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. 2023 Aug 7;57(47):18765–18774. doi: 10.1021/acs.est.3c00719

Amino Acids as Potential Precursors to Odorous Compounds in Tap Water during Spring Runoff Events

Caley B Craven †, Nicholas J P Wawryk †, Kristin Carroll †, Wendell James ‡, Zengquan Shu ‡, Jeffrey WA Charrois ‡, Steve E Hrudey †, Xing-Fang Li †,*
PMCID: PMC10690712  PMID: 37549310

Abstract

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The onset of spring runoff in northern climates and tap water odor events are difficult to predict because common water quality parameters cannot fully explain the intermittent odor events that occurred over past decades. Studies have shown that small polar water-soluble compounds, such as amino acids (AAs), leach first from ice/snowmelt. AAs are known to produce odorous compounds, such as aldehydes and chloroaldimines, upon chlorination. Therefore, we proposed that AAs may serve as markers for small and soluble organics that contribute to the odor of chlorinated tap water. Here, we studied the occurrence of AAs in source water collected at two water treatment plants and the odor profiles of tap water at >300 homes during the 2021 and 2022 spring runoff events. AA concentrations were at baseline levels (<100 ng/L) during the 2021 runoff but much higher (up to 5500 ng/L) in 2022 and associated with an escalation in odor complaints. AA concentrations peaked at the onset of the 2022 spring runoff and corresponded with the strongest reported odor intensities in tap water. We obtained high resolution MS and MS/MS spectra of chloroaldimines and confirmed the formation of chloroaldimines under chlorination of the six AAs detected in source water. The results indicate that AAs signal the onset of spring runoff and represent small polar water-soluble compounds that may contribute to tap water odor problems.

Keywords: amino acids, water treatment, odor profile, chlorinated amino acids, odor problems

Short abstract

This paper discusses the occurrence of amino acids during spring runoff and their association with the odor profile of home taps over two consecutive years.

Introduction

Water treatment is essential for safe drinking water free of waterborne pathogens to prevent the transmission of waterborne diseases.1,2 However, an unintended consequence of conventional water treatment is the formation of disinfection byproducts (DBPs) from the reactions of natural organic matter (NOM) with disinfectants, commonly chlorine and chloramine.3−9 The most effective way to limit DBP formation is to remove NOM prior to disinfection.10 Although physical filtration and coagulation can remove a large portion (∼65%) of the total amino acids (AAs) (including combined AAs, i.e., peptides), they cannot efficiently remove smaller compounds (<1000 Da).11−13 This is important because naturally occurring AAs, depending on geographical location, can make up a significant portion (possibly as high as 75%) of dissolved organic nitrogen.11,14

In much of the developed world, regulations on finished water quality have meant that drinking water is routinely monitored and consistently meets chemical and microbial maximum contaminant limits.15 However, chlorinous taste and odor are a leading cause of complaints to water utilities.16−22 Most consumers judge the safety of drinking water based on tangible qualities, such as odor, that are easily perceived.23 Odor complaints remain a major challenge for the drinking water utilities, particularly because of the difficulty to establish and resolve the cause.24 Common parameters of source water quality include color, turbidity, pH, dissolved organic carbon (DOC), total organic nitrogen (TON), and ammonia (NH3–N), and are routinely monitored. However, these common parameters are unable to predict the onset of odor events. If water utilities could predict the likelihood of an odorous event based on water quality parameters of source water, they could make timely and targeted changes to the treatment process to help remove precursors prior to disinfection. For example, increasing powdered activated carbon (PAC) doses may remove taste- and odor-causing compounds to prevent, or reduce, odorous events.10,25,26 There is an economic trade-off because PAC dosing can be very expensive.27

To help understand and potentially prevent odorous events, EPCOR (Edmonton’s drinking water provider) has established a unique home analyst program called SHARP (Spring Home Analysis Runoff Program), different from other Flavor Profile Analysis (FPA) methods that attempt to empirically define a characteristic attribute (e.g., flavor, smell) of water. The SHARP program trains over 300 home analysts from across the city to monitor the intensity and odor descriptors in cold and hot tap water samples. Odor descriptors are important because they provide insight into the type of odorous DBPs being formed.17,19−21 For example, geosmin is a well-studied compound that causes musty/earthy odor problems.28 The geographical location impacts the source water and the types of odorous DBPs that can form. EPCOR has experienced intermittent annual complaints, predominantly about chlorinous issues, for over 30 years.29,30 SHARP is especially useful because it provides timely customer feedback to water treatment plant operators so that informed decisions can be made about taste and odor control measures during critical periods of spring runoff. Temperature increases cause snow and ice to melt, creating runoff with increased debris and organic matter that flows into surface water sources.31−33 Most odor complaints in recent years are believed to be attributed to the formation of odorous DBPs arising from precursors that increase during spring runoff. To investigate potential contributors to taste and odor events, we proposed AAs as precursors and as markers of small and water-soluble organics signaling the onset of spring runoff.

AAs may serve as good markers for precursors to odorous DBPs and to indicate the onset of spring runoff, based on the following rationale. Previous studies have demonstrated that laboratory chlorination and chloramination of AAs can produce aldehydes, N-chloroaldimines, and nitriles, which are odor-causing compounds that are often characterized as chlorinous odors.16−18,20,22,34−40 Compounds formed from AAs, particularly phenylalanine (Phe), leucine (Leu), isoleucine (Ile), and valine (Val), have been suggested as sources of odor concerns because their odor thresholds are as low as 150 ng/L.16,22,36,38 Several studies reported that AAs in environmental water occurred at ranges of 500 to 30,000 ng/L.11,13,16,41 Environmental levels of AAs in source water could potentially produce odorous DBPs at levels above the odor threshold during chlorination. For example, previous studies of laboratory chlorination of mg/L of Phe have identified N-chlorophenylacetaldimine with an odor threshold of 3–4 μg/L, and it has been linked with odor events in drinking water.36,42 However, AAs make up a small portion of the total organic matter present in water, and thus a change in AA concentration may not result in changes to common water quality parameters which are used by operators to guide water treatment performance. No study has systematically investigated the occurrence of AAs in source water and odor profiles in home taps, because odor events are unpredictable. Therefore, we systematically determined the occurrence of free AAs in source water at intakes for two water treatment plants, E.L. Smith and Rossdale, as well as the odor acceptability profiles of tap water at over 300 homes during spring runoff in two years (2021 and 2022). Using our established solid phase extraction (SPE) with hydrophilic interaction chromatography and tandem mass spectrometry (HILIC-MS/MS) method,43 we determined the concentrations of 20 AAs in source water samples from the two water treatment plants between February and May in 2021 and 2022, periods that included spring runoff. At >300 homes, tap water odor was measured daily and assigned both an intensity rating and odor descriptor. We compared AA occurrence to changes in commonly tracked water quality parameters and the odor acceptability profile of tap water over both 2021 and 2022 spring runoff periods. Although previous studies reported the formation of chloroaldimines from Phe, Val, Ile, and Leu, accurate mass and MS/MS spectra of these chloroaldimines are not available to date.18,20,35,36 Therefore, we performed laboratory experiments of these AAs and used a high performance liquid chromatography (HPLC) with tandem quadrupole time-of-flight mass spectrometry (HPLC-QTOF-MS) method to obtain accurate MS and MS/MS spectra of chloroaldimines, to confirm the formation of N-chloroaldimines. Further, we developed a targeted HPLC-MS/MS method to evaluate the stability of N-chloroaldimines under various chlorination conditions. This study aimed to determine whether AAs could serve as markers of small water-soluble organics that contribute to the odor of tap water, signaling the onset of problematic spring runoff.

Materials and Methods

Chemicals and Materials

Formic acid (FA, 98%), ammonium formate (AF), sodium hypochlorite (NaOCl, reagent grade, 10–15% available chlorine), ammonium chloride (≥99.5%), polyvinylidene difluoride (PVDF) syringe filters (0.22 and 0.45 μm), and nylon disk filters (0.45 μm) were purchased from Sigma-Aldrich (St. Louis, MO). Optima grade water, Optima grade acetonitrile (ACN), Optima grade methanol (MeOH), aqueous ammonium hydroxide (30 wt %), ammonium chloride, potassium sulfate ACS grade, copper sulfate ACS grade, sulfuric acid reagent grade, sodium phosphate dibasic heptahydrate ACS grade, NaOH ACS grade, ammonium chloride ACS grade, type I reagent water (ASTM D1193), potassium hydrogen phthalate ACS grade, Whatman filter paper #40, and AA standards were purchased from Fisher Scientific (Fair Lawn, NJ). The Oasis MAX cartridges (6 mL, 150 mg) were purchased from Waters (Milford, MA). The platinum–cobalt color standard stock solution was purchased from Anachemia. Cellulose nitrate membrane filters (47 mm, 0.45 μm) were purchased from Thermo Fisher. A StablCal turbidity primary standard kit, containing formazin standard solutions, was purchased from HACH (London, ON, Canada). Salicylate and hypochlorite were purchased from VWR. The ammonia calibration solution (1000 mg NH3/L) was purchased from Lab Chem.

Water Treatment Processes

The water treatment process for both water treatment plants includes the following steps: coagulation (alum with or without PAC), flocculation, clarification, chlorination, filtration, UV disinfection, chloramination, fluoridation, and pH adjustment. Further details on each step of the water treatment process, as well as a schematic of the overall process, can be found in the Supporting Information (Text S1 and Figure S1).

Water Quality Parameters

Source water collected from the intake was routinely measured for common quality parameters at the treatment plants. These water quality parameters included color, turbidity, TON, DOC, NH3–N, total Kjeldahl N (TKN), and specific ultraviolet absorbance (SUVA at 254 nm). The treatment dose of PAC was recorded, and its dosing was dependent on the values of color and turbidity. The treatment dose for alum was recorded, and its dosing was determined by a model taking into account the raw water color, turbidity, and temperature. These water quality parameters were analyzed following the procedures set in the Standard Methods for Examination of Water and Wastewater.44−47 The specific details of the measurements of individual parameters are described in Texts S2–S8 and Table S1.

Sample Collection

Source water samples were collected from two sites along the North Saskatchewan River at the entrance to the E.L. Smith and Rossdale water treatment plants in Edmonton, Alberta, Canada. Samples were collected prior to, during, and after spring runoff over two consecutive years, 2021 and 2022. The 2021 source water samples were collected on March 1, March 4, March 8, March 15, March 17, March 18, and March 19. The 2022 source water samples were collected on February 15, February 23, February 28, March 7, March 14, March 16, March 21, March 24, March 28, and March 31. Differences in sampling dates are due to differences in the starting date of spring runoff, which is defined as the first occurrence when raw water color exceeds 10 TCU at E.L. Smith. Clean 4-L amber glass bottles were used for sample collection and rinsed three times before being filled with no remaining headspace. All source water samples were filtered by 1.5 μm glass microfiber filters and followed by 0.45 μm nylon membrane disks and stored at 4 °C before SPE and HPLC-HILIC-MS analysis. The samples were processed within 24 hr after collection.

SPE of AAs in Source Water

Triplicate 500 mL aliquots of source water samples were extracted by SPE using the same procedures previously reported.43 Briefly, each sample (500 mL) was first prepared with addition of 2 mL of an aqueous ammonium hydroxide solution (30 wt %). Oasis MAX cartridges (6 cc, 150 mg) were preconditioned with methanol (2 mL), followed by an aqueous ammonium hydroxide solution (4 mL, 0.5 wt %). Water samples were then loaded and passed through the MAX cartridge at approximately 1–2 mL/min. Following loading, SPE cartridges were washed with an aqueous ammonium hydroxide solution (2 mL, 0.5 wt %) and then eluted with methanol (10 mL, containing 0.2% FA). The eluate was concentrated to 0.1 mL under a gentle nitrogen stream (<5 psi) (TurboVap LV Concentration Workstation, Caliper Life Sciences, Waltham, MA). The extracted samples were reconstituted with 0.4 mL of ACN to a final volume of 0.5 mL and filtered using 0.25 μm PVDF syringe filters before HILIC-MS/MS analysis. The SPE recoveries are summarized in Table S2.

Targeted HILIC-MS/MS Analysis of AAs

The extracted samples were analyzed for AAs using the same HILIC-MS/MS method previously reported.43 Briefly, an Agilent 1290 series HPLC system (Agilent, Santa Clara, CA) coupled with a triple quadrupole mass spectrometer (Sciex 5500; Sciex, Framingham, MA) was used for HILIC separation and MS/MS targeted analysis of AAs. The column was an InfinityLab Poroshell 120 HILIC-Z column (2.7 μm × 100 mm × 2.1 mm ID) (Agilent). The specific parameters for the HILIC separation including mobile phase, gradient program, autosampler conditions, and column temperature are described in Table S3. The MS parameters were first optimized using direct injection of the standard solutions and then optimized again using the HILIC-MS/MS method. The optimized MS ionization parameters are summarized in Table S4, and the optimized MRM transitions and parameters are given in Table S5. In this study, standards of 20 AAs were used for method development. Table S6 presents the limit of detection (LOD), limit of quantification (LOQ), linear range, and relative standard deviation (RSD) of signals and retention times of the SPE-HILIC-MS/MS method for analysis of 20 AA standards (n = 3).

HPLC-QTOF-MS Nontargeted Analysis of N-Chloroaldimines from AAs

A high resolution quadrupole time-of-flight mass spectrometer (QTOF-MS) (Sciex x500R) was coupled with an Agilent 1290 series HPLC system with a Luna C18(2) column (100 × 2.0 mm i.d., 3 μm particle size; Phenomenex, Torrance, CA) for nontargeted analysis of the AA chlorination reaction solutions. The Supporting Information provides the details of the conditions for the HPLC separation (Table S7) and the QTOF-MS with the information dependent acquisition (IDA) mode (Table S8), including instrument parameters, system control, data collection, and data analysis (Text S9).

HPLC-MS/MS Targeted Analysis of N-Chloroaldimines

A triple quadrupole mass spectrometer (Sciex 5500) was coupled with an Agilent 1290 series HPLC system with a Luna C18(2) column (100 × 2.0 mm inner diameter, 3 μm particle size; Phenomenex) for targeted analysis of the AA chlorination reaction solutions. The Supporting Information provides the conditions for the HPLC separation (Table S9), optimized MS ionization parameters (Table S10), and optimized MRM transitions and parameters (Table S11).

Chlorination of AAs

The six most abundant AAs (Phe, Thr, Leu, Ile, tyrosine (Tyr), and tryptophan (Trp)) detected in the 2022 samples were subjected to simulated chlorination experiments. Val was also chlorinated under the same conditions, because it is known to produce odorous DBPs. First, each individual AA (1 mM) solution (2 mL) was chlorinated at a molar ratio (2.4:1) of NaOCl to AA (2.4 mM as Cl2). The reaction mixtures were allowed to react for 20 min. The chlorine dose was chosen to produce the highest yield of chloramine according to a study by Freuze et al.20 The reaction samples were directly analyzed using the HPLC-QTOF-MS nontargeted analysis method. Experimental controls of AAs in ultrapure water were analyzed by using the same procedure. The concentration of free chlorine as Cl2 in the NaOCl solution was determined with a chlorine amperometric titrator (Autocat 9000; HACH).

Next, we studied N-chloroaldimines formed from Phe, Leu, Ile, and Val. Individual solutions of Phe (10 μM), Leu (100 μM), Ile (100 μM), and Val (100 μM) were used to detect a sufficient signal during analysis without preconcentration. First, Phe, Leu, Ile, and Val were chlorinated under various NaOCl:AA molar ratios (2.4:1, 100:1, and 1000:1). Second, we examined their stability in a simulated distribution system, following laboratory chlorination reactions. Individual AAs were prepared in the source water. An appropriate chlorine dose was added to each sample to reach a chlorine residual of 2–3 mg/L (free chlorine, as Cl2). After 20 min of free chlorine contact time, ammonium chloride was added to the reaction solutions (0.7 Cl/N molar ratio) to form chloramines.48 Samples were directly analyzed at various time points before and after the addition of ammonium chloride using the HPLC-MS/MS targeted method. All of the reaction solutions were prepared in amber HPLC vials (2 mL) without any headspace and stored at 4 °C during analysis.

Satisfaction Rating (%) Program for Odor of Tap Water

An independent consultant, hired by EPCOR, conducted SHARP to monitor customers’ odor intensity ratings in home taps. More than 300 home analysts were trained to analyze their home tap water, both cold and hot. Intensity ratings of odor were assigned: 0 – No odor detected; 0.25 – Trace odor (difficult to identify); 0.5 – Very slight odor (identifiable, not objectionable); 1.0 – Slight but definite odor (slightly objectionable); 1.5 – Slight to moderate odor (somewhat objectionable); 2.0 – Moderate, very noticeable odor (objectionable); 2.5 – Very strong odor (strongly objectionable); 3.0 – Severe odor (so objectionable that the water is deemed undrinkable). The odor descriptor is assigned if the intensity is 0.5 or higher. The odor descriptor was recorded: A – musty, earthy, moldy; B – chlorine, bleach; C – other. Each home analyst analyzed their home tap water and assigned a rating and an odor descriptors. The home analysts reported online the daily values, and the average from all participants was used for a daily report. Customer acceptance, or % satisfaction rating, was determined by the percent of responses at or below 0.5 – very slight odor (identifiable, not objectionable). Any value above 0.5 – very slight odor (identifiable, not objectionable) was determined to be unsatisfactory by home analysts. Because home analysts were a large and diverse group representing all areas of the distribution system, the percentage reporting unsatisfactory odor intensity ratings for cold and hot water samples was used as an overall indicator of tap water odor. Alternatively, the percentage reporting satisfactory ratings was used as an indicator of customer satisfaction. Further details on the satisfaction rating program and the number of responses assigned to each intensity rating of odor are described in Figure S2.

Results and Discussion

Odor Profile of Tap Water and Quality Parameters of Source Water

SHARP consisted of more than 300 home analysts in 2021 and 2022. Although FPA at water treatment plants is common, having a city-wide tap water FPA program, with daily online reporting to track customer satisfaction, is unique. The average values of daily ratings provided by more than 300 home analysts account for variations in home analyst odor sensitivity. Figure 1 shows the average satisfaction ratings (%) of cold and hot water at home taps of more than 300 home analysts during the 2021 and 2022 spring runoff periods, which were calculated from the individual home rating values (Figure S2). As shown in Figure 1, the home analyst satisfaction rating of tap water remained consistently close to 95% in 2021. Comparatively, the home analysts’ satisfaction rating showed a major decline during 2022 spring runoff. The odor descriptor reported by home analysts was predominantly chlorine or bleach. Figure 1 shows a satisfaction rating near 95% between February 22 and March 18 before spring runoff, followed by an odor peak (total odor ratings shown in Figure S2), between March 20 and 26, correlating to a dip in home analyst satisfaction to ∼67% on March 23. To explain the occurrence of the odor event, we first examined the water quality parameters measured at E.L. Smith.

Figure 1.

Figure 1

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Home analyst % satisfaction rating for hot and cold tap water in 2021 and 2022.

Figure 2 shows the water quality parameters tracked over the 2022 spring runoff at E.L. Smith and the home analyst % satisfaction rating at home taps. As shown in Figure 2A, the initial baseline of water quality parameters, along with alum and PAC dose, remained nearly consistent from March 16 to March 18, followed by large changes in the water quality parameters from March 19 to April 12. PAC and alum were added to remove color and taste- and odor-causing compounds, as well as turbidity in source water. The doses of PAC and alum were added based on changes in raw water color and turbidity. Thus, the majority of odor-causing compounds was expected to be removed with increasing doses of PAC. However, without identifying the agents responsible for odor, it is not possible to optimize treatments such as PAC for removal of the responsible agents. Thus, tap water odor problems continuously present a challenge to the oxidative water treatments, including chlorination and chloramination.

Figure 2.

Figure 2

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Common water quality parameters of source water measured at E.L. Smith over the 2022 spring runoff compared to the home analysts’ % satisfaction measured for cold and hot tap water.

Figure 2A shows the lowest % satisfaction rating of odorous tap water on March 23 after the initial small increase in water quality parameters (March 22) and the corresponding increase in alum and PAC doses. It is important to note that water quality parameters were measured in source water before any treatment at the water treatment plants. It takes an estimated 1–3 days for source water to undergo the entire treatment process, travel through the distribution system, and reach home taps. The satisfaction ratings were performed at home taps. It is interesting to note that the minimum % satisfaction rating occurred before the peaks in water quality parameters. Furthermore, as shown in Figure 2B, the peaks of DOC and TON in E.L. Smith source water occurred after the tap water home analyst % satisfaction rating bottomed, indicating that these parameters are not leading predictors for odor events. Generally, the common water quality parameters measured at water treatment plants cannot explain the specific agents responsible for odor events at home taps.

We suspected that small polar water-soluble organic compounds may contribute to the odor events because studies have shown that smaller, polar, and water-soluble organic compounds are released first during snow melts.31−33 This led us to investigate AAs, which could leach out first at the beginning of spring runoff when snow starts to melt. AAs may also serve as potential markers of soluble organics that contribute to odor events. AAs are ubiquitous in nature and highly soluble in water, are ineffectively removed during water treatment, and typically exist in source water at ng/L levels. AAs by themselves do not produce an odor in finished water. However, AAs, such as Phe, Val, Ile, and Leu, can produce odorous DBPs including aldehydes and chloroaldimines under chlorination.16−18,20,22,34−40 When these odorous DBPs reach a certain concentration, they can exceed their median odor threshold resulting in odor problems.49 It is important to note that individuals have different sensitivities to odor thresholds, meaning that two individuals can perceive odor threshold concentrations with 100-fold or more difference.50 In light of these findings, we investigated the occurrence of AAs in source water during two consecutive spring runoff periods at two separate water treatment plants, E.L Smith and Rossdale.

Occurrence of AAs in Source Water during 2021 and 2022 Spring Runoff

SPE-HILIC-MS/MS analysis of AAs showed a significant difference in the occurrence of AAs in the 2022 spring runoff compared to that in the 2021 spring runoff. The concentrations of AAs determined in 2021 samples were 0.01–70 ng/L (n = 3) (Figure S3A and Table S12) for E.L. Smith and 0.02–98 ng/L (n = 3) (Figure S3B and Table S13) for Rossdale. The concentrations of AAs were at baseline levels and showed little change during the 2021 spring runoff period. This coincided with no change in the odor profile in 2021, supported by the consistent % satisfaction rating of 95%, as well as the limited changes in the 2021 water quality parameters, especially when compared to the 2022 trends, as shown in Figures S4 and S5.

During the 2022 spring runoff, the concentrations of AAs in source water at E.L. Smith were in the range of 0.2–4993 ng/L (n = 3), as shown in Figure S6A. The concentrations of AAs in source water at Rossdale were in the range of 4–5556 ng/L (n = 3), as shown in Figure S6B. When the individual AA concentrations were compared, those detected in the 2022 samples were up to 45 times higher than those detected in the 2021 samples (Tables S12–15).

Table 1 presents the six most abundant AAs detected in 2021 at both water treatment plants, which were glycine (Gly), Phe, threonine (Thr), Trp, asparagine (Asn), and serine (Ser). Gly was the highest detected AA at 72 ng/L at E.L. Smith, and Phe was the highest detected AA at Rossdale with 98 ng/L. It is important to note that there were only small changes in AA concentrations over the course of spring runoff in 2021. Comparatively, the top six AAs detected in 2022 samples at both water treatment plants were Phe, Thr, Leu, Ile, Tyr, and Trp. Phe was the highest detected AA at both water treatment plants, with concentrations of 4993 and 5556 ng/L, respectively. The peak of the top six AAs at both water treatment plants was detected on March 21, 2022. The 2022 occurrence of AAs in source water at both water treatment plants shows a large change in concentration at the beginning of spring runoff in 2022.

Table 1. Comparison of 2021 and 2022 Spring Runoff at Two Different Water Treatment Plants of the Top Six Detected AAsa.

2021
 2022
Amino Acid Lowest Conc Highest Conc Peak Date Amino Acid Lowest Conc Highest Conc Peak Date
E.L. Smith (ng/L) E.L. Smith (ng/L)
Gly 16 72 19-Mar Phe 42.8 4992.9 21-Mar
Phe 17 60 18-Mar Thr 428.3 4261.9 21-Mar
Thr 20 55 18-Mar Leu 9.4 2986.8 21-Mar
Trp 7.8 51 17-Mar Ile 0.2 1938.1 21-Mar
Asn 49 39 19-Mar Trp 22.2 1827.0 21-Mar
Ser 4.2 34 19-Mar Tyr 35.8 1218.6 21-Mar
Rossdale (ng/L) Rossdale (ng/L)
Phe 26 98 17-Mar Phe 42.5 5555.9 21-Mar
Ser 12 98 15-Mar Thr 238.1 5059.2 21-Mar
Thr 27 83 17-Mar Leu 4.1 3793.0 21-Mar
Trp 7.3 77 18-Mar Ile 9.2 2303.1 21-Mar
Gly 16 72 15-Mar Trp 29.8 1679.8 21-Mar
Asn 23 35 8-Mar Tyr 37.6 1135.1 21-Mar
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a

The top six AAs for each water treatment plant for 2021 and 2022 are listed with their highest and lowest concentration during spring runoff as well as the peak date when the highest concentration occurred.

Total AAs, Water Quality Parameters, and Odor Profile at Home Taps

Figure S7 shows a direct comparison of the total AAs detected at both water treatment plants over the 2021 and 2022 spring runoff. Similar baseline concentrations were determined between February 14 and March 5 for both years with a significant increase in AA concentration during the 2022 spring runoff (between March 5 and March 25). These changes in AA concentrations are indicative of the onset of spring runoffs and are followed by large changes in water quality parameters (Figure S4 and Figure S5). The total AAs will be used in the comparison to water quality parameters and % satisfaction rating to better represent small, polar, and water-soluble organic compounds.

Figure 3A shows the occurrence of total AAs in 2022 at E.L. Smith, as well as the water quality parameters typically used to determine organic content and DBP formation potential. The first increase in the AAs occurred from March 1 to 7, with further increases until March 21. The AA peak on March 21 was detected before the peak of water quality parameters (TON, DOC, SUVA, and color). While an initial increase was detected in both the water quality parameters and AAs on March 21, the water quality parameters continue to increase, while AAs begin to decrease. This difference in peak dates between AAs and water quality parameters shown in Figure 3A (other water quality parameters over spring runoff are shown in Figure 2) could be explained by the small polar water-soluble compounds that leach from snow first. This would explain the initial peak of AAs at the initial onset of spring runoff and then the continuing increase in water quality parameters while the remainder/other larger compounds enter source water with increasing snowmelt. However, the water quality parameters’ further increase does not explain the odor profile.

Figure 3.

Figure 3

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Comparison of the 2022 E.L. Smith results: (A) total AA concentrations versus common water quality parameters (TOC, TON, SUVA, and color) in source water samples collected at E.L. Smith during spring runoff and (B) total AA concentration at E.L. Smith versus satisfaction ratings (%) at home taps across the city. Note that total AA concentrations were scaled at ×10–2 for comparison.

Figure 3B shows the total AAs detected during spring runoff in 2022 at E.L. Smith against the home analyst % satisfaction rating. The peak of AAs during the spring runoff was detected on March 21. Comparatively, the % satisfaction rating (odor level) begins to decrease on March 18 and reaches its lowest satisfaction, or lowest reported rating, on March 23. It is difficult to correlate the date because of the time difference between sampling intake vs home detection of odor. The water distribution system can take an estimated 1–3 days to reach home taps from the water treatment plants (Figure S8). However, when considering the potential time delay in Figure 3B, the decrease in the % satisfaction rating matches well with the increase in the concentration of AAs.

As mentioned previously, PAC is used to remove color and odor-causing compounds and is dose dependent on the color and turbidity of source water. However, this means PAC dosing occurred (March 25 peak dosing, Figure S5A) after the peak for AAs (March 21). To determine PAC removal efficiency of AAs from water, we performed a quick test of PAC on removing AAs at neutral pH (details in Text S10). While PAC was able to remove some AAs in water, a large portion still remained which can cause taste and odor issues. Some studies have shown that varying the pH or using different PAC types can improve the removal efficiency.26 While PAC may not efficiently remove AAs in source water, the addition of PAC is likely to reduce odor by removing the other odor agents.

High Resolution Mass Spectrometry (HRMS) Detection of N-Chloroaldimines from Chlorination of AAs

ESI-MS and MS/MS spectra of chloroaldimines resulting from the chlorination of AAs have not been reported. Previous studies on chlorination of mg/L of individual AAs (e.g., Phe, Val, Ile, and Leu) support the formation of aldehydes, chloroaldimines, and nitriles using gas chromatography (GC) with either flame ionization detection (FID) or GC-MS, HPLC-UV, and/or proton nuclear magnetic resonance (1H NMR analysis).34−38 We used HPLC-QTOF-MS to investigate the formation of chloroaldimines from the chlorination of individual AAs. The top six AAs (Phe, Thr, Leu, Ile, Tyr, Trp) detected in the 2022 samples were reacted with free chlorine (NaOCl) and analyzed using high resolution HPLC-QTOF-MS with IDA. Val was also chlorinated and analyzed using the same procedures because it has been shown to produce odorous products.20,22,34,39 Using the built-in peak picking software (described in Text S9), a single peak of a chemical feature containing a monochlorine isotopic pattern was detected in each of the Phe, Leu, Ile, and Val samples after chlorination. We used the exact mass to determine a molecular formula and manually interpreted the MS and MS/MS spectra of the chlorination reaction products. Figure 4 shows the HPLC-QTOF detection of N-chloroaldimine produced after the chlorination of Phe. The exact mass of the feature detected in the Phe chlorination sample was m/z 154.0421, which matches that of N-chlorophenylacetaldimine ([C8H8ClN + H]+, 154.0418, mass error 1.9 ppm). Figure 4A shows that the XIC of m/z 154.0421 was only detected in the reaction solution and not in the Phe control. The experimental MS spectrum of m/z 154.0421 (black trace) in Figure 4B matches the theoretical monochlorinated isotope distribution (red trace). Figure 4C shows the MS/MS spectrum of m/z 154.0421. The fragmentation pattern is consistent with the structure of N-chlorophenylacetaldimine, supporting its putative identification. An N-chloroaldimine was also putatively identified during the chlorination of Leu, Ile, and Val (Figures S9–S11). No features matching the expected N-chloroaldimine of Tyr, Trp, or Thr were found.

Figure 4.

Figure 4

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HPLC-QTOF detection of N-chloroaldimine produced after chlorination of 1 mM Phe at a NaOCl:AA (as Cl2) molar ratio of 2.4:1. HRMS information for each N-chloroaldimine includes (A) XIC in a chlorination solution (black trace) and the control (red trace) in ultrapure water, (B) MS spectrum (black trace) and theoretical isotopic distribution of the proposed N-chloroaldimine (red trace), and (C) MS/MS spectrum with labeled major fragments.

Targeted Analysis of N-Chloroaldimines in Simulated Water Treatment Plant Conditions

Using the exact masses of the parent and fragment ions observed in the MS and MS/MS data obtained using HPLC-QTOF MS, we developed a targeted HPLC-MS/MS method to investigate the formation and stability of N-chloroaldimines from Phe, Leu, Ile, and Val. First, we studied the formation of N-chloroaldimines under various NaOCl:AA molar ratios (2.4:1, 100:1, and 1000:1). We found that under each of these conditions, N-chloroaldimines are formed rapidly (Figures S12–S14). However, the stability of the N-chloraldimines decreased as the NaOCl:AA molar ratio increased. Next, we studied the stability of N-chloroaldimines in a simulated water treatment plant distribution system. Source water spiked with AAs was chlorinated with NaOCl to reach a free chlorine residual of 2–3 mg/L. After 20 min, ammonium chloride (NH4Cl) was added to the reaction solution (0.7 Cl/N molar ratio) to form chloramines. Chloroaldimines formed in these samples were determined after 20 min of reaction with free chlorine and at various time points (0, 0.5, 12, 24, 48, and 72 h) after NH4Cl addition. Figure 5 shows the HPLC-MS/MS chromatograms for N-chloroaldimine formed from (A) Phe, (B) Leu, (C) Ile, and (D) Val in these samples. The N-chloroaldimine peak from each AA was still detected at 72 h. These results suggest that N-chloroaldimines may reach home taps up to 72 h later, as an estimation shown in Figure S8. These results further support AAs as markers of small soluble nitrogenous organic compounds that may contribute to odor issues.

Figure 5.

Figure 5

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Stability of N-chloroaldimine formation from (A) Phe (10 μM), (B) Leu (100 μM), (C) Ile (100 μM), and (D) Val (100 μM) in a simulated water treatment plant distribution system.

Implications

Chlorination is a strong oxidative treatment that effectively inactivates microbial pathogens.1,2 Complex mixtures of organics in source water can undergo various oxidative reactions during chlorination to produce odorous and non-odorous DBPs.6,8,16−18,20,22,34,36,38,39,51−54 Small water-soluble organics, such as AAs, are ubiquitously present in source water and are difficult to remove by common water treatment processes. Low levels of AAs, which cannot be detected by commonly tracked water quality parameters, can form odorous DBPs under chlorination reactions that can result in significant complaints about tap water quality.16,18−22,30,36 This can cause significant challenges for water treatment plants to optimize treatment processes in a timely manner. Thus, AA occurrence in source water indicating the onset of spring runoff may be useful, both for timely adjustments to water treatment procedures and as predictors for the formation of odorous DBPs.

While this study closely examined the occurrence of AAs over spring runoff, it is important to recognize that these are only a representative group of small water-soluble compounds that can form a wide range of DBPs. Further elucidation of these compounds and their effect on the quality and safety of drinking water is needed. This is especially true when considering the effects of climate change on the composition of source water.55−59 Some solutions involve the integration of water reuse, which drastically alters the composition of source water and may increase the low molecular weight small soluble compounds (small water-soluble, <1000 Da) represented by AAs.60,61 Complex changes to the composition or type of source water dramatically alter the formation of DBPs; thus, it is important that we further understand the occurrence and effect of small, polar, and water-soluble compounds such as AAs in oxidative water treatment.

Acknowledgments

This research was supported by grants from the Natural Sciences and Engineering Research Council of Canada, Alberta Innovates, EPCOR Water Services, and the Canada Research Chairs Program.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.est.3c00719.

  • Solid phase extraction recovery; HPLC parameters; MS parameters; MRM parameters; figures of merit; water quality parameter methods; odor profile responses; spring runoff occurrence of AAs; Tables S1–S15, Figures S1–S14 (PDF)

Author Contributions

C.C. and N.W. contributed equally to this project.

The authors declare no competing financial interest.

Supplementary Material

es3c00719_si_001.pdf (2.3MB, pdf)

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