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EPA Self-Audit and Inspection Guide
Organic Finishing of Metals

Table of Contents

Dip, Flow, and Curtain Coating: Regulatory Requirements

The Clean Air Act regulates the emission of volatile organic 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 requirements.

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 materials.

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 the local 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?
  • 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 Coating: Common Causes of Violation
  • 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 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 Coating: Sources of Pollution
  • 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 become waste.
  • Dip, flow and curtain systems require regular cleaning which creates solvent and/or water wastes.
  • 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.
Dip, Flow, and Curtain Coating: Pollution Prevention Alternatives
  • 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 recirculated.
  • 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.

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