Test
Data on Thermal and Catalytic Oxidizers
(9-1-05)
Department
of Toxic Substances Control (DTSC) test data on chlorinated dibenzodioxin
emissions, expressed as toxic equivalents (TEQ) per dry standard cubic meter
(dscm) of exhaust gas, from 3 thermal and 7 catalytic oxidizers treating
chlorinated solvents at 10 different SVE sites are summarized in figure 1
below. The data are plotted as a
function of inlet temperature to the first bed of the catalytic oxidation unit
or the outlet temperature of the thermal oxidizer versus the international TEQ
(I-TEQ). A total of 57 measurements at
the outlet of the units were made with 12 at greater than 700 oC
(thermal oxiders) and 45 at temperatures below 600 oC. (Many of the data points near the axis
overlap and do not appear as separate points.)
These data may not be representative of all thermal oxidization units
treating chlorinated wastes and additional testing has been underway.
Figure 1. All data compiled from field tests of operating units receiving dilute chlorinated solvent emissions from SVE; Dioxin/furan stack emissions expressed as toxic equivalents of 2,3,7,8-TCDD (with non-detects taken at 0.5 of detection limit), plotted against the operating temperature of the catalyst or thermal oxidizer.
The California Air Resources Board has tested
the inlets and outlets of two additional thermal oxidizers for dioxin
concentrations. Testing was performed
to assist the DTSC with their studies on dioxin emissions from catalytic and
thermal oxidizers used in site cleanups.
The main unanswered question of DTSC’s studies was whether dioxins at
the inlet of the oxidizers serve as a significant source of dioxins at the
stack. (Note that the test data for
Figure 1 are all outlet concentrations.)
Whittaker-Bermite Electrically-heated catalytic
oxidizer:
Dioxin testing occurred in August, 2002, on a
King Buck manufactured catalytic oxidizer.
The operating temperature was a steady 800 oF (427 oC)
during the testing. Each dioxin and
furan isomer concentration substantially increased from the inlet to the
outlet. The average concentrations for
the inlet, outlet, and percentage increase are listed in Table 1.
Table 1
|
|
INLET |
OUTLET |
|
|
|
Average |
Average |
increase |
|
|
ng/m3 |
ng/m3 |
% |
|
TCDF |
4.36E-04 |
2.31E-03 |
430 |
|
PeCDF |
3.97E-05 |
6.47E-04 |
1528 |
|
PeCDF |
4.60E-04 |
1.72E-03 |
273 |
|
HxCDF |
1.38E-04 |
1.98E-03 |
1340 |
|
HxCDF |
2.01E-04 |
2.19E-03 |
990 |
|
HxCDF |
2.33E-05 |
9.30E-05 |
299 |
|
HxCDF |
6.95E-04 |
3.93E-03 |
465 |
|
HpCDF |
1.40E-03 |
1.70E-02 |
1116 |
|
HpCDF |
1.60E-04 |
1.60E-03 |
899 |
|
OCDF |
6.30E-04 |
7.71E-03 |
1124 |
|
TCDD |
2.60E-05 |
4.26E-05 |
64 |
|
PeCDD |
2.70E-05 |
2.10E-04 |
679 |
|
HxCDD |
3.78E-05 |
1.91E-04 |
405 |
|
HxCDD |
5.78E-05 |
1.29E-03 |
2137 |
|
HxCDD |
8.29E-05 |
7.80E-04 |
841 |
|
HpCDD |
7.15E-04 |
9.26E-03 |
1195 |
|
OCDD |
2.23E-03 |
2.08E-02 |
830 |
Several isomers (or congeners) increased by more
than an order of magnitude, which leads to a greater than 1,000 % increase in
the isomer. These isomers are
1,2,3,7,8-pentachlorodibenzofuran, 1,2,3,4,7,8-hexachlorodibenzofuran,
1,2,3,4,6,7,8-heptachlorodibenzofuran, octachlorodibenzofuran,
1,2,3,6,7,8-hexachlorodibenzo-p-dioxin
and 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin. Based on international TEFs (US EPA, 1989),
the average dioxin I-TEQ increased 465%.
Southland Oil thermal oxidizer:
Dioxin testing occurred in December, 2003, on a
Soil Therm manufactured thermal oxidizer.
The operating temperature averaged 1455 oF (790 oC)
during the testing. All but three
dioxin and furan isomers increased in concentration from the inlet to the
outlet, but the overall percentage increase was much less. There was also a small decrease in
concentration at the outlet as compared to the inlet for
octachlorodibenzofuran, 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin, and octachlorodibenzo-
p-dioxin. The average
concentrations for the inlet, outlet, and percentage increase are listed in
Table 2.
Table 2
|
|
INLET |
OUTLET |
|
|
|
Average |
Average |
increase |
|
|
ng/m3 |
ng/m3 |
% |
|
TCDF |
3.13E-04 |
4.91E-04 |
57 |
|
PeCDF |
1.50E-04 |
4.43E-04 |
195 |
|
PeCDF |
6.42E-04 |
8.72E-04 |
36 |
|
HxCDF |
2.30E-04 |
5.74E-04 |
149 |
|
HxCDF |
1.58E-04 |
5.06E-04 |
221 |
|
HxCDF |
8.63E-05 |
1.30E-04 |
51 |
|
HxCDF |
5.46E-04 |
7.33E-04 |
34 |
|
HpCDF |
1.18E-03 |
1.48E-03 |
26 |
|
HpCDF |
1.75E-04 |
2.72E-04 |
56 |
|
OCDF |
9.98E-04 |
8.57E-04 |
-14 |
|
TCDD |
4.25E-05 |
8.24E-05 |
94 |
|
PeCDD |
5.96E-05 |
1.75E-04 |
194 |
|
HxCDD |
6.92E-05 |
1.81E-04 |
162 |
|
HxCDD |
1.45E-04 |
2.81E-04 |
94 |
|
HxCDD |
1.23E-04 |
3.59E-04 |
191 |
|
HpCDD |
1.44E-03 |
8.12E-04 |
-44 |
|
OCDD |
4.74E-03 |
1.71E-03 |
-64 |
Based on international toxicity equivalence
factors (US EPA, 1989), the average dioxin I-TEQ increased 63%. These data indicate that although there may
be a contribution of inlet dioxin concentrations to that observed at the
outlet, depending upon operational unit type and temperature, outlet dioxin
concentrations can increase significantly across the unit.
To
provide perspective on the observed values, consider that the cancer potency
value of chlorinated dioxins, expressed as TEQ, is 38 [ug/m3]-1. The cancer potency, when multiplied by the
concentration in the air, provides an estimate of the increased lifetime cancer
risk when a person is continuously exposed to that level. The average of the three measured values in
the exhaust of the highest emitting catalytic oxidizer unit (ca. 300 oC)
is 900 X 10-6 ug/m3 expressed as TEQ. Thus the calculated increased cancer risk of
an individual constantly breathing that exhaust over a 70-yr period is 3.4 in
100. That unit has been shut-down. For the thermal oxidizers operating above
700 oC, the estimate is less than 8 in 100,000 (the actual value is
estimated to be lower because of the assumption of one-half of the detection
limits for non-detected samples).
Dilution of the exhaust will also occur in the atmosphere (typically by
a factor of greater than 100 by the time a property line is reached) further
reducing those estimates. For
comparison, the actual cancer incidence in the U.S. population is about 1 in 4
over an individual's lifetime. Thus,
while there is on-going scrutiny and effort within our public agencies to
understand the role of thermal oxidation in the formation chlorinated dioxins
and to further reduce those emissions, there is no immediate threat to human
health.
The
USEPA maintains a website where the public can access information about dioxins
in the environment and their effects on health. You may follow the link to that site National Center for
Environmental Assessment - Dioxin and Related Compounds or download a
Questions and Answers PDF file prepared by the USEPA by clicking here http://www.epa.gov/ncea/pdfs/dioxin/dioxin
questions and answers.pdf. The
California Air Resources Board has also instituted a monitoring program for
dioxins. Data for ambient levels of
dioxins at several locations throughout the state can be accessed by following
the link to that site Dioxin Ambient
Air Monitoring Program.
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