Odor characteristics of source water and effluent of DWTP
Fig.1 shows the odor categories and intensities of source water and effluent from QT DWTP. Four odor categories, including medicinal, chemical, musty, and septic odors, with intensities of 11~12, 11.4~12, 10.7~11.7, and 11.3~11.9, respectively, were detected in source water, suggesting severe and complicated odor characteristics in the QT River. After conventional coagulation, sedimentation, filtration, and disinfection, the odor characteristics were changed to medicinal, chemical, and musty odors with intensities of 6.4~9.2, 6.7~9.7, 6.8~9.2, respectively. Odor intensities of source water reached a strong scale (the maximum odor intensity), while odor intensities of effluent reached a moderate scale, indicating the critical odor situations in the QT River. Generally, musty odor is associated with the metabolism of microbes such as algae and bacteria activities (Li et al. 2012a; Zhang et al. 2013; Li et al. 2018), while medicinal, chemical, and septic odors are more related to exogenous pollutions (Suffet et al. 2004; Watson 2004; Agus et al. 2012). The odor characteristics are further elaborated with odor-causing compounds in the next section.
Odorants identified in source water of QT River and effluent of DWTP
Sensory evaluation of source water and effluent from DWTP located at QT River detected medicinal, chemical, musty, and septic odors with moderate or strong intensities (Fig. 1). It is noteworthy that the GC/O detected more olfactory odor peaks, as shown in Fig. 2, 3, and Table 1. Fourteen olfactory odor peaks, including four medicinal peaks, three chemical peaks, two musty peaks, four septic peaks, and one fishy peak, were detected in the QT River source water. Post conventional processes, there were still nine olfactory odor peaks, including four medicinal peaks, two chemical peaks, two musty peaks, and one fishy peak, detected in the effluent of DWTP. The GC/O detected olfactory odor peaks were almost consistent with the sensory evaluation outcomes. Among identified olfactory odor peaks, peak number (No.) 6, 7, 8, 11, 13, and 14 were all detected both in the source water and effluent, while, No. 1, 2, 3, 4, 5, 9, 10, 12 were only identified in the source water, and No. 15, 16, 17 in the effluent. Total seventeen olfactory odor peaks, including five medicinal peaks, four chemical peaks, two musty peaks, four septic peaks, and two fishy peaks, were detected in the QT River source water and DWTP effluent. The GC/GC analysis (Fig. S1) successfully screened and identified seventeen odor-causing compounds (Table 1), corresponding to the GC/O detected seventeen olfactory odor peaks.
Table 1 shows the identified relevant odorants for medicinal peaks No.1, 6, 7, 13, and 17 as styrene, phenol, 2-chlorophenol, 2-tert-butylphenol, and 2-methylphenol, respectively. Usually, phenolic odorants with medicinal odors associated with agriculture and industry are frequently detected in drinking and wastewaters (Davi et al. 1999; Agus et al. 2012). Especially, 2-tert-butylphenol, which is a typical industrial product of antioxidant, medical intermediate, pesticide intermediate, perfume raw material, etc., has been detected as a typical odor-causing compound in the cross-linked polyethylene pipes used for drinking water supply (Liu et al. 2017; Kalweit et al. 2019). The studies reported that the phenolic compounds detected in the source water are related to industrial sewage discharge (Chen et al., 2018). Fortunately, these compounds are rarely identified in the source and finished waters without industrial contamination.
The chemical odors for olfactory peaks No. 3, 8, 9, 15 were identified as cyclohexanone, 1,4-dichlorobenzene, nitrobenzene, and cis-3-hexenol. Benzene compounds are usually deemed essential chemical products and mainly obtained from coking, gasoline, and petroleum industries (Botalova et al. 2009). In contrast, cyclohexanone and cis-3-hexenol mainly involved with industrial solvents, additives for perfumes, and wastewater discharge (Mayuoni-kirshinbaum et al. 2012; Sheng et al. 2016). The odor descriptor for cis-3-hexenol reported green, grassy, melon rind-like with a pungent freshness (Watson. 2004). However, the olfactometry peak descriptor reported grassy, chemical, plastic by the FPA panel. The results might be related to odor characteristics' variation at different impact factors such as exposed subjects, naïve subjects, and concentration of odorants (Gallagher et al. 2015; Guo et al. 2019a).
The septic odor peaks of No. 2, 5, 10, and 12 were identified as propyl sulfide, diethyl disulfide, propyl disulfide, and indole. In the preceding odor studies across China, thioethers were discovered as significant septic/marshy odor-causing compounds (Guo et al. 2015; Wang et al. 2019b). These septic odor compounds were mainly associated with domestic pollution, industrial sewages, agricultural operations, etc (Schiffman et al. 2001; Liu et al. 2012; Lu et al. 2013).
The detection of various artificial compounds, such as phenols, benzenes, sulfides, etc., indicated that the water matrix of QT River was suffered from exogenous pollutions (Hu et al. 2020; Liu et al. 2021). The QT river is situated near the most agricultural, urban, and industrial areas in China, resulting in easy water contamination by exogenous pollutions. Supplementary protection procedures need to be adopted to alleviate the future risk of odor occurrence in the QT River. Significantly, the polluting factories should strictly adhere to the effluent standards. Also, cleaner production and end treatment should be strengthened in the production processes of polluting factories and enterprises. Otherwise, the risk of source water contamination from the QT River could emerge.
The olfactory peaks No. 11 and 14 with musty odors were recognized as 2-MIB and geosmin, respectively, the principal musty/earthy odorants in the worldwide water matrix (Lin et al. 2002; Watson 2004; Li et al. 2019), mainly produced by cyanobacteria and actinomycetes (Suffet et al. 1999; Li et al. 2012b; Sun et al. 2013). The olfactory peaks No. 4 and 16 with fishy odors were identified as heptanal and 2,4-heptadienal, respectively, which were reported to be found in fishy odor events and mainly associated with algal metabolites (Watson 2004). The presence of 2-MIB, geosmin, heptanal, and 2,4-heptadienal indicated that the microbial metabolism existed in the QT River. The detection of the above odorants, involved with exogenous pollution and microbial metabolism, suggested a high risk of odor incidents in the QT River and the subsequent need for further attention and protection in the future.
Evaluation and confirmation of identified odorants
Table 1 listed the identified odorants in source water and effluent of DWTP located at QT River in December 2013. A total of seventeen detected compounds were quantified (Fig. S2), and OAVs were measured to evaluate the identified odorants' contribution to source water and effluent odors. Fig. 4 demonstrates the identified compounds sorted based on their odor features and material structures as medicinal odorants (styrene, phenol, 2-chlorophenol, 2-tert-butylphenol, and 2-methylphenol), chemical odorants (cyclohexanone, 1,4-dichlorobenzene, nitrobenzene, and cis-3-hexenol), stink odorants (propyl sulfide, diethyl disulfide, propyl disulfide, and indole), musty odorants (2-MIB and geosmin) and fishy odorants (heptanal and 2,4-heptadienal).
Among the identified odorants in the source water of the QT River, the concentrations of styrene, phenol, 2-chlorophenol, and 2-tert-butylphenol with medicinal odors were 255~345, 252~342, 186~252, and 264~358 ng/L, respectively. The conventional coagulation, sedimentation, filtration, and disinfection, reduced the concentrations of the residual phenol, 2-chlorophenol, and 2-tert-butylphenol to 170~230, 160~216, and 179~243 ng/L, respectively. Usually, compounds with OAVs > 1 are considered crucial to the corresponding odor characteristics (Benkwitz et al. 2012). Based on measured OAV and rank of identified odorants in the QT River, 2-chlorophenol and 2-tert-butylphenol were considered major medicinal odor compounds. Geosmin and 2-MIB with OAVs of 8~10 and 7~9 in the source water and 6~8 and 4~6 in the effluent were significant compounds responsible for musty odor. OAVs of propyl sulfide, diethyl disulfide, propyl disulfide, and indole in the source water were 5~6, 1~2, 3~4, and 0.8~1.0, respectively, as major septic odorants in the QT River. Interestingly, the concentrations of chemical odorants were lower than their corresponding OTCs. However, the chemical odor intensities in the source water and effluent were strong and moderate, respectively. The observed result could be associated with the synergistic effect among odorants (Watson 2004; Guo et al. 2019a). Thus, it is necessary to verify the identified odorants' mutual odor effect by reconstitution test.
After the DWTP's conventional process, the effluent exhibited 2-methylphenol with a medicinal odor, cis-3-hexenol with a grassy/chemical odor, 2,4-heptadienal with a fishy odor. The observed concentrations of 2-methylphenol, cis-3-hexenol and 2,4-heptadienal in the effluent were 150~204 ng/L, 178~240 ng/L, and 101~137 ng/L, respectively. Compared with high OTCs of 14700 ng/L (2-methylphenol), 1200 ng/L (cis-3-hexenol), and 5000 ng/L (2,4-heptadienal), OAVs of these three emerged odorants were less than 1, confirming these compounds could not be major odor contributors.
Fig. 5 and Table S3 demonstrate the comparison of odor characteristics of primary samples and reconstituted water samples. Odor intensities of medicinal, chemical, septic, and musty odors in the reconstituted water sample could explain 87%, 87%, 89%, and 94% of corresponding odors in the source water from the QT River. Similarly, reconstituting identified odorants could explain 90%, 87%, and 88% of medicinal, chemical, and musty odors, respectively, in the effluent. Almost all odorants in the odor incident in the QT River were screened and identified in this study. The musty odor was mainly caused by geosmin and 2-MIB, medicinal odor was mainly caused by 2-chlorophenol and 2-tert-butylphenol, septic odor was caused by propyl sulfide, propyl disulfide, diethyl disulfide, and indole. The result was consistent with the OAVs' evaluation to identify odorants (Fig. 4). 87% of chemical odors' explanation in the reconstituted samples suggested that the major compounds causing chemical odor were identified and verified. Considering OAVs < 1 for compounds causing chemical odors, the synergistic odor effect of other odorants on chemical odor is possible (Watson 2004). Even though all identified odorants were added in the reconstituted water samples, odor intensities of primary samples could not be explained 100%, indicating that the odor incident of QT River was complicated and might be affected by other materials. As observed, odor-causing compounds under the limit of detection concentrations might be undetected, and other co-existing organic matters without odors could influence the whole odor profiles (Li et al. 2020).
Effect of PAC on removing odors in source water of QT River
Fig. 1 illustrates the odor characteristics of effluent from DWTP after the coagulation, sedimentation, filtration, and disinfection process. The results showed intensities for medicinal odor as 6.4~9.2, chemical odor as 6.7~9.7, and musty odor as 6.8~9.2. Fig. 4 shows the OAVs of geosmin with a musty odor, 2-MIB with a musty odor, 2-chlorophenol with medicinal odor, and 2-tert-butylphenol with medicinal odor in the effluent were still greater than 1. Thus, these odorants contribute significantly to the effluent's odor profile. These results indicated that the effect of conventional processes on odors' removal is limited. For emergency control of odors occurring in the source water of QT River, laboratory batch tests of activated carbon adsorption of odors of the source water from the QT River were performed. As shown in Fig. 6, an increase in PAC dosage improved the effect of PAC on odor removal. The addition of 15 mg/L of PAC followed by 30 min oscillation exhibited no odors detected in the source water sample from QT River. Immediately, the emergency control measure of 15 mg/L PAC was fed to the DWTP, which resulted in the complete removal of effluent odors (Fig. S3). PAC has proved to be an effective way to control emergency odor problems. The factors influencing the PAC adsorption, including pH, natural organic matter (NOM), operating conditions of PAC addition, etc., should be recognized in practical situations (Li et al. 2015). However, considering the pressing circumstances of emergency treatment and management of odor incidents in the source water, the specific conditions to use PAC and the associated problems need to be investigated in the future.