# Fate of N-nitrosodimethylamine and its precursors during a wastewater reuse trial in the Llobregat River (Spain)

Josep Sanchís, Wolfgang Gernjak, Antoni Munné, Núria Catalán, Mira Petrovic, Maria José Farré

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## ► To cite this version:

Josep Sanchís, Wolfgang Gernjak, Antoni Munné, Núria Catalán, Mira Petrovic, et al.. Fate of N-nitrosodimethylamine and its precursors during a wastewater reuse trial in the Llobregat River (Spain). Journal of Hazardous Materials, 2021, 407, pp.124346. 10.1016/j.jhazmat.2020.124346. hal-03183923

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Submitted on 11 Jun 2021

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Research paper

# Fate of N-nitrosodimethylamine and its precursors during a wastewater reuse trial in the Llobregat River (Spain)

<figure><img src="image_3.png" /></figure>

Josep Sanchís<sup>a, b</sup>, Wolfgang Gernjak<sup>a, c</sup>, Antoni Munné<sup>d</sup>, Núria Catalán<sup>e, f</sup>, Mira Petrovic<sup>a, c</sup>, Maria José Farré<sup>a, b, *</sup>

## ARTICLE INFO

### Keywords:

Disinfection byproduct

N-nitrosamines

HRMS

Water reuse

Chloramine

## ABSTRACT

In summer 2019, a full-scale trial was carried out to investigate the effects in drinking water quality when tertiary treated wastewater was discharged into the Llobregat River upstream of the intake of one of the major drinking water treatment plants of Barcelona and its metropolitan area. Two scenarios were investigated, i.e. discharging the reclaimed water with and without chemical disinfection with chlorine. This study investigates the concentration of N-nitrosodimethylamine (NDMA) as the specific disinfection conditions employed in this trial may favor its formation. To this aim, both NDMA and NDMA formation potential, were measured. The river contained NDMA at very low concentrations, but the concentration of NDMA precursors was already high. The NDMA concentration was reduced from discharge to the river to drinking water intake probably due to a combined effect of dilution and photolysis. The formation potential was also reduced probably due to dilution and biodegradation. The concentration of NDMA in the drinking water was always low (<7.3 ng/L), although the formation potential was above 10 ng/L in one sample. Dissolved organic matter characterization by high resolution mass spectrometry revealed differences between the nature of the organic matter in the river before and after reclaimed water discharge.

## 1. Introduction

Globally increasing water scarcity impacts environmental flows of rivers and threatens the supply of drinking water in many highly populated areas. Considering the possibility of implementing water reuse in such areas is becoming a reality (WHO, 2017a) and research investigating the effects on water quality are crucial to ensure that potable water reuse projects protect the health of communities adequately (USEPA, 2017).

In the summer of 2019, the Catalan Water Agency (ACA) designed an experiment that consisted of introducing reclaimed water into the Llobregat River upstream of a drinking water treatment plant (DWTP). The Llobregat River is one of the main rivers that supply drinking water to the city of Barcelona and its metropolitan area. It has a total length of 175 km, a basin surface area of 4,957 km², and a mean water discharge

of 20 m³/s at the lower part where around 3 m³/s of water is withdrawn to provide drinking water supply for more than 45% of approximately four million people living in the Barcelona metropolitan area (Mujeriego et al., 2017). Most of the Llobregat basin has semi-arid climatic conditions, highly variable between seasons. It is impacted by several sewage and industrial effluents in the middle and lower parts, as well as by potash-mining activities (Munné et al., 2012). Therefore, the water quality is variable due to the extremely complex, dense, diverse, and numerous sets of activities upstream. Independently of this water reclamation trial, it is accepted that the degree of impact from wastewater effluents (de-facto reuse) varies between 8% and 82% due to the 63 urban wastewater plants that discharge treated effluent into the Llobregat River and its tributaries upstream of the last abstraction point at the Sant Joan Despí DWTP (Drewes et al., 2017; Mujeriego et al., 2017). Sant Joan Despí is one of the most advanced DWTP in Europe that

<footnote><sup>a</sup> Catalan Institute for Water Research (ICRA), C/ Emili Grahit, 101, 17003 Girona, Spain</footnote>

<footnote><sup>b</sup> University of Girona, 17071 Girona, Spain</footnote>

<footnote><sup>c</sup> Catalan Institution for Research and Advanced Studies (ICREA), Passeig Lluís Companys 23, 08010 Barcelona, Spain</footnote>

<footnote><sup>d</sup> Catalan Water Agency, C/ Provença 204-208, 08036 Barcelona, Spain</footnote>

<footnote><sup>e</sup> United States Geological Survey, Boulder, CO, USA</footnote>

<footnote><sup>f</sup> Laboratoire des Sciences du Climat et de l'Environnement, LSCE, CEA, CNRS, UVSQ, 91191 Gif-Sur-Yvette, France</footnote>

<footnote>* Corresponding author at: Catalan Institute for Water Research (ICRA), C/ Emili Grahit, 101, 17003 Girona, Spain. E-mail address: mjfarre@icra.cat (M.J. Farré).</footnote>
includes two parallel treatment lines: one employs ozone and active carbon filtration and the other ultrafiltration followed by reverse osmosis. Both are blended before final disinfection with chlorine.

The experiment by the ACA investigated two different scenarios and lasted two months (June and July 2019). The first experiment consisted of discharging reclaimed water into the river without chemical disinfection, while the second experiment included the addition of chlorine to the treated tertiary effluent to reduce discharge of pathogens to the river. However, as it is well known, the reaction of chemical disinfectants with organic matter can generate disinfection byproducts (DBPs) which include potential carcinogens (Richardson et al., 2007). In fact, while the primary public health concern in potable reuse are pathogens (Pecson et al., 2015), the generation and control of DBPs also requires attention (Furst et al., 2018). Among the organic chemicals present in potable reuse, the American National Research Council (2012) concluded that DBPs deserve special attention in water reuse projects because they represent a more serious human health risk than pharmaceuticals and personal care products do.

Compared to traditional drinking water sources without significant anthropogenic impact, treated effluents used in water reuse projects have very specific properties and tend to contain increased concentrations of organic matter, ammonia, bromide or iodide which can lead to the formation of diverse, and sometimes unique, DBPs in water reuse schemes. The most significant factors affecting the structure and concentrations of DBPs are (i) the level of nitrification at the wastewater treatment plant as well as efficacy of treatment processes to remove organic matter prior to disinfection (Krasner, 2009), (ii) the presence of high concentrations of anthropogenic or natural iodide and bromide that can affect the speciation of halogenated DBPs formed which may be different to the DBPs generated during drinking water production using groundwater or surface waters, and (iii) the use of different disinfectants and disinfection parameters such as temperature, pH and contact times during advanced water treatment and in distribution systems (WHO, 2017a).

One very specific group of DBPs related to water reuse practices are N-nitrosamines and in particular, N-nitrosodimethylamine (NDMA) (National Research Council, 2012), although previous research has also shown that other nitrosamines can be relevant in reclaimed water such as N-nitrosomorpholine (Schreiber and Mitch, 2006), N-nitrosodienanolamine (Dai et al., 2015), and other tobacco-specific nitrosamines (Wu et al., 2014). NDMA is a DBP formed by the reaction of chloramines (or chlorine in the presence of ammonia) with amines and is classified as "2B carcinogen - reasonably anticipated to be a human carcinogen" by the United States Environmental Protection Agency (USEPA). NDMA is included in the Contaminant Candidate List 4 of the USEPA (2016). California's Department of Public Health has established a 10 ng/L notification level for NDMA in drinking water (California Environmental Protection Agency, 2014), while the California's Office of Environmental Health Hazard Assessment (2006) has defined 3 ng/L as a public health goal. Massachusetts has a guideline value of 10 ng/L (Massachusetts Department of Environmental Protection, 2014), Health Canada (2019) has set a 40 ng/L national guideline value, whereas the Ontario Ministry of the Environment (2003) has a maximum allowable concentration for NDMA of 9 ng/L in drinking water. The World Health Organization (WHO) guideline was established at 100 ng/L for NDMA in drinking water (WHO, 2017b), which was adopted as a target value by Japan and Australia.

The majority of NDMA precursors have low molecular weight, are electron-rich, hydrophilic and contain at least one dimethylamino moiety (Dotson et al., 2009; Krauss et al., 2010). However, the percentage of NDMA formation potential identified in wastewaters by individual NDMA precursors such as pesticides (Chen and Young, 2008), pharmaceuticals (Farré et al., 2016; Shen and Andrews, 2011), cationic polymers and ion exchange resins (Kohut and Andrews, 2003), quaternary amines (Kemper et al., 2010) or amines present in human waste (Mitch et al., 2003b) is low. Therefore, new strategies to investigate

NDMA precursors with more holistic approaches are required. Current approaches to structurally characterize dissolved organic matter (DOM) focusing on NDMA precursors include fluorescence spectroscopy (Roccaro et al., 2020) or high-resolution mass spectrometry (HRMS) (Hani gan et al., 2017). Orbitrap™-MS, although working at lower mass resolution than other established HRMS methods such as high field strength Fourier Transform Ion Cyclotron Resonance (FTICR-MS), has proven suitable to characterize DOM (Hawkes et al., 2016, Patriarca et al., 2018), and to investigate NDMA precursors in natural waters based on fingerprinting (Farré et al., 2019). Farré et al. (2019) study showed that the molecular composition of DOM across the studied drinking water reservoirs correlated with high hydrogen saturation compounds (H/C ≥ 1.5), present in reservoirs with higher background nutrient concentrations and wastewater indicators.

Previous research has also investigated the changes in DOM across wastewater treatment plants (WWTP), advanced wastewater treatment plants (AWTPs) and DWTPs (Gonsior et al., 2014; Lavonen et al., 2015; Phungsai et al., 2016, 2018; Maziel and Remucal, 2017). However, none of them is performed in a full-scale reuse system involving a natural buffer, neither being related to the formation of NDMA.

The present manuscript investigates NDMA and NDMA precursors in a full-scale water reuse trial and addresses three research needs that are: (i) Understand the formation of hazardous transformation products such as NDMA during advanced water treatment for reuse, (ii) develop a better understanding of contaminant attenuation in environmental buffers (attenuation of NDMA in the Llobregat River in Spain), and (iii) identify better indicators and surrogates to monitor process performance in water reuse scenarios, leading to the development of online real-time or near real-time monitoring techniques for their measurement (DOM characterization with Orbitrap™-MS analysis). Although some of these research questions have been investigated individually earlier, this study addresses the challenges related to formation, attenuation, and development of monitoring techniques together in a full-scale reuse trial. The two scenarios investigated (i.e., chlorination vs non chlorination of reclaimed water) generate unique data that will inform utilities about the expected different impacts between practices.

## 2. Materials and methods

### 2.1. Chemical and reagents

NDMA (CAS 62-75-9) standard for gas chromatography mass spectrometry (GC-MS) analysis (5000 µg/mL in methanol, Supelco) had a purity of >99.9%. Deuterated d<sub>6</sub>-NDMA (>98% Cambridge Isotope Laboratories, Inc.) and d<sub>14</sub>-NDPA (N-nitrosodipropylamine, >99% Restek) were used as surrogate and internal standard, respectively. NH<sub>4</sub>Cl (>99.5%, Sigma-Aldrich), NaOH (ACS, ISO, Scharlau) and NaClO (reagent grade, available chlorine ≥4%, Sigma-Aldrich) were used for the NDMA formation potential test. KH<sub>2</sub>PO<sub>4</sub> (>99%, Sigma-Aldrich) and Na<sub>2</sub>HPO<sub>4</sub> (>99%, Sigma-Aldrich) were used to prepare pH buffer solutions. Na<sub>2</sub>SO<sub>3</sub> (>98%, Sigma-Aldrich) was employed to quench residual disinfectant in the NDMA formation potential tests. Commercial DPD test kits (LCK310, Hach Lange) were used for the analysis of free and total chlorine using a Hach DR2800 spectrophotometer. Ultrapure water and methanol (Optima® LC/MS grade) were purchased from Fisher Chemical. Formic acid 98–100% (ACS/Reag. pHEur grade) was acquired from Merck (Darmstadt, Germany). Sulphuric acid 95–97% (Reag. pHEur grade) was obtained from Scharlau. Nitrogen (99.995% purity) for extract drying was purchased from Abelló-Linde. Glass fibre filters (GF/F, 47 mm diameter, 0.7 µm mesh size) were obtained from Whatman. Bond Elut PPL ("Priority Pollutant") cartridges (500 mg, 3 mL; reference 12105006) were purchased from Agilent Technologies. Activated charcoal cartridges (6 mL, 2 g) optimized for NDMA extraction were purchased from Restek.
## 2.2. Description of sampling campaigns

The study was carried out in the lower part of the Llobregat River basin between the towns of Molins de Rei and Sant Joan Despí, located ca. 8.5 km upstream of the inlet of one of the main DWTP that supply water to the city of Barcelona and its metropolitan area. In order to take comparable samples along river, the hydraulic retention time between sampling sites was estimated by using HEC-RAS model (US Army Corps of Engineers, 2016). The ACA, that is in charge of managing water flows and flood risk events in the Llobregat river, use this model based on a steady flow water surface profile through several transects along the river bed on this purpose. The water velocity is calculated by knowing the water discharge, river width and depth profiles, the Manning coefficient (rugosity) and river slope. The hydraulic retention time between sampling sites is estimated knowing water velocity and distance between them. Grab samples were therefore taken accordingly to estimated hydraulic retention time in the individual components of the whole system (including the river), meaning that sampling commenced at 7 a.m. at the WWTP, whereas the last sample, the drinking water produced, was taken at 10 p.m. Table SI1 shows the details of the sampling times that were the same during the 6 sampling events. Samples were sequentially taken at the secondary effluent from the WWTP (site ST, at 7:00 a.m.) and at the tertiary treated effluent from the AWTP El Prat the Llobregat (site TT, at 9:00 a.m.). When chlorine was added to the reclaimed water during the second phase of the trial, the TT sample was taken before chlorine addition. Sample at the point of discharge (pipeWR, at 12:00 p.m.) was taken at the end of a 16 km upstream pipe from the AWTP (residence time approximately 3 h) just before discharge to the river. Samples were also taken in the river 0.2 km upstream of the reclaimed water point of discharge (RpreWR, at 12:00 p.m.) and next to the intake for drinking water production (DWTP_in, at 4:00 p.m.). Finally, a drinking water sample (DWTP_out) was taken at 10:00 p.m. of the same sampling day at the outlet of the DWTP. Fig. 1 shows the sampling points described here.

Sampling was carried out weekly during June and July 2019, during low flow conditions in the Llobregat River. The river flow in the section of the river researched was controlled at around 3–5 m³/s during the trial by managing reservoirs located upstream, whereas discharged reclaimed water was set at 1.5 m³/s in order to achieve a dilution rate of about 0.3–0.5 (reclaimed water/river flow) simulating the worst-case scenario in future severe drought conditions (Table SI2). Six sampling

<figure><img src="image_4.png" /><figcaption>Fig. 1. Sampling diagram. Sampling points are marked in purple. ST corresponds to secondary treated effluent, TT corresponds to tertiary treated effluent, pipeWR corresponds to the treated wastewater discharged to the Llobregat River after 16 km of pipe, RpreWR corresponds to the sample taken from the river prior to treated wastewater discharge, DWTP_in is the intake of the drinking water treatment plant and DWTP_out is the drinking water.</figcaption></figure>

campaigns were carried out (C1–C6). Campaigns 1, 2 and 3 did not employ chlorine to disinfect the reclaimed water prior to pumping to the river discharge point (named hereinafter strategy 1). On the other hand, during sampling campaigns 4, 5 and 6, reclaimed water was disinfected by the addition of sodium hypochlorite at the AWTP (strategy 2) at doses between 10 and 14 mg Cl₂/L, also chosen due to pump capacity limitations, which did not allow adding the dose required to reach break point chlorination (experimentally measured around 30 mg Cl₂/L) and leave free chlorine as residual disinfectant. Therefore, chloramines were generated during reclaimed water disinfection.

To minimize any risk to the population due to unknown NDMA formation, the disinfected reclaimed water during C4 (which was the first campaign that added chlorine to disinfect the reclaimed water) was not pumped upstream and instead discharged to the river at the AWTP, i. e. downstream of the DWTP. Therefore, not all the samples could be taken (DWTP_in and DWTP_out not taken, pipeWR sample taken at the outlet of the AWTP and not 16 km upstream). Campaign C5 and C6 corresponded to a complete sampling camping carried out during strategy 2.

Tables SI3 and SI4 of the Supporting Information summarize the chemical parameters of all samples, including ammonia concentration. A description of the WWTP, AWTP and DWTP part of this study can be found in Text 1 of the Supporting Information. Summarizing, the WWTP consists of a primary settlement, followed by a biological treatment by activated sludge including nitrification and denitrification. The AWTP is comprised of chemical phosphorous removal, sand filtration and water disinfection. The latter is conducted with UV at a dose of 52 mJ/m² and chlorine addition. The DWTP includes settling tanks, peroxidation with chlorine dioxide, chemical clarification, and later disinfection by ozonation followed by activated carbon. Additionally, microfiltration and reverse osmosis are employed to filter part of the treated water (ca. 50%) and blended before final distribution with chlorine.

## 2.3. Sampling and standard chemical analysis

2.5 litre grab samples were taken in amber glass bottles, immediately refrigerated, and shipped to ICRA on the same day. Once received, the samples were kept at 4 °C and filtered through 0.7 µm mesh-size glass fiber filters and then extracted for both NDMA analysis and DOM analysis within 24 h. Samples used to measure actual NDMA in the system were quenched immediately after sampling with sodium sulfite (Sacher et al., 2008). Formation potential tests were started immediately upon reception (less than 24 h after sampling) with unfiltered samples to capture all possible precursors, although previous research showed that low molecular weight compounds (<1 kDa and <2.5 kDa) are the most abundant precursors in wastewater effluents and in surface water, respectively (Chen et al., 2014; Pehlivanoglu-Mantas and Sedlak, 2008; Kristiana et al., 2013).

In all filtered samples the following parameters were also measured: Total organic carbon and total nitrogen (TOC, TN Shimadzu TOC-VCSH analyzer with a TNM-1 unit), ammonium (BUCHI B-324 distiller, Titrino 719S Metrohm), total Kjeldahl nitrogen (BUCHI B-324 distiller, Titrino 719S Metrohm), and finally nitrite (NO₂-N) and nitrate (NO₃-N) nitrogen with ion chromatography (ISC5000 from Dionex). Sodium, potassium, magnesium, calcium, sulfate and chloride ions were also measured with ion chromatography. Results are shown in Tables SI3 and SI4 of the Supporting Information.

## 2.4. NDMA formation potential test, extraction and analysis

NDMA formation potential (FP) was quantified following the protocol published by Mitch et al. (2003a, 2003b). NDMA precursors were quantified by subtracting the actual NDMA concentration measured at the time of sampling to the NDMA formation potential. For the NDMA formation potential test, 10 mM phosphate buffered samples were disinfected with a 140 mg/L freshly made monochloramine concentration
and kept in the dark and at ambient conditions (T = 21 ± 1 °C) for seven days. After this contact time, chloramines were quenched with 2.5 g/L of sodium sulfite and filtered prior to NDMA extraction for analysis by GC-MS/MS in duplicate (Sacher et al., 2008). The solid phase extraction method used to concentrate NDMA is based on USEPA Method 521 (Munch and Bassett, 2004), described elsewhere (Farré et al., 2011a) and summarized in Text 2 of the Supporting Information.

## 2.5. DOM extraction and analysis

For characterization of DOM with high resolution mass spectrometry aliquots of each sample were filtered and extracted by SPE according to the method described elsewhere (Dittmar and Koch, 2006) with minor modifications, as previously implemented in Farré et al. (2019) and Sanchís et al. (2020). The extraction process has been described in detail in Text 3 of the Supporting Information.

For the characterization of DOM, an Orbitrap LTQVelos (Thermo Scientific) hosted at ICRA was used. The Orbitrap™ equipment was externally calibrated for mass accuracy on the day of analysis using the manufacturer's guidelines and negative calibration solution. DOM was analyzed by direct infusion adapting the methodology employed elsewhere (Hawkes et al., 2016; Patriarca et al., 2018). Farré et al. (2019) compared the results obtained employing direct infusion to the OrbitrapTM-MS to the results obtained with liquid chromatography. The same conclusions were reached with both methods, but the computational analysis was considerably reduced using direct infusion. Therefore, we selected the direct infusion method for the present study. Moreover, despite the fact that matrix effects can be increased with this methodology, direct injection also allows for the analysis of compounds that are not retained by the chromatographic column, resulting in peaks with non-gaussian shapes, which are not always easy to detect, deconvolute or interpret, compounds that are too large to elute with conventional liquid chromatography techniques, or compounds that are adsorbed onto the pre-column or column.

Electrospray ionization (ESI) in negative mode was selected as ionization source (Reemtsma, 2009). This choice was made because the resolution of this mass spectrometer allowed for peaks with width at half height of 3–4 mDa (m/z 401), and the split between NaH and C₂ is only 2.4 mDa, making these peaks coalesce. The ion optics and ESI source settings were tuned to maximize the intensity of the peak at m/z 397.1141 in low-resolution linear ion trap mode. The resulting settings were as follows: spray voltage: -3.1 kV; sheath gas flow rate: 28 a.u.; capillary temperature: 275 °C; S-lens RF level: 69%; syringe pump flow rate: 8 µl min⁻¹. Each scan was internally calibrated using lock mass mode in the Xcalibur software package (Thermo Electron Corporation) with a list of compounds commonly found in DOM. Complete details of the instrumental tuning and analytical methodology can be found elsewhere (Farré et al., 2019).

Each sample mass list obtained with the HRMS was individually processed to remove noise peaks, which occurred below a mass-dependent detection limit. This variable detection limit was calculated in Excel by considering the first 95 percentile of peaks with a mass defect between 0.4 and 0.8 Da (no organic formulae can fall in this range within our mass window according to Riedel and Dittmar, 2014), and calculating the linear dependence of their intensity with m/z. Formula assignment was done with an R routine allowing formulas up to C₄–₄₀H₈₀O₄₀NS with and without one ¹³C. Formulas were allowed according to the following rules: O/C < 1, H/C 0.3–2.5, m/z 200–800, double bond equivalents minus oxygen (DBE-O) > -10 or DBE-O < 10. We kept only those masses present in the six samples across the dataset. Signal intensities were normalized to the sum of all signals after also removing the peaks detected in the blank. The whole data set was processed through R 3.6.3 (Team, 2017) as described elsewhere (Farré et al., 2019; Hawkes et al., 2016).

Many different chemical class divisions using the characteristic clustering of DOM data on van Krevelen diagrams exist (Kellerman et al.,

2014; Rivas-Ubach et al., 2018; Sleighter and Hatcher, 2008). In this study, we used cluster compounds in the van Krevelen diagram according to five broad chemical compound classes. These classes are based on O/C, H/C, and the modified aromaticity index AI<sub>mod</sub> (Koch and Dittmar, 2006) and correspond to the ones used in the international laboratory comparison in Hawkes et al. (2020). The AI<sub>mod</sub> was calculated as:

$$AI_{\text{mod}} = (1 + C - 0.5O - S - 0.5(N + P + H)) / (C - 0.5O - S - N - P)(1)$$

The classes chosen were “aliphatic” (H/C ≥ 1.5), “low O unsaturated” (H/C < 1.5, AI<sub>mod</sub> < 0.5, O/C < 0.5), “high O unsaturated” (H/C < 1.5, AI<sub>mod</sub> ≥ 0.5, O/C ≥ 0.5), “aromatics” (0.5 < AI<sub>mod</sub> < 0.67) and “condensed aromatics” (AI<sub>mod</sub> ≥ 0.67). These classes represent broad compositional groups from HRMS data projected on van Krevelen diagrams. They highlight key chemical differences between the samples and demonstrate the inter-instrument variability in interpreting these results. Since an individual formula could occur as multiple isomeric structures, these classifications only serve as a guide to the structures present within DOM. Additionally, separately for the two groups of samples derived from the six campaigns with and without chlorination of the tertiary effluent, we obtained the mean relative change of each compound between two sites along the sampling system (i.e. (Intensity site<sub>n</sub> – Intensity site<sub>n-1</sub>)/ Intensity site<sub>n-1</sub>).

The statistical software R was also used to assess differences across samples and fractions using Bray-Curtis dissimilarities (vegdist function), where signal intensity is compared between each pair of samples for each molecular mass. The dissimilarities were compiled into a matrix and used as the input for a principle coordinates analysis (PCoA). PCoA (cmdscale function) was used to investigate the variability of DOM constituents across samples. The relationship between the scores of the PCoA with NDMA and other environmental factors, was done using Pearson correlations. Spearman rank correlations between individual DOM constituents and the PCoA canonical axes and NDMA formation potential corrected by multiple comparisons (function p-adjust) were investigated and represented in the Van Krevelen diagrams. Vegan package for R was used for statistical multivariate analysis (Oksanen, 2015).

# 3. Results and discussion

## 3.1. NDMA measurement

Fig. 2 shows the NDMA concentration measured at the different sampling points. The NDMA concentration in the secondary treatment effluent ranged between 20 and 40 ng/L, except for sampling campaign C4, for which the concentration was 72.1 ± 4.1 ng/L. The tertiary treatment, which included UV disinfection at a dose of 52 mJ/m², did not significantly affect the concentration of NDMA since the reclaimed effluent from the AWTP decreased only slightly compared to the measured concentration in the secondary effluent (average 8% NDMA removal). Despite direct photolysis of NDMA being recommended as standard treatment technology, the UV doses required to photolyze NDMA to a large degree are high. Sharpless and Linden (2003) reported a photolysis constant of 2.32 × 10⁻³ cm²/mJ for 253.7 nm photons emitted by low pressure lamps. At a dose of 52 mJ/cm² as applied in the UV disinfection in the AWTP, this corresponds to a theoretical NDMA degradation of 11%.

Yoon et al. (2013) reported NDMA concentrations measured in treated wastewater effluents from less than 10 ng/L to 73 ng/L, depending on the treatment used in Japan. This agrees with our results and the first data published by Sedlak et al. (2005) who reported average concentrations of NDMA in secondary effluent before disinfection of 46 ng/L investigating different WWTPs in the USA. Concentrations measured in another USA study found values between <2–320 ng/L in secondary and tertiary treated effluents (Zeng et al., 2016). In contrast,
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** NDMA (ng/L) measured at the different sampling points during different sampling campaigns (C1-C6). ST: secondary effluent, TT: tertiary effluent, pipeWR: corresponds to the treated wastewater discharged to the Llobregat River after 16 km of pipe, RpreWR: river water before treated wastewater discharge, DWTP_in: inlet at the DWTP, DWTP_out: outlet at the DWTP. Error bars correspond to the mean deviation of a duplicate measurement. In C4 samples for DWTP_in and DWTP_out were not taken and pipe_WR sample was taken after chlorination in the TT sampling point and not after 16 km of pipe.</figcaption></figure>

the values were higher and up to 1161 ng/L in a secondary effluent from WWTPs of Shanghai (Wang et al., 2014), which may have been caused by high NDMA concentration at the inlet of the WWTP from industrial wastewater discharge.

A recent study suggested the formation of an NDMA artefact during analysis of secondary effluents if nitrite was present in solution (Chuang et al., 2019). The authors concluded that NDMA formation resulted from the reaction of nitrite and secondary amines catalyzed by the activated carbon SPE cartridges employed by the extraction method. However, the concentration of both is typically low in secondary effluents (Gerecke and Sedlak, 2003; Mitch and Sedlak, 2004) and therefore the potential for NDMA formation during the analysis of wastewaters is low (Chuang et al., 2019). In the present study there was no relation between NDMA and nitrite measurements in samples from secondary and tertiary treatment (Fig. SI1 of the Supporting Information). Also, the lowest NDMA concentration was measured when the concentration of nitrite was the highest. Therefore, we conclude that this effect has not impacted our results in a statistically significant manner.

The concentration of NDMA in sample pipeWR, which was taken after a hydraulic retention time of 3 h approximately, remained constant in comparison to TT samples during campaigns C1, C2 and C3. In contrast, NDMA concentration increased during sampling campaigns C5 and C6 because of chlorine addition to the reclaimed water. Ammonia concentration in TT sample varied from 0.4 to 3 mg/L and TOC varied from 8 to 12 mg/L (Tables SI3 and SI4). Chlorine was added just after to the tertiary effluent at doses between 10 and 14 mgCl₂/L, which was not enough to overcome the dose required to reach break point chlorination (experimentally measured around 30 mgCl₂/L) and leave free chlorine as residual disinfectant. Therefore, chloramines were generated after the tertiary effluent which are responsible for NDMA formation (Mitch and Sedlak, 2002). As seen in Fig. 2, a high increase in NDMA formation after chlorine addition was observed during campaign C5, reaching the maximum concentration measured in the study of 138.5 ± 2.1 ng/L (230% increase when comparing sample TT and pipeWR). In C6, an increase was also observed although in this occasion the NDMA concentration measured was 74.2 ± 3.2 ng/L (137% increase when comparing sample TT and pipeWR). We did not observe an increase in NDMA formation in pipeWR sample after chlorination during sampling campaign C4, which is explained by the lower hydraulic retention time between chlorination and the sampling site on this specific campaign. As

we already explained, campaign C4 (which was the first campaign that added chlorine to disinfect the reclaimed water) was performed considering to minimize any risk to the population due to the unknown possible NDMA formation, and water from tertiary treatment was not boosted upstream in order to avoid the intake of the DWTP. Therefore, the tertiary effluent only was few minutes in contact with chlorine before sample collection and disinfectant quenching in C4 campaign, which did not allow enough time to form NDMA.

NDMA concentration in the river before treated wastewater discharge was always below 4 ng/L. NDMA values measured at the inlet of the DWTP (DWTP_in) were lower than 3 ng/L during C1, C2 and C3 and lower than 14.5 ng/L during the campaigns in which the reclaimed water was chlorinated. Although high concentrations of NDMA were discharged, especially during strategy 2, when chlorine was used to disinfect the reclaimed water, the concentrations measured at the intake of the DWTP (ca 8.5 km downstream of the river, DWTP_in) were 90% lower than the values measured at the discharge sampling point (pipeWR). This reduction was probably due to the environmental dilution and photolysis in roughly equal parts. Photolysis is one of the main mechanisms for NDMA degradation in shallow rivers (Plumlee and Reinhard, 2007) and applied to remove NDMA in water (Stefan and Bolton, 2002). Biodegradation is another possible, albeit slower, mechanism for the natural abatement of NDMA (Qiu et al., 2019; Wijekoon et al., 2013). The average daily accumulated solar radiation in Barcelona on a horizontal surface in the summer months ranged between 7.4 and 7.6 kWh/m², which is an extremely high value. Hence, direct photolysis in the shallow river can be expected to occur as an important degradation mechanism. In our case, photolysis can be expected to be around 60% given the residence time, water depth and solar irradiance based on the photolysis data published by Plumlee and Reinhard (2007). Also, an average 30% dilution could be quantified by establishing mass balances of chloride and sodium ions in samples pipeWR, RpreWR and DWTP_in (Tables SI3 and SI4). These values matched with the calculated dilution based on flow measurements (Table SI2). Therefore, the average 90% NDMA degradation in the river, can be entirely explained by these two factors. It would be interesting to evaluate NDMA decay in the river during the night or the winter months when the average daily accumulated solar radiation falls to 1.9 kWh/m² values (Adrase, 2020). In fact, very recently Zhao and co-authors already showed that NDMA could persist a long time and distance in winter and especially during the night (Zhao et al., 2020). It has been shown that hydrolysis, volatilization, and sorption do not play a relevant role in NDMA degradation (Jin et al., 2012; Qiu et al., 2019). NDMA in the final drinking water (DWTP_out) was generally below 2 ng/L, except for C5 and C6 where a concentration of 7.3 ± 2.4 ng/L and 3.8 ± 0.2 ng/L was measured, respectively.

## 3.2. NDMA formation potential measurement

Fig. 3 shows the concentration of NDMA precursors for the different samples taken in the study. The concentration of NDMA precursors corresponds to the NDMA formation potential subtracting the actual NDMA concentration measured at the time of sampling. The tertiary treatment employed did not significantly remove nitrogen while it effectively removed phosphate (Tables SI3 and SI4). Nitrogen is already removed during the secondary treatment by activated sludge treatment and anoxic denitrification, whereas phosphate concentrations decrease through the tertiary treatment due to chemical coagulation/flocculation followed by microfiltration (45 mg PAX/L treated water, 18% aluminium polychloride in PAX). Therefore, and as expected, tertiary treatment did not significantly reduce NDMA precursors (Farré et al., 2011a, 2011b; Mitch and Sedlak, 2004). Concentrations of NDMA precursors at the point of treated wastewater discharge (pipeWR) ranged between 200 and 706 ng/L. Other studies have noted that WWTPs performing nutrient removal are able to significantly reduce the total nitrogen content of the treated water and therefore have a lower
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** NDMA precursors (ng/L) measured at the different sampling points during different sampling campaigns (C1–C5). ST: secondary effluent, TT: tertiary effluent, pipeWR: corresponds to the treated wastewater discharged to the Llobregat River after 16 km of pipe, RpreWR: river water before treated wastewater discharge, DWTP_in: inlet at the DWTP, DWTP_out: outlet at the DWTP. Error bars correspond to the mean deviation of a duplicate measurement. In C4 samples for DWTP_in and DWTP_out were not taken and pipe_WR sample was taken after chlorination in the TT sampling point and not after 16 km of pipe.</figcaption></figure>

concentration of nitrosamines precursors in their effluents compared to non-nitrifying WWTPs (Gerrity et al., 2015; Mamo et al., 2016; Sgroi et al., 2016; Zeng and Mitch, 2016). However, we could not entirely evaluate this effect in this study because samples before secondary treatment were not taken and therefore the calculation of the net NDMA precursors removal by the secondary treatment was not possible.

Interestingly, although the NDMA concentration at the Llobregat River before discharge (RpreWR) was negligible, the concentration of NDMA precursors ranged between 90 and 270 ng/L, which may be due to the *de-facto* potable reuse already performed in that river (Mujeriego et al., 2017). Similar concentrations were also found in rivers that were also impacted by wastewater discharges such as the Tokyo River where concentrations of NDMA precursors between 11 and 185 ng/L were measured (Van Huy et al., 2011). Subsequently, the concentration at the intake of the DWTP (DWTP_in) was relatively similar to the concentration measured before reclaimed water discharge showing the buffer capacity of the river to reduce NDMA precursors (range 110–280 ng/L). Natural attenuation mechanisms to reduce NDMA precursors involve biotic and abiotic processes such as hydrolysis, photodegradation, irreversible sorption and redox reactions (Lin et al., 2014). Woods and Dickenson (2016) also investigated the decay of NDMA precursors in Las Vegas Wash and provided evidence that NDMA precursors derived from wastewater effluent could be significantly removed from natural water systems over a relatively short period of time, and that longer residence times in the natural environment, prior to introduction in DWTPs, could help to reduce NDMA in finished drinking water. They also concluded that photolysis and especially biodegradation were the main mechanisms for NDMA precursor removal in natural environments (Woods and Dickenson, 2016). Chen et al. (2009) found that NDMA formation potential degraded by ~50% at a location in the river 23 km downstream of the WWTP, while conservative parameters (i.e., conductivity, chloride, sulfate) remained the same, indicating no dilution effects. In our study, an average dilution factor of 3 was estimated based on the concentration measurement of conservative ions such as chloride and sodium of the pipeWR, RpreWR and DWTP_in samples. However, dilution alone was not able to explain the attenuation of NDMA precursors in the river between sampling points.

Despite the relatively high concentration of NDMA precursors at the inlet of the DWTP named DWTP_in (i.e., (110–280 ng/L), the concentration of NDMA precursors in the drinking water was below 12.5 ng/L. Sant Joan Despí is one of the most advanced DWTP in Europe that apart

from conventional treatment with ozonation followed by activated carbon filtration also treats part of the water with microfiltration followed by reverse osmosis. Both streams are blended before final disinfection with chlorine. We recently investigated the fate of NDMA and NDMA precursors at 5 DWTPs of central Spain and found that the concentration of NDMA precursors in the treated water could be as high as 24 ng/L (Farré et al. 2020). However, the actual NDMA concentration measured in the finished drinking water prior to distribution and in the distribution systems were never above 4.2 ± 0.2 ng/L, evidencing the conservative nature of the NDMA formation potential test that overestimates NDMA formation in the 7 days compared to real distribution systems. This can be expected due to the higher concentration of chloramines used in the test (Linge et al. 2017). Similar to these results, the NDMA measured at the outlet of the DWTP was below 2 ng/L, except for C5 and C6 where 7.3 ± 2.4 ng/L and 3.8 ± 0.2 ng/L were measured, respectively. Krasner et al. (2018) found similar results, especially when investigating DWTPs including ozonation, as is the case for the Sant Joan Despí DWTP. Also, although the concentration of NDMA formation potential at the inlet of the DWTP named DWTP_in was relatively high (110–280 ng/L), NDMA concentration did not increase from the inlet to the outlet of the plant. In particular, the average net removal of NDMA across the DWTP was 40 ± 20% (NDMA values measured in the drinking water between 1.1 and 7.3 ng/L). The individual percentages of NDMA removal at the DWTP were 57, 31.5, 13.0, 50.1, 53.4% for C1, C2, C3, C4, C5 and C6, respectively.

### 3.3. Dissolved organic matter characterization with high resolution mass spectrometry

DOM fingerprinting based on high resolution mass spectrometry data was investigated also to further evaluate if there was a group of compounds correlating with NDMA precursors, as previously suggested (Farré et al., 2019). In an initial step, a Bray-Curtis dissimilarity test showed pair-wise differences between the samples (Fig. 4). Samples from the secondary treatment (ST), tertiary treatment (TT) and reclaimed water right before river discharge (pipeWR) clustered together and within this group, by sampling campaign. Samples from the river prior to the reclaimed water discharge (RpreWR) and from the finished drinking water (DWTP_out) clustered together and were grouped by site and not by sampling campaign. These showed the greatest differences in DOM from the rest of the samples. The reason may be that DWTP barriers remove well the DOM coming from the reclaimed water. DWTP_in, in agreement with its character of being a mixture between RpreWR and pipeWR is located between both groups of samples. We further visualized the data obtained from the Orbitrap by using van Krevelen (VK) diagrams where the element ratio H/C is plotted against the O/C ratio for each determined molecular formula. In the VK diagrams, features can be assigned to compositional spaces (“regions”) that have been traditionally related to certain chemical structures. To assess the change in the molecular signatures between sampling sites, the relative change of each compound between samples was calculated. Fig. 5 plots the relative change of each compound between samples of consecutive sites. The relative frequency distribution plot of the intensity changes across the treatments is shown in Fig. 6. A specific compound was classified as enriched if the relative change was > 0 or as depleted if relative change < 0. In addition, the main molecular features for each site and group of campaigns, are presented in Table 1.

As seen in Table 1, wastewater related samples (ST, TT and pipeWR) contain a high proportion of saturated compounds and present low O/C ratios. They show no remarkable differences between them (Fig. 4) especially during C1–C3 (Fig. 5a). When chlorine was added to TT (i.e., C5), aromatic compounds seem to be lost while unsaturated and aliphatic compounds appear to increase (Fig. 5b). C4, was not represented in Fig. 5 as this sample was taken just after chlorine addition, as explained earlier. The reduction in aromatic compounds, which did not occur during the first campaigns, might be linked to the effect of
<figure><img src="image_7.png" /><figcaption>Fig. 4. Heat map built on Bray-Curtis dissimilarities between the samples studied. Higher values denote more dissimilar samples.</figcaption></figure>

chlorination reactions (Phungsai et al., 2016; Gonsior et al., 2014). When compared with wastewater related samples, RpreWR is highly enriched in high and low oxygen unsaturated formula and aromatics, compounds related with DOM of natural sources, like terrestrial vegetation, that seem to be depleted in both groups of campaigns at the DWTP input. It is important to note that RpreWR presents a higher number of compounds, in agreement with a higher diversity of natural DOM sources, but that the sum of intensities for each compound group is much lower than in wastewater related samples (Table 1). The dilution effect of the river is clear when we look to the fate of aliphatic compounds from pipeWR. These compounds appear to be depleted in DWTP_in due to the mixing with the natural DOM in RpreWR, which contains increased aliphatic and oxygen unsaturated compounds as well as aromatics. Interestingly, the only enriched category in the final drinking water (DWTP_out) are the high oxygen unsaturated compounds in agreement with the effects of DWTP treatments reported in Lavonen et al. (2015) and Phungsai et al. (2016) evidencing the effect of the conventional and advanced treatment to remove aliphatic and aromatic compounds, but also the production of more oxidized molecules (high O/C) due to ozonation.

The variability of the DOM constituents plotted in the VK diagrams was further investigated across samples using principal coordinates

analysis (PCoA). PCoA is an unconstrained ordination method that builds the multi-dimensional space based on linear relationship between the dissimilarities among samples, allowing us to unravel relationships between our samples as a function of their DOM constituents. PCoA including the different DOM samples resulted in a first principal coordinate, the axis of major compositional variation of DOM, explained 81% of the variability, separating again DWTP_out and RpreWR from the rest (Fig. SI2). That axis is related on its positive side with NDMA, NDMA formation potential and other environmental variables, thus, indicating that DOM in samples ST, TT and pipeWR is linked to higher NDMA formation potential. In a previous study, Farré et al. (2019) investigated the relation between HRMS fingerprints of natural DOM and the potential of NDMA precursors. It was concluded that NDMA was preferentially related with compounds with high hydrogen saturation (H/C ≥ 1.5), corresponding also to reservoirs with higher background nutrient concentrations and wastewater indicators. In agreement with this, these samples presented higher saturated character (see H/C and DBE in Table 1) as well as higher intensity in the aliphatic group of compounds in both groups of campaigns. Additionally, Spearman rank correlation was used to investigate the relation between each of the individual formulae, the first axis of the PCoA and, the NDMA formation potentials of unfiltered samples as the majority of the precursors in this
<figure><img src="image_8.png" /><figcaption>Fig. 5. Mean relative change of each compound between two sites (i.e. (Intensity site_n – Intensity site_{n-1})/ Intensity site_{n-1}) for campaigns C1–C3 (mean of the three campaigns) (a) and C5 (b). The number of compounds enriched (mean relative change >0) or depleted (mean relative change <0) is indicated in each panel. Compound category labels for delineation in right-most panels also apply to delineated regions in the rest of the panels.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 6. Relative frequency and relative cumulative frequency distribution of the intensity changes between two sites (i.e. (Intensity site_n – Intensity site_{n-1})/Intensity site_{n-1}) (a) for campaigns C1–C3 and (b) for campaign C5.</figcaption></figure>

type of samples are expected to be <2.5 kDa (Chen et al., 2014; Pehlivanoglu-Mantas and Sedlak, 2008; Kristiana et al., 2013). Results are plotted in a VK diagram (Figs. SI3a and 7, respectively). Both figures show that the compounds that appear positively correlated with the first axis and NDMA formation potential respectively, correspond to compounds with a H/C > 1.5 (ca. aliphatic) in agreement with previous results reported (Farré et al., 2019), but also with low O unsaturated compounds. These regions where NDMA precursors may be present deserve further attention.

## 4. Conclusions

This study investigates NDMA and NDMA precursors measured at different sampling points of a water reuse full-scale pilot trial in the Llobregat River carried out during the summer of 2019. Two different strategies were investigated (1) no disinfection of the reclaimed tertiary treated effluent before discharge into the river, and (2) addition of chlorine before the transport to the upstream discharge point without reaching break point chlorination. The following conclusions can be
<caption>**Table 1**</caption>

Molecular signatures of compounds present in each site for the two groups of campaigns (without chlorination: C1 to C3 and with chlorination: C4–C5. C4 was sampled just after chlorine addition and was not subjected to 3 h contact time as C5.

<table><thead><tr><th rowspan="2">Campaign</th><th colspan="2">ST</th><th colspan="2">TT</th><th colspan="3">pipeWR</th><th colspan="2">RpreWR</th><th colspan="2">DWTP_in</th><th colspan="2">DWTP_out</th></tr><tr><th>C1–C3</th><th>C4–C5</th><th>C1–C3</th><th>C4–C5</th><th>C1–C3</th><th>C4</th><th>C5</th><th>C1–C3</th><th>C4–C5</th><th>C1–C3</th><th>C5</th><th>C1–C3</th><th>C5</th></tr></thead><tbody><tr><td>Aliphatic</td><td>548<br>(1.27)</td><td>562<br>(1.45)</td><td>557<br>(1.42)</td><td>499<br>(1.62)</td><td>526<br>(1.41)</td><td>416<br>(2.22)</td><td>360<br>(2.46)</td><td>652<br>(0.969)</td><td>487<br>(1.6)</td><td>561<br>(1.16)</td><td>441<br>(0.919)</td><td>615<br>(0.923)</td><td>491<br>(1.24)</td></tr><tr><td>Aromatics</td><td>314<br>(2.04)</td><td>389<br>(1.85)</td><td>312<br>(2.26)</td><td>326<br>(2.07)</td><td>289<br>(2.39)</td><td>265<br>(2.48)</td><td>188<br>(2.57)</td><td>610<br>(0.895)</td><td>494<br>(1.29)</td><td>420<br>(1.44)</td><td>341<br>(0.807)</td><td>422<br>(0.447)</td><td>296<br>(0.736)</td></tr><tr><td>Condensed<br>Aromatics</td><td>120<br>(2.31)</td><td>164<br>(0.661)</td><td>105<br>(3.14)</td><td>114<br>(0.665)</td><td>100<br>(2.53)</td><td>90<br>(0.906)</td><td>60<br>(2.39)</td><td>304<br>(0.452)</td><td>213<br>(0.576)</td><td>186<br>(0.948)</td><td>143<br>(0.289)</td><td>180<br>(0.134)</td><td>128<br>(0.165)</td></tr><tr><td>HighO<br>Unsaturated</td><td>1192<br>(2.53)</td><td>1302<br>(2.72)</td><td>1140<br>(2.6)</td><td>1206<br>(2.87)</td><td>1121<br>(2.85)</td><td>1002<br>(3.12)</td><td>936<br>(3.30)</td><td>2415<br>(1.94)</td><td>2075<br>(2.44)</td><td>1803<br>(2.05)</td><td>1801<br>(1.35)</td><td>2367<br>(2.77)</td><td>2032<br>(3.3)</td></tr><tr><td>LowO<br>Unsaturated</td><td>992<br>(5.27)</td><td>1146<br>(4.15)</td><td>980<br>(5.19)</td><td>986<br>(4.93)</td><td>959<br>(5.21)</td><td>815<br>(6.23)</td><td>637<br>(8.25)</td><td>1792<br>(2.22)</td><td>1435<br>(2.3)</td><td>1367<br>(3.49)</td><td>994 (2)</td><td>1343<br>(1.99)</td><td>1152<br>(2.12)</td></tr><tr><td>DBE</td><td>7.35</td><td>7.68</td><td>7.31</td><td>7.43</td><td>7.30</td><td>7.28</td><td>6.98</td><td>8.75</td><td>8.58</td><td>8.15</td><td>8.06</td><td>8.52</td><td>8.08</td></tr><tr><td>O/C</td><td>0.49</td><td>0.48</td><td>0.48</td><td>0.49</td><td>0.48</td><td>0.49</td><td>0.51</td><td>0.51</td><td>0.51</td><td>0.50</td><td>0.53</td><td>0.53</td><td>0.53</td></tr><tr><td>H/C</td><td>1.21</td><td>1.19</td><td>1.21</td><td>1.20</td><td>1.21</td><td>1.21</td><td>1.23</td><td>1.14</td><td>1.14</td><td>1.17</td><td>1.17</td><td>1.18</td><td>1.20</td></tr><tr><td>Al<sub>mod</sub></td><td>0.27</td><td>0.28</td><td>0.26</td><td>0.27</td><td>0.26</td><td>0.26</td><td>0.24</td><td>0.30</td><td>0.30</td><td>0.28</td><td>0.27</td><td>0.27</td><td>0.25</td></tr></tbody></table>

Note: Values represent number of compounds (intensity) for each molecular category.

<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Representative van Krevelen diagrams showing all formula classes for that correlate with NDMA FP (red corresponds to high positive correlation) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).</figcaption></figure>

drawn:

- Chemical disinfection with chlorine increased the formation of NDMA in the reclaimed tertiary treated effluent in comparison to the concentration of NDMA measured in the non-disinfected reclaimed water. The disinfection conditions and the ammonia concentration in the effluent did not allow reaching break point chlorination and promoted chloramine formation.

- NDMA concentration was reduced in the river prior to drinking water intake probably due to both dilution and photolysis. It would be interesting to repeat the study at night or during winter months where the UV radiation is lower.

- NDMA precursors in the river before treated wastewater discharge were already significantly high due to the *de-facto* potable reuse performed in that river. Attenuation of NDMA precursors in the river reduced the concentration to the levels found before treated wastewater discharge probably due to both, dilution and biodegradation.

- NDMA net removal at the drinking water treatment plant was 40 ± 20%. As a result, the maximum NDMA concentration measured

in the finished drinking water was 7.3 ± 2.4 ng/L. This was measured in an event when the tertiary treated effluent was chemically disinfected.

- Drinking water treatment heavily reduced the concentration of NDMA precursors due to the advanced technology involved at the DWTP that includes microfiltration and reverse osmosis to treat part of the water. Only in one sample NDMA precursors could be measured above 10 ng/L (specifically 12.3 ± 4 ng/L). More studies would be needed involving the distribution networks to investigate further formation of NDMA in distribution systems.

- Aliphatic compounds were present in wastewater derived samples while river water was enriched in aromatic compounds. High oxygen unsaturated compounds were the only category enriched in the final drinking water probably related with the use of ozone. Non-targeted analysis to reveal the nature of these compounds is warranted.

- Compounds with low O/C, mostly present in the aliphatic and low O unsaturated region of the van Krevelen diagram, and compounds with a H/C > 1.5 (ca. aliphatic) were correlated with NDMA formation potential and deserve further attention to understand the
nature of NDMA precursors and the use of HRMS to predict NDMA
formation.

CRediT authorship contribution statement

**Josep Sanchís:** Methodology, Software, Investigation, Writing - review & editing. **Wolfgang Gernjak:** Conceptualization, Writing - review & editing. **Antoni Munné:** Conceptualization, Writing - review & editing. **Núria Catalan:** Methodology, Software, Writing - review & editing, Visualization. **Mira Petrovic:** Writing - review & editing, Supervision. **María José Farré:** Conceptualization, Methodology, Investigation, Visualization, Writing - original draft, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.

Acknowledgements

This project was funded by the Spanish Ministry of Science, Inno-
vation and Universities, AEI-MICIU, and the European Fund for Regional
Development under the National Program for Research Aimed at the
Challenges of Society, through the project NDMA_Predict
(CTM2017-85335-R). The authors thank Generalitat de Catalunya
through Consolidated Research Groups ICRA_ENV 2017 SGR 1124 and
ICRA_Tech 2017 SGR 1318. ICRA researchers thank funding from the
CERCA program. Maria José Farré acknowledges her Ramón y Cajal
fellowship (RyC-2015-17108), from the AEI-MICIU. Núria Catalan ac-
knowledges the funding received from the European Union's Horizon
2020 research and innovation program under the Marie Skłodowska-
Curie grant agreement No. 839709. The authors would like to thank
the staff of the Scientific and Technical Services of the Catalan Institute
of Water Research for their assistance, especially Sara Insa, the AGBAR
(Aigües de Barcelona) technicians, especially Miquel Paraira (Water
Quality Director & Laboratory Manager), for their help providing sam-
ples from the Drinking Water Treatment Plant, and Mercè Aceves (Area
Metropolitana de Barcelona - AMB) who was in charge of providing
samples from the wastewater treatment plant and tertiary treatment
(reclaimed water).

**Appendix A. Supporting information**

Supplementary data associated with this article can be found in the
online version at doi:10.1016/j.jhazmat.2020.124346.

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