Relationship between intensity, concentration, and temperature for drinking water odorants

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Water Research 38 (2004) 1604–1614

Relationship between intensity, concentration, and temperature for drinking water odorants Andrew J. Wheltona,*, Andrea M. Dietrichb a

US Army Center for Health Promotion and Preventive Medicine, Water Supply Management Program, Aberdeen Proving Ground, MD 21010-5403, USA b Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA 24061-0246, USA Received 23 January 2003; received in revised form 18 November 2003; accepted 18 November 2003

Abstract Odor analyses experiments indicated that, for the concentrations and temperatures tested, odor intensity was a function of both aqueous concentration and water temperature for water containing 1-butanol, free available chlorine, geosmin, n-hexanal, 2-methylisoborneol, and trans-2, cis-6 nonadienal. At weak odorant concentrations (approximately 4 on the flavor profile rating scale) the perceived odor intensity of these six chemicals was greater when the temperature was 45 C than was 25 C. Both of these temperatures are commonly encountered by consumers when they use tap water. Odor response to water containing isobutanal was affected by concentration but not water temperature. Experiments also revealed that reduction in aqueous concentration did not consistently reduce odor intensity; for some aqueous concentrations and chemicals an increase in odor intensity occurred at lower concentrations. r 2004 Elsevier Ltd. All rights reserved. Keywords: Drinking water; Odor; Temperature; Geosmin; 2-Methylisoborneol (MIB); Chlorine; Flavor profile analysis

1. Introduction Odor is a common drinking water problem that occurs throughout the world in countries that include Australia, Canada, France, Japan, South Africa, and the United States of America. Odor problems can be related to microbial byproducts, disinfectants, and disinfection byproducts. Geosmin (earthy-odor), 2-methylisoborneol (MIB, musty-odor), trans-2, cis-6-nonadienal (nonadienal, cucumber-odor), and n-hexanal (lettuce-heart, grassy odor) have been identified in numerous water supplies and tap waters [1]. These compounds are metabolites excreted by actinomycetes, cyanobacteria, or algae, and their ease of removal from water varies.

*Corresponding author. Tel.: +1-410-436-3919; fax: +1410-436-8104. E-mail addresses: [email protected] (A.J. Whelton), [email protected] (A.M. Dietrich).

Geosmin and MIB, which occur in water at concentrations from a few ng/L [2] to >800 ng/L [3], require ozone [4] or activated carbon [5] and are not readily removed by conventional water treatment processes (chlorination, coagulation, sedimentation, filtration). nHexanal is also difficult to remove by conventional treatment and requires either ozone or ozone in combination with hydrogen peroxide [4]. Nonadienal, on the other hand, can be removed by the application of free available chlorine [6] or permanganate [7]. The chlorinous odor from free chlorine (Cl2, HOCl and OCl ), which is typically present in drinking water at concentrations of 0–2 mg/L, is one of the most common drinking water complaints [8,9]. Free chlorine odor has the ability to mask earthy and musty odors, and this is one of the reasons earthy and musty odors are better detected when chlorine levels are lower [10]. Isobutanal, which has a sweet/fruity [11] or malty-odor [12], has been identified as a byproduct from ozonation, chlorination, and chloramination [13–15].

0043-1354/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2003.11.036

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Table 1 Aqueous odor threshold concentrations, reported aqueous concentrations for FPA values, and Henry’s law constants Odorant

Aqueous odor threshold concentration (mg/L)

Concentration yielding a FPA intensity of 4 at 45 C (mg/L)

Concentration yielding a FPA intensity of 8 at 45 C (mg/L)

Dimensionless Henry’s law constants

1-Butanol

200–500b

—

—

Free Chlorine (pH 5.0, 10.0)

280, 360d

1500, 1800 at 25 Cd

—

0.0003a (25 C)c —

Geosmin

0.006–0.01e,f

0.015–0.100e,f

0.056–0.920e,f

n-Hexanal

0.06–1.9e,h

19–68e

1000–9300e

Isobutanal

0.15–2.0j,k

—

—

2-Methylisoborneol

0.002–0.02f

0.005–0.100e,f

0.042–0.230e,f

trans-2, cis-6-Nonadienal

0.002–0.013e,f

0.050, 0.300f

—

0.0023 (25 C)g 0.0109 (25 C)i 0.0080 (25 C)h 0.0027 (25 C)g 0.0045 (25 C)l

a

Nirmalakhandan et al. [33]. Lillard and Powers [23]. c Brennan et al. [24]. d Krasner and Barrett [25]. e Burlingame et al. [26]. f Rashash et al. [27]. g Lalezary et al. [28]. h Mallevialle and Suffet [1]. i Syracuse Research Corporation [29]. j Guadagni et al. [30]. k Guadagni et al. [31]. l Esimated by Zander and Pingert [32]. b

Odors are detected by the olfactory system—or sense of smell—which is extremely complex. Odorants arrive at the olfactory receptors by either entering the nasal cavity through the nostrils or by traveling up passages from the nasopharyx to the olfactory cleft. Odorants stimulate the nervous system and cause the transfer of electrical signals from the olfactory receptors to the brain [16]. One of the significant issues when detecting and quantifying odor intensity is that perception is not only influenced by odorant contact with receptors but also the flow rate through the nose, the duration of odorant contact, and the individual’s sensitivity to odorants [17]. In the human population, a common olfactory abnormality is anosmia in which the ability to smell specific or all odors is lost [12]. Adaptation is a normal response that involves a loss in sensitivity due to continued exposure to an odorant [18–20]. This reduction in smell can allow individuals to become accustomed to an odor and be unable to detect it [21]. Before an odor can be perceived, the concentration in air must be greater than an individual’s odor threshold concentration (OTC). The OTC is defined as the

minimum concentration required for an individual to detect an odorant; OTCs are reported as a range of values and vary from person to person by factors of 10 or more [22]. Typical OTCs for the seven compounds used in this study are shown in Table 1. The amount of odorant that can be detected in the vapor phase is related to the aqueous phase concentration by Henry’s Law. Although, Henry’s Law constants were not available for the odorants in Table 1 at the temperatures tested, Henry’s Law predicts a higher vapor phase concentration as the temperature increases for compounds which have positive values for the enthalpy of solution. Flavor profile analysis (FPA) is used to quantify the taste-and-odor intensity of water samples and requires a minimum of four trained panelists [34]. This method is one of the most popular drinking water odor sensory methods and is described in Standard Method 2170B [34]. Panelists assign an intensity rating to each water sample taste-and-odor attribute using a 7-point category scale: 0=not detectable odor, 2=very weak, 4=weak, 6=weak-to-moderate, 8=moderate, 10=moderate-to-

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strong, 12=strong. After individual ratings are conducted, the panel then comes to a consensus to obtain a final water sample taste-and-odor profile. As shown in Table 1, the range of FPA intensities for each compound can vary significantly due to sensitivity of individual panel members. FPA experimental data can be described using the Weber-Fechner model. This model relates odor intensity to the logarithm of odorant concentration: Intensity =m log (concentration)+b. Although consumers use drinking water at temperatures that vary from cold to hot, few researchers have quantitatively analyzed how water temperature affects odor response. Two previous studies reported that dichloramine odor intensities were greater at 40 C than at 25 C, and the odor intensities for geosmin, MIB, and n-hexanal solutions were greater at 45 C than at 25 C [26,35]. Additionally, [36] found that water temperature influenced how subjects liked their drinking water. These researchers also concluded that chilling drinking water increased consumer palatability and acceptance. Food industry research has indicated that sample temperature influences consumer thresholds and responses [37,38,39]. Sizer and Harris [37] determined that serving temperature affected the rate at which an individual felt the chemical burn from capsaicin, a common food additive. In the chemical industry, [40] conducted a study whereby odor thresholds were measured for three industrial chemicals dissolved in water at 20 C, 40 C, and 60 C (Table 2). Results demonstrated that as temperature increased, a lower concentration of the chemical was required to reach the odor threshold concentrations. Specifically, there was a greater change in concentration between 20 C and 40 C. The objectives of this research were to determine how temperature and/or concentration affected odor response for geosmin, 2-methylisoborneol, trans-2, cis-6nonadienal, n-hexanal, free available chlorine, isobutanal, and 1-butanol solutions. Although 1-butanol is not an odorant in drinking water, it was included in this study because of its important history in sensory Table 2 Mean aqueous odor threshold concentrations of 1-butanol, diethanolamine, and propylene glycol methyl ether at 20 C, 40 C, and 60 C Chemical

1-Butanol Diethanolamine Propylene glycol methyl ether a

Mean odor threshold concentrations (mg/L)a 20 C

40 C

60 C

800 4,000,000 16,000

300 160,000 7400

270 160,000 4000

Adapted from Alexander et al. [40].

research [26,34,41–43]. Specific objectives were: (1) determine if odorant solutions at 25 C had the same odor intensities as solutions at 45 C; (2) determine the FPA rating of odorant solutions at different concentrations and two temperatures; and (3) determine differences in FPA ratings of geosmin solutions at 5 C, 25 C, and 45 C.

2. Materials and methods 2.1. Reagents The following high-purity chemicals were purchased from Sigma-Aldrich Chemical (St. Louis, MO): 1butanol (CAS 71-36-3), geosmin (CAS 16423-19-1), nhexanal (CAS 66-25-1), isobutanal (CAS 78-84-2), trans2, cis-6-nonadienal (CAS 557-48-1), citric acid (CAS 7792-9), and quinine monohydrochloride dihydrate (CAS 6119-47-7). Also used in this research were 2-methylisoborneol (CAS 2371-42-8; Supelco, Bellefonte, PA) and calcium carbonate A.C.S. grade (CAS 1317-65-3; Fisher Scientific, Pittsburgh, PA). Consumer grade sucrose (sugar) and sodium chloride (table salt) were also used in these experiments. Reagent water was obtained from a Nanopures ultrapure water system (Barnstead/Thermolyne, model #D4744, Dubuque, IA). 2.2. Glassware and plastic ware A 10% nitric acid solution was used to soak all of the glassware for 48 h. Sparkleens odor-free detergent (Fisher Scientific) was used to wash all glassware. Before all experiments, glassware was rinsed 10 times with tap water, and rinsed three times with reagent water. This process was repeated until no odors remained. Two hundred milliliters of reagent water was added to each 500 mL wide mouth Erlenmeyer flask to prevent the formation of chalky odors. Sixteen ounce DARTs Kresin cups and lids were used as received from the manufacturer without further treatment (Dart Container Corporation, Mason, MI). 2.3. Free available chlorine solutions Cloroxs bleach was used to prepare free available chlorine solutions. These solutions consisted of 50 mg/L calcium carbonate reagent water. Ten percent hydrochloric acid solution was used to adjust solution pH to 7.5. A pH of 7.5 was chosen because water pH typically varies between 7.0 and 9.0 in public water supplies and is a mixture of both HOCl and OCl . The free available chlorine concentration was measured using a free available chlorine Hach Kit (Hach Company, Loveland, CO).

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2.4. Panelists The Institutional Review Board at Virginia Tech approved the study protocol. All subjects were ages 20 to 45, either students or faculty and consisted of 6 males and 7 females. Prior to training and testing, panelists were advised not to wear perfumes, hand lotions, scented soaps, or similar odorous products. These panelists were also asked to refrain from eating, drinking, and smoking at least 30 min before testing. Panelists first underwent an introductory training as recommended for new subjects [44]. The Sense of Smell Kit (Carolina Biological Supply Company, Burlington NC) was used to familiarize the panelists with their sense of smell. Following this training, panelists were trained in flavor profile analysis (FPA) according to the American Waterworks Association (AWWA) manual [45] using solutions of citric acid (sour), quinine monohydrochloride dehydrate (bitter), sodium chloride (salty), and sucrose (sweet) plus a variety of common foods. After FPA training, the subjects were screened for their ability to detect specific odorants using triangle testing. This method is explained in further detail by [46] and [47]. Only persons who were shown to be able to detect a specific odorant were allowed to participate in data generation for that odorant.

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were evaluated in a particular sensory session and were presented in random order to promote impartial and unbiased results. Panelists were directed to wait 2 min between sample analyses and encouraged to sniff odor free water. During each FPA session, panelists were provided weak (FPA 4) and moderate (FPA 8) sour taste standards to calibrate their senses. To specifically calibrate their sense of smell, panelists were provided a 30 ng/L geosmin solution (FPA 4) in an Erlenmeyer flask at 45 C. After calibrating with both the taste and the odor standards, panelists evaluated odors in water samples. Panelists assigned an FPA odor rating value to each water sample, ranging from 0 to 12, even numbers [45]. Similar to Quantitative Data Analysis (QDA) [22], a mean and standard deviation was calculated for each water sample that the panelists rated. 2.6. Computational and statistical analyses A 2-tailed t-test was applied to directional difference data for geosmin, MIB, and nonadienal responses with an alpha value of 0.10 [49]. For FPA data, 1-tailed ttests were performed with an alpha value of 0.05. Trend line slopes for geosmin and MIB Weber-Fechner plots were compared statistically to determine whether or not they were different at an alpha value of 0.05 [49].

2.5. Sensory test sessions 3. Results All experiments were performed under negligible background odors. Directional difference testing involved the presentation of two 200 mL water samples (A and B) in 16 ounce DARTs-brand, translucent, plastic cups. DARTs-brand cups were previously shown to be odor-free and to perform similar to Erlenmeyer flasks [48]. Prior to testing, the 16 ounce plastic cups were placed inside clean ceramic coffee mugs to prevent the panelist from detecting the temperature of the water sample. During testing, panelists were presented with the cup (inside the mug), but were not allowed to touch it. The test administrator opened and closed the samples for all panelists. Panelists were asked to determine which sample had greater odor intensity. In this experiment the concentrations of geosmin, MIB, and nonadienal were equal. The water temperature of one sample was 2571.5 C, while the other was 4571.5 C. Flavor profile analysis was conducted using 200 mL odorant solutions in 500 mL wide-mouth Erlenmeyer glass flasks. Concentrations of odorant solutions were selected such that the lowest concentration yielded a weak FPA rating of about 4 and that the higher concentrations yielded FPA ratings of approximately 6– 10. Samples were either warmed inside a water bath (4571.5 C) or allowed to remain on the tabletop at room temperature (2571.5 C). No more than 6 samples

3.1. Screening odor intensities When the thirteen panelists were screened for their ability to detect the test odorants at concentrations used in this research, many panelists could not detect the odor either because they possessed a higher individual OTC or specific anosmia. Only 9 individuals could detected MIB, 10 detected n-hexanal or isobutanal, and 12 individuals could detect geosmin. All 13 panelists detected chlorine, nonadienal, and 1-butanol. Only panelists who passed the screening test were allowed to participate in research for that odorant. Directional difference tests demonstrated that when an odorant of weak intensity (4 on the FPA rating scale) was presented to panelists, a solution at 45 C was perceived as more intense than the same concentration of the same odorant at 25 C (Table 3). Because this procedure indicated an effect, further testing of temperature and concentration were conducted using FPA. 3.2. FPA rating of geosmin and MIB at 25 C and 45 C For geosmin solutions of equal concentration, the FPA ratings of earthy odor intensity were greater at 45 C than at 25 C (Table 4). The odor intensity

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Table 3 Directional difference experiments with single odorant concentrations constant and temperature varied at either 25 C or 45 C Odorant and Concentration

Trial number

Number of panelists choosing 45 C solution as more intense

Significant at 90% confidencea

1 2 3 4 5 6 7 8

10/12 11/12 10/12 10/11 10/11 10/11 12/12 10/11

Yes Yes Yes Yes Yes Yes Yes Yes

1 2

7/8 7/8

Yes Yes

1 2

11/13 11/13

Yes Yes

Geosmin (30 ng/L)

MIB (30 ng/L)

Nonadienal (40 ng/L)

a

2-tailed t-test.

Table 4 FPA rating of geosmin and MIB solutions at 25 C and 45 C Odorant

Concentration (ng/L)

Odor intensitya of solution at 25 C

Odor intensitya of solution at 45 C

Difference in odor intensity (45 C–25 C)

Significant at 95% confidenceb

Geosmin

30 200 400 600

2.9 3.4 3.8 3.0

4.5 5.0 5.8 5.8

1.6 1.6 2.0 2.8

Yes Yes Yes Yes

MIB

50 100 400 600

2.7 4.4 4.9 4.3

4.4 6.0 6.9 6.3

1.7 1.6 2.0 2.0

Yes Yes Yes Yes

a The value for the odor intensity is the mean of the FPA ratings by individual panelists; n=9–12; Standard error values were between 0.1 and 0.3 for all means. b One tailed t-test.

difference between the 25 C and 45 C geosmin solutions ranged from 1.6 to 2.8 FPA units with the odor intensity difference increasing as the concentration increased. Weber-Fechner plots for these data are presented in Fig. 1. A test of the 25 C and 45 C regression lines determined that the slopes were not statistically different and the lines were parallel. The data for the 600 ng/L geosmin solution were not included in the regression analyses because the QDA intensities were equal to or less than those for the 400 ng/L solutions, suggesting that adaptation occurred. All musty odor intensities of MIB solutions were statistically greater at 45 C than at 25 C (Table 4). Regression and statistical analysis of the 25 C and 45 C trend lines indicated that the lines were parallel (Fig. 2). Odor intensity differences for the

solutions at both temperatures varied from 1.6 to 2.0 FPA units. 3.3. FPA rating of nonadienal, n-hexanal, isobutanal, free available chlorine, and 1-butanol at 25 C and 45 C Cucumber odor intensity data for nonadienal are reported in Table 5. A temperature difference was found for only the 50 ng/L nonadienal solutions; a 1-tailed ttest demonstrated that the odor intensities of the 100 and 200 ng/L solutions at 45 C were not greater than at 25 C (Table 5). As concentration increased, the odor intensity difference decreased from 1.2 to 0 FPA units between nonadienal solutions. Similar to the nonadienal solution, a 1-tailed t-test indicated that the lettuce heart odor intensity of the

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7.0 Mean FPA Intensity

45oC

y = 1.04x + 2.89

6.0

1609

R2 = 0.85

5.0

25oC

4.0 3.0 2.0

y = 0.80x + 1.75

1.0

2 R = 0.96

0.0 0

0.5

1

1.5 2 Log Concentration (ng/L)

2.5

3

Fig. 1. Weber-Fechner plot of geosmin solutions at 25 C and 45 C. The regression line is for the 3 points connected by solid line.

8.0 Mean FPA Intensity

45oC

y = 1.78 x + 1.86

7.0

R2 = 0.71

6.0

25oC

5.0 4.0 3.0

y = 1.42 x + 0.86

2.0

R2 = 0.56

1.0 0.0 0

0.5

1

1.5

2

2.5

3

Log Concentration (ng/L) Fig. 2. Weber-Fechner plot of MIB solutions at 25 C and 45 C. The regression line is for the 3 points connected by solid line.

Table 5 FPA rating of Nonadienal, n-Hexanal, Isobutanal, free available Chlorine and 1-Butanol solutions at 25 C and 45 C Odorant

Concentration (mass/volume)

Odor intensitya of solution at 25 C

Odor intensitya of solution at 45 C

Nonadienal

50 ng/L 100 ng/L 200 ng/L 50 mg/L 800 mg/L 10 mg/L 1600 mg/L 1.88 mg/L 20 mg/L 100 mg/L

2.6 5.2 4.5 2.3 5.0 5.0 8.0 4.5 2.3 4.2

3.8 5.8 4.5 4.0 3.8 4.5 7.8 5.9 3.8 6.2

n-Hexanal Isobutanal Chlorine 1-Butanol

Difference in odor intensity (45 C–25 C) 1.2 0.6 0.0 1.7 1.2 0.5 0.2 1.4 1.5 2.0

Significant at 95% confidenceb Yes No No Yes No No No Yes Yes Yes

a The value for the odor intensity is the mean of the FPA ratings by individual panelists, n=9–13; Standard error values were between 0.1 and 0.3 for all means. b One tailed t-test.

50 mg/L n-hexanal solution was greater at 45 C than at 25 C. The 45 C 800 mg/L solution of n-hexanal did not have greater odor intensity than the 25 C solution (Table 5). FPA odor intensity differences were 1.7 FPA units at 50 mg/L and –1.2 FPA units at 800 mg/L.

Interestingly, the FPA intensity increased with concentration at 25 C (2.3 to 5.0), but did not increase at 45 C. A Weber-Fechner plot of isobutanal at 45 C is provided in Fig. 3. FPA data and 1-tailed t-test results indicated that the malty odor intensities of solutions at

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Mean FPA Intensity

8.0 6.0 4.0

y = 0.57 x + 2.99 R2 = 0.72

2.0 0.0 0

2

4 Log Concentration (ug/L)

6

8

Fig. 3. Isobutanal Weber-Fechner plot of solutions at 45 C (n=9).

Mean FPA Intensity

10.0 8.0 6.0 y = 2.82 x + 1.58

4.0

2 R = 0.97

2.0 0.0 0

0.5

1

1.5

2

2.5

3

Log Concentration (mg/L) Fig. 4. 1-Butanol Weber-Fechner plot of solutions at 45 C (n=9).

45 C were not greater than solutions at 25 C (Table 5). The differences observed were –0.5 FPA units at 10 mg/L concentration and 0.2 FPA units at 1600 mg/L concentration. Free available chlorine and 1-butanol FPA results are provided in Table 5. As the 1-tailed t-test results indicated, all free available chlorine and 1-butanol solutions at 45 C had greater odor intensities than solutions prepared at 25 C. The chlorinous odor intensity difference between the 25 C and 45 C solutions of free available chlorine (1.88 mg/L, pH 7.5) was 1.4 FPA units. The rancid odor intensity differences of 1-butanol were 1.5 and 2.0 FPA units in the 20 and 100 mg/L solutions. A Weber-Fechner plot at 45 C is shown for 1-butanol in Fig. 4.

intensity of the 30 ng/L geosmin sample at 25 C was not greater than the odor intensity at 5 C (p=0.134). Further testing and statistical analyses determined that when panelists were presented again with two 30 ng/L geosmin solutions, one at 5 C and one at 25 C, but were told that the 30 ng/L geosmin reference standard had an FPA =6 rating, the odor intensity of the 25 C solution was not greater than the 5  C solution (p=0.216) (Table 6). Due to author concerns for limited FPA value choices (possibly clustering the FPA intensity values), panelists were directed to ‘‘recalibrate’’ their senses using a 30 ng/L geosmin reference standard with a FPA =6 rating. Results from lower temperature testing indicated that although scaling affected the absolute intensity, the affect on relative odor intensity was not statistically significant.

3.4. FPA rating of geosmin at 5 C When panelists rated two 30 ng/L geosmin solutions, one cooled to 5 C and the other warmed to 45 C, the mean FPA ratings were 2.0 and 4.5 FPA units, with standard errors of 0.2 and 0.1 FPA units, for the 5 C and 45 C samples, respectively. A 1-tailed t-test indicated that the odor intensity of the 30 ng/L solution at 45 C was greater than at 5 C (p=0.007). When panelists compared 30 ng/L geosmin samples at 5 C and 25 C, statistical analyses demonstrated that the odor

4. Discussion 4.1. Relation to the literature FPA ratings of geosmin, MIB, nonadienal, n-hexanal, and free available chlorine from this research were consistent with FPA reported in the drinking water literature [25–27,50]. Similar to previous sensory research [40], this research found that as the aqueous

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Table 6 Geosmin FPA rating at 5 C and 25 C using either a FPA 4 or FPA 6 odor reference standard Odor intensity Rating with a FPA 4 reference standarda

Rating with a FPA 6 reference standarda

Aqueous Concentration (ng/L)

Solution at 5 Cb

Solution at 25 Cb

Solution at 5 Cb

Solution at 25 Cb

30 100

2.0 —-

2.9 —-

2.5 2.4

3.5 3.1

a b

Geosmin reference standard: 30 ng/L geosmin warmed to 45 C. Standard errors values were between 0.1 and 0.2 FPA units for all values.

concentration of 1-butanol was increased and the temperature of the 1-butanol solution was increased, the perceived odor intensity increased as well. Analogous to food and beverage industry research on product temperature [37–39], and limited research within the drinking water industry [26,35,36] results from this study support that drinking water temperature can affect drinking water odor response.

increased at the same rate as 45 C solutions (Figs. 1 and 2). Also, the odor intensities at 400 ng/L for these odorants were greater than the odor intensities at 600 ng/L concentrations. Figs. 1 and 2 can only be used to demonstrate adaptation and temperature effects at studied concentrations since geosmin and MIB WeberFechner plots were overly weighted near the mid-to-high end.

4.2. Temperature effects at 25 C and 45 C

4.3. Temperature effects at 5 C

Although the results of this study demonstrated that the odor intensities of many drinking water odorants were enhanced at the higher water temperature, the effect of water temperature on odor response was not consistent for all compounds or all concentrations tested. The human senses could only perceive an increased odor for selected aqueous concentrations. An increase in odor intensity was observed at the higher solution temperature for geosmin, MIB, nonadienal, nhexanal, free available chlorine, and 1-butanol when the concentration was about equivalent to that producing an odor intensity of FPA 4–6 at 45 C. This discovery is very important to the water industry because geosmin, MIB, and free available chlorine are three of the most common odorants and consumers will notice these odors more readily in warm water at weak odor intensities, typical concentrations found at consumers taps [25,51]. Indications of panelist adaptation were found for analyses of geosmin and MIB at concentrations of 400 ng/L and above, (Table 4), nonadienal for concentrations at or above 100 ng/L (Table 5), and n-hexanal for concentrations at or above 50 ng/L (Table 5). This finding is important from a water treatment standpoint because water treatment plant personnel and consumers may be unable to detect odor differences at higher concentrations or at higher water temperatures for certain odorants. Isobutanal odor response was unaffected by temperature at 10 and 1600 mg/L at 25 C and 45 C. Temperature affected odor response to geosmin and MIB solutions similarly. As concentration increased, both earthy and musty odor intensities at 25 C

Odor responsiveness to water containing geosmin demonstrated that a greater odor intensity difference was found between 5 C and 45 C solutions than between either 25 C and 45 C or 5 C and 25 C solutions. One implication of this finding is that when consumers compare odorous drinking water with greater temperature differences (5 C and 45 C vs. 25 C and 45 C) they are more likely to find a greater odor intensity difference. According to Henry’s Law, this greater temperature difference could result in more odorant volatilized into air and be available for sensory perception. Panelists were unable to detect a statistically significant odor intensity difference between lower temperature waters (5 C and 25 C), containing the same concentration of geosmin. Similarly, the panelists’ inability to detect an odor intensity difference could be attributed to Henry’s Law. Since lower water temperatures do not as fully promote the transfer of geosmin from water to air, panelists might not have been able to effectively determine whether or not the water samples were different. 4.4. Consumer odor complaints, odor monitoring, and water treatment strategy To better understand consumer odor complaints, water utilities should keep in mind that consumer odor thresholds and odorant anosmias differ between customers. This was observed in this research where only 69% of the people tested could detect 2-methylisoborneol. Individual thresholds and anosmias must be recognized for both water utility personnel and consumers. A

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consumer odor complaint may be filed for the reason that the affected consumer has an increased sensitivity to that odorant and other customers do not. Also, sensory analysts at the water treatment plant should be screened to be sure that they are not anosmic to the odorants that they are supposed to evaluate. Water utilities should recognize that consumer perception of drinking water odor might be quite different than the perception of the water plant operators during routine testing because of water temperature (this research) or chlorine levels [10]. At customer taps, consumers are exposed to drinking water from below ambient temperature to as high as 65 C and odor testing at the plant does not necessarily represent similar exposure conditions. For example, Standard Methods recommends water temperatures of 25 C and 45 C for flavor profile analysis [34]. Results of this research indicate that the water treatment strategy should consider the specific odorant present, its water concentration, and the likely water temperature consumers will be using. Failure to consider all of these characteristics, and subsequent lowering of the odorant concentration may or may not have a positive affect on odor intensity. For instance, by reducing geosmin and MIB from 400 ng/L to less than 30 ng/L, odor strength is decreased for both 25 C and 45 C samples. However, for a severe odor episode, reducing geosmin and MIB concentrations from 600 ng/ L to only 400 ng/L odor strength may actually increase odor strength. While a 400 ng/L concentration is nonetheless high, the resulting increase in odor strength could cause an increased complaint volume and consumer dissatisfaction.

5. Conclusions *

*

*

For odorant solutions prepared at weak odor intensities (approximately an FPA rating of 4), water temperature affected the perception of solutions containing geosmin, MIB, nonadienal, n-hexanal, free available chlorine, and 1-butanol. Panelists perceived more odor at 45 C than at 25 C for these compounds at FPA=4 concentrations. The response to isobutanal in aqueous solution was affected by concentration but not water temperature. Adaptation occurred for water containing between 400 and 600 ng/L geosmin, 400 and 600 ng/L, MIB, 100 and 200 ng/L nonadienal, 50 and 800 mg/L nhexanal, and 10 and 1600 mg/L isobutanal. At the higher concentrations cited for these odorants, an increase in odor intensity could not be perceived. For the same concentration of an aqueous solution of geosmin, reducing the water temperature for 45 C to 25 C reduced the perceived odor intensity. Reducing water temperature from 45 C to 5 C reduced the

*

perceived odor even more, while reducing temperature from 25 C to 5 C had no statistically significant effect. Particular attention must be made to ensure a wide range of FPA odor intensities are used to develop Weber-Fechner plots.

Acknowledgements The authors would like to thank the Awwa Research Foundation for the financial support, and specifically Djanette Khiari and Jarka Popovicova, for their technical advice. The authors valued the water treatment and water quality expertise of Gary Burlingame (Philadelphia Water Department) and the statistical guidance of Daniel Gallagher (Virginia Tech). The authors would also like to thank Robert Hoehn, Susan Duncan, and John Little of Virginia Tech for their guidance on this project. The following panelists are thanked for their participation: Borbala Boda, Brian Brazil, Judith Eggink, Vishal Gandhi, Charissa Harris, Cortney Itle, Ryan Kelly, Yan Kuang, Steve Kvech, Sandy Robinson, Paolo Scardina, and Brad Shearer.

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