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. 2024 Jun 26;58(27):11958–11969. doi: 10.1021/acs.est.4c01894

Climate-Driven Increases in Source Water Natural Organic Matter: Implications for the Sustainability of Drinking Water Treatment

Ryan Swinamer †, Lindsay E Anderson †,*, Dave Redden †, Paul Bjorndahl ‡, Jessica Campbell §, Wendy H Krkošek §, Graham A Gagnon †
PMCID: PMC11238540  PMID: 38922292

Abstract

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This study presents an updated analysis spanning over two decades (1999–2023) of climate, water quality, and operational data from two drinking water facilities in Atlantic Canada that previously experienced gradual increases in the natural organic matter (NOM) concentration and brownification. The goal was to assess the impact of recent extreme weather events on acute NOM concentration increases and drinking water treatment processes. In 2023, a dry spring combined with a warm and wet summer caused NOM in the water supplies to increase by >67% (as measured by color). To mitigate increased NOM concentration, the alum dose nearly doubled in 2023 compared to that in 2022. Disinfection byproducts were elevated following the event but remained within the compliance levels. From 1999 to 2023, the two plants responded to gradual climate change impacts and brownification, with alum dose increases of between 4.1 and 8.3 times. Equivalent CO2 emissions were estimated for alum usage, which increased by 3 to 7-fold in 2023 compared to when the plants were commissioned decades prior. The plants were not only adversely impacted by climate change but also contributed to the global CO2 burden. Thus, a paradigm shift toward sustainable alternatives for NOM removal is required in the water sector, and climate change adaptation and mitigation principles are urgently needed.

Keywords: sustainable development goals, brownification, climate change, natural organic matter, drinking water treatment

Short abstract

Climate change is forcing raw water organic matter to new extremes and is challenging the sustainability of conventional water treatment practices.

Introduction

For several decades, there have been increases in the concentration of natural organic matter (NOM) measured as dissolved organic carbon (DOC) or color in surface waters, a phenomenon sometimes referred to as brownification.1−7 Brownification has been widespread throughout the northern hemisphere, with one of the most reported drivers being the reversal of atmospheric acid (e.g., SO4) deposition,5,8 which increases the solubility of NOM.5,9,10 The authors have previously demonstrated brownification trends through the reversal of acid deposition in the context of surface waters and drinking water supplies in Atlantic Canada2,9,11 and globally.10 More recently, the importance of climate change12−18 and changing land use1,19,20 have been highlighted as other drivers of increased NOM export to surface water, particularly as deposition stabilizes. However, the individual importance of these factors has been difficult to disentangle.12,14,19,21,22 Many climate projections estimate that global temperature will continue to warm and that precipitation events will become more intense and frequent,23 which will undoubtedly promulgate these water quality trends.

The efficacy of most drinking water treatment processes including the selection, design, and operation is dependent on the concentration of NOM, quantified through metrics like DOC, color, and UV absorbance at 254 nm (UV254).24,25 In low turbidity sources, NOM is responsible for most of the chemical demand and is known to be an important parameter affecting coagulant dose.26−28 More specifically, humic-like NOM fractions with higher molecular weight (MW), aromaticity, and hydrophobicity are most amenable to removal via enhanced coagulation, which is the major NOM removal process used in drinking water treatment.27,29,30 The removal of NOM is typically driven by minimizing the formation of regulated disinfection byproducts (DBPs), including trihalomethanes (THMs) and haloacetic acids (HAAs), which are formed during chlorination when residual NOM is present.

Due to both regulatory and design considerations, most surface water treatment facilities in North America are designed with coagulation processes for NOM and turbidity removal.31,32 For example, the Ten State Standards dictate that direct or conventional filtration facilities require the use of a primary coagulant, while the Disinfectants and DBP (D/DBP) rule sets standards for NOM removal prior to disinfection primarily through enhanced coagulation. The reliance on coagulation is exacerbated by brownification, which has a significant impact on surface drinking water treatment practices,10 notably increased coagulant demand, and sludge production.9,29,33

Aluminum-based coagulants are widely used with nearly 800 million kg consumed domestically in the United States in 2019, 45% of which is attributed to the water sector.34 Alum (Al2(SO4)3) is traditionally produced from bauxite,35 which is mined in the primary aluminum industry.36 This industry is energy intensive, accounting for 4% of global industrial CO2 emissions and 3% of the global industrial energy demand.37 According to Shen and Zhang,38 the greenhouse gas emissions of the primary aluminum industry nearly doubled from 2005 to 2021 (570 to 1120 Mt),39 and carbon emissions through electricity consumption are the greatest source of emissions from this industry. In 2022, they were approximately 65% of the total emissions for the primary aluminum industry (approximately 6.6 ton CO2/ton Al).38 The water sector also contributes to emissions directly through energy usage—approximately 2% of total emissions from the US electricity sector are from drinking water and wastewater processes,40 where pumping is often the major contributor.40,41

As climate pressures exaggerate elevated NOM trends and brownification, the impact on engineered drinking water infrastructure will become more severe. Recent extreme weather events (e.g., drought, wildfires, and floods) in Atlantic Canada and other brownification-prone regions highlight the necessity to assess the additional impact of climate change on raw water quality and drinking water treatment practices, especially as other drivers (e.g., SO4 deposition) stabilize. This work presents an updated analysis spanning over two decades (1999–2023) of climate, water quality, and operational data from two drinking water facilities in Atlantic Canada that have previously undergone gradual raw water brownification over several decades, which was mainly attributed to the reversal of SO4 deposition.9 The overall goal is to understand climate impacts and to assess the acuteness of recent extreme weather events on NOM concentration changes and treatability. The impact of raw water NOM trends on the sustainability of treatment processes at these facilities was also assessed through an analysis of the indirect carbon footprints from chemical consumption. Collectively, this knowledge of NOM trends and the associated treatment impacts will shape how surface drinking water providers address operational challenges and future sustainability considerations.

Materials and Methods

Study Locations

Both sites used in this study are protected watershed areas, are not impacted by wastewater discharges, and consist of forested land. Lake A and Lake B have surface areas of approximately 800 and 343 ha, respectively, and maximum depths of 47 and 65 m.42,43 Both lakes are drinking water supplies located in Nova Scotia, Canada, characterized as having low pH (pH < 6), turbidity (<0.5 nephelometric turbidity units), and alkalinity (<5 mg CaCO3/L). Lake A is the water supply for Plant A, a direct filtration facility commissioned in 1977 with an average daily flow of approximately 91 million liters per day (MLD) [24 million gallons per day (MGD)] with a design capacity of 227 MLD (60 MGD).44 Lake B is the water supply for Plant B, which has an average daily production of 32 MLD (8.5 MGD) with a design capacity of 94 MLD (25 MGD). Plant B is a conventional filtration plant with up flow clarification that was commissioned in 1999.44Table S1 summarizes typical water quality parameters for Lake A and Lake B for the study period.

Data Acquisition

Water Quality and Operational Data

Raw water quality data for both lakes were obtained from historical records provided by the local water utility for the period January 1, 1999, to December 31, 2023. Raw water color and UV254 were measured daily by operations staff using a benchtop spectrophotometer (Hach Company, Loveland CO). Color data are presented in true color units (TCU) and UV254 are presented as cm–1. DOC data were obtained from a historical data set developed by the Centre for Water Resources Studies (CWRS) at Dalhousie University.9,11 Although there is no long-term DOC data set with high-frequency data for Lake B, additional DOC data for this supply was obtained from the utility’s sampling program as well as other unpublished data from the CWRS. Samples for DOC analysis were collected in headspace-free vials acidified to pH < 2 with phosphoric acid and were quantified with a total organic carbon (TOC) analyzer (TOC-V CPH, Shimadzu Corporation, Kyoto, Japan). Fluorescence excitation emission matrix (FEEM) spectra were collected per the method described by Brophy et al.45 to assess any changes in raw water NOM quality in Lake A following climate events in 2023 (August to December, n = 23) and were compared to data for the same period in 2022 (n = 25). Fluorescence regional integration was used following the method described by Chen et al.46 to detect five NOM regions including humic, fulvic, microbial, and two aromatic proteins. FEEM data were unavailable for Lake B.

Distribution system DBPs including total THMs (TTHMs) [chloroform, bromodichloromethane, dibromochloromethane, and bromoform] and HAA5 (monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, monobromoacetic acid, and dibromoacetic acid) were obtained from the utility’s compliance data set for 2022 and 2023. Samples were collected quarterly throughout the distribution systems fed by Plants A and B (February, May, August, and November) for TTHMs (n = 11 Plant A, n = 7 Plant B) and HAA5 (n = 9 Plant A, n = 5 Plant B) in both 2022 and 2023.

Daily alum dosages (mg/L) from January 1, 2004, to December 31, 2023, for Plant A and from January 1, 1999, to December 31, 2015, and January 1, 2019, to December 31, 2023, for Plant B were also obtained from the utility’s operational records. There were no records of alum dosing before 2003 for Plant A, and electronic records could not be obtained from 2016 to 2018 for Plant B. Unit filter run volume data for Plant A were also obtained from the utility’s records. Daily raw water flow rates (MLD) were also obtained to calculate alum consumption at each facility, although electronic flow data were missing from 1999 to 2003, and therefore, the average of the first complete year on record was used for missing flow values to calculate alum consumption for these years. Daily alum consumption (kg) was calculated by multiplying the daily raw water flow (106 L/day) and the corresponding paired alum dose (mg of Al2SO4/L). Alum consumption values were added to the previous day, starting January 1, to obtain a cumulative alum consumption value (kg) that spanned each year of the data set. The annual alum consumption for each facility was also used to estimate associated equivalent CO2 (CO2-eq) emissions by multiplying the total consumption by the “carbon footprint” of each product, estimated through life cycle analysis of alum production at 0.148 kg CO2-eq/kg alum product.47

Climate Data

Daily total precipitation and temperature data for the area from January 1, 1999, to December 31, 2023, were obtained from Environment Canada’s historical climate database.48 Nearby monitoring stations Shearwater A (Climate ID 8205090) and RCS (Climate ID 820592) were approximately 15 km from Plant B and 40 km from Plant A. Stations Shearwater A and Shearwater RCS had daily temperature and precipitation data ranging from January 1, 1999, to December 12, 2007, and July 2, 2008, to December 31, 2023, respectively. The two data sets were combined to represent the local data set for the study area. The frequency of the daily mean temperature exceeding 0 °C during colder months (December to March) and 20 °C, the typical midsummer daytime temperature for the region, for warmer months (May to September) was evaluated to assess trends in temperature. Monthly and annual cumulative total precipitation data were used to understand precipitation trends.

Statistical Analyses

All data were analyzed using R (Version 1.4.1717).49 The Seasonal Mann Kendall Test (SMKT)50 was applied to monthly means to statistically evaluate temporal trends in historical water quality parameters such as raw water color for updated trend comparison with our previous work.9 This test was also applied to assess trends in operational data (e.g., alum dose). Briefly, MKT is a form of nonparametric monotonic trend analysis for temporal data with serial dependence and is used to determine the statistical significance of the identified trend. The Sen’s slope estimation51 was used to calculate the magnitude of significant trends identified using SMKT. Differences in the various FEEM regions (e.g., humics, fulvics, microbial, and proteins) in 2022 versus 2023 were assessed using a nonparametric Mann–Whitney U test. Annual trends in total daily precipitation and mean monthly temperature were assessed by using a Fligner-Killeen test of unequal variances. All statistical analyses were tested at a significance level of α (0 05) at minimum.

Results and Discussion

Climate Trends and the Impact on NOM Concentration

Acute Climate Impacts in 2023

The area studied in this work and other parts of Atlantic Canada experienced extreme weather events during 2023. Figure 1 depicts historical daily (a) and monthly (b) total precipitation data over the study period.

Figure 1.

Figure 1

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Percentile distribution of daily precipitation (a) and total monthly precipitation (b) for the study area since 1999. Years 2007–2008 excluded due to incomplete data for these years.

Considering the daily total precipitation data for the study years (1999–2023), there were generally consistent trends in terms of precipitation amounts as well as the distribution of data. However, from an acute perspective, 2023 was anomalous. Specifically, the first half of 2023 was one of the driest in recent history—aside from January, which had higher than normal total precipitation, there were lower than normal levels for the remainder of winter (e.g., February to March), which continued into spring. Specifically, there was only 45.8 mm of total precipitation in April 2023 and only 84.8 mm in May 2023 (Table S2), which includes one rain event of 68 mm that occurred near the beginning of the month. Additionally, in May 2023, there were 29 consecutive days of <10 mm of total precipitation compared to 15–20 days in recent years (e.g., 2020–2022). These dry conditions led to the largest wildfires in Nova Scotia’s history, where over 800 ha in the region burned near the end of May 2023.52 The affected areas were in proximity to the boundaries of the study lakes, but no burning occurred directly in the watersheds. This was followed by the wettest summer in recent decades—severe rainfall and flood events experienced in June, July, and August resulted in record precipitation levels (Figure 1). Specifically, a flash flooding event in late July 2023 with reports of ∼250 mm in 12 h in the region resulted in a state of emergency;53 however, smaller amounts were captured at the monitoring station used for this study (100 mm in 24 h). This was followed by another significant rainfall event in early August of over 120 mm. Overall, the months of June to August had record rainfalls of over 650 mm total, with many days of heavy rain. Other parts of Atlantic Canada also reported more than double the typical rainfall during this period, and this was considered the wettest modern-day summer in the area.53 In addition to more intense rainfall events, our data suggest a shift in the distribution of seasonal rainfall patterns. Tests for unequal mean and variance confirmed that 2023 had significantly (p < 0.05) lower winter and spring (Feb to May) precipitation, while summer (June to August) precipitation was significantly (p < 0.05) higher compared to prior study years, indicating an anomalous variance in precipitation.

The year (2023) also had significantly (p < 0.05) warmer mean winter and summer temperatures and was the warmest on record for the study period, with 44 days above 20 °C (Figure 2a). Specifically, July 2023 was record breaking, with 25 days above 20 °C (Figure 2b), while the next closest year was 2020, which had only 15 days above this temperature. On an average basis, July was also the warmest on record (21 ± 2.1 °C) for the study period (Table S3). The winter months were also significantly (p < 0.05) warmer (Table S3) and had the most days above 0 °C in December 2022 (25 days) and January 2023 (20 days) since 1999 (Figure S1). Accordingly, 2023 was one of the warmest and wettest years in recent decades, which is consistent with global challenges identified by climate change.54,55

Figure 2.

Figure 2

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Number of days where the mean temperature was above 20 °C the entire year (a) and for May to September (b) for the study area since 1999. Years 2007–2008 excluded due to incomplete data for these years. Note that there were no days above 20 °C in April.

Water Quality Response to Climate Extremes

The reported climate events in Nova Scotia had a significant impact on water quality in both study lakes. A significant and acute increase in NOM concentration was experienced in Summer 2023 following the extreme rainfall events (Figure 3a). Color was selected as an indicator for NOM, as consistent daily DOC data were not available for the entire study duration for these lakes—color has been shown to correlate strongly with DOC in Atlantic Canadian surface waters.9,11,56 Specifically, in the 4 days following the precipitation event in July 2023, the lake color rose from an average of 27 to 47 TCU for Lake A and from ∼60 to above 100 TCU for Lake B. These values were sustained for several months before gradually decreasing in December; however, from a seasonal perspective, the value remained higher than normal compared to the previous year.

Figure 3.

Figure 3

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Time series (a) and percentile distribution (b) of daily lake color for Lake A and Lake B since 1999.

The percentiles (Figure 3b) demonstrate the distribution of lake color over the past 25 years as well as the extremes that were experienced in 2023. The median color rose to 35 TCU in Lake A, an increase of 11 TCU from the 2022 median, and a > 3-fold increase in median color since 1999. Similarly, in Lake B, the color rose from a median value of 52–61 TCU from 2022–2023 and increased by nearly 3-fold since 1999. Overall, the mean color in both Lakes A (0.4 TCU/year) and B (1.1 TCU/year) increased significantly (p < 0.05) over the study period (1999–2023).

Other NOM metrics, including UV254 and DOC, also showed significant increases following the extreme rainfall events (Figures S2 and S3). The DOC in Lake A (Figure S3) rose to a maximum of ∼7 mg/L after rainfall and stabilized at ∼4 mg/L throughout the fall and winter. This was higher than in recent years and nearly 2 mg/L more than just over a decade ago (Figure S3). There is no current long-term data set of DOC for Lake B; however, DOC values of above 11 mg/L were recorded during lake sampling in August and September following the event after which it stabilized at between 6.5 and 7.5 mg/L later in the fall. For reference, our previous work9 reported raw water DOC for Lake B between 4 and 5 mg/L. Regional integration of FEEM data from Lake A (data were unavailable for Lake B) was also used to quantify the five NOM regions (e.g., humic, fulvic, microbial, and two aromatic proteins) in 2022 and 2023 (August to December). Samples from 2023 had a significantly higher (p < 0.05) integrated volume for the humic region compared to those from 2022, while the others showed no change. This provides further evidence that the elevated NOM following the climate events in 2023 was dominated by humic substances.

Prior to 2023, both lakes experienced chronic increases in lake color over several decades, as described previously by the authors.9,10 This was attributed to gradual reductions in atmospheric acid (e.g., SO4) deposition through more stringent air pollution controls. Recent atmospheric and water quality data suggest that deposition has effectively stabilized in the region.44,57

It is anticipated that instead of gradual increases in NOM over decades, trends will shift toward more short-term and acute increases because of climate-related events such as changes in precipitation amount, intensity, and seasonal distribution. A previous review by the authors provides in-depth description of climate event impacts on source water DOC, which becomes the dominant driver as deposition stabilizes.10 Briefly, increases in precipitation can result in the mobilization and export of DOC from the catchment by increased hydrologic connectivity within the watershed,58 where runoff is routed through organically rich surficial areas.59 As evidenced in our study, extreme rainfall events flushed significant quantities of NOM to both drinking water supplies. This was also demonstrated by Raymond & Saiers,21 who found that 57% of annual DOC transportation in 30 streams and rivers in the Eastern United States occurred during large storm events that occurred during only 5% of the year. Strock et al.60 analyzed a database of over 80 remote lakes (0 to 50% wetland cover, 0 to 36% watershed development) throughout the Northeastern United States over a 30 year period and observed DOC increases during extreme wet years. Others have also noted significant pulses in NOM that were higher than normal seasonal values following extreme precipitation events in drinking water sources and lakes with varying watershed activity including agriculture, forestry, recreation, and urban use.61−64 However, some authors63,65 have observed more short-term pulses (lasting only days to weeks), while others have documented no or lagged responses of up to a year.63,66 Conversely, Warner et al.67 found that an early summer precipitation event (∼30 mm in 24 h) caused a greater change in NOM quality compared to concentration, which was correlated with land cover. Overall, responses would be impacted by watershed and lake characteristics, as well as lake dynamics, differences in local climate, and the magnitude and intensity of the precipitation events themselves.

The dry spring conditions in the region as well as warmer than normal summer temperatures may have also contributed to elevated NOM. Warming temperatures are said to stimulate biological decomposition of organic carbon and release from soil organic matter leading to increased DOC mobilization.68−70 As reported elsewhere,68,71 an increased DOC release has been observed during drought recovery, particularly during the first flush.

The extreme events described in this work are expected to become more frequent and intense as climate change continues. For the study area, predictions indicate that annual precipitation could increase from a baseline of 1440 mm (1976–2005) to 1781 mm for the period 2021–2050, with precipitation increasing each season under a high-emission scenario.72 Under a low-emission scenario, precipitation predictions are still high at >1760 mm. Mean temperatures are also projected to increase under both scenarios as annual temperature is expected to go from a mean of 6.8 °C (1976–2005) to 9.9 °C (2021–2050) under a high-emission scenario and to 9.7 °C under a low-emission scenario.72 Accordingly, regardless of the emission scenario, significant changes to precipitation patterns and temperature in the region can be expected, which will undoubtedly continue to impact the concentration of NOM in surface waters throughout the region.

Impact to Treatment Processes and Plant Operations

The authors previously demonstrated how the treatment processes at these plants have been impacted significantly by gradual or chronic NOM increases in their sources.9 Specifically, Plant A increased their alum dose to 12 mg/L after operating at roughly 8 mg/L for the previous 35 years and experienced reduced filter run times as a result.9 Similarly, at Plant B, alum dosing increased by 4-fold (12.9–49.5 mg/L) over the same period. Previous increases in coagulant dose were correlated to increases in raw water color.9 The recent climate events caused drastic and immediate changes in raw water NOM in both lakes, challenging these facilities to new extremes. Figure S4 depicts daily alum dosing at Plant A and Plant B from 1999 to 2023, while Figure 4 shows the cumulative alum consumption for each year.

Figure 4.

Figure 4

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Cumulative alum consumption at Plant A and Plant B (calculated based on raw water flow) since 1999. Note that Plant A only has data beginning in 2003, and data for Plant B were unavailable from 2016 to 2018.

In summer 2023, alum dosing increased significantly as a result of climate events. Specifically, the alum dose at Plant A increased from ∼20 mg/L to around 27 to 33 mg/L to satisfy the coagulant demand caused by elevated NOM of humic character and to minimize DBP formation. The alum dose was gradually reduced starting in September as the lake color decreased.

Total alum consumption at Plant A in 2023 was approximately 700,000 kg (Figure 4) compared to ∼570,000 kg in 2022 (1.2-fold increase) and ∼240,000 kg (3-fold increase) in 2003, which was the first full year of available data. By June 2023, Plant A consumed an equivalent amount of alum as it did for the entire year of 2003. It should also be noted that changes in production at Plant A were not responsible for increases in alum consumption as raw water flow (Figure S5) did not increase throughout the study period. In fact, raw water flow at Plant A decreased throughout the study period until 2016 due to the implementation of a water loss control program, after which it gradually returned to levels observed in earlier years due to population growth. Although increasing lake color was the main driver of alum dosing over the years, there were other factors including process optimization activities. For example, from 2003 to 2015, Plant A operated at a low alum dose of 8 mg/L and low coagulation pH of 5.8–5.9 with a goal of maximizing filter run time at this direct filtration facility. From 2016 to 2020, alum dosing ranged between 8 and 12 mg/L. In 2021, a coagulation pH of 6.2–6.3 was selected to achieve minimum aluminum solubility, and the alum dose was increased to 17.5 mg/L to improve NOM removal.

As a result of recent climate events, Plant A is now operating well beyond the design thresholds for direct filtration (e.g., TOC < 4 mg/L, color <20 TCU, alum dose <15 mg/L),9 and as a result, the UFRVs were impacted, gradually decreasing through July and August to ∼100 from ∼150 m3/m2 immediately prior to the precipitation event. Other remedies including polymer addition and prechlorination were implemented to recover UFRVs, which eventually returned to pre-event levels throughout the fall months.

Alum dosing at Plant B also increased gradually over several decades (Figure S4). Our previous study estimated that alum dosing at Plant B increased at a rate of 1.7 mg/L/year (1999–2015).9 From 2016 to 2022, there were further increases in alum due to gradual increases in raw water color. An updated analysis of data (1999–2022) revealed that alum increased at a rate of 2.8 mg/L/year. Prior to the 2023 event, Plant B was operating at approximately 70 mg/L, increasing to >100 mg/L after the rainfall. A maximum alum dose of 125 mg/L was sustained in September and October and was gradually reduced to ∼100 mg/L in December; however, the alum dose at Plant B did not return to typical seasonal values of the previous year.

Alum consumption at Plant B (Figure 4) reached over 1,100,000 kg in 2023, and by April 2023, the alum consumption exceeded annual totals from 1999. Like that in Plant A, these increases were not because of increased production, as flow rates at Plant B decreased throughout the study period due to water loss controls (Figure S5). There were also process changes at Plant B around 2012 that were driven by improving finished water quality while maximizing water production, which contributed to the increase in alum consumption. However, changes in raw water color accounted for the majority of the increase at this facility.

Others have also documented the impacts of increasing NOM levels on coagulation processes. Eikebrokk et al.33 demonstrated experimentally that an increase in raw water color from 20 to 35 mg Pt/L resulted in a coagulant increase of over 60%, causing sludge production to increase by a similar rate, and also increased the number of daily backwashes by 87%. Sharp et al.29 also observed coagulant demand increases in a drinking water supply (UK moorland) with increasing DOC and noted a change in the composition of NOM, where an increase in the hydrophobic fraction was responsible for the coagulant increase, which is in line with the increased humic NOM observed in this work. Hurst et al.73 showed that in a UK river source with agricultural activity in the catchment, a rainfall event increased raw water DOC from 3.9 to 14.4 mg/L, resulting in impaired coagulation and decreased turbidity removal efficiency during clarification. Also, Delpla et al.74 observed that in a river source with forested land and agricultural activity in the watershed, there were significant increases in raw water UV254 during rainfall events that occurred after a dry period, which caused increased NOM in the filtered water quality and the elevated formation of DBPs. However, in their study, there was no consistent pattern in coagulant adjustment as a result of the increased NOM in the raw water from the rainfall event.74

At the study facilities, increasing coagulant dose was an immediate measure taken to maintain finished water with low NOM content (e.g., DOC < 2 mg/L) and minimize DBP formation. Trends in distribution system TTHM and HAA5 concentrations were assessed using a bulk snapshot of quarterly (Q) compliance samples collected from several locations in the distribution systems fed by Plants A and B (Figure 5).

Figure 5.

Figure 5

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DBP data for compliance samples collected from distribution systems fed by Plant A and Plant B. Samples were collected quarterly (February, May, August, and November) for TTHMs (n = 11 Plant A, n = 7 Plant B) and HAA5s (n = 9 Plant A, n = 5 Plant B).

Considering all sample locations, median TTHM and HAA5 concentrations in Q1 and Q2 were lower in 2023 compared to those in 2022 for both treatment plants. The first set of compliance samples collected following the summer climate event (Q3-2023) had 1.5- and 2-fold higher median TTHM and HAA5 concentrations at Site A and 1.2-fold higher concentrations at Site B for both DBPs compared to 2022 data, which was likely attributed to elevated residual NOM in the finished water. In Q4-2023, Site A levels were 1.2- and 2-fold higher for TTHMs and HAA5s, respectively. This is attributed to direct filtration facilities being limited in their ability to increase the coagulant dosing. In comparison, at Site B, Q4 TTHMs recovered to lower levels than in 2022, which was likely due to increased alum dosing to overcome elevated NOM. However, at Site B, Q4 HAA5 levels did not recover to 2022 levels, although they were lower than Q3, which is a typical seasonal pattern. Despite elevated DBPs, both systems remained within regulatory compliance at below 100 μg/L for TTHMs and 80 μg/L for HAA5s, which is determined by averaging the last four quarterly sample results separately for each sample location.

Others have studied the impacts of extreme weather events on DBP formation during water treatment and observed similar findings to this study.74−77 For example, Delpla and Rodriguez76 assessed the influence of climate on DBP formation, capturing significant rainfall events (amounts of up to ∼47 to ∼56 mm), which yielded an increase in 7 day DBP formation potential, and specifically, it was observed that THM and HAA concentrations could double following rainfall. Nguyen et al.77 also observed more hydrophobic, high MW NOM post storm event, which implies greater THM formation potential. On the other hand, Delpla et al.78 did not observe any substantial increase in THM formation potential following a rainfall event despite significant increases in DOC.

Environmental Implications

The trends in rising NOM are occurring globally,10 and the associated treatment challenges will become more prevalent with climate change pressures. The drastic increases in alum consumption described above are costly and have substantial implications for supply chain planning and also demonstrate significant environmental implications.

Considering both direct and indirect energy, chemical production contributes to a significant portion of the carbon footprint for a treatment facility.79−81 For example, Santana et al.81 estimated that chemical usage was responsible for 37% of the total embodied energy consumption, while 63% was due to energy associated primarily with the pumping of finished water. A life cycle analysis for various coagulants reported a carbon footprint of manufacturing liquid alum at 148 kg CO2-eq/ton Al2(SO4)3 (or 0.148 kg CO2-eq/kg Al2(SO4)3).47 Using this estimate, the annual CO2 emissions from alum manufacturing based on the total annual alum consumption for the two facilities were calculated. It should be noted that these represent indirect energy consumption, in contrast to direct onsite sources such as pumping and mixing energy. Additionally, the emission factor is a general estimate as there are site-specific variables including plant proximity to alum manufacturer that would influence carbon emissions.

The alum usage at Plant A remained consistent between 2004 and 2018, which translates to the indirect CO2 emissions of approximately 36,000 to 46,000 kg of CO2/year. From 2019 onward, increasing alum usage contributed to increases in indirect CO2 emissions at Plant A despite gradual decreases in production (92–81 MLD) due to water loss controls. The indirect CO2 emissions in 2023 (104,000 kg of CO2/year) were approximately three times greater than that in 2004. For Plant B, indirect CO2 emissions during the first years of operation post commissioning (1999–2002) amounted to approximately 23,000 kg CO2/year. In 2023, the drastic increases in alum dose required to address the challenging water quality during the summer translated to emissions of >160,000 kg CO2. Overall, there would have been seven times more CO2 emitted in 2023 compared with that when Plant B was commissioned.

Others have reported on indirect CO2 emissions associated with drinking water production. For an 80 MLD direct filtration plant, Pellikainen et al.80 reported an annual carbon footprint of 305,000 kg CO2 associated with aluminum-based PACl coagulant (0.537 kg CO2/kg) usage. After accounting for in-plant direct emissions due to electricity consumption, chemical manufacturing made up ∼60% of CO2 emissions. The estimate for emissions from coagulant production from this study were comparable to our estimates for 2023 (∼100,000 for Plant A and ∼164,000 kg of CO2 for Plant B). The differences are likely attributed to the coagulant type and the associated emissions factors (PACl 0.537 kg CO2/kg vs alum 0.148 kg CO2/kg), raw water alkalinity, and coagulation pH. Another study,79 which estimated equivalent CO2 emissions from a conventional water treatment plant (200 MLD) also using alum as a coagulant, estimated that over 90% of the total embodied CO2 emissions generated by the water treatment process was attributed to offsite chemical production. However, it should be noted that this study used much higher emission factors (2.779 kg CO2/kg Al2(SO4)3) compared to those used in our study (0.148 kg CO2/kg Al2(SO4)3), and as a result, their estimates were significantly higher. Others also have noted that the operational phase and specifically chemical consumption contribute significantly to the overall environmental footprint, with coagulation and pH adjustment being some of the dominant sources.82,83

Other aspects of treatment that would have been impacted by elevated NOM in the study lakes and associated elevated coagulant consumption include pH adjustment chemicals, filter backwashing, and on-site residuals management. Lime (Ca(OH)2) is a commonly used chemical to increase alkalinity and adjust the pH for optimal coagulant performance and has a reportedly high emission factor (0.774–0.94 kg CO2/kg), more than five times that of alum.80,84 Increased alum usage would require additional lime that would further impact indirect CO2 emissions during the NOM removal process. Elevated coagulant consumption caused increased filter backwashing frequency at both treatment facilities and generated increased residuals (e.g., clarifier sludge and backwash water) that require on-site management (e.g., dewatering) and eventual disposal offsite, which would also contribute to energy consumption. However, a follow-up study that would include an in-depth life cycle assessment at each facility would be required to quantify these emissions.

This study highlights the elevated demand and continued pressure on the aluminum industry to supply raw materials for aluminum-based coagulant products. The drinking water industry is inherently biased toward elevated coagulant consumption particularly in the United States where the D/DBP rule outlines TOC removal requirements with enhanced coagulation based on raw water TOC and alkalinity.85 The rule identifies enhanced coagulation as the best available technology for minimizing DBPs,85 which promotes increased coagulant use for facilities to meet compliance levels.

A paradigm shift to more sustainable practices is required in the aluminum industry86,87 and more broadly in the water sector. Switching to coagulants (e.g., iron-based) that have a lower carbon footprint80 may offer short-term relief, but ultimately, there is a need to shift toward more sustainable, low chemical treatment technologies, in addition to a transition to clean energy to reduce carbon emissions.88 In the face of this transition, the water industry requires a rapid modernization of technologies to improve energy efficiency within the water–energy nexus. A comprehensive review of available water treatment technologies and their associated carbon footprints is recommended to inform the selection of sustainable technologies and to assist water professionals in the design and operation of treatment facilities as well as for supply chain planning.

Accordingly, climate change adaptation and mitigation principles are urgently required for the water industry. Our data shed new light on immediate and long-term source water quality and water treatment challenges brought on by climate change. Globally, climate leaders at the 2023 United Nations Climate Change Conference (COP28) emphasized that water should be more systematically integrated into global climate policy at all levels and urged swift action at all levels (local to global) to realize the climate-resilient supply of water.89

Acknowledgments

We would like to acknowledge funding from the Natural Sciences and Engineering Research Council (NSERC) Alliance “Partnership for Innovation in Climate Change Adaptation in Water & Wastewater Treatment” (grant ALLRP 568507-21), with supporting industry organizations: Halifax Water, LuminUltra Technologies Ltd., Cape Breton Regional Municipality, Mantech Inc., City of Moncton, AquiSense Technologies, AGAT Laboratories, and CBCL Ltd. We also thank Halifax Water staff for facilitating the collection of operational and water quality data.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.est.4c01894.

  • Summary of raw water characteristics for both lakes; summary of monthly total precipitation and mean temperature data for the region; summary of days with a mean temperature above 0 °C; historical raw water UV254 for both lakes and DOC for Lake A; and historical alum dosing and flow data for both treatment plants (PDF)

The authors declare no competing financial interest.

Supplementary Material

es4c01894_si_001.pdf (929.7KB, pdf)

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