4.1 Introduction

Clean and safe drinking water is a basic need for human beings. Taste and odor in drinking water can be directly perceived by human, thus are often associated with the perception of water safety for consumers (Lin et al. 2019). In recent years, despite our ability to provide safe drinking water was improved substantially, the frequently occurring emerging pollutants in source drinking water led to poor taste and odor continues to be a culprit issue for water suppliers globally. As reported, a wide range of emerging pollutants including algal and/or bacterial metabolites and industrial pollutants can induce odor issues in drinking water. Two widely documented musty/earthy odor compounds, 2-methylisoborneol (2-MIB) and geosmin, are mainly produced by cyanobacteria and actinomycetes (Suffet et al. 1999; Suurnäkki et al. 2015). Thioethers (Wang et al. 2019a; Watson and Jüttner 2016), and indoles (Suffet et al. 1999) were reported to cause swampy/septic odor or similar odors. Chemicals generated from industrial activities, such as substituted benzenes, ethers, dioxanes, and dioxolanes, could cause chemical, solvent-like, sweet or aromatic odors (Quintana et al. 2015). These compounds with various structural features have different odor characteristics as well as diverse sources.

In China, public concern has been evoked recently due to the outbreak of a serious drinking water odor crisis in Wuxi City in 2007 (Yang et al. 2008), in Qinghuangdao City (2007), as well as in Lanzhou City (2014, 2015). An investigation of source waters among 34 major cities in China showed that 80% of samples exhibited odor issues (Sun et al. 2014), while the knowledge about the composition and sources of corresponding odorants is rather limited for most odor issues, except the most widely documented earthy/musty 2-MIB and geosmin. More systematic investigation is urgently needed to choose suitable treatment technologies and improve water quality standards for delivering much tasty water. This chapter presents the occurrence and potential sources of 95 odorants in source drinking water, based on a nationwide investigation of 98 drinking water treatment plants in China using a high-throughput quantification method of gas chromatography-triple quadrupole tandem mass spectrometry (GC–MS/MS) and Flavor Profile Analysis (FPA). The investigated odorants include aldehydes, compounds containing benzene-rings and no oxygen, cyclic acetals, ethers, indoles, terpenes and terpenoids, musks, phenols, pyrazines and thiazoles, thioethers. The results provide an informative reference for water quality management in the drinking water industry and would benefit the setting of related water quality standards for drinking water in China.

4.2 Simultaneous Quantification of 95 Odorants Using GC–MS/MS

A sensitive method combining liquid–liquid extraction with gas chromatography-triple quadrupole tandem mass spectrometry (GC–MS/MS) was established to simultaneously analyze 95 odor causing compounds in drinking water. Water samples underwent liquid–liquid extraction with dichloromethane by a factor of 1000. Three deuterated analogs of target analytes, dimethyl disulfide-d6, benzaldehyde-d6, o-cresol-3,4,5,6-d4 and 1,4-dioxane-d8 were used to correct the variations in recovery, five isotope-labeled internal standards (4-chlorotoluene-d4, 1,4-dichlorobenzene-d4, naphthalene-d8, acenaphthene-d10, phenanthrene-d10 respectively) were used prior analysis to correct the variations arising from instrument fluctuations and injection errors among different samples. Analyses were performed on a GCMS-TQ8040 (Shimadzu Co., Japan) equipped with a VF-624 ms column (length, 60 m; diameter, 0.32 mm; thickness, 1.8 μm; Agilent Technologies, USA). Multiple reaction monitoring (MRM) mode was used for the quantification of the target chemicals, as shown in Table 4.1. The procedure has been described in detail in a previous study (Wang et al. 2019b).

Table 4.1* Parameters of GC–MS/MS in multiple reaction monitoring mode
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Of the 95 compounds, the detection limits of the developed quantitative method were in the range of 0.10–100 ng/L, most of which were well below the odor threshold concentration of each odorant. The detection limits of most odorants were much improved by GC–MS/MS compared to other methods. The average recoveries of the most analytes in tap water samples were between 65 and 119%, and the method was reproductive (RSD < 20%, n = 5).

4.3 Odor Characteristics of Source Water

Odor characteristics of water samples were evaluated by Flavor Profile Analysis based on Standard Methods for the Examination of Water and Wastewater (American Public Health Association 2012). Source water samples were collected from 98 different drinking water treatment plants (DWTPs) in 31 cities throughout China from September 2015 to December 2018, covering ten watersheds including Songhua River, Liaohe River, Haihe River, Huaihe River, Yellow River, Yangtze River, Pearl River, Taihu Lake, Chaohu Lake and Dianchi Lake (Wang et al. 2021). Results showed that more than 90% source water samples exhibited odor problems, and earthy/musty (31.8%) and swampy/septic (45.4%) odors were dominant odor descriptors (Fig. 4.1). Swampy/septic odor was the major odor type in Pearl River (FPA intensity ≤ 9), Taihu Lake (FPA intensity ≤ 6), and Yangtze River (FPA intensity ≤ 7.3), with FPA average intensity of 2–3. Except Chaohu Lake and Dianchi Lake, earthy/musty odors were detected widely with odor intensity of less than 6.7. The detection frequency of swampy/septic odors and earthy/musty odors are comparable in Yellow River, Haihe River, and Huaihe River.

Fig. 4.1
figure 1

Source Authors

Odor characteristics of water samples in raw water samples (a) and distribution of earthy/musty and swampy/septic odors in water samples from major watersheds (b).

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4.4 Occurrence of the Investigated Odorants in Source Water

A total of 75 odorants were found in raw water with concentration ranges for individual odorants from not detected (n.d.) to hundreds of or thousands of ng/L (Table 4.2). About 30 odorants showed detection frequencies of above 30%, and 19 odorants were frequently detected (freq. > 50%). In lake/reservoir source water, 66 odorants were detected with 22 odorants, including two aldehydes, six non-oxygen benzene-containing compounds, one cyclic acetal, three terpenes and terpenoids, two phenols, five pyrazines, two thiazoles, and two thioethers, being frequently detected. In contrast, 67 odorants were detected with 17 odorants, including one aldehyde, four non-oxygen benzene-containing compounds, one cyclic acetal, one musk, one phenol, five pyrazines, two thiazoles, and two thioethers, being frequently detected in river source water.

Table 4.2* Concentrations of odorants detected in raw water samples obtained for 140 sampling events
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4.4.1 Odorants Related to Earthy/musty Odor

The occurrence of earthy/musty odors in drinking water has been attributed to the presence of several organic chemicals, including geosmin, 2-MIB, 2-isobutyl-3-methoxy pyrazine, 2-isopropyl-3-methoxy pyrazine and 2,4,6-trichloroanisole in literature (Pirbazari et al. 1992). These odorants could cause earthy/musty odor at low ng/L levels (Lin et al. 2019). In the current investigation, 2-isobutyl-3-methoxy pyrazine, 2-isopropyl-3-methoxy pyrazine, and 2,4,6-trichloroanisole were rarely detected during the investigation period (freq. < 5%), indicating their limited odor contribution.

2-MIB and geosmin have been reported to be the most emerging pollutants that contribute to the majority of earthy/musty odors in drinking water, and are produced by cyanobacteria and actinomycetes (Bruce et al. 2002). 2-MIB was detected in raw water at a concentration range of n.d.-250.97 ng/L with a detection frequency of 53.79%. Relatively higher concentrations (n.d. − 251 ng/L) of 2-MIB were observed with a detection frequency of 72% in lake/reservoir source water compared to river source water (n.d. − 148 ng/L). Geosmin, mainly exhibiting earthy odor, was detected at concentrations of < 11 ng/L and a detection frequency above 50%. By using Pearson correlation analysis, 2-MIB was identified as the major earthy/musty odor-causing compound in source water of China (p < 0.05). 2-MIB has been more prominent recently and has been commonly detected globally, especially in China. Some species of Oscillatoria, Planktothricoides, Pseudanabaena, and Leptolyngbya bijugata have been reported to be 2-MIB producers in China (Wang et al. 2015, 2011; Zhong et al. 2011).

In addition, concentrations of alkyl pyrazines were detected with a detection frequency of more than 50% (n.d. − 62.2 ng/L), these odorants typically possess earthy/musty/moldy/nutty odors, and might also contribute to earthy/musty odor by synthetic effects between multiple odorants (Wang et al. 2020).

4.4.2 Odorants Related to Swampy/septic Odor

Some small molecule organic compounds, such as thioethers, exhibit odors described as swampy/septic, rotten, rancid and stink ones, which could be perceived by human beings at low levels of ng/L or even less (Guo et al. 2016; Watson and Jüttner 2016). In China, concerns about swampy/septic odor problems caused by thioethers have greatly increased after the water crisis in Wuxi in 2007 (Yang et al. 2008).

Eight thioethers were detected in the major watersheds of China, among which dimethyl disulfide (DMDS) and dimethyl trisulfide (DMTS) were detected co-occurred widely in raw water (r = 0.93, p < 0.05), with concentrations of n.d.-714 ng/L (freq. 86%) and n.d.-84.4 ng/L (freq. 60%). DMDS and DMTS were reported to be commonly found in the decay of cyanobacteria, algae, or vegetation, wastewater discharges, and anoxic sediments (Watson and Jüttner 2016). Diisopropyl disulfide was also detected with a maximum concentration of 27.2 ng/L though with a very low detection frequency (2.1%). Microcystis flos-aquae was ever reported to be the causative agent for the generation of isopropyl-sulfur compounds (Hofbauer and Jüttner 1988; Jenkins et al. 1967; Xu et al. 2017). All other thioethers were less frequently detected (<10%).

The Pearson correlation analysis showed that thioethers were the major swampy/septic odor-causing compounds in source water of China (r = 0.45, p < 0.05). Thioethers were distributed widely in major watersheds, and higher concentrations were detected in the east and south parts of China. Based on Classification and regression tree analysis, TOC input and geographic location (watershed), were two major factors affecting the occurrence levels of thioethers (Fig. 4.2). Higher levels of DMDS in Taihu Lake, Yangtze River, and Yellow River at TOC of > 2.58 mg/L were observed. For DMTS, TOC was at the first level in the hierarchy, and samples taken from Taihu Lake, Pearl River, and Yellow River showed higher concentrations at TOC of ≥ 2.08 mg/L. Especially, DMTS at NH4-N ≥ 0.456 mg/L showed higher concentration compared to lower NH4-N in samples when TOC < 2.08 mg/L, illustrating the higher organic sulfides might originate from sulfur-containing amino acid (e.g., methionine or cysteine). The results showed that TOC input affected the distribution of thioethers in source waters predominantly. For example, dramatic increases in nutrient input of TOC and NH4-N were observed in Taihu Lake (China) during the odor crisis in June 2007, which might be caused by urban and agricultural development in the watershed (Zhang et al. 2010).

Fig. 4.2*
figure 2

Source Wang et al. 2021

Classification and regression tree of the explanatory factors regarding (a) DMDS concentrations, ng/L, and (b) DMTS concentrations, ng/L. Note: R., the abbreviation of “River”; L., the abbreviation of “Lake”; M, mean concentration in ng/L is shown at the top of each node and is marked with “□” in each box plot (outliers are not drawn).

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Besides, indoles are also reported to be strongly associated with septic odors in natural waters, with odor thresholds in the range of 0.1 − 300 μg/L (Lin et al. 2019). Indole was detected in raw water at concentrations of n.d. − 1025 ng/L with a detection frequency of 21%. 3-Methylindole was not found in any of the water samples. Indoles are largely present in livestock and municipal wastewaters (Kim et al. 2016).

4.4.3 Odorants Related to Fishy Odor

Fishy odor problems often occur in cold water, even under ice-covered water with low nutrient concentrations. The occurrence of fishy odor has been associated with algal metabolites, mainly polyunsaturated aldehydes (PUAs), such as 2,4-heptadienal, 2,4-decadienal, and 2,4,7-dectridienal (Pohnert 2002; Watson 2010; Wendel and Jüttner 1996). Some saturated aldehydes, e.g., hexanal, heptanal, and benzaldehyde, were also assumed to contribute to fishy odor (Li et al. 2016; Venkateshwarlu et al. 2004). As sporadically reported, fishy odor issues often occur in low-temperature period with low nutrient concentrations. The potential fishy odor-causing algae include chrysophytes, diatoms, cryptophytes, and dinoflagellates (Jüttner 1981; Li et al. 2016; Shinfuku et al. 2022; Watson et al. 2001; Zhao et al. 2013).

A total of nine aldehydes was investigated, and seven kinds of aldehydes were detected in raw water. Hexanal (n.d.-211 ng/L), benzaldehyde (n.d.-351 ng/L), nonanal (n.d.-150 ng/L), heptanal (n.d.-158 ng/L), and decanal (n.d.-135 ng/L) were the dominant aldehydes detected (freq. > 40%). Relatively higher average concentrations in lake/reservoir source water samples were observed for hexanal, benzaldehyde, nonanal, and decanal compared to river source water samples. 2,4-Heptadienal, 2-octenal, 2,6-nonadienal and 2,4-decadienal were rarely detected.

The odor activity values (ratio of odorant concentration and odor threshold concentration) were in the range of 0–0.81, 0–0.13, 0–0.20, 0–0.19, and 0–0.19 for hexanal, benzaldehyde, nonanal, heptanal, and decanal, with average values of 0.10, 0.01, 0.02, < 0.01, and 0.02. Hexanal was described as grassy, herbal, or fatty odor, with odor thresholds of 0.3–14,000 µg/L (Ömür-Özbek and Dietrich 2008; Pripdeevech and Wongpornchai 2013). Benzaldehyde has cherry, almond odor, and odor threshold concentrations of 0.5–4.5 µg/L (Lin et al. 2019). Nonanal and decanal exhibit orange-fruity odor dominantly, with odor thresholds of 1 µg/L and 0.1–2.53 µg/L, respectively (Dietrich and Burlingame 2020; Lin et al. 2019). Heptanal exhibits oily-fatty odor and might has a fishy odor in the co-occurrence with (E, Z)-3,5-octadien-2one, another oxylipin common during and after algae bloom episodes (Malnic et al. 2004). Overall, hexanal and heptanal might contribute to fishy odor by synthetic effects in the simultaneous presence of multiple odorants.

4.4.4 Odorants Related to Chemical/hydrocarbon Odor

Chemical/hydrocarbon odor problems caused by emerging chemical pollution are common in some open water bodies like river source water. Certain emerging pollutants not only pose health risks but also contribute to unpleasant odors. Serious odor incidents induced by sudden chemical spills could disrupt consumers’ normal lives for several days, bring about psychological fear and loss of confidence regarding the safety of drinking water, and even evolve into severe social public events (Gallagher et al. 2015). About 40 compounds were detected in the investigation that were associated with industrial pollution, including benzene-containing compounds, ethers, cyclic acetals, phenols, etc.

Several non-oxygen benzene-containing compounds, including ethylbenzene, para-xylene, indan, biphenyl, and 1-methylnaphthalene, were detected at concentrations of less than 3644 ng/L with detection frequencies of > 40% in raw water and correlated well. The co-occurrence of these substituted benzenes illustrated their similar source. Benzenes are important industrial chemicals, exhibiting sweet, varnish, gasoline, paint/putty, and solvent odors (Suffet et al. 2019), were reported mainly from discharging by petrochemical industries, like gasoline (Satoshi 2003).

Cyclohexanone was detected at concentrations of n.d.−351 ng/L with a detection frequency of 70%. The co-occurrence with benzaldehyde was observed in source water (r = 0.81, p < 0.05). Aldol condensation of cyclohexanone and benzaldehyde is an important industrial process manufacturing many fine chemicals of commercial interest (Tang et al. 2013). For example, α,α´–bisbenzylidene cyclohexanone is an important intermediate in the pharmaceutical industries, optics, rocket engineering, photolithography, and liquid crystalline polymers (Tabrizian et al. 2015; Vashishtha et al. 2015).

Bis (2-chloro-1-methylethyl) ether (2,2'-dichlorodiisopropyl ether, DCIP), is a b-haloether that is used as an extracting solvent in the chemical industry and as an organochlorine pesticide in agriculture (Iordache et al. 2009). It is also an undesirable by-product in the industrial production of propylene oxide and epichlorohydrin by the chlorohydrin process (Moreno Horn et al. 2003). Previous studies demonstrated the global detection of DCIP in the surface water of Ohio River of the U.S. (0.5–5 μg/L) (Kleopfer and Fairless 1972), the Rhine and Scheldt rivers of the Netherlands (Moreno Horn et al. 2003), the Elbe river of Germany (< 0.8 μg/L) (Franke et al. 1995), Odra River in Central Europe (0.8 μg/L) (Kuczyn´ska et al. 2004). DCIP was detected at concentrations of n.d.-1280 ng/L (freq. 42%) with a mean concentration of 35.8 ng/L in raw water of China (Wang et al. 2021). It is mainly distributed in the eastern and southern regions with higher concentrations detected in water samples from watersheds of Yellow River, Haihe River, Yangtze River, and Taihu Lake (Fig. 4.3). The occurrence of DCIP was in accordance with the distribution of industrial production of propylene oxide and epichlorohydrin, indicating DCIP might be a by-product of industrial activities related to epichlorohydrin/propylene oxide according to industrial distribution analysis.

Fig. 4.3*
figure 3

Source Wang et al. 2023. Note L, lake and reservoir source water; R, river source water; The extreme values were not plotted

Distribution of bis (2-chloro-1-methylethyl) ether in raw water samples of major watersheds and in different source water types (embedded plot)

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Cyclic acetals, including dioxanes and dioxolanes, which are normally formed as byproducts during resin manufacturing, have been reported to be involved in several drinking water odor incidents (Carrera et al. 2019; Quintana et al. 2015; Schweitzer et al. 1999). Among this group of compounds, 2-alkyl-5,5-dimethyl-1,3-dioxanes and 2-alkyl-4-methyl-1,3-dioxolanes have been particularly focused because of their low odor thresholds (5 − 10 ng/L) with sickening sweet, olive, latex paint, varnish, solvent, green apple, marshy/sulfurous/decaying vegetation and fishy/algal odors (Crump et al. 2014). The first documented odor incident with dioxanes and dioxolanes was in 1977. Since then, these chemicals intermittently reported as imparting odor issues in surface or ground waters globally (Bruchet et al. 2007; Quintana et al. 2015). Some recent odor incidents occurred in January-March, 2010 in North-East UK London and in Barcelona in the year of 2013–2014 (Quintana et al. 2015). In China, five cyclic acetals including 1,4-dioxane, 1,3-dioxolane, 2-methyl-1,3-dioxolane (2MDL), 2-ethyl-4-methyl-1,3-dioxolane (2E4MDL), and 2-ethyl-5,5-dimethyl-1,3-dioxane (2EDD) were detected according to the national investigation. 1,4-Dioxane (n.d.-7757 ng/L), 2MDL (n.d.-1644 ng/L), and 2E4MDL (n.d.-61.8 ng/L) were the predominant ones, with detection frequencies of 81%, 22% and 12%. 2EDD was rarely detected at concentrations of n.d.-12.6 ng/L, though it was the most identified malodor dioxane in literature.

In particular, relatively high occurrence levels and detection frequency were observed for dioxanes and dioxolanes in Huangpu River, China. Eight chemicals were detected, including 1,4-dioxane, 1,3-dioxane, 2,5,5-trimethyl-1,3-dioxane (TMD), 2EDD, 1,3-dioxolane, 2MDL, 2-ethyl-2-methyl-1,3-dioxolane (2E2MDL) and 2E4MDL. 1,4-Dioxane was the predominant dioxane (212 − 8310 ng/L, freq. = 100%) with a mean concentration of 1958 ng/L, followed by TMD (n.d. − 133 ng/L, freq. = 56.7%). 1,3-Dioxane (n.d. − 71.9 ng/L) and 2EDD (n.d. − 48.3 ng/L) were rarely detected in the river samples (< 5%). 2MDL (49.5 − 2278 ng/L), 2E4MDL (n.d. − 167 ng/L) and 1,3-dioxolane (n.d. − 225 ng/L) were the major dioxolanes as exhibited by their high detection frequencies (100%, 85.0% and 73.3%, respectively). 2E2MDL was less frequently detected (24.5%), with an average concentration of 2.6 ng/L. At the same time, DCIP was detected and co-occurred with the detected cyclic acetals at the concentration range of n.d. − 1094 ng/L with a detection frequency above 90%. The maximum odor activity values of 2E4MDL, TMD, 2EDD were 0.19 − 33.4, 13.3, and 4.8 − 76.7, respectively, should have contributed to the septic/chemical odor profiles in the river water.

4.4.5 Odorants Related to Medicinal/phenolic Odor

Phenols, especially chloro, and bromo substituted halophenols are reported to be associated with medicinal odors (Dietrich and Burlingame 2020; Suffet et al. 1999). Seven phenols were detected, and o-cresol (n.d.-11.4 ng/L), p(m)-cresol (n.d.-645 ng/L), o-nitrophenol (n.d.-422 ng/L), 2,6-dimethylphenol (n.d.-7.9 ng/L), 2-chlorophenol (n.d.-5.4 ng/L) were the dominated ones (> 20%) in raw water. Phenols are widely used in household products and as intermediates for the industrial synthesis of plastics, pesticides, insecticides, and petroleum (Murray et al. 2010), leading to their potential to cause odor issues in water sources. There was a serious plastic/chemical odor problem in the tap water of Hangzhou City, which was confirmed to be caused by the pollution of phenolic substances such as o-tert-butylphenol (Sun and Xiao 2014).

Specifically, p(m)-cresol showed that it co-occurred with geosmin in source waters (r = 0.64, p < 0.05). The two compounds were reported to co-occur in municipal wastewaters (Bylinski et al. 2019), industrial wastewaters (e.g. pulp and paper mill) (Cook and Hoy 2008), as well as in juices or wine when grapes containing fungal pathogens were processed (Welke 2019). Thus, the ability of microbes to produce geosmin and m-cresol suggests that their co-occurrence in raw water is due to natural causes.

Halophenols (chlorophenols, bromophenols) exhibit similar iodoform, phenolic, and medicinal odors, with odor thresholds in ng/L level (Young et al. 1996). The formation of halophenols in drinking water is usually related to the reaction of phenols and halogen ions during the chlorine process in water treatment plants as well as in water distribution system (Khiari et al. 1999; Whitfield et al. 1988).

4.5 Conclusion

The nationwide investigation of odor distribution characteristics across China showed widespread odor problems in source water samples. Earthy/musty, swampy/septic, fishy, chemical/hydrocarbon, and medicinal/phenolic odors were detected, and the potentially responsible odorants for each odor type were summarized. 2-Methylisoborneol and thioethers were identified as the main earthy/musty and swampy/septic odor-causing compounds, respectively. Some cyclic acetals were detected to contribute to chemical odor episodes in Huangpu River source water. Their co-occurrence with bis(2-chloro-1-ethylethyl) ether indicates the association with resin-related industrial pollution. The results would be helpful for the management of aesthetic quality of drinking water and provide references for setting water quality standards in China.