United States
 Environmental Protection
 Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
 Research and Development
 EPA-600/S7-81-125 Dec. 1981
 Project Summary
Spray Charging  and Trapping
Scrubber for  Fugitive  Particle
Emission  Control

Shui-Chow Yung, Julie Curran, and Seymour Calvert
  The control of fugitive particle
emissions (FPEs) with a Spray
Charging and Trapping (SCAT) scrub-
ber was evaluated both theoretically
and experimentally. The system uses
an air curtain and/or jets to contain,
convey,  and divert the FPEs into a
charged-spray scrubber.
  Experiments  were performed on a
225 mVmin bench-scale spray
scrubber to verify the theory and
feasibility of collecting fugitive par-
ticles with  charged water spray. The
effects of charge levels on drops and
particles, nozzle type, drop size, gas
velocity, and  liquid/gas ratio on
collection efficiency were determined
experimentally. The results of the
experiments and  the  comparison
between theory and data are presented.
  An air curtain  was developed for
conveying the FPEs to the spray
scrubber. The design and air flow field
for the air curtain are presented.
  A prototype  SCAT scrubber was
built to study the effects of crosswind
and hot buoyant plume. Available data
revealed that  the air curtain was
successful in deflecting crosswind up
to 15 mph and containing a hot
buoyant  plume. Theories were devel-
oped for predicting the trajectories of
the  air curtain jet stream and the hot
buoyant  plume.
  This Project Summary was developed
by EPA's Industrial Environmental
Research Laboratory. Research Tri-
angle  Park. NC, to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report
ordering information at back).

Introduction
  The Spray Charging and Trapping
(SCAT) scrubber system is a simple and
inexpensive way to control fugitive
particle emissions (FPEs). The SCAT
system  uses air curtains or air jets to
contain, divert, and convey FPEs into a
charged spray scrubber located near the
source.
  The SCAT system has two sections
arranged in a push-pull configuration
with the fugitive particle  emission
source located between them (Figure 1).
The fugitive particles are contained by
air curtains and are pushed from the
source  into the spray scrubber. The
scrubber  has a  low-pressure-drop
entrainment separator to remove the
spray drops.
  Water from the entrainment separator
can be passed through a separation
process, such as a filter, to remove the
collected dust particles. The water may
then be recycled and the dust may be
disposed of to prevent its redispersion.
Alternatively, a blowdown stream  of
dirty liquid may be directed to a disposal
system.
  The major SCAT system feature,
suiting it to FPE control, is the use of air
curtains and/or air push jets. The use of
air curtains minimizes the requirement
for solid boundaries and enables access
to the source. Air curtains could also be
used to deflect the wind or to deflect a

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buoyant plume from a hot source. The
SCAT  system  is very  compact  and
portable.

Preliminary Experiments
  The  SCAT system has three basic
functions, to: (1) contain and convey the
fugitive emissions to the scrubber, (2)
remove the particles with charged
water sprays,  and (3) collect  the
particles and water drops. There  is
insufficient  published information on
the design  and performance of an air
curtain and a charged-water spray
scrubber. To generate design data, the
air eurtain and the charged spray
scrubber were  studied individually  in
separate bench-scale experiments.  A
prototype SCAT system  was built and
used to study the effects of hot buoyant
plumes and crosswinds.

Air Curtain
  An air curtain is a sheet of moving air
formed by round or rectangular jets. Air
curtains have  been widely used  in
industrial  and  commercial plants,
mainly to provide constant access or to
isolate a warm interior  from the  cold
outdoors or vice versa.
  Most published information on air
curtain performance and design relates
to air  conditioning  and ventilation.
There  is little published literature on
dust containment even though it has
been used for this purpose in industry.
The design of the SCAT system requires
information on the jet expansion angle,
air entrainment ratio, mixing of particles
in the curtain,  and the effects  of
crosswind and hot sources. Jet expan-
sion angle and particle mixing determine
the overall  cross-sectional dimensions
of the  spray scrubber. The air entrain-
ment  ratio  determines  the volumetric
flow rate. Crosswinds and heat effects
dictate the nature and placement of air
curtains and sprays.
  Ideally, the air curtain should  have
small  expansion angle,  small  air en-
trainment ratio, and a uniform velocity
distribution.

Experiment
  The  air jet nozzle of  the air  curtain
used in this study was a continuous slot
2.1 m (7 ft) long. The slot was formed by
two parallel plates which protruded
22.9 cm (9 in.) from one side of the duct.
The distance between the plates, which
is the slot width, could be adjusted. The
slot was divided by thin cross-plates at
5.1 cm (2 in.) apart, so that the air would
discharge perpendicularly to the longi-
tudinal axis of the duct.
  The  discharge distribution  for this
manifold was uniform and the discharge
angle close to 90°.
  The  air  curtain flow field was
measured  for several  slot widths and
slot exit velocities with the slot vertical.
Linear  velocity was measured for three
vertical levels at several locations
downstream of the slot. The  jet ex-
pansion angle and the air entrainment
ratio were calculated from the measured
velocity distribution. The results were
compared  with  the  equations  derived
from a two-dimensional jet exhausting
into  still surroundings and jets with
two-sided  expansion. Figures 2 and 3
show the  measured centerline axial
velocity decay and entrainment ratio,
respectively.  The  measurements  lie
between the predictions by Abramovich
and the present study and are equal t<
the average of the two predictions.
  The  measured jet expansion angk
was  20-28°, depending on air  exi
velocity. The  average of measured je
expansion  angles  agrees  with tha
calculated for a  pure momentum jet.

Charged-Spray Scrubber
  For a spray  system,  collection b\
drops is the principal collection mecha
nism and the particle penetration for E
given size particle depends on the drop
diameter, the collection efficiency of a
single drop,  and the ratio of liquid-to-
gas flow rates.
  The  instantaneous single-drop  col-
lection efficiency has been determined
experimentally  for  uncharged drops
collecting uncharged particles. There is
no explicit expression for single-drop
efficiency  when electrostatic,  inertial.
      Q=
    Air Curtain



Hot Fugitive
Emission
Source

o

o
                                                SCAT
                                                Spray
                                                Scrubber
                             0
             SCA T system arrangment.
                                                               '-27 cm
                                                            A 2.54 cm
                                                               3.81 cm
                                                       300
                       500
                                  2L-- Dimensionless
                                   W
Figure 2.    Measured and predicted centerline axial velocity decays.
TOOL
                                                                                                                 I

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 and viscous forces are simultaneously
 present. The equations of motion have
 been solved  numerically for the
 collection of fine particles by a single
 spherical collector with various combi-
 nations  of charge: the results can be
 used for predicting particle penetration.
 Experiment
   Charged-spray scrubbers have been
 studied experimentally by  several
 researchers. However,  their data  are
 not suitable for verifying a mathematical
 model because several important elec-
 trical parameters were not defined.
   To obtain design data under well
 defined  conditions, the  charged-spray
 scrubber system shown in Figure 4 was
 built. The  system consisted of a flow
 straightening section, an  inlet particle
 sample  section, a particle charging
 section, a spray section, an entrainment
 separator, and  an outlet  sampling
 section.
   The particle charger section consisted
 of two rows of corona wires and ground
 electrode  tubes. The spray  section
 included two spray banks: the water
 was charged by induction.
   Experiments were performed  to
 determine  the minimum water require-
•ments,  to  evaluate spray nozzles,  to
 study various drop charging methods, to
 determine  the effect of drop and/or
 particle charging on particle collection
 efficiency, and  to verify published
 theories. The particle collection effi-
 ciency,  of  the charged-spray scrubber
 was determined by injecting redispersed
 dust to the blower inlet and by simul-
 taneously  measuring the  particle size
 distribution and mass concentration at
 the inlet and outlet of the  scrubber.
   Particle charge was measured with a
 Faraday  cup  which  consisted of  an
 isolated, shielded, glass-fiber filter
 connected  to an electrometer. The filter
 collected the particles and their charges
 which were measured by leaking them
 to the ground through the electrometer.
 Charge/mass ratio was calculated from
 the measured charge and particle mass
 on the filter.
   Drop charge was measured by placing
 a drop collector  in the  scrubber. The
 collector collected the drops and their
 charges which  were  measured  by
 leaking  the charge  to the  ground
 through an  electrometer.  Thus,  by
 monitoring the current  and sampling
 time, and  measuring the amount  of
 water collected, the charged level can
 be calculated.
  Figure 5 shows the measured charge
level on drops. Nozzles were hook-type
hollow-cone nozzles. Curve A is for a
water flow rate of 9.5 x 1CT4m3/s (0.3
gpm) per nozzle and a pressure of 450
kPa (50 psig). Curve B' is for a water flow
rate of 7.2 x lO^mVs (0.25 gpm) per
nozzle  and a pressure of 380 kPa (40
                            psig). The drop diameter, measured and
                            sized photographically, was about 0.24
                            mm for both conditions.
                            Results
                              The scrubber was operated for four
                            conditions:

     700
      50
      40

      30


      20
       10




       5

       4

       3


       2
      I      I    I   I   I
A  7.27 cm Slot Width

Q  3.8 cm Slot Width

     5.1 cm Slot Width
                                                   i      I    I   i
                                             Derived from Prandtl
                                             Eddy Viscosity Theory
           -= 0.76 (—)05
            McElroy
                        1
                            I
                                  I  i
                                                                I  i  t
        70
     20    30 40 50
    100

x/w, m/m
200  300 400 500    1000
figure 3.    Measured and predicted air entrainment ratio.
                                          Power Supply
              Flow
           Straightening
             Section
                     Inlet
                   Sampling
                    Section
                       Spray
                     IS action
 Blower
            Particle
            Charging
            Section
                    Outlet
                  Sampling
                   Section
                                                        I Vent
   u
                                             Sump
     'Entrainment
      Separator
                                     Pump
Figure 4.    Experimental apparatus for studying charged-spray section of SCAT
            scrubber.

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  (1) Uncharged particles/neutral drops
     (UP/ND).
  (2) Charged  particles/neutral  drops
     (CP/ND).
  (3) Uncharged  particles/charged
     drops (UP/CD).
  (4) Charged particles/charged drops
     (CP/CD).
  Figures 6 and 7 show data for nozzles
A and B,  respectively. The measured
mass median drop  diameter,  spray
angle, and discharge coefficient for
nozzle A  (hooktype)  were 0.24  mm,
100°, and 0.63, respectively. They were
0.35 mm,  87°, and 0.69, respectively,
for nozzle B (pigtail type). Only one spray
bank was  used in all experiments and
nozzle pressure was maintained at 432
kPa (48 psig). One power supply was
used to charge both the particles and
the drops.  The applied voltage was -10
kVDC. The measured charge/mass
ratio was -1.5 x 10~5 C/g for particles
with a mass median diameter of 3 /urn
aerodynamic diameter and a geometric
standard deviation of 2.5.
  The collection efficiency of the spray
scrubber is improved by charging either
the water or  the particles.  Further
enhancement was measured when the
water and particles were oppositely
charged and  it is greatest with sub-
micron  particles. For particles  with
diameters larger  than 5 //m aerody-
namic diameter,  charging the water
and/or particles  has little effect on
efficiency.
  The scrubber with nozzle A has better
collection  efficiency at a lower liquid/
gas ratio  than that with nozzle  B. A
possible explanation Is that nozzle A
produced finer drops.


Comparison Between Theory
and Experimental Results
  The drops from a hollow-cone nozzle
are localized at the edge of the spray
cone and do not travel parallel to the gas
stream, but at an angle which depends
on the spray  orientation with the gas
stream. Therefore, in calculating the
single-drop collection efficiency, the
resultant relative velocity between the
gas and the  drop must  be  used for
calculating the initial impaction param-
eter.
  The spray nozzles in the spray
scrubber were equally spaced in the
duct  so the  drops  travelled various
distances  before striking  the  wall. To
simplify the  calculation of  particle
penetration, the  average range of all
drops was used.  Figure 8 shows the
predicted and  the measured grade
penetration for the UP/NP condition. As
can be seen, the agreement is good. For
CP/ND  and  UP/CD conditions,  the
theory predicted  no improvement in
particle  collection efficiency, which is
contrary to experimental findings.
  When drops and particles are oppo-
sitely charged, the theory predicted an
increase in the collection  efficiency.
Figure 9 shows the predicted scrubber
penetration along with that measured.
The  agreement is good for particles
larger than 3 fjm aerodynamic diameter,
but not for those smaller than 3 /jm.
  These discrepancies could be due to
the use of average drop range. Figure 10

5x70'6
        ~i—i	1—i	1	1	1—i	1—
      -UG = 2.9 m/s
      -1 Spray Bank
       Nozzle/Grid Spacing —1.3 cm
       Nozzle 'A'
              B. QL= 7.2 x 70~4 m3/s
                Pressure - 380 kPa-
       :             A
% 10'
I
5x10'
          A. QL = 9.5 x 1CT* nf/s
          Pressure = 450 kPa
      024681012 1416 1820

          Applied Voltage. -kV
Figure 5.    Measured charge  level
             on drops.
  7.0

  0.5
                           c UP/ND '-
       UG = 2.9 m/s
       Ql/0G = 4 x 1
       1 Spray Bank
       Nozzle 'A'
       Hydrated Lime Particles
  0.05
 0.01      	
    0.7       0.5  7          5  70
 Aerodynamic Particle Diameter, fjmA


Figure  6.    Experimental spray scrub-
            ber penetration.
                                       CJ
                                       CB
  1.0


  0.5





:  0.7
I

,0.05

i



 0.07
                                                    UP/ CD
                                                     CP/ND'

                                                     CP/CD
                                                                 ,UP/ND
                                               G — 2.9 m/s
                                              QL/QG =7 x 10'* m3/mj
                                              1 Spray Bank
                                              Nozzle 'B-
                                              Hydrated Lime Particles
                                            0.1       0.5   1         5   10
                                        Aerodynamic Particle Diameter, /jmA

                                        Figure 7.    Experimental spray scrub-
                                                    ber penetration.
                                          1.0

                                          05
                                       
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shows a plot of the predicted single-
drop  collection  efficiency  for  1  //m
diameter particles vs drop  range.
Electrostatic augmentation does not
have  much  effect  until the drop  has
been  slowed down; i.e., drop range is
large. In performance predictions, drop
range was limited to 70 cm, which was
the average. In reality, drop range varied
between 9 and 97 cm, depending on the
location of the  nozzle. Therefore, the
use of average drop range could cause
underestimation of electrostatic effects.
Other possible  explanations are  that
drops are not of uniform diameter and
charge level.

Prototype SCAT System
  A two-section SCAT scrubber system
was  designed and built for studying
crosswind deflection and  hot source
control.  One section housed the spray
scrubber; the other section had three air
curtains and one push jet. Both sections
were  on casters so the distance be-
tween the air curtains and the spray
scrubber could be adjusted.
  The spray scrubber had  a  cross
section of 2.44 x 1.83  m (8 x 6 ft). The
bottom 0.61  m (2 ft) was the scrubber
sump. Therefore, the  active scrubber
cross  section was 1.83 x 1.83 m (6 x 6
ft). There were 36 large pigtail nozzles
set concurrent with the gas  flow at the
scrubber front surface. A zigzag baffle
entrainment separator was used to
remove the water drops.
  The scrubber was designed for an air
velocity of 4.5 m/s (10 mph), at which
                                       the pressure drop for the entrainment
                                       separator is approximately 1.3 cm W.C.
                                       (0.5 in. W.C.): an induced draft fan was
                                       needed to  overcome  the entrainment
                                       separator  pressure   drop.   In  some
                                       applications (e.g., spray scrubber located
                                       downwind from the FPE source), the fan
                                       may be unnecessary.
                                         In the other section, two air curtains
                                       were vertical and about 1.83 m (6 ft)
                                       apart; the third was horizontal and 2.7 m
                                       (9 ft) above the ground. The air curtains
                                       could be swivelled as needed to deflect
                                       crosswinds and bouyant smoke plumes.
                                       A propeller fan was at the center.
                                         Even  though the air curtain section
                                       had three air curtains and one air-push
                                       jet, they need  not be operated  simul-
                                       taneously. Under calm conditions only
                                       the air-push jet may be required to move
                                       the dust into the spray scrubber. When
                                       there is crosswind, one air curtain may
                                       be  enough to deflect  the wind and to
                                       convey the dust into the scrubber.
                                       Crosswind Experiments
                                         Under windy conditions, the SCAT
                                       system spray scrubber  can  be put
                                       downwind of the fugitive particle source
                                       and the wind will carry the particles to
                                       the spray scrubber. If the spray scrubber
                                       cannot be put downwind, the wind can
                                       be deflected from the FPE source with
                                       wind  screens  or air curtains. Some-
                                       times one air curtain can be used to both
                                       deflect the wind and convey the particles.
                                         Complete wind deflection, required to
                                       maintain dust containment,  occurs
    0.4
i   O-3
ti
I   0.2
.o
S   0.1
                                                                T
               Drop Diameter = 250 fjrn
               Particle Diameter — 1 fjmA
                do = 1,800 cm/s
                  = 300 cm/s
                         Charged Particle/Charged Drop
                                             Neutral Particle/Neutral Drop
               10        20        30       40

                                 Drop Range, cm
                                                     50
60
70
Figure 10.    Single-drop collection efficiency as a function of drop range.
when the resultant air flow of crosswind
and SCAT air curtain jet flow bypasses
the SCAT scrubber. At this  point the
blocking distance (range produced by
the SCAT air curtain) is just larger than
the distance between the air curtain and
the spray scrubber.
  For wind deflection, the momentum
of the air  curtain flow in the direction
opposing the wind must be equal to or
greater  than the momentum of the
wind. Wind deflection depends on
several SCAT operating parameters: the
incident angle at which the air curtain
meets the wind is the most important.
Several formulas for  correlating the
parameters mentioned above are avail-
able in  the literature. Indoor experi-
ments were intended for identifying the
best correlation. Experiments were
performed  by fixing the  range,  wind
speed,  air curtain slot width, and slot
exit velocity, and varying the incident
angle for wind deflection. Actually, both
the wind speed and  direction fluctuate.
For this reason, additional  crosswind
experiments were done outdoors.
  One  outdoor  experiment  was for
determining the air  curtain range. The
scrubber  and  air  curtain  were so
arranged that the wind direction  was
perpendicular to the  spray scrubber and
the air curtain  jet  discharged at 45°
against the wind.  Smoke  or tracer
particles  were  injected  at various
locations for flow pattern observations.
The air curtain range  was determined
visually as the  distance from the air
curtain nozzle to the  nearest location
where the tracer plume is disturbed by
the cross wind.
   In a second experiment the jet stream
trajectories for  various wind  speeds,
incident angles, air  curtain slot widths,
and exit velocities were determined.
   If the air curtain is also to convey the
dust, then the spray scrubber needs to
be rotated to intercept the curved jet
stream. The third experiment, involved
measurement of the angle between the
spray scrubber  frontal face  and  wind
direction  for various crosswind and
wind/air curtain incident angles.
   Measured air curtain  range and
trajectory were  compared with predic-
tions  in  Figures  11 and 12.  The
agreement is good,  so one can predict
the location of the air curtain and spray
scrubber relative to  the crosswind and
fugitive emission source.

Hot-Source Experiments
  Some metallurgical  processes, such
as iron and steel manufacturing, emit

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  750
fc/30
'-5 110
I
   90
O
     90      110       130      150
            Measured, degree


 Figure 11.    Predicted and measured
               incident angle for wind
               deflection.
   8
 c
T3
 0)
    02468
    Measured Air Curtain Range, m


 Figure  12.    Predicted and measured
              air curtain range (out-
              door data).
 very hot plumes containing  high con-
 centrations of  particles.  The  most
 efficient and economical way to clean
 these plumes is to capture them at the
 source where the  concentration  is
 highest and the gas volume smallest. In
 many cases, practical reasons (e.g., the
 presence of overhead cranes) make it
 impossible to capture the plume at the
 source or even vertically above the
 source with fume  hoods. In these
 situations, an air curtain could be used
 as the "ceiling" to contain the fumes
 and dust and to horizontally displace the
 plume into a receiving hood or scrubber.
   Experiments were  done to study the
 feasibility of containing hot plumes with
 air curtains. The  hot  source was
simulated with an open-top furnace
with an open-flame burner. The furnace
was at the center of the SCAT system
with 3.1  m (10 ft) between the air
curtain and the spray scrubber. Since
the operation of the burner was fixed,
the ceiling air curtain  location  was
adjustable so that the jet stream could
meet the hot plume at different tem-
peratures.
  Most of the experiments were  per-
formed with the ceiling air curtain exit
axis 61 cm (2 ft) above the top of the
furnace, where the peak plume rising
velocity and temperature were about
190cm/s(4.3 mph)and471°C(800°F),
respectively.
  Experiments were done for three air-
curtain slot  widths and  four slot exit
velocities. Except for small slot width
(2.5 cm or less) coupled with low exit
velocity (20 m/s or less), the air curtain
contained the hot plume. Experimental
observation  could be described by a
correlation for predicting the hot plume
trajectory which  accounted for the
buoyancy and momentum of the plume.

Conclusions
  FPEs can be controlled by  using air
jets to contain and convey the emissions
into a nearby spray scrubber.  The
collection efficiency of a spray scrubber
can be improved by charging the water
and/or the particles. Measured particle
penetration can be predicted for the un-
augmented scrubber but not very well
for the electrostatically augmented
scrubber.
  The air curtain developed in this study
can achieve a smaller expansion angle
and a lower entrainment ratio than
those reported in  the literature. Small
expansion angles and  entrainment
ratios are beneficial to the control  of
FPEs with the SCAT system.
  A prototype SCAT system has been
built to study the  effects of crosswind
and containment of hot buoyant plume.
Reasonable predictions of experimental
data on air curtain range and trajectory
in the presence of crosswind can be
made. The air curtain was successful in
containing a hot buoyant plume and the
trajectory of the plume can be predicted.

Recommendations
  The theories and experimental  data
presented in  this  research permit the
design of a SCAT system. However,
additional  studies are required  to
optimize the SCAT design. Future
research  and development work  is
needed in the following areas:
  (1) The  effect of  obstacles on  air-
     curtain flow field. One of many
     SCAT features which suit it for
     fugitive  emission control is  un-
     obstructiveness. Workmen and
     equipment (e.g., cranes) can pass
     freely and work on  the  source
     during SCAT system operation.
     The  presence  of workmen and
     equipment may create turbulence
     and  change the  air-curtain flow
     field.
  (2) The optimal design of the receiving
     hood to intercept the air-curtain
     jet stream.
  A pilot  study on an actual fugitive
emission  source  is recommended to
demonstrate the feasibility of using the
SCAT system  for controlling the emis-
sions. Since electric arc furnaces, coke
ovens, copper converters, etc. are the
major fugitive emission sources and the
plumes from these sources are hot, an
ideal demonstration would be on one of
these.

Nomenclature
QG  =  volumetric gas flow rate, mVs
QG, =  volumetric gas flow rate at
       nozzle  exit, mVs
QG« =  average gas flow rate at "x"
       meters downstream from
       nozzle, mVs
QL  =  liquid volumetric flow rate,
       mVs
UG  -  gas velocity, m/s
UGC -  centerline gas velocity, m/s
UGJ  =  gas velocity at nozzle exit, m/s
UG» =  average jet velocity at "x"
       meters downstream from
       nozzle, m/s
w  =  slot width, m
x   =  distance downstream from
       slot, m

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5. Yung, J. Curran, and S. Calvert are with Air Pollution Technology, Inc., 4901
  Morena Blvd., Suite 402, San Diego, CA 92117.
Dennis C. Drehmel is the EPA Project Officer (see below).
The  complete report, entitled "Spray Charging and Trapping Scrubber for
  Fugitive Particle Emission Control."(Order No. PB 82-115 304; Cost: $21.00,
  subject to change)  will be available only from:
        National  Technical Information Service
        5285 Port Royal Road
        Springfield, VA 22161
        Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
        Industrial Environmental Research Laboratory
        U.S.  Environmental Protection Agency
        Research Triangle Park, NC 27711
                                                                        •tt U.S.  GOVERNMENT PRINTING OFFICE : 1 981 --559-092/3360

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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
             PS   0000329
             U  S  ENVIfc  PROTECTION
             REGION  5 UiaHA
             ,230  S  DEARBORN
             CH1CAGU  II  60604
           AGENCY

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