Table of Contents
Dip, Flow, and Curtain Coating: Regulatory Requirements
The Clean Air Act regulates the emission of
compounds (VOCs) (40 CFR Part 60) and hazardous
air pollutants (HAPs) (40 CFR Part 61 and
40 CFR Part 63), and provides specific standards
of performance to control emissions from various types of coating operations
(40 CFR Part 60). Depending on the solvent
content of the coating material used with dip, flow, and curtain coating
methods, solvents can evaporate and produce sufficient VOC and HAP emissions
to subject an operator to major source
requirements and Title V permitting
Controlling VOC emissions from dip, flow, and curtain coating areas
can be accomplished in two ways. First, a coating material with a
lower VOC content can be used. Second, air
pollution control equipment can be attached to the ventilation system
to capture VOCs prior to their release into the atmosphere.
As part of the Clean Water Act, Effluent
Guidelines and Standards for Metal Finishing (40 CFR Part 433) have
been established that limit concentrations of toxic
organics in wastewater streams. The organic solvents often contained
in liquid coatings used with dip, flow, and curtain coating application
methods may be classified as toxic organics. Actual limits for effluent
constituents depend on the size of the operation and the amount of wastewater
generated from the facility. If the facility discharges directly to receiving
waters, these limits will be established through the facility's National
Pollutant Discharge Elimination System (NPDES) permit (40
CFR Part 122). Facilities which are indirect dischargers releasing
to a POTW must meet limits in the POTW's discharge agreement. Wastewater
streams with concentrations exceeding permit limits will require pretreatment
prior to discharge to receiving waters
or to a publicly
owned treatment works. Pretreatment may include separation of
liquid wastes to remove solvents and settling or precipitation of solid
Solid and Hazardous Waste
Under the Resource Conservation and Recovery
Act (RCRA), organic finishing facilities are required to manage listed
and characteristic hazardous wastes (40 CFR Part
261). Liquid coatings used with dip, flow, and curtain coating
methods may contain constituents listed or characterized as hazardous wastes. Materials contaminated with the coatings, such as masking materials for
coating area light fixtures and floors, conveyor system components, and
rags used for cleaning, may require treatment as hazardous waste depending
on their formulation. Hazardous waste management (40
CFR Part 262) includes obtaining permits for the facility in order
to generate wastes, meeting accumulation limits for waste storage areas,
and manifesting waste containers for off-site disposal.
The Emergency Planning and Community Right-to-Know
Act (EPCRA) requires facilities to notify employees, customers and
the surrounding community of certain hazardous chemicals and materials
(40 CFR Parts 355 and 370)
that are present on-site. Dip, flow, and curtain coating operations
may use hazardous materials in sufficient quantities to subject a facility
to several EPCRA requirements. Facilities may be required to inform
emergency planning committee (LEPC) and the state
emergency response commission (SERC) of the materials stored and used
on-site, devise emergency
response plans for reacting to spills, and notify authorities of accidental
spills and releases (40 CFR Parts 302 and 355). The materials used in dip, flow, and curtain coating operations may also
require facilities to submit Material
Safety Data Sheets (MSDS) for these materials to state, regional, and
local organizations, while disposed volumes of the material may have to
be documented on annual Toxic Release Inventory reports (40
CFR Part 372).
Health and Safety
While not directly regulated by the EPA, several conditions exist that
should be considered when using dip, flow, and curtain coating methods. Workers should be aware of their responsibilities when handling coating
materials during equipment preparation and cleaning activities. Workers
should also know the risks associated with inhaling the VOCs
emitted from the coating material. Finally, workers should be trained
properly to avoid accidents and injuries when working with dip, flow, and
curtain coating equipment.
Do exhaust air streams have air pollution control equipment attached? Is that air pollution control equipment working properly? Does the
final exhaust air have concentrations of pollutants below required levels?
Does the dip, flow, and curtain coating system produce a liquid waste stream? Do concentrations of pollutants in the waste stream exceed limits established
by the facility NPDES permit or the POTW discharge agreement?
Dip, Flow, and Curtain Coating: Common Causes of Violation
Are solid wastes handled to separate hazardous and non-hazardous wastes? Are wastes labeled and packaged in accordance with 40
CFR part 262, subpart C? Are manifest forms completed for hazardous
wastes to be shipped for disposal?
Dip, flow and curtain systems apply coating materials which may include
solvents classified as volatile organic compounds and/or hazardous air
pollutants. The solvents evaporate and may accumulate above limits
allowed by Clean Air Act Title V permits. Ventilation and exhaust systems must operate properly to ensure the vapors
are removed from the coating area. Air pollution control equipment
should be attached to exhaust systems to recover or destroy volatile organic
compounds instead of releasing them to the air.
Dip, flow and curtain systems utilize liquid coating materials which can
contaminate water streams. Contamination may occur when cleaning
tanks and reservoirs or from accidental spills or leaks from equipment. Contaminated water streams may contain pollutants or heavy metals in concentrations
that exceed the limits established by facility NPDES permits or POTW discharge
agreements. As a result, effluent may not be directly released to
water systems or to publicly owned treatment works without pretreatment.
Dip, Flow, and Curtain Coating: Sources of Pollution
Dip, flow and curtain systems may use liquid coating materials with organic
solvents which must be properly stored, manifested and disposed of according
to RCRA standards if classified as hazardous waste.
Dip, flow and curtain systems apply liquid coating materials that contain
components classified as volatile organic compounds, hazardous air pollutants,
and/or ozone-depleting substances. In addition, the large surface
area of dip tanks and the constant movement of coatings in flow and curtain
systems exposes much of the coating to the air which promotes evaporation
of the solvents.
Dip, flow and curtain systems can generate large volumes of coating material
waste. Coating materials may be contaminated by dirt, dust, or other
debris that falls into the dip tanks. Coating materials in dip tanks
may not maintain uniform consistency of pigments and additives if not properly
mixed. In all three systems, excess coating material is necessary
to meet tank and reservoir depth requirements. These coating materials
Dip, flow and curtain systems require regular cleaning which creates solvent
and/or water wastes.
Dip, Flow, and Curtain Coating: Pollution Prevention Alternatives
Flow and curtain systems may leave bare spots on work pieces if the coating
material experiences a break in the stream. The may require the work
piece to be reworked or discarded, resulting in additional waste.
Use liquid coating materials with low organic solvent content to minimize
the amount of volatile organic compounds that will be volatized and to
reduce the volume of solid and liquid hazardous waste created.
Dip, flow and curtain systems provide greater opportunities for pollution
prevention than traditional spray application systems due to the higher
transfer efficiency (above 90%), and lower volatilization of organic solvents.
Reduce or eliminate contamination of coatings by enclosing and covering
tanks, reservoirs, and coating streams. Surround systems with a semi-open
structure which allows operation of the process, but does not fully expose
the coatings to the air and contaminants from the rest of the facility. Securely cover dip tanks when not in use to maintain coating purity and
prevent solvent evaporation.
Orient parts to minimize areas that would hold excess coating material
when removed from tanks or covered with coating.
Increase the drain time of parts over the coating tanks or catch basin
so that excess material runs back into system and can be reused.
Schedule paint jobs to minimize changing colors in dip, flow, or curtain
equipment. Paint with light colors first, then darker ones; lighter
coating does not need to be completely removed from the equipment, but
can blend into the darker coating. Since most dip lines apply only
one color, this is typically not an issue.
Modify consecutive flow and curtain streams applying different coating
materials to prevent coatings from mixing together. Increasing the
distance between the two streams and having separate catch basins for each
will keep the different coating materials separate and allow them to be
Clean dip, flow, and curtain equipment regularly to prevent coating materials
from drying inside tanks and fluid lines. Use water in cleaning steps
to reduce the amount of organic solvents used and amount of hazardous waste
generated. Perform initial cleaning with used solvent, saving fresh
solvents for final cleaning stages.
Segregate non-hazardous coating solids and water from hazardous solvents
and thinners, and label containers to prevent mixing. Separation
of the materials reduces the amount of hazardous waste that is produced. Coating material solids can be dried and treated as a solid waste, allowing
for disposal in a landfill.
Maintain dip, flow, and curtain equipment to sustain proper operation. Make sure valves, gauges, pumps, and filters are in proper working order.
Keep dip, flow, and curtain areas clean so that problems with equipment
can be found and fixed quickly, and accidents can be prevented.
Train employees on the safe handling of materials and wastes and encourage
continuous improvement. Training familiarizes workers with their
responsibilities, which reduces spills and accidents.