United States
                     Environmental Protection
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
                     Research and Development
EPA/600/SR-94/127    September 1994
4/EPA       Project  Summary
                     Bicarbonate  of Soda Blasting
                     Technology  for Aircraft Wheel
                     Abraham S. C. Chen, Lawrence A. Smith, and Robert F. Olfenbuttel
                       This  evaluation addressed product
                     quality, waste reduction/pollution pre-
                     vention, and economics in  replacing
                     chemical solvent strippers with a bi-
                     carbonate of soda blasting technology
                     for removal of paint  from aircraft
                     wheels. The evaluation was conducted
                     in the Paint Stripping Shop at Ellington
                     Field, National Aeronautics and Space
                     Administration/Lyndon  B.  Johnson
                     Space Center (NASA/JSC), in Houston,
                     TX. The evaluation used limited new
                     test  data, information from previous
                     tests by NASA/JSC as part of their pro-
                     gram to adopt this process  as a non-
                     destructive inspection  of aircraft
                     wheels,  cost estimates for the chemi-
                     cal stripping and bicarbonate blasting
                     based on facility records. Because the
                     paint being removed contained hazard-
                     ous  metal constituents, the  liquid and
                     solid wastes as well as the cloud of
                     spray generated were  evaluated  for
                     metal concentrations present and their
                     teachability. Analyses for Cd, Cr,  Cu,
                     Pb, Mn, Ni, and Zn were made as well
                     as total  metals concentrations, pH, to-
                     tal  suspended  solids, and oil and
                     grease. The blasting technology is ef-
                     fective for removing paint from aircraft
                     wheels without significant damage to
                     the anodized surface under  the paint.
                     Engineering improvements that avoid
                     the need of respirators, reduce noise
                     levels,  and  minimize water  use could
                     enhance the application. Applications
                     that  do not  contain hazardous materi-
                     als in the coating being removed could
                     be significantly more lucrative. In com-
                     parison to solvent depainting, this tech-
nology reduced the amount of hazard-
ous waste generated as well as cost
savings due to reduced operating and
disposal costs, resulting In a 15% re-
turn on Investment in about 4 years.
  This Project Summary was developed
by the EPA's Risk Reduction Engineer-
Ing Laboratory, Cincinnati, OH,  to an-
nounce key  findings of the  research
project that Is fully documented In a
separate report of the same  title (see
Project Report ordering Information at

  The objective of the U.S. EPA's Waste
Reduction Innovative Technology Evalua-
tion (WRITE) Program was to evaluate, in
a typical  work place environment, ex-
amples of prototype technologies with the
potential for reducing wastes at the source
or for preventing pollution. The goal of
this study was to evaluate a bicarbonate
of soda depainting technology that uses
sodium bicarbonate-based blasting media
to replace chemical  solvents, such  as
trichloroethylene (TCE), for stripping paints
from aircraft wheels. Specifically, this study
evaluated (1) the effectiveness of this tech-
nology, (2) the waste  reduction and pollu-
tion prevention potential, and (3) the
  Bicarbonate of soda blasting is a rela-
tively  new process that  is commercially
available. Compressed air delivers sodium
bicarbonate media from a pressure pot to
a  nozzle  where the  media mix  with a
stream of water. The media/water mixture
impacts the coated surface and removes
old coatings from the substrate. The wa-
                                                                       Printed on Recycled Paper

Tab/0 1.  Oil and Grease. TSS, pH, and Metal Contaminants in Wastewater Collected
         from the Vat
Oil and grease (mg/L)
TSS (mg/L)
C(f (mg/L)
C/* (mg/L)
Cif (mg/L)
P& (mg/L)
Mrf (mg/L)
Ni " (mg/L)
Zrf> (mg/L)
1 240
Discharge Limit
   Maximum allowable limits for grab samples. Industrial Waste Permit No 1030, City of Houston,
   Texas, March 10, 1989
   Total metal
Table 2.  Total and Leachable Metals in Solid Waste that Settled to the  Vat Bottom
                                                  Leachable Metal (mg/L)
0 056
Data have been corrected with field blank.
operator  was measured in terms of air-
borne metal concentrations.  Noise levels
were  measured  on a sound-level meter
and a dosimeter.
  Air  emissions were  measured in the
breathing zone  of the operator  and ana-
lyzed  for Cd, Cr,  Cu,  Pb, and  Zn. The
cloud  of mist created around the blasting
activity was maintained within  the  work
area and removed by a ventilation system
consisting  of an  exhaust hood and  a
rotoclone dust separator.
  The results of the airborne metal expo-
sure  study indicated  that  8  hr time-
weighted average (TWA) exposure to the
airborne  metals  were below  specified
OSHA and  American Conference of Gov-
ernmental  Industrial  Hygienists (ACGIH)
limits. Sound levels measured periodically
in the operator's hearing zone during the
two blasting sessions  ranged  from 76.8
decibels  (dB) on the "A"-weighted scale
(dBA) to  120.0  dBA. Dosimetry  samples
integrated cumulative noise exposures of
106.6 and 101.7 dBA for the first and the

second  blasting  session,  respectively.
These samples were based on 8-hr TWA
calculated from dosimetry results recorded
during the period sampled. If the actual
work period were increased to a full 8 hr,
the projected 8-hr TWAs would  be 121.3
for  the first  test and  115.9 dBA  for  the
second lest. A peak level of 146 dBA, the
maximum level the  dosimeter is capable
of measuring,  was  recorded during both
periods sampled.

Economic Evaluation
  Cost comparisons were made for bicar-
bonate blasting vs. chemical stripping.
Blasting times to strip each wheel were
measured during the test. NASA/JSC his-
torical data were used to determine chemi-
cal  stripping times. The capital investment,
operating costs and payback period were
calculated according  to the worksheets
provided  in the U.S. EPA  Waste  Minimi-
zation Opportunity  Assessment Manual.
The results of the economic analysis indi-
cated that a  return  on investment (ROI)
greater  than  15% (which  is the cost of
capital)  could be  obtained in 4 years, or
that  the payback  period for  NASA/JSC
would be 4 years.

  Based primarily  on two depainting cycles
and  a previous NASA/JSC study, the bi-
carbonate of soda blasting evaluation con-
cludes that the blasting technology can
effectively strip paint from aircraft wheels.
The  anodized surface damage, as a re-
sult of blasting, is considered minimal.
  The  blasting technology substantially
reduced the number of man-hours required
for paint stripping  in comparison to chemi-
cal  stripping. The time  saved was more
than 95%. The liquid waste accumulated
in the vat contained higher-than-discharge-
limit  Cr and could not be  disposed of to
the  POTW.  The  quantity  to be  shipped
away as hazardous waste was about 7.5
gal/T-38 aircraft wheel. The solid  waste in
the vat  contained paint  chips  and debris,
most of which was  insoluble  under the
TCLP conditions. The wastewater in the
rotoclone separator could be sewered with-
out treatment. Although convenient for this
application and within the existing  local
limits, the source reduction of this waste
as well as reuse/recycling should be in-
vestigated in greater depth.
  Although the exhaust ventilation system
kept the  heavy metal concentrations in
the  workspace below OSHA  and NASA
limits, the opportunities  for source reduc-
tion  to minimize  rotoclone  wastewater
should be explored.
  The operator of the blasting equipment
was required to wear a full-face air-purify-
ing respirator and protective clothing. Al-
though  the  present test results did not
make this an OSHA  requirement,  previ-
ous  testing  of  this  system  produced
chrome particulate concentrations that did.
The added precautions are recommended
until a better understanding of the system
is developed. Improved  lighting for better
visibility at the work surface  also is  rec-
  The noise measurements indicated that,
under the conditions encountered during
this study, hazardous noise exposures can
result.  Therefore,  engineering control of
noise  exposure should  be investigated.
Hearing protection devices for all person-
nel who operate or work in the vicinity of
the operation should be  provided with the
present configuration. Evaluation of the
hearing protectors used  during the  actual
times worked during this  study indicated
that  the protectors reduced exposures to
below  the OSHA and NASA  permissible
exposure limits. For compliance with the

NASA NHS/H-1845.4, work durations us-
ing the blasting equipment and the hear-
ing protectors assigned should not exceed
5 hr in an 8-hr work shift.  NHS/IH-1845.4
requires use of both plugs and muffs when
exposures  equal  or  exceed  110 dBA.
NASA  Environmental Health  Services
(EHS) also requires all personnel who rou-
tinely  operate  the blasting equipment to
be placed in a hearing testing and evalua-
tion program at the NASA/JSC clinic.
  The blasting technology has good po-
tential for reducing waste and consequent
waste disposal costs. For the application
studied, this is due primarily  to changing
the waste from a RCRA hazardous cat-
egory to a nonhazardous category.  Paint
stripping shops may find this technology
beneficial, especially as more stringent fed-
eral and local regulations are implemented
for the disposal of the toxic solvent-con-
taminated wastes. When no  hazardous
contaminants are  present in  the paint,
elimination of all hazardous waste may be
  The full report was submitted in fulfill-
ment of Contract No.  68-CO-0003  by
Battelle Memorial Institute under the spon-
sorship of the U.S. Environmental Protec-
tion Agency.
 A.S.C. Chen, LA. Smith, and P.P. Olfenbuttel are with Battelle Memorial
   Institute, Columbus, OH 43201-2693.
 Ivars Llcls is the EPA Project Officer (see below).
 The complete report, entitled "Bicarbonate of Soda Blasting Technology for
     Aircraft Wheel Depainting," (Order No. PB94-193323; Cost: $19.50,
     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:
         Risk Reduction Engineering Laboratory
         U.S. Environmental Protection Agency
         Cincinnati, OH 45268
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268

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 ter used dissipates the heat generated by
 the abrasive  process, aids the paint re-
 moval  by hydraulic action, and  reduces
 the amount of dust in the air. As another
 convenience, the workers do not need to
 prewash  or mask the surface. The dust,
 unlike that of plastic media, is not an ex-
 plosive hazard, nor is sodium bicarbonate
 toxic in this form. The airborne particulates
 generated from the  stripping  operation,
 however, can contain toxic elements from
 the paint  being removed.
   The effectiveness of bicarbonate of soda
 blasting depends on optimizing a number
 of operating parameters including nozzle
 pressure, standoff distance, angle of  im-
 pingement, media flow rate, water pres-
 sure, and traverse speed.
   The present study evaluated the bicar-
 bonate  of soda  blasting technology,
 ARMEX®/ACCUSTRIP™ (see Figure  1)*,
 marketed by  the CDS Group  (Houston,
 * Mention of trade names or commercial products does
  not constitute endorsement or recommendation for
Texas). The evaluation was conducted at
NASA/JSC's  Ellington Field, which main-
tains and repairs a fleet of 37 aircraft and
has  adopted this method for inspecting
aircraft wheels.

Product Quality Evaluation
  Although the quality of the work for this
bicarbonate blasting  system  was based
on paint removal without damage to the
wheel surface that either modified metal
performance  or masked any  cracks dur-
ing inspection, an additional issue was
the anodized  layer below the paint.
  The  previous  method included scrub-
bing  and scraping that produced signifi-
cant damage  to the anodized layer. As an
added feature of the bicarbonate blasting
technology, the  effect on the  anodized
layer was determined by inspection of the
condition of the  anodized layer for two
cycles of depainting.
  A special test method was developed to
qualitatively assess the anodized surface
damage resulting from bicarbonate blast-
                                               Water line
Figure 1.  ACCUSTRIP SYSTEM™ with wet blast head.
 ing. The method  required visual inspec-
 tion of the same wheel pieces after they
 were  first stripped  and after they were
 repainted and  restripped under the same
 stripping conditions.
   The results of the inspection suggested
 minimal, if  any,  damage as  a  result of
 bicarbonate  blasting.  Most damage  ob-
 served occurred  in the areas around the
 slots,  ridges, and bead rim and was attrib-
 uted to  mechanical wear caused by tool
 contact and wear and tear.
   This study did not evaluate the effects
 of  blasting  on metal  substrate damage
 and crack closure  because the  literature
 has already established the negligible dam-
 age and crack closure caused by the blast-

 Waste Reduction  and Pollution
 Prevention Potential Evaluation
   The waste reduction was measured in
 terms of volume reduction  and  pollutant
 reduction. Volume reduction addressed the
 gross  wastestream such as liquid and solid
 wastes in the vat and wastewater in  the
 rotoclone separator. Pollutant reduction
 involved individual pollutants, such  as oil
 and grease, total suspended solids (TSS),
 and   heavy   metals,  in   the   gross
 wastestream.  Pollutant  reduction ad-
 dressed the specific hazards of individual
  About 30 gal of wastewater was gener-
 ated and collected in a vat during  each of
 the two blasting sessions. The mean val-
 ues for the measured pollutants are pre-
 sented in Table 1.  The Cr concentration
 did not meet the local discharge limits, so
 the wastewater could not be disposed of
 to the Publicly Owned Treatment Works
  About 8 gal of solid waste  settled to the
 bottom of the  vat.  Metal  concentrations
 measured are presented in Table  2. Only
 a very small fraction of these metals was
 leachable under the Toxicity Characteris-
 tic Leaching Procedure (TCLP) conditions
 (see Table 2). TCLP requires the waste to
 meet limits of  1.0 mg/L  Cd,  5.0 mg/L Cr,
 and  5.0  mg/L  Pb.  No  regulations  have
 been set for Cu,  Mn,  Ni,  and Zn, The
 wastewater in the rotoclone separator con-
 tained  less than  detection  limit  of TSS
 and a very small amount of heavy  metals,
 ranging from 0.005  mg/L  of Cd to 0.39
 mg/L of Zn. For the particular case tested,
 the  wastewater could be sewered  without
  Other considerations  were hazards that
the  stripping  technology might pose  to
workers.  These included  toxic airborne
particulates and unsafe noise exposures.
Air  quality in the vicinity of the  blasting