered and unfiltered samples, hypothesizing that ozone reacted with dissolved precursors. This effect was more dramatic in samples with high level of total suspended solids (TSS). Similarly, TSS hindered the degradation of dissolved chromophores in wastewater organic matter resulting in less decreased UV absorbance, suggesting relationships between NDMA formation and spectroscopic measurements. Furthermore, previous researches showed that NDMA formation potential during chloramination of natural and drinking water was correlated with differential specific UV absorbance (SUVA) value at 272 nm (Chen and Valentine, 2007), and with the tryptophan-like component of dissolved organic matter (DOM) obtained by parallel factor analysis of fluorescence excitation-emission matrices (Yang et al., 2015). In the present study, UV absorbance at 254 nm (UV254) removal and total fluorescence (FT) removal were correlated to NDMA formation during ozonation. Excellent correlations were observed for all the tested wastewater as reported in Table 4 and Figs. S1eS3. Results from WWTP 4 were excluded due to the absence of significant NDMA formation. Linear regressions with square correlation factor (R2) higher than 0.9 have always been observed, but the

regression slope was different for different waters, and it was higher in wastewater with more elevated concentration of NDMA precursors (i.e. water with higher NDMA formation) and higher values of TN. Hence, in all the studied wastewaters the degradation of chromophores and fluorophores in effluent organic matter (EfOM) by ozonation seemed to be well correlated to the oxidation of NDMA precursors and to the consequent NDMA formation. This result confirms that differential absorbance and fluorescence are good surrogates to predict formation of DBPs during chlorination or ozonation according to prior researches (Korshin et al., 1997; Roccaro et al., 2009; Roccaro and Vagliasindi, 2010; Roccaro et al., 2011; Liu et al., 2015). 4. Conclusions This study was mainly focused on NDMA formation in wastewater during ozonation. Based on the results of the study, the following conclusions can be presented:
wastewater ozonation often produced high level of NDMA not in compliance with California's potable reuse requirements (i.e. limit of 10 ng/L);
wastewater treated by extended biological process and nitrogen removal showed low formation of NDMA during ozonation;
hydroxyl radicals did not have an important role in NDMA formation during ozonation of wastewater;
water treatment polymers, including polyDADMAC, anionic polyacrylamide, and cationic polyacrylamide did not increase the NDMA formation during ozonation in wastewater;
UV absorbance at 254 nm and total fluorescence removal showed excellent correlation with NDMA formation in all the tested wastewaters Acknowledgments Funding for M. Sgroi and P. Roccaro was partially supported by the Italian Ministry of Instruction, University, and Research (MIUR), through the Research Projects of National Interest “Reuse of Wastewater in Agriculture: Emerging Pollutants and Operational Problems” (PRIN 2009 e grant 20092MES7A_002). P. Roccaro acknowledges the US-Italy Fulbright Commission for supporting his research in the US through the “Fulbright Scholar Program Advanced Research and University Lecturing Awards in the United States”. Views expressed in this paper do not necessarily reflect those of the funding agencies. The authors are also grateful to Joe Weitzel from Agilent Technologies (Santa Clara, California) and Jens Scheideler of Wedeco (Herdford, Germany) for their equipment and technical support used in this study. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.chemosphere.2015.10.023.

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