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septic odor and possible odorants in source and finished drinking water of major cities across China
Environmental Pollution 249 (2019) 305e310
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Occurrence of swampy/septic odor and possible odorants in source and finished drinking water of major cities across China* Chunmiao Wang a, b, Jianwei Yu a, b, *, Qingyuan Guo a, c, Daolin Sun a, Ming Su a, b, Wei An a, Yu Zhang a, b, Min Yang a, b a Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China b University of Chinese Academy of Sciences, Beijing, 100049, China c College of Environmental Science & Engineering, Yancheng Institute of Technology, Yancheng, 224051, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 November 2018 Received in revised form 9 March 2019 Accepted 11 March 2019 Available online 14 March 2019
Swampy/septic odors are one of the most important odor types in drinking water. However, few studies have specifically focused on it compared to the extensive reported musty/earthy odor problems, even though the former is much more offensive. In this study, an investigation covering the odor characteristics, algal distribution and possible odorants contributing to swampy/septic odor, including dimethyl disulfide (DMDS), dimethyl trisulfide (DMTS), diisopropyl sulfide (DIPS), dipropyl sulfide (DPS), dibutyl sulfide (DBS), 2-methylisoborneol (2-MIB) and geosmin (GSM), was performed in source and finished water of 56 drinking water treatment plants (DWTPs) in 31 cities across China. While the musty/earthy and swampy/septic odors were dominant odor descriptors, the river source water exhibited a higher proportion of swampy/septic odor (38.5%) compared to much higher detection rate of musty/earthy odor (50.0%) in the lake/reservoir source water. The occurrence of swampy/septic odor, which was much easier to remove by conventional drinking water treatment processes compared to musty/earthy odors, was decreased by 62.9% and 46.3% in river and lake/reservoir source water respectively. Statistical analysis showed that thioethers might be responsible for the swampy/septic odor in source water (R2 ¼ 0.75, p < 0.05). Specifically, two thioethers, DMDS and DMTS were detected, and other thioethers were not found in all water samples. DMDS was predominant with a maximum odor activity value (OAV) of 2.0 in source water and 1.3 in finished water. The distribution of the thioethers exhibited a marked regional characteristics with higher concentrations being detected in the east and south parts of China. The high concentrations of thioethers in lake/reservoir source water samples could be partly interpreted as the bloom of the cyanobacteria. This study provides basic information for swampy/septic odor occurrence in drinking water and will be helpful for further water quality management in water industry in China. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Swampy/septic odor Thioethers Drinking water Odor activity value
1. Introduction Unpleasant odor occurring in drinking water has long been a big € € issue for water utilities throughout the world (OmürOzbek and Dietrich, 2008). According to a nationwide survey in China, approximately eighty percent of source water exhibited odor
*
This paper has been recommended for acceptance by Charles Wong. * Corresponding author. Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China. E-mail address: [email protected] (J. Yu). https://doi.org/10.1016/j.envpol.2019.03.041 0269-7491/© 2019 Elsevier Ltd. All rights reserved.
problems, with the musty/earthy and swampy/septic odor being the dominant ones (Sun et al., 2014). Compared to the extensive reports on musty/earthy odor, which usually caused by algal or fungi metabolites including 2-MIB and GSM, few studies have specifically focused on swampy/septic odor though it is much more offensive and intolerable for its unpleasant sensory experiences for consumers (Cheng et al., 2005). The swampy/septic odor problems have been a major concern for drinking water in some countries. An intermittent strong swampy/septic odor was noted on source water abstracted from the River Dee (north-west England) and groundwater in some areas of Perth (Western Australia), and some volatile organic sulfur
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compounds (VOSCs) like DMDS and DMTS were identified as the main source of the swampy/septic odor in reports involving odor in water (Campbell et al., 1994; Franzmann et al., 2001). In China, concerns on swampy/septic odor problems caused by DMTS have greatly increased after the water crisis in Wuxi in 2007 (Yang et al., 2008). Most frequently encountered thioethers in natural water included dimethyl sulfide (DMS), DMDS and DMTS, exhibiting odors described as swampy/septic, rotten, rancid and stinky (Guadayol et al., 2016; Ma et al., 2013; Moreira et al., 2013), which could be perceived by human beings at concentration levels of ng/L or even less (Guo et al., 2016b; Zhou et al., 2017). Thioethers could be produced both under oxic and anoxic environmental conditions, involving in a wide array of abiotic or biological process (Watson and Jüttner, 2016; Findlay, 2016), including the biomethylation and polymerization of thiols (Chin and Lindsay, 1994; Stets et al., 2004). Methionine was indicated as the main precursor of thioethers in algae-induced bloom in some natural waters (Kiene et al., 1990). Up to now, sporadic detections of thioethers have been reported in previous studies in China. For example, an average DMDS concentration of 23.4 ng/L was reported in the source water of Huangpu River with swampy/septic odor intensity of 5e8 (Guo et al., 2016a). Even though DMTS and related alkyl sulfide were identified in some drinking water, the association of the thioethers with the widely distributed swampy/septic odor problems is actually not clear. In this study, characterization of the odors and detection of the potential swampy/septic odorants were performed in source and finished water of 56 DWTPs in 31 cities across China. Five typical thioethers of DMDS, DMTS, DIPS, DPS, DBS, and two typical musty/ earthy odor compounds of 2-MIB and GSM were determined to explore the association of thioethers with the swampy/septic odor. At the same time, the distribution of algae in source water was also investigated to clarify the possible contribution of algae to the swampy/septic odor. The results would provide a support for further management and coping with the swampy/septic odor problems in drinking water.
2. Material and methods 2.1. Chemicals A total of seven authentic standards including DMDS, DMTS, DIPS, DPS, DBS, 2-MIB and GSM were purchased from SigmaAldrich Co. (USA) at high level of purity (>99%). NaCl and ascorbic acid of guaranteed reagent purity grade was obtained from Beijing Chemicals Ltd. (China), and NaCl was heated to 450 C for 2 h before use. Ultrapure deionized water (>18 MU cm) was produced by a Milli-Q purification system. Stock standard solutions of 100 mg/L for all target compounds were prepared in ultra-pure water and stored at 4 C. The odor characteristics and odor threshold concentration (OTC) values of the odorants are listed in Table 1.
2.2. Sampling and pretreatment Source water and corresponding finished water samples were collected from 56 DWTPs in 31 major cities in China from July to December 2011. For each DWTP, the source and finished water samples were collected on the same day. Samples were refrigerated and transported to the laboratory in 500 mL amber glass bottles (without headspace). An excess of ascorbic acid (below 100 mg) was added to each water sample to eliminate the effect of chlorine. Among the 56 DWTPs, 51.8% adopt river water as source water and the remaining 48.2% relied on lake/reservoir water.
2.3. Odor evaluation Flavor profile analysis (FPA) was employed to evaluate odor characteristics of water samples. The FPA panelist training and application procedure was adopted from the Standard Methods for Water and Wastewater (American Public Health Association (APHA), American Water Works Association, Water Environment Federation, 2012). In this study, the panel consisted of five panelists for each test. The FPA protocol was described in detail in a previous study (Guo et al., 2016b). The odor activity value, calculated from ([detected concentration]/[threshold concentration]), was employed to evaluate the contribution of different odorants to the odor profile (Burdack-Freitag and Schieberle, 2012). 2.4. Algal enumeration Except for 15 river water samples with high turbidity, algal taxa and cell counts were performed for the other 41 surface water samples with a 1 mL phytoplankton counter chamber using a microscope (BX 51 Olympus, Japan) under a 20 objective lens. 100 mL samples for cell enumeration were preserved with 5% Lugol's iodine and stored for 48 h, and then the samples were pre-concentrated to 10 mL and kept in dark for cell counting. The algae identification and enumeration was described in detail in a previous study (Su et al., 2017). 2.5. HS-SPME/GCeMS procedure Analysis of 2-MIB and GSM was performed on a gas chromatography mass spectrometry (GC-MS, HP 6890/5975, USA) equipped with a 30 m capillary column (HP-5ms, 30 m 0.25 mm0.25 mm, Agilent Technologies, USA) combined with solid phase microextraction (SPME) (30/50-mm DVB/Carboxen/PDMS, No.57348, Supelco, USA), as described in a previous study (Sun et al., 2014). The detection limits were 0.60 ng/L for 2-MIB and geosmin, respectively. All the thioethers were analyzed by headspace SPME using an 85 mm Carboxen/PDMS fiber (No. 57334-U, Supelco, USA), followed by GC-MS analysis. Before the SPME procedure, the fiber was conditioned in the injection port of the gas chromatograph at 250 C for 30 min. SPME was performed using a manual SPME device (Supelco, USA). The sample volume used for SPME was 50 mL, in a 75 mL extraction vial. After adding 12.5 g NaCl to the samples, the SPME fiber was introduced to the headspace of the vial and extraction was carried out for 30 min at 55 C with constant stirring at 200 r/min. Then the fiber was desorbed at 250 C for 3 min in the injector port of the gas chromatograph. Thioether analysis was performed on a GC-MS (HP6890/5975, USA) equipped with a HP5ms column (60 m 0.25 mm0.25 mm, Agilent Technologies, USA). Helium (99.999%) was used as carrier gas at a constant flow of 2 mL/min. The oven temperature program was as follows: 35 C held for 5 min, then increased at a rate of 10 C/min to 110 C, finally ramped to 250 C at 20 C/min, held for 1 min. The mass spectrometer was equipped with an electron ionization source (EI, 70 eV) and maintained at 230 C. Parameters for selected ion mode (SIM) and odor characteristics of all the targets are shown in Table 1. 3. Results and discussion 3.1. Odor characteristics of the source and finished water Odor characteristics of source and finished water samples categorized by FPA are shown in Fig. 1 and Tables S1eS2. The dominant odor characteristics of the source and finished water were musty/earthy and swampy/septic odor. The occurrence of
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Table 1 Parameters for the SPME-GC/MS program (SIM) applied to the experiments. Compounds
IUPAC name
2-methylisoborneol 1,2,7,7-tetramethylbicyclo [2.2.1] heptan-2-ol geosmin (4R,4aR,8aS)-4,8a-dimethyl1,2,3,4,5,6,7,8-octahydronaphthalen4a-ol dimethyl disulfide (methyldisulfanyl)methane diisopropyl sulfide 2-propan-2-ylsulfanylpropane dipropyl sulfide 1-propylsulfanylpropane dimethyl trisulfide (methyltrisulfanyl)methane dibutyl sulfide 1-butylsulfanylbutane a
Abbreviation Target ion Qualifier ions Recovery % Quantification Odor description OTC (Guo et al., limits ng/L 2016b) ng/L 2-MIB
95
107, 43
102
1.80
musty/earthy
10
GSM
112
55, 41
106
1.80
musty/earthy
4
DMDS DIPS DPS DMTS DBS
94 103 89 126 146
79, 61 118, 61 118, 76 79, 64 90, 61
90 83 110 109 120
1.90 2.65 3.82 2.97 3.92
swampy/septic swampy/septic swampy/septic swampy/septic swampy/septic
30 n.a. 19 10 30 a
odor threshold determined by 3-alternative forced choice method.
Table 2 Concentrations of the detected odorants in the source and finished water samples. Category
Analytes Mean (SD) ng/L Max ng/L Detection frequency %
Source water
2-MIB GSM DMDS DMTS
Finished water 2-MIB GSM DMDS DMTS
Fig. 1. Odor characteristics categorized by FPA in the source and finished water samples.
musty/earthy odor in lake/reservoir source water (50.0%) was higher with an average FPA intensity of 4.7 compared with river source water (35.2%) with an average odor intensity of 4.1. Limited removal of musty/earthy odor was observed in the investigated samples (<10%), indicating that a typical sequence of water treatment routes (i.e., coagulation-sedimentation-filtrationchlorination) is not efficient to remove this odor from water (Cook et al., 2001). For swampy/septic odor, a higher detection rate (38.5%) was observed in the river source water with an average odor intensity of 4.5 compared with a detection rate of 29.2% and odor intensity of 5.0 in the lake/reservoir source water. The occurrence of swampy/septic odor, which was much easier to remove compared to musty/earthy odor, was decreased by 62.9% and 46.3% in river and lake/reservoir source water, respectively. Besides, chemical odor was also detected in river source samples, with an occurrence of 13.2% and an average odor intensity of 3.2, further indicating the complexity of odor problems in river source water due to the likely presence of some industrial or domestic wastes (Gallagher et al., 2015; Whelton et al., 2015). 3.2. Occurrence of typical odorants in source and finished water Table 2 summarizes the occurrence of the four odorants detected in source and finished water samples. The average 2-MIB concentrations in source and finished water samples were 10.8 ng/L and 7.3 ng/L, with detection frequency of 65.8% and 47.5% respectively. Compared to 2-MIB, GSM showed lower detection rate (28.9% and 34.4%) and average concentrations (0.69 ng/L and 0.85 ng/L) in source and finished water samples, respectively. Both compounds are reported to be difficult to remove by conventional
10.8 (20.7) 0.69 (1.3) 5.1 (9.6) 0.80 (4.0)
104 5.5 60.9 28.2
65.8 28.9 42.3 8.7
7.3 (20.3) 0.85 (2.0) 3.5 (9.3) 0.23 (1.4)
115 14.0 38.0 10.0
47.5 34.4 29.7 3.1
water treatment processes, thus activated carbon are frequently used for their control during water treatment process (Jo et al., 2011). The maximum concentration of 2-MIB was 104 ng/L in source water samples with an OAV of 10.4 much higher than the maximum OAV of GMS (1.4), indicating 2-MIB was the main cause of the musty/earthy odor in the investigated water samples of China, which was in accordance with a previous study (Sun et al., 2014). It was reported that 2-MIB and GSM were the most seasonable encountered musty/earthy odor compounds in drinking water throughout the world, which are usually related to proliferation of some cyanobacteria in lakes and reservoirs (Nam-Koong et al., 2016; Su et al., 2015; Lee et al., 2017). Specifically, Lin et al. (2002) reported that the musty/earthy odor of the two source waters of Taiwan was most likely contributed from 2-MIB (Lin et al., 2002). GSM was reported as a major musty/earthy odor compound in lakes of Kansas, USA (Dzialowski et al., 2009). On the other hand, DMDS and DMTS were detected among the investigated thioethers in this study. DMDS was the major detected thioethers in source water, with a relatively high detection frequency (42.3%) and concentrations of n.d.-60.9 ng/L, of which the detection frequency was 29.7% in finished water. DMTS was less frequently detected (<10%) both in source and finished water with average concentrations of 0.80 ng/L and 0.23 ng/L, respectively. The calculated OAV results showed that DMDS was predominant with a maximum OAV of 2.0 in source water and 1.3 in finished water. And the relationship between the total OAVs of thioethers and swampy/ septic odor intensity in source water samples followed a logarithmic linear equation (Fig. 2). Using Pearson correlation analysis, the total OAVs for thioethers showed a high correlation (R2 ¼ 0.75, p < 0.05) with the swampy/septic odor, suggesting that thioethers might be responsible for the swampy/septic odor in source water. As further indicated in Fig. 3, thioethers could be detected in 48.3% source water samples and 31.1% finished water samples. The lower detection ratio in finished water samples might be due to the prechlorination or chlorine disinfection process, which have been reported to be able to remove a variety of thioethers to some extent (Krasner, 1988; McGuire and Gaston, 1988). Furthermore, 3.6% of
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density of 6.5 105 cells/L, 1.9 105 cells/L, and 5.5 105 cells/L, respectively. And the average algal density of lake/reservoir source water samples (1.4 106) was higher than the river source water samples (3.2 105). According to Fig. 4 (a), the main odor type in source water in the Yellow River basin was musty/earthy odor, while swampy/septic odor as the main type in the Yangtze River and Pearl River basins. Fewer odor problems were detected in the northeast of China. Fig. 4(b) further describes the occurrence of thioethers in source water. As shown, DMDS was distributed more widely, and showed significant regional differences, with some of the highest concentrations being detected in the east and south parts of China. The highest total concentrations of thioethers in source water occurred in Wuxi City, where the concentrations of two investigated DWTPs were 55.9 ng/L and 64.4 ng/L respectively. In a similar manner, two typical thioethers of DMDS and DMTS were reported in the range of 3.29e78.83 ng/L and n.d.-46.88 ng/L, respectively (Chen et al., 2010; Huang et al., 2018; Deng et al., 2019). Pearson correlation analysis between the detected algae and the
Fig. 2. Relationship between the total OAVs of thioethers and swampy/septic odor intensity in source waters.
the source water samples showed total OAVs of thioether exceeding one, which was far less than the detection rate of swampy/septic odor, indicating that other than thioethers, some other chemicals might also have contributions to the swampy/septic odor. Notably, previous studies showed that different odorants might have mutual interaction, causing odor enhancement or suppression (Curren et al., 2009; Wilson and Stevenson, 2003). Guo et al. (2019) have confirmed that the musty/earthy odorants 2-MIB and GSM could enhance the septic odor caused by the typical septic odorants (Fig. S1) (Guo et al., 2019), a fact which could be used to interpret some of higher swampy/septic odor intensity detected with lower concentrations of thioethers in some samples. The synthetic effects of odorants should be further studied in the future. 3.3. The possible factors contributing to the occurrence of thioethers According to Fig. S2, cyanobacteria, chlorophyta, and diatoms were the dominant algae in the source water, with an average algal
Fig. 3. Cumulative frequency of the total OAVs of thioethers in the investigated samples.
Fig. 4. Distribution of odor characteristics of musty/earthy and swampy/septic odor strength (FPA intensity) (a) and detected thioether concentrations (b) in source water of 31 cities across China.
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total concentrations of thioethers using 27 groups of lake/reservoir source water data showed that DMDS was found to be correlated with cyanobacteria (R2 ¼ 0.69, p < 0.05) (Fig. S3). It has been reported that the anaerobic decomposition of massive cyanobacteria blooms could promote production of some thioethers like DMTS in some waters (Zinder et al., 1977; Yang et al., 2008; Zhang et al., 2010; Jiang et al., 2016). And the methionine was reported as the main precursor of thioethers in some algae-induced bloom natural water (Hu et al., 2007; Lu et al., 2013). Besides, Microcystis flosaquae was reported to be responsible for the generation of organic sulfides during algal blooms in Lake Neusiedl, Austria (Hofbauer and Jüttner, 1988) and Chabot Reservoir, Oakland, California (Jenkins et al., 1967). On the other hand, domestic wastes, agricultural and industrial emissions, such as bio-industry and fermentation industry, were reported as important source of thioethers (Landaud et al., 2008; Marie et al., 2015). For example, the emission of some volatile organic sulfur compounds occurred in a river heavily polluted with domestic and industrial wastewater in Guangzhou (Sheng et al., 2008) and other surface water systems (Muezzinoglu, 2003). In this study, high concentrations of thioethers and swampy/septic odor intensity were detected in the investigated river source water of Shanghai, which is also known for its high population densities and intensive industrial activities. Further study should be conducted to investigate the source and formation mechanism of thioethers in this area. 4. Conclusions The results showed that odor problems occurred widely in source and finished water in the investigated cities of China. The odor type was dominated by musty/earthy and swampy/septic odor. Two kinds of thioethers were detected, and DMDS was predominant, with a maximum OAV of 2.0 in source water and 1.3 in finished water, which was responsible for the most of the swampy/ septic odor (R2 ¼ 0.75, p < 0.05). Besides, the occurrence of thioethers showed a significant regional difference, which was higher in East and South China. And the high concentrations of thioethers in lake/reservoir source water samples might be related to the bloom of the cyanobacteria. This study will help to provide a more comprehensive understanding of distribution characteristics of swampy/septic odor and alkyl sulfide odorants in drinking water and guide taste and odor problem management. Acknowledgements This work was supported by Funds for the National Natural Science Foundation of China (Nos. 51778602, 21707117), the Major Science and Technology Program for Water Pollution Control and Treatment (Nos. 2015ZX07406001, 2017ZX07207004), and the Major Project of Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental Sciences, CAS (17Z02KLDWST). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.envpol.2019.03.041. References American Public Health Association (APHA), American Water Works Association, Water Environment Federation, 2012. In: Rice, E.W., Baird, R.B., Eaton, A.D., Clesceri, L.S. (Eds.), Standard Methods for the Examination of Water and Wastewater, twenty-second ed. American Public Health Association, Washington, D.C.
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of taste and odor by deep-living cyanobacteria in drinking water reservoirs by regulation of water level. Sci. Total Environ. 574, 1477e1483. Su, M., Yu, J.W., Zhang, J.Z., Chen, H., An, W., Vogt, R.D., Andersen, T., Jia, D.M., Wang, J.S., Yang, M., 2015. MIB-producing cyanobacteria (Planktothrix sp.) in a drinking water reservoir: distribution and odor producing potential. Water Res. 68, 444e453. Sun, D.L., Yu, J.W., Yang, M., An, W., Zhao, Y.Y., Lu, N., Yuan, S.G., Zhang, D.Q., 2014. Occurrence of odor problems in drinking water of major cities across China. Front. Environ. Sci. Eng. 8 (3), 411e416. Watson, S.B., Jüttner, F., 2016. Malodorous volatile organic sulfur compounds: sources, sinks and significance in inland waters. Crit. Rev. Microbiol. 43 (2), 210e237. Whelton, A.J., McMillan, L., Connell, M., Kelley, K.M., Gill, J.P., White, K.D., Gupta, R., Dey, R., Novy, C., 2015. Residential tap water contamination following the Freedom Industries chemical spill: perceptions, water quality, and health impacts. Environ. Sci. Technol. 49 (2), 813e823. Wilson, D.A., Stevenson, R.J., 2003. The fundamental role of memory in olfactory perception. Trends Neurosci. 26 (5), 243e247. Yang, M., Yu, J., Li, Z., Guo, Z., Burch, M., Lin, T.-F., 2008. Taihu Lake not to blame for Wuxi's woes. Science 319 (5860), 158-158. Zhang, X.-j., Chen, C., Ding, J.-q., Hou, A., Li, Y., Niu, Z.-b., Su, X.-y., Xu, Y.-j., Laws, E.A., 2010. The 2007 water crisis in Wuxi, China: analysis of the origin. J. Hazard Mater. 182 (1), 130e135. Zhou, X., Zhang, K., Zhang, T., Li, C., Mao, X., 2017. An ignored and potential source of taste and odor (T&O) issuesdbiofilms in drinking water distribution system (DWDS). Appl. Microbiol. Biotechnol. 101 (4), 1e14. Zinder, S.H., Doemel, W.N., Brock, T.D., 1977. Production of volatile sulfur compounds during the decomposition of algal mats. Appl. Environ. Microbiol. 34 (6), 859e860.
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Occurrence of swampy/septic odor and possible odorants in source and finished drinking water of major cities across China* Chunmiao Wang a, b, Jianwei Yu a, b, *, Qingyuan Guo a, c, Daolin Sun a, Ming Su a, b, Wei An a, Yu Zhang a, b, Min Yang a, b a Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China b University of Chinese Academy of Sciences, Beijing, 100049, China c College of Environmental Science & Engineering, Yancheng Institute of Technology, Yancheng, 224051, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 November 2018 Received in revised form 9 March 2019 Accepted 11 March 2019 Available online 14 March 2019
Swampy/septic odors are one of the most important odor types in drinking water. However, few studies have specifically focused on it compared to the extensive reported musty/earthy odor problems, even though the former is much more offensive. In this study, an investigation covering the odor characteristics, algal distribution and possible odorants contributing to swampy/septic odor, including dimethyl disulfide (DMDS), dimethyl trisulfide (DMTS), diisopropyl sulfide (DIPS), dipropyl sulfide (DPS), dibutyl sulfide (DBS), 2-methylisoborneol (2-MIB) and geosmin (GSM), was performed in source and finished water of 56 drinking water treatment plants (DWTPs) in 31 cities across China. While the musty/earthy and swampy/septic odors were dominant odor descriptors, the river source water exhibited a higher proportion of swampy/septic odor (38.5%) compared to much higher detection rate of musty/earthy odor (50.0%) in the lake/reservoir source water. The occurrence of swampy/septic odor, which was much easier to remove by conventional drinking water treatment processes compared to musty/earthy odors, was decreased by 62.9% and 46.3% in river and lake/reservoir source water respectively. Statistical analysis showed that thioethers might be responsible for the swampy/septic odor in source water (R2 ¼ 0.75, p < 0.05). Specifically, two thioethers, DMDS and DMTS were detected, and other thioethers were not found in all water samples. DMDS was predominant with a maximum odor activity value (OAV) of 2.0 in source water and 1.3 in finished water. The distribution of the thioethers exhibited a marked regional characteristics with higher concentrations being detected in the east and south parts of China. The high concentrations of thioethers in lake/reservoir source water samples could be partly interpreted as the bloom of the cyanobacteria. This study provides basic information for swampy/septic odor occurrence in drinking water and will be helpful for further water quality management in water industry in China. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Swampy/septic odor Thioethers Drinking water Odor activity value
1. Introduction Unpleasant odor occurring in drinking water has long been a big € € issue for water utilities throughout the world (OmürOzbek and Dietrich, 2008). According to a nationwide survey in China, approximately eighty percent of source water exhibited odor
*
This paper has been recommended for acceptance by Charles Wong. * Corresponding author. Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China. E-mail address: [email protected] (J. Yu). https://doi.org/10.1016/j.envpol.2019.03.041 0269-7491/© 2019 Elsevier Ltd. All rights reserved.
problems, with the musty/earthy and swampy/septic odor being the dominant ones (Sun et al., 2014). Compared to the extensive reports on musty/earthy odor, which usually caused by algal or fungi metabolites including 2-MIB and GSM, few studies have specifically focused on swampy/septic odor though it is much more offensive and intolerable for its unpleasant sensory experiences for consumers (Cheng et al., 2005). The swampy/septic odor problems have been a major concern for drinking water in some countries. An intermittent strong swampy/septic odor was noted on source water abstracted from the River Dee (north-west England) and groundwater in some areas of Perth (Western Australia), and some volatile organic sulfur
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compounds (VOSCs) like DMDS and DMTS were identified as the main source of the swampy/septic odor in reports involving odor in water (Campbell et al., 1994; Franzmann et al., 2001). In China, concerns on swampy/septic odor problems caused by DMTS have greatly increased after the water crisis in Wuxi in 2007 (Yang et al., 2008). Most frequently encountered thioethers in natural water included dimethyl sulfide (DMS), DMDS and DMTS, exhibiting odors described as swampy/septic, rotten, rancid and stinky (Guadayol et al., 2016; Ma et al., 2013; Moreira et al., 2013), which could be perceived by human beings at concentration levels of ng/L or even less (Guo et al., 2016b; Zhou et al., 2017). Thioethers could be produced both under oxic and anoxic environmental conditions, involving in a wide array of abiotic or biological process (Watson and Jüttner, 2016; Findlay, 2016), including the biomethylation and polymerization of thiols (Chin and Lindsay, 1994; Stets et al., 2004). Methionine was indicated as the main precursor of thioethers in algae-induced bloom in some natural waters (Kiene et al., 1990). Up to now, sporadic detections of thioethers have been reported in previous studies in China. For example, an average DMDS concentration of 23.4 ng/L was reported in the source water of Huangpu River with swampy/septic odor intensity of 5e8 (Guo et al., 2016a). Even though DMTS and related alkyl sulfide were identified in some drinking water, the association of the thioethers with the widely distributed swampy/septic odor problems is actually not clear. In this study, characterization of the odors and detection of the potential swampy/septic odorants were performed in source and finished water of 56 DWTPs in 31 cities across China. Five typical thioethers of DMDS, DMTS, DIPS, DPS, DBS, and two typical musty/ earthy odor compounds of 2-MIB and GSM were determined to explore the association of thioethers with the swampy/septic odor. At the same time, the distribution of algae in source water was also investigated to clarify the possible contribution of algae to the swampy/septic odor. The results would provide a support for further management and coping with the swampy/septic odor problems in drinking water.
2. Material and methods 2.1. Chemicals A total of seven authentic standards including DMDS, DMTS, DIPS, DPS, DBS, 2-MIB and GSM were purchased from SigmaAldrich Co. (USA) at high level of purity (>99%). NaCl and ascorbic acid of guaranteed reagent purity grade was obtained from Beijing Chemicals Ltd. (China), and NaCl was heated to 450 C for 2 h before use. Ultrapure deionized water (>18 MU cm) was produced by a Milli-Q purification system. Stock standard solutions of 100 mg/L for all target compounds were prepared in ultra-pure water and stored at 4 C. The odor characteristics and odor threshold concentration (OTC) values of the odorants are listed in Table 1.
2.2. Sampling and pretreatment Source water and corresponding finished water samples were collected from 56 DWTPs in 31 major cities in China from July to December 2011. For each DWTP, the source and finished water samples were collected on the same day. Samples were refrigerated and transported to the laboratory in 500 mL amber glass bottles (without headspace). An excess of ascorbic acid (below 100 mg) was added to each water sample to eliminate the effect of chlorine. Among the 56 DWTPs, 51.8% adopt river water as source water and the remaining 48.2% relied on lake/reservoir water.
2.3. Odor evaluation Flavor profile analysis (FPA) was employed to evaluate odor characteristics of water samples. The FPA panelist training and application procedure was adopted from the Standard Methods for Water and Wastewater (American Public Health Association (APHA), American Water Works Association, Water Environment Federation, 2012). In this study, the panel consisted of five panelists for each test. The FPA protocol was described in detail in a previous study (Guo et al., 2016b). The odor activity value, calculated from ([detected concentration]/[threshold concentration]), was employed to evaluate the contribution of different odorants to the odor profile (Burdack-Freitag and Schieberle, 2012). 2.4. Algal enumeration Except for 15 river water samples with high turbidity, algal taxa and cell counts were performed for the other 41 surface water samples with a 1 mL phytoplankton counter chamber using a microscope (BX 51 Olympus, Japan) under a 20 objective lens. 100 mL samples for cell enumeration were preserved with 5% Lugol's iodine and stored for 48 h, and then the samples were pre-concentrated to 10 mL and kept in dark for cell counting. The algae identification and enumeration was described in detail in a previous study (Su et al., 2017). 2.5. HS-SPME/GCeMS procedure Analysis of 2-MIB and GSM was performed on a gas chromatography mass spectrometry (GC-MS, HP 6890/5975, USA) equipped with a 30 m capillary column (HP-5ms, 30 m 0.25 mm0.25 mm, Agilent Technologies, USA) combined with solid phase microextraction (SPME) (30/50-mm DVB/Carboxen/PDMS, No.57348, Supelco, USA), as described in a previous study (Sun et al., 2014). The detection limits were 0.60 ng/L for 2-MIB and geosmin, respectively. All the thioethers were analyzed by headspace SPME using an 85 mm Carboxen/PDMS fiber (No. 57334-U, Supelco, USA), followed by GC-MS analysis. Before the SPME procedure, the fiber was conditioned in the injection port of the gas chromatograph at 250 C for 30 min. SPME was performed using a manual SPME device (Supelco, USA). The sample volume used for SPME was 50 mL, in a 75 mL extraction vial. After adding 12.5 g NaCl to the samples, the SPME fiber was introduced to the headspace of the vial and extraction was carried out for 30 min at 55 C with constant stirring at 200 r/min. Then the fiber was desorbed at 250 C for 3 min in the injector port of the gas chromatograph. Thioether analysis was performed on a GC-MS (HP6890/5975, USA) equipped with a HP5ms column (60 m 0.25 mm0.25 mm, Agilent Technologies, USA). Helium (99.999%) was used as carrier gas at a constant flow of 2 mL/min. The oven temperature program was as follows: 35 C held for 5 min, then increased at a rate of 10 C/min to 110 C, finally ramped to 250 C at 20 C/min, held for 1 min. The mass spectrometer was equipped with an electron ionization source (EI, 70 eV) and maintained at 230 C. Parameters for selected ion mode (SIM) and odor characteristics of all the targets are shown in Table 1. 3. Results and discussion 3.1. Odor characteristics of the source and finished water Odor characteristics of source and finished water samples categorized by FPA are shown in Fig. 1 and Tables S1eS2. The dominant odor characteristics of the source and finished water were musty/earthy and swampy/septic odor. The occurrence of
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Table 1 Parameters for the SPME-GC/MS program (SIM) applied to the experiments. Compounds
IUPAC name
2-methylisoborneol 1,2,7,7-tetramethylbicyclo [2.2.1] heptan-2-ol geosmin (4R,4aR,8aS)-4,8a-dimethyl1,2,3,4,5,6,7,8-octahydronaphthalen4a-ol dimethyl disulfide (methyldisulfanyl)methane diisopropyl sulfide 2-propan-2-ylsulfanylpropane dipropyl sulfide 1-propylsulfanylpropane dimethyl trisulfide (methyltrisulfanyl)methane dibutyl sulfide 1-butylsulfanylbutane a
Abbreviation Target ion Qualifier ions Recovery % Quantification Odor description OTC (Guo et al., limits ng/L 2016b) ng/L 2-MIB
95
107, 43
102
1.80
musty/earthy
10
GSM
112
55, 41
106
1.80
musty/earthy
4
DMDS DIPS DPS DMTS DBS
94 103 89 126 146
79, 61 118, 61 118, 76 79, 64 90, 61
90 83 110 109 120
1.90 2.65 3.82 2.97 3.92
swampy/septic swampy/septic swampy/septic swampy/septic swampy/septic
30 n.a. 19 10 30 a
odor threshold determined by 3-alternative forced choice method.
Table 2 Concentrations of the detected odorants in the source and finished water samples. Category
Analytes Mean (SD) ng/L Max ng/L Detection frequency %
Source water
2-MIB GSM DMDS DMTS
Finished water 2-MIB GSM DMDS DMTS
Fig. 1. Odor characteristics categorized by FPA in the source and finished water samples.
musty/earthy odor in lake/reservoir source water (50.0%) was higher with an average FPA intensity of 4.7 compared with river source water (35.2%) with an average odor intensity of 4.1. Limited removal of musty/earthy odor was observed in the investigated samples (<10%), indicating that a typical sequence of water treatment routes (i.e., coagulation-sedimentation-filtrationchlorination) is not efficient to remove this odor from water (Cook et al., 2001). For swampy/septic odor, a higher detection rate (38.5%) was observed in the river source water with an average odor intensity of 4.5 compared with a detection rate of 29.2% and odor intensity of 5.0 in the lake/reservoir source water. The occurrence of swampy/septic odor, which was much easier to remove compared to musty/earthy odor, was decreased by 62.9% and 46.3% in river and lake/reservoir source water, respectively. Besides, chemical odor was also detected in river source samples, with an occurrence of 13.2% and an average odor intensity of 3.2, further indicating the complexity of odor problems in river source water due to the likely presence of some industrial or domestic wastes (Gallagher et al., 2015; Whelton et al., 2015). 3.2. Occurrence of typical odorants in source and finished water Table 2 summarizes the occurrence of the four odorants detected in source and finished water samples. The average 2-MIB concentrations in source and finished water samples were 10.8 ng/L and 7.3 ng/L, with detection frequency of 65.8% and 47.5% respectively. Compared to 2-MIB, GSM showed lower detection rate (28.9% and 34.4%) and average concentrations (0.69 ng/L and 0.85 ng/L) in source and finished water samples, respectively. Both compounds are reported to be difficult to remove by conventional
10.8 (20.7) 0.69 (1.3) 5.1 (9.6) 0.80 (4.0)
104 5.5 60.9 28.2
65.8 28.9 42.3 8.7
7.3 (20.3) 0.85 (2.0) 3.5 (9.3) 0.23 (1.4)
115 14.0 38.0 10.0
47.5 34.4 29.7 3.1
water treatment processes, thus activated carbon are frequently used for their control during water treatment process (Jo et al., 2011). The maximum concentration of 2-MIB was 104 ng/L in source water samples with an OAV of 10.4 much higher than the maximum OAV of GMS (1.4), indicating 2-MIB was the main cause of the musty/earthy odor in the investigated water samples of China, which was in accordance with a previous study (Sun et al., 2014). It was reported that 2-MIB and GSM were the most seasonable encountered musty/earthy odor compounds in drinking water throughout the world, which are usually related to proliferation of some cyanobacteria in lakes and reservoirs (Nam-Koong et al., 2016; Su et al., 2015; Lee et al., 2017). Specifically, Lin et al. (2002) reported that the musty/earthy odor of the two source waters of Taiwan was most likely contributed from 2-MIB (Lin et al., 2002). GSM was reported as a major musty/earthy odor compound in lakes of Kansas, USA (Dzialowski et al., 2009). On the other hand, DMDS and DMTS were detected among the investigated thioethers in this study. DMDS was the major detected thioethers in source water, with a relatively high detection frequency (42.3%) and concentrations of n.d.-60.9 ng/L, of which the detection frequency was 29.7% in finished water. DMTS was less frequently detected (<10%) both in source and finished water with average concentrations of 0.80 ng/L and 0.23 ng/L, respectively. The calculated OAV results showed that DMDS was predominant with a maximum OAV of 2.0 in source water and 1.3 in finished water. And the relationship between the total OAVs of thioethers and swampy/ septic odor intensity in source water samples followed a logarithmic linear equation (Fig. 2). Using Pearson correlation analysis, the total OAVs for thioethers showed a high correlation (R2 ¼ 0.75, p < 0.05) with the swampy/septic odor, suggesting that thioethers might be responsible for the swampy/septic odor in source water. As further indicated in Fig. 3, thioethers could be detected in 48.3% source water samples and 31.1% finished water samples. The lower detection ratio in finished water samples might be due to the prechlorination or chlorine disinfection process, which have been reported to be able to remove a variety of thioethers to some extent (Krasner, 1988; McGuire and Gaston, 1988). Furthermore, 3.6% of
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density of 6.5 105 cells/L, 1.9 105 cells/L, and 5.5 105 cells/L, respectively. And the average algal density of lake/reservoir source water samples (1.4 106) was higher than the river source water samples (3.2 105). According to Fig. 4 (a), the main odor type in source water in the Yellow River basin was musty/earthy odor, while swampy/septic odor as the main type in the Yangtze River and Pearl River basins. Fewer odor problems were detected in the northeast of China. Fig. 4(b) further describes the occurrence of thioethers in source water. As shown, DMDS was distributed more widely, and showed significant regional differences, with some of the highest concentrations being detected in the east and south parts of China. The highest total concentrations of thioethers in source water occurred in Wuxi City, where the concentrations of two investigated DWTPs were 55.9 ng/L and 64.4 ng/L respectively. In a similar manner, two typical thioethers of DMDS and DMTS were reported in the range of 3.29e78.83 ng/L and n.d.-46.88 ng/L, respectively (Chen et al., 2010; Huang et al., 2018; Deng et al., 2019). Pearson correlation analysis between the detected algae and the
Fig. 2. Relationship between the total OAVs of thioethers and swampy/septic odor intensity in source waters.
the source water samples showed total OAVs of thioether exceeding one, which was far less than the detection rate of swampy/septic odor, indicating that other than thioethers, some other chemicals might also have contributions to the swampy/septic odor. Notably, previous studies showed that different odorants might have mutual interaction, causing odor enhancement or suppression (Curren et al., 2009; Wilson and Stevenson, 2003). Guo et al. (2019) have confirmed that the musty/earthy odorants 2-MIB and GSM could enhance the septic odor caused by the typical septic odorants (Fig. S1) (Guo et al., 2019), a fact which could be used to interpret some of higher swampy/septic odor intensity detected with lower concentrations of thioethers in some samples. The synthetic effects of odorants should be further studied in the future. 3.3. The possible factors contributing to the occurrence of thioethers According to Fig. S2, cyanobacteria, chlorophyta, and diatoms were the dominant algae in the source water, with an average algal
Fig. 3. Cumulative frequency of the total OAVs of thioethers in the investigated samples.
Fig. 4. Distribution of odor characteristics of musty/earthy and swampy/septic odor strength (FPA intensity) (a) and detected thioether concentrations (b) in source water of 31 cities across China.
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total concentrations of thioethers using 27 groups of lake/reservoir source water data showed that DMDS was found to be correlated with cyanobacteria (R2 ¼ 0.69, p < 0.05) (Fig. S3). It has been reported that the anaerobic decomposition of massive cyanobacteria blooms could promote production of some thioethers like DMTS in some waters (Zinder et al., 1977; Yang et al., 2008; Zhang et al., 2010; Jiang et al., 2016). And the methionine was reported as the main precursor of thioethers in some algae-induced bloom natural water (Hu et al., 2007; Lu et al., 2013). Besides, Microcystis flosaquae was reported to be responsible for the generation of organic sulfides during algal blooms in Lake Neusiedl, Austria (Hofbauer and Jüttner, 1988) and Chabot Reservoir, Oakland, California (Jenkins et al., 1967). On the other hand, domestic wastes, agricultural and industrial emissions, such as bio-industry and fermentation industry, were reported as important source of thioethers (Landaud et al., 2008; Marie et al., 2015). For example, the emission of some volatile organic sulfur compounds occurred in a river heavily polluted with domestic and industrial wastewater in Guangzhou (Sheng et al., 2008) and other surface water systems (Muezzinoglu, 2003). In this study, high concentrations of thioethers and swampy/septic odor intensity were detected in the investigated river source water of Shanghai, which is also known for its high population densities and intensive industrial activities. Further study should be conducted to investigate the source and formation mechanism of thioethers in this area. 4. Conclusions The results showed that odor problems occurred widely in source and finished water in the investigated cities of China. The odor type was dominated by musty/earthy and swampy/septic odor. Two kinds of thioethers were detected, and DMDS was predominant, with a maximum OAV of 2.0 in source water and 1.3 in finished water, which was responsible for the most of the swampy/ septic odor (R2 ¼ 0.75, p < 0.05). Besides, the occurrence of thioethers showed a significant regional difference, which was higher in East and South China. And the high concentrations of thioethers in lake/reservoir source water samples might be related to the bloom of the cyanobacteria. This study will help to provide a more comprehensive understanding of distribution characteristics of swampy/septic odor and alkyl sulfide odorants in drinking water and guide taste and odor problem management. Acknowledgements This work was supported by Funds for the National Natural Science Foundation of China (Nos. 51778602, 21707117), the Major Science and Technology Program for Water Pollution Control and Treatment (Nos. 2015ZX07406001, 2017ZX07207004), and the Major Project of Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental Sciences, CAS (17Z02KLDWST). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.envpol.2019.03.041. References American Public Health Association (APHA), American Water Works Association, Water Environment Federation, 2012. In: Rice, E.W., Baird, R.B., Eaton, A.D., Clesceri, L.S. (Eds.), Standard Methods for the Examination of Water and Wastewater, twenty-second ed. American Public Health Association, Washington, D.C.
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