Treatment of Phenolic Aircraft Paint
Stripping Wastewater
RONALD H. KROOP, First Lieutenant
United States Air Force
Water Resources Section
Environics Branch
Air Force Weapons Laboratory
Kirtland AFB, New Mexico 87117
INTRODUCTION
Purpose
The purpose of this research was to find a satisfactory treatment process for large
volumes of phenolic aircraft paint stripping wastewater. This treatment process would be
geared toward "on-site" treatment of just the paint stripping wastewater or for treating the
entire industrial wastewater flow where phenols represented the major portion of the
organic contaminants. To determine what treatment process to employ, laboratory
investigations were conducted using three unit processes with known capabilities for the
removal of phenols and other organic contaminants. These processes were oxidation with
ozone, oxidation with potassium permanganate, and adsorption with granular activated
carbon.
Background
Paint stripping (depaint) of aircraft and ground equipment is done periodically for the
prevention of corrosion to aircraft surfaces. The frequency of depainting varies. For
example, a B-52 is depainted every 3'/2 years. Paint stripping is accomplished by brushing or
spraying on a viscous paint remover/ stripper. The paint remover is allowed to dwell on the
paint for a period of time while it swells, wrinkles, and softens the paint, thus lifting the
paint from the surface. The paint remover and paint particles are then rinsed from the
aircraft with a high-pressure water stream. This constitutes the source of the wastewater.
The type of paint system (topcoat and primer) on an aircraft dictates the type of paint
remover required. In the past, most aircraft in the Air Force inventory have had an acrylic
lacquer topcoat with a zinc chromate primer. The depainting of these paint systems
involved the use of paint removers containing primarily methylene chloride
(dichloromethane, CH2C12) and hexavalent chromium with additional amounts of
surfactants, thickeners, and wetting agents. Treatment of the wastewater evolving from the
use of these paint removers have not been a significant problem, or at least not documented
to be, in that methylene chloride is highly volatile, and much of it evaporates before being
carried off in the wastewater in which it has only limited solubility anyway. At bases having
industrial wastewater treatment facilities the hexavalent chromium in the paint stripping
wastewater would be reduced to trivalent and precipitated as chromium hydroxide
Cr(OH),.
In recent years the Air Force has started using paint systems having polyurethane
topcoats with epoxy primers. Depainting of these paint systems necessitates the use of paint
removers containing, in addition to the above mentioned, significant concentrations of
phenols (12 to 22 percent).
Depainting of aircraft is done when an aircraft is undergoing major maintenance and
is, therefore, primarily accomplished at Air Force Air Materials Areas (AMAs). Tinker
AFB (OCAMA), Oklahoma, had been and was expected to depaint approximately 60 B-
52's each fiscal year. Each B-52 requires approximately 3,350 gallons of paint stripper.
1071