Determination of Absolute Odor Intensities of
Emissions From a Wastewater Treatment Plant
Paper # 613
Roger Lewis, Ph.D., Keith Magers, Ph.D., Joseph Khoury, Ph.D.,
Sally Mathison, Ph.D., Leo Gallardo, and Jean Lee, Ph.D.
County Sanitation Districts of Los Angeles County
Joint Water Pollution Control Plant Water Quality Laboratory
24501 South Figueroa Street
Carson, CA 90745
ABSTRACT
Odorous gas streams are usually evaluated in terms of the threshold dilution ratio, which
is a measure of the odor concentration as determined by the dilution factor necessary to
reduce the odor of a sample to the detection threshold.
However, the threshold dilution ratio does not give a true measure of the perceived odor
intensity of the sample or gas stream. It is well known that the perceived intensity of an
odorous sample is not a linear function of the sample odorant concentration, nor of the
threshold dilution ratio. The Stevens Power Law states that the perceived intensity of an
odor varies as the odorant concentration raised to some exponent, with the exponent
typically having a value between 0.3 and 0.8. Atmospheric dispersion models cannot
accurately predict the perceived intensity of an odor at a given point without knowledge
of both the perceived intensity of the undiluted gas stream and the Stevens Law exponent
for that gas stream.
An odor panel has been used to determine the absolute odor intensities of odorous gas
streams by means of intensity matching to a seven-point scale based on 1-butanol. The
absolute odor intensities of successive dilutions of the sample are also determined. From
these data, the Stevens Law exponents for the odorous gas streams can be determined.
The absolute intensities of the undiluted samples can also be related to the threshold
dilution ratios of the samples. These techniques have been used to characterize odorous
emissions from biofilters and other odor control equipment at a large wastewater
treatment plant.
INTRODUCTION
Odorous gas streams are usually evaluated in terms of their odor concentration, or
threshold dilution ratio, which is defined as the dilution factor necessary to reduce the
odor strength to a barely perceptible level, where the odor concentration is defined as
being equal to one odor unit. The threshold dilution ratio of an air/gas stream can be
related to the perceived intensity of its odor1,2. However, the perceived odor intensity of
an air/gas stream is not a linear function of the concentrations of the odorants present in
the stream. The Stevens Power Law3 shows that the perceived intensity of an odorous
sample varies as the odorant concentration raised to some power.
Equation 1. Stevens Power Law
I = k C
where:
I = the perceived odor intensity of the sample
k = a proportionality constant
C = the odorant concentration
The exponent usually lies between 0.3 and 0.8.
Atmospheric dispersion models cannot accurately predict the potential impact of odorous
emissions at a downstream site without knowledge of both the perceived intensity of the
undiluted gas stream and the Stevens Law exponent for that stream. The odor removal
efficiencies of control equipment cannot be accurately predicted from odorous compound
removal efficiencies without knowledge of this exponent.
The Stevens Law exponent can be determined by measuring the perceived odor intensity
of a sample as a function of the sample dilution factor or sample concentration.
Conceptually, the Stevens Law exponent of any odorous air/gas stream can be measured
in this manner. However, a multi-component odorant mixture may not behave in the
same manner as a single component mixture, especially if the Stevens Law exponents of
the various odorants in the mixture are significantly different from each other.
The problem becomes more complex if the perceived odor intensities of the odorants are
not additive in some simple relationship. It is likely that the intensities of odorants of the
same chemical class are additive for a mixture of known concentrations, but it is
unknown if odors show the same proportional additivity as the gas stream is diluted.
The goal if this study, then, is to determine the concentration dependence of the perceived
odor of single odorous compounds and of mixtures of these compounds. These data are
used to characterize and predict the concentration dependence of odorous emissions from
biofilters employed at a large wastewater treatment plant.
Odor intensities were measured using an odor panel that determined the odor intensities
of biofilter air streams and standard single- and multi-component mixtures of the same
odorants by matching the sample intensities to those of a set of 1-butanol odor intensity
standards4.
EXPERIMENTAL
Materials
Methyl mercaptan, dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) were
obtained as single-component 100 ppm v/v (nominal) mixtures in nitrogen from Scott
Specialty Gases. Dimethyl trisulfide (DMT3S) was obtained as the neat liquid from
Aldrich. Chromatographic internal standards were obtained as a 50 ppb v/v (nominal)
multi-component mixture from Scott Specialty Gases containing 1,1-difluoro-2chloroethylene, 1-fluror-2-chloroethylene, halothane, fluorobenzene, 2,3-dicloropropene,
4-fluorotoluene, 1-chloro-3-fluorobenzene, 4-fluoro-meta-xylene, and 1-bromo-4fluorobenzene. 1-butanol used to make odor standards was obtained as the neat liquid
from Aldrich. UHP nitrogen (Praxair) was used as a diluent gas.
GC/MS System
Standards and samples were analyzed using a Leco Pegasus III time-of-flight GC/MS
system. The Agilent N6890 gas chromatograph was equipped with a 60 m x 0.32 mm
o.d. fused silica column (J&W DB-1, one micron film thickness). The helium carrier gas
was controlled at a constant flow of 2 mL/min (nominal) by means of an electronic
pressure control (EPC) system operating in the programmed pressure mode. Oven
temperature program consisted of a three-minute hold at 5o C, followed by a 5o C/minute
ramp to a temperature of 200oC, followed by a 40oC/minute ramp to a final temperature
of 250o, where the temperature was held for three minutes. Chromatographic column
effluent was routed to an open split interface (OSI, SGE Ltd.) before entering the mass
spectrometer. The OSI ensured that a constant flow of effluent was introduced to the
ionization region, thus eliminating response differences due to variable column flow rate.
Sample introduction was effected using an Entech 7100 cryogenic preconcentrator
equipped with an Entech 7032 autosampler. The preconcentrator is equipped with three
cryogenically cooled traps. The first trap consists of a fused silica-lined open tube, and is
cooled to approximately -20°C in order to remove water vapor from the gas sample
through condensation. The second trap is filled with Tenax TA and is cooled to
approximately -50°C, allowing the analytes to be trapped while allowing the permanent
gases to pass through the system. In each chromatographic analysis, a fixed amount of a
gaseous internal standard mixture is introduced and trapped on the Tenax adsorbent.
After the internal standards and sample components have been fully loaded onto the
Tenax trap, it is heated to 180°C, and the analytes are thermally desorbed and carried into
the third, cryofocusing-type trap. This small, open tubular trap is cooled to -160° C and
condenses the analytes in a narrow zone along its inner walls. After a four-minute
transfer time, the cryofocusing trap is rapidly heated, allowing the analytes to be injected
onto the chromatographic column in a tight band.
Sampling
Samples were taken in 6- and 15-liter fused silica-lined stainless steel canisters that were
prescreened for stability with sulfur compounds. It was found that some canisters
contained active sites that catalyzed the conversion of methyl mercaptan to dimethyl
disulfide. Canisters were screened by introducing 150 ppb methyl mercaptan in air into
the canister, then determining the actual methyl mercaptan concentration. The canister
was then reanalyzed after a 24-hour holding period, and the recoveries of the two
analyses were compared. Canisters with less than 90% methyl mercaptan recovery after
24 hours were deemed unsuitable for this study and were not used.
Biofilter samples were taken as grab samples at the common inlet to a series of biofilters
used to test various support media. Canisters were evacuated before sampling and were
filled to less than atmospheric pressure (typically 0.8-0.9 atm.) to prevent water vapor
condensation in the canister. Polytetrafluoroethylene (PTFE) tubing was used to connect
the canisters to the sample port. The samples were pressurized with UHP nitrogen to a
final pressure of approximately two atmospheres (1500 torr) and were allowed to mix for
several hours before analysis or further dilution. The highest concentration samples are
therefore diluted by a factor of two relative to the biofilter gas stream. Sample dilutions
were carried out by transferring either a known pressure or known volume of the original
sample into a second canister of known volume, then pressurizing the canister to
approximately 1500 torr with UHP nitrogen. The overall dilution factors used were
2,10,50, 250 and 1000 for all samples. After dilution, the canisters were fitted with a
sniffing port which consists of a length of 1/8” o.d. fused silica-lined stainless steel
tubing equipped with a needle valve to control flow. The samples were presented to the
odor panelists through glass nose cones affixed to the exit of the tubing. Samples were
presented at a flow rate of approximately 150 mL/min, as specified by ASTM method
544-994. Certain samples were also analyzed using the ASTM E-697-91 procedure5.
These samples were transferred to Tedlar bags and were analyzed promptly after
transfer.
Odor Intensity Standards
1-butanol odor intensity standards were made by injecting known amounts of the neat
liquid through a heated stainless steel “tee” into a flowing stream of UHP nitrogen where
it was swept into a 15-liter fused silica-lined stainless steel canister. The canisters were
pressurized to a final pressure of about 1500 torr. The standard canisters were equipped
with flow controllers (Veriflo model A202-3), which were connected to the canisters via
1/8” o.d. fused silica-lined stainless steel tubing. Each panelist sniffed the flowing
standards using a glass nose cone that was placed at the exit end of the tubing. Odor
standards were presented to the panelists at a flow rate of 150 mL/minute.
Odor Intensity Scale
A seven-step intensity scale was used in all odor intensity measurements in this work. 1butanol standards of 5.5 (level 1), 15, 40, 110, 300, 800 and 2100 ppm (level 7) were
designed to increase in concentration by a factor of approximately 2.7 with each intensity
level. Given that the Stevens Law exponent for 1-butanol is about 0.74, each successive
standard concentration increase should result in an increase in the perceived odor
intensity of about a factor of two. Level seven (2100 ppm 1-butanol) is judged to be
overpowering by all panelists in this study and level one (5.5 ppm 1-butanol) was
perceived as a weak odor by all panelists. The intensity scale can be described as:
Equation 2. Odor intensity scale
I = k’ (2n)
where:
I = the perceived odor intensity
k = a proportionality constant
n = the numerical value of the odor intensity level on the seven-point scale.
METHODOLOGY
For a single-component sample of known concentration, equations 1 and 2 can be
combined to yield:
Equation 3.
k’(2n) = kC
Or, in logarithmic form:
Equation 4.
log k” + n log 2 =  log C
A plot of the perceived odor intensity value n versus the logarithm of the concentration
should yield a straight line of slope 3.3. This relationship can also be expressed in terms
of the dilution factor Zi of a diluted sample1.
Equation 5. Relationship between perceived odor intensity and sample dilution factor.
Ii = k(Ci) = k(CoZi) = K(Zi)
where:
Ii = the perceived odor intensity of a diluted sample
Ci = the concentration of the diluted sample
Co = the concentration of the undiluted sample.
Combining equations 2 and 5 yields (in logarithmic form):
Equation 6.
log K” + n log 2 =  log Zi
Therefore, the actual concentration of a sample need not be known in order for the
Stevens Law exponent to be found. A plot of the perceived intensity value n versus the
dilution factor Zi of a sample should again yield a straight line with the slope 3.3 .
A sample was analyzed by determining its odor intensity at several dilution levels. Odor
intensities were determined by an odor panel that matched the perceived sample
intensities to the odor intensity of one of several 1-butanol standards.
In a typical experiment, the panelists were asked to match the odor intensity of each
sample dilution level to the odor intensity of one of the 1-butanol standards. If a panelist
determined that a sample odor intensity was less than that of the lowest standard (level 1),
the sample intensity was assigned a value of 0.5. Any sample found to have an
overpowering intensity was assigned an intensity value of 7. If a panelist could not
determine if a sample intensity was closer to one standard or another, a value midway
between the two was assigned; for example, if an intensity was found to be greater than
that of the level 3 standard, but less than that of the level 4 standard, it was assigned a
value of 3.5. The mean value of the panelists’ responses to each sample dilution level
was used as the perceived odor intensity at each dilution level. After the odor intensities
of each sample dilution were determined, each dilution level was analyzed by GC/MS to
determine the concentrations of the odorants in the sample.
RESULTS
Samples were taken during the months of October and November 2004 from the common
inlet of a series of biofilters. It was found that almost all of the significant odorants were
sulfur compounds. Four sulfur compounds were found in all samples: methyl mercaptan
(MeSH), dimethyl sulfide (DMS), dimethyl disulfide (DMDS) and dimethyl trisulfide
(DMTS). In a few samples, iso-propyl mercaptan and tert-butyl mercaptan were also
observed at very low concentrations. The sulfur gas concentrations determined for the
undiluted samples are listed in Table 1. In most samples methyl mercaptan was the
dominant odorant and was typically found at a concentration of two to five times higher
than dimethyl sulfide and five to ten times higher than dimethyl disulfide. Dimethyl
trisufide was found at concentrations less than 10 ppb in all samples. Also shown in
Table 1 are data for synthetic mixtures of the four sulfur odorants. These samples were
prepared to approximate the odorant concentrations observed in biofilter samples. As
expected, the slopes, intercepts and  values are similar to those of the biofilter samples.
Table 1. Sulfur Gas Odorant Concentrations (ppb v/v)
Date
14-Oct
22-Oct
26-Oct
29-Oct
2-Nov
5-Nov
9-Nov
12-Nov
16-Nov
Type
MeSH
Biof inlet
500
Biof inlet
840
Biof inlet
14
Biof inlet
0
Biof inlet
107
Mix 11/02(a) 130
Mix 10/22(b) 750
Biof inlet
690
11/12 repeat 690
DMS
380
480
5
325
24
22
410
280
280
DMDS
310
45
1
30
20
13
170
80
80
iso-PrSH tert-BuSH DMTS
5
2.5
<0.2
5
2
3
4
4
9
2
5
2
5
(a) Synthetic mixture equivalent to November 02 sample
(b) Synthetic mixture equivalent to October 22 sample
Odor intensity data are shown in Table 2. The intensities listed are the mean numerical
intensity levels calculated from the responses of the odor panelists. A typical plot of
perceived odor intensity versus dilution factor is shown in Figure 1. Sample dilution
factors ranged from 2 to 1000. In most cases, the highest dilution factors resulted in an
odor barely perceivable by the panel while the other dilution steps were chosen to result
in measurable differences in the odor intensities of the samples. In general, the odor
intensity plots have slopes lying between 1.5 and 2.0, which result in  values between
0.45 and 0.61. Linearity is good in most plots, although the lowest level point (highest
dilution factor) may reflect the canister background odor in some cases. The observed
slopes, intercepts and  values of the individual plots are shown in Table 2.
Figure 1: Intensity vs. Log Z 10/29
5
4.5
y = 1.6781x + 5.8512
R2 = 0.9811
4
3.5
Intensity
3
2.5
2
1.5
1
0.5
0
-4
-3
-2
Log Z
-1
0
The odor intensities as a function of concentration were also determined for each of the
four main sulfur odorants. Methyl mercaptan, dimethyl disulfide and dimethyl disulfide
odor intensities were measured over the concentration range of 1 ppm to 1 ppb, and
dimethyl trisulfide odor intensities were determined over the range of 0.1 ppb to 10 ppb.
At the highest concentration studied, methyl mercaptan typically had an odor intensity
equivalent to approximately 300 ppm 1-butanol (level 5), 1 ppm DMS was equivalent to
approximately 110 ppm 1-butanol (level 4), and DMDS was equivalent to about 40 ppm
1-butanol (level 3). DMTS was found to be a much stronger odorant than the others, and
had an odor intensity of approximately level 5 (300 ppm 1-butanol) at a concentration of
10 ppb—a concentration 100-fold lower than the concentration of methyl mercaptan with
the same odor intensity. The Stevens Law exponents derived from these studies range
from about 0.44 (DMTS), 0.40 (MeSH and DMS) to about 0.23 (DMDS). The plots are
shown in Figures 2a-d. There is a significant amount of scatter in the plots and it is seen
that the average odor intensity determined for a given concentration evaluated on
different days can be subject to large variations. There may or may not be a significant
difference in the slopes of the MeSH, DMS and DMTS plots, but the slope found for
DMDS does appear to be significantly lower than that of the others.
Table 2. Observed Odor Intensities and  Values.
Date
Type
14-Oct
22-Oct
26-Oct
29-Oct
2-Nov
5-Nov
9-Nov
12-Nov
16-Nov
2
Biof inlet
5.21
Biof inlet
6.25
Biof inlet
3.56(c)
Biof inlet
4.43
Biof inlet
4.06
Mix 11/02(a) 5.3
Mix 10/22(b) 5.57
Biof inlet
5.13
11/12 repeat 5.17
10
3.5
5
3
2.85
3.125
4
4.21
3.5
3.08
Dilution Factor
50
250
2.07
1.07
3
1.5
2.13
0.87
1.64
1
1.81
0.813
2.4
1.7
3
2.07
2.38
1.63
2.17
1
slope
I(o)

1.98
2.16
1.66
1.68
1.58
1.58
1.72
1.6
1.63
5.6
6.9
4.88
5.85
4.59
5.55
6.02
5.34
5.16
0.60
0.65
0.50
0.51
0.48
0.48
0.52
0.48
0.49
1000
0.67
0.563
1.1
0.79
0.63
0.7
(a) Synthetic mixture equivalent to November 02 sample.
(b) Synthetic mixture equivalent to October 22 sample.
(c) Dilution factor of 8.
DISCUSSION
The odor intensity plots for the four target odorants appear to be reasonable. The high
concentration (1 ppm) odor intensities are determined with reasonable accuracy and the
slopes are within the range of values expected for the Stevens Power Law. Odor
detection thresholds can be calculated by extrapolating the observed lines to level zero
(no odor present). The extrapolated detection threshold concentrations are in reasonable
agreement with values reported for MeSH, DMS and DMTS, but the extrapolated
threshold concentration for DMDS is significantly lower than reported elsewhere6,7. This
implies that the concentration dependence of DMDS odor intensity is somewhat stronger
than observed here.
Figure 2b: Dimethyl Sulfide
6
6
5
5
4
4
Intensity
Intensity
Figure 2a: Methyl Mercaptan
3
2
y = 1.3357x + 0.9382
R2 = 0.7397
1
y = 1.3426x - 0.0227
R2 = 0.609
3
2
1
0
0
0
1
2
3
4
0
1
Log C
3
4
Log C
Figure 2c: Dimethyl Disulfide
Figure 2d: Dimethyl Trisulfide
6
6
5
5
y = 0.751x + 0.7546
R2 = 0.6572
4
Intensity
Intensity
2
3
2
1
y = 1.4599x + 3.1143
R2 = 0.9871
4
3
2
1
0
0
0
1
2
Log C
3
4
-3
-2
-1
0
Log C
1
2
The slopes and Stevens Law  values shown in Table 2 are higher than the slopes and 
values for the individual components of the mixtures. This infers that there is some
degree of odor additivity in the odorant mixtures. Due to the varying concentrations of
the odorants, however, the contributions of the individual odorants to the overall intensity
behavior are not weighted equally. The concentration dependence of the odor intensity of
a mixture will necessarily reflect the behavior of the most intense odorants. If odor
intensities of the individual components of a mixture are calculated from the observed
intensity versus concentration functions and then summed, the contribution of each
odorant to the total odor intensity of the sample can be estimated. In these samples,
methyl mercaptan and dimethyl trislfide are the major contributors to the odor (40% and
35%, respectively) while dimethyl sulfide and dimethyl disulfide are lesser contributors
(15% and 10%, respectively). It is likely that the odor concentration dependence will be
driven by the concentration dependence of the major odorants.
If the total sample odor intensity is calculated from the intensity versus concentration
functions and it is assumed that the odorant intensities obey simple arithmetic additivity,
the concentration dependence of the odor intensity may be calculated. In all samples
evaluated in this work, the calculated concentration dependence has a much lower slope
than is observed experimentally. The calculated slopes and intercepts are compared with
the observed values in Table 3.
Table 3. Comparison of Measured and Calculated Odor Intensities and  Values
Date
14-Oct
22-Oct
26-Oct
29-Oct
2-Nov
5-Nov
9-Nov
12-Nov
16-Nov
Type
Biof inlet
Biof inlet
Biof inlet
Biof inlet
Biof inlet
Mix 11/02(a)
Mix 10/22(b)
Biof inlet
11/12 repeat
slope
1.98
2.16
1.66
1.68
1.58
1.58
1.72
1.60
1.63
I(o)
5.6
6.9
4.88
5.85
4.59
5.55
6.02
5.34
5.16

0.60
0.65
0.50
0.51
0.48
0.48
0.52
0.48
0.49
I(o) Calc
4.87
6.23
6.42
5.59
5.78
5.52
5.54
6.30
 Calc
0.37
0.38
0.39
0.40
0.38
0.39
0.39
0.39
(a) Synthetic mixture equivalent to November 02 sample.
(b) Synthetic mixture equivalent to October 22 sample.
There are several possible causes of this behavior. If the measured  value for methyl
mercaptan (the major odorant) is lower than the true value, calculated slopes would be
lower than expected. The experimentally observed slopes can be replicated if a  value
of about 0.58 (corresponding to a intensity versus concentration plot with a slope of 1.9)
is used. While use of this value will result in a line with a slope matching the observed
slope, the predicted odor intensities of the undiluted samples exceed the observed
intensities by one to two levels. Alternatively, if the observed odor intensity of
approximately level 5 at 1 ppm methyl mercaptan is used to anchor the intensity function
while keeping a  value of 0.58, the predicted detection threshold for methyl mercaptan is
much too high. The experimentally observed slope can also be replicated by increasing
the slopes of all intensity functions by about 23%; however this also results in calculated
undiluted intensities much higher than those observed.
It is unlikely that there is another undetected odorant present that has a significant odor
intensity and higher  value that could serve to increase the slope. Synthetic mixtures of
the odorants--which clearly cannot contain unknown odorants--also exhibit calculated
slopes that are much lower than those observed experimentally.
The most likely explanation for the difference, then, is that the odors do not exhibit
simple arithmetic additivity. A more complex additive relationship exists which results
in the odor intensity decreasing at a much faster rate than predicted as the mixture is
diluted. It is also possible that a lower intensity odorant masks the intensity of the
principal odorants in a manner that results in a faster intensity drop off. The existing data
do not allow any conclusions to be drawn as to the exact reason for the discrepancy
between observed and calculated odor intensity behavior.
The existing data for samples containing methyl mercaptan as the principal odorant show
an average Stevens Law  value of about 0.53. Any odor control strategy must
necessarily require any odorant removal device to operate at very high efficiency if
significant odor removal is to be achieved. For example, a ten-fold reduction in odor
intensity (90% odor removal efficiency) would require 98.7% odorant removal
efficiency. An odorant removal efficiency of 75% would result in a barely perceptible
odor intensity decrease of a factor of two. The relatively low Stevens Law exponents
found for the individual compounds suggest that gas streams containing a single odorant
such as methyl mercaptan must have extremely high removal efficiencies to achieve high
odor removal efficiencies. A gas stream containing methyl mercaptan only would require
an odorant removal efficiency of 99.6% in order to achieve 90% odor removal. A methyl
mercaptan removal efficiency of 88% would be required to reduce the odor intensity by a
barely perceptible factor of two.
ACKNOWEDGEMENTS
Enlightening discussions with the JWPCP Engineering Research staff are gratefully
acknowledged.
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