United States
Environmental Protection
Agency
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
Depainting
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
back).
Introduction
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
economics.
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
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Tab/0 1. Oil and Grease. TSS, pH, and Metal Contaminants in Wastewater Collected
from the Vat
Parameter
Oil and grease (mg/L)
TSS (mg/L)
pH
C(f (mg/L)
C/* (mg/L)
Cif (mg/L)
P& (mg/L)
Mrf (mg/L)
Ni " (mg/L)
Zrf> (mg/L)
Mean
Concentration
49.1
253
8367
0.033
80690
1 240
1.430
0.022
0.006
5990
locaf
Discharge Limit
200
365
6-10
0.2
5.0
2.0
1.5
3.0
3.0
6.0
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)
Metal
Cd
Cr
Cu
Pb
Mn
Ni
Zn
Field
Blank
0.50
0.69
1.30
1.70
0.19
0.50
1.90
Mean
Concentration
2.73
14607
3297
70.87
277
0.72
281.33
Field
Blank
0005
0012
0003
0019
0005
0.005
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
Mean
Concentration
0.0303
22006
03927
05397
0.0023
00017
4.2840
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.
Conclusions/Recommendations
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-
ommended.
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
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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
possible.
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
Official Business
Penalty for Private Use
$300
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POSTAGE & FEES PAID
EPA
PERMIT No. G-35
EPA/600/SR-94/127
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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
use.
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-
ing.
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
pollutants.
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
(POTW).
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
treatment.
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
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