For more than 20 years, the U.S. finishing industry has been subject to increasing regulation of its air and water emissions. The aim has been to minimize the potential health and environmental impacts of releasing potentially contaminated wastewater and other effluent produced by finishing processes, such as heavy metals and organic compounds, into the environment.
These regulations are not going to go away; in fact, they continue to multiply on the federal, state, and local levels. The U.S. Environmental Protection Agency (EPA), for example, is advancing a new round of air emission controls for plating and polishing. EPA is currently surveying plating and polishing facilities in preparation for publication of a regulation that is scheduled aimed at air emissions of heavy metals.
Few rational people would argue that environmental regulations of all types have had the desired effect—-in general, air and water and water pollution from all sources, including finishing operations, is significantly lower today than it was 40 or even 20 years ago.
Many finishing shops have worked hard to stay in compliance with increasing regulations at the local, state and federal levels. And, suppliers have been more than happy to develop technologies that can help shops keep up. One result of this has been confusion as shops try to figure out which potential technological solution is best for their situation.
AIR POLLUTION CONTROL
For paint shops, compliance with the EPA’s MON MACT directive, scheduled to take effect this month, is a major concern. Of more than 120 organic and inorganic hazardous air pollutants identified by the EPA, eight account for approximately 75% of total hazardous air pollutant (HAP) emissions: methanol, hydrochloric acid, cresols, methylene chloride, methyl ethyl ketone, toluene, xylene and vinyl acetate.
The directive is designed to regulate HAP emissions from process vents, storage tanks, wastewater streams and transfer operations as well as fugitive emissions.
Requirements are strict. For example, 98% of HAP emissions from transfer operations and process vents must be controlled; as must 95% of emissions from storage tanks. The new ruling determines a facility’s source category based on both current and potential emissions; therefore, it’s possible the rule could apply to facilities that fall below current emission thresholds.
That short list of HAPs given above obviously includes several compounds used as solvents used in many liquid paint formulations. Shops may be able to comply by switching to powder coating or to waterborne or high-solids paint formulations, which eliminate organic solvents.
First developed in the 1950s, waterborne coatings have come into wide use. Typically, waterborne coatings have VOC levels well below 2 lb/gal. They can be applied to most substrates using most types of application equipment. However, they are not appropriate for every application, and they may be more expensive than other types of coating systems.
High-solids materials for general spray finishing are usually defined as coatings containing more than 60% of solids by volume, although some high-solids formulations may contain 80% solids or higher. These coatings have evolved steadily over the past 30 years or so. As more and more chemicals were classified as HAPs over the years, more coatings with lower solvent content were developed. High-solids coatings are one of the principal coating types used today to comply with air emission regulations.
Switching from liquid paint to powder coating is sometimes preferred by finishers in the market for a new system, because the cost for new equipment for either powder or liquid is nearly the same. Quality and environmental benefits then can tip the scales in favor of powder.
All powder coatings are VOC-free materials, and most of them are considered non-hazardous. Powders contain no solvents and in most cases no heavy metals. Filters and worn gun parts can be thrown away with your normal plant garbage, and powder coating materials can combust only in a very narrow concentration of powder and air. Insurance underwriters rate powder systems much safer than liquid systems, resulting in lower premiums.
Painters who are reluctant or unable to switch materials can select from a variety of technologies to eliminate the HAPs resulting from their paint process before process air is vented to the atmosphere. Thermal oxidation is a process that can handle a variety of vent stream characteristics and provide high destruction efficiencies. Included in this category are thermal oxidizers, regenerative thermal oxidizers and catalytic oxidizers. For larger air streams, a hybrid rotary concentrator/oxidizer system could be designed to reduce the oxidizer size and minimize operating costs.
Regardless of design, thermal oxidation systems burn HAPs and VOCs with oxygen to destroy pollutants in the air stream by converting them to carbon dioxide, water and heat. Environmental compliance is achieved as more than 99% of the HAPs or VOCs present in the original exhaust stream are destroyed before release to the outside atmosphere.
All of these options involve capital outlays—in some cases, major capital expenditures—and/or changes in basic process operations. For this reason a good first step in deciding how to comply with emission regulations is a thorough emissions inventory. Selection of a control technology is often determined by the actual makeup of the emission sources and related vent streams.
WATER POLLUTION CONTROL
For platers, water is the most important process material in terms of both quantity and quality. Almost all platers operate wastewater treatment systems in an effort to assure that the effluent they produce meets local, state and federal standards. These systems involve two components: treating the water to remove heavy metals and other hazardous wastes before discharging it, and legally disposing of the treatment residues, which are commonly referred to as sludge.
Many plating firms also employ technologies to minimize the amount of water they use. The most effective way of reducing wastewater flow is to convert parallel rinses to counterflow rinses, and add additional counterflow rinses if possible. Spray rinsing above process tanks or rinse tanks also can take the place of a rinse tank and can reduce the dragout of process solutions into succeeding tanks.
There are also a number of technologies available to electroplaters that can result in “zero” wastewater discharge to sewer or stream and generate only a small volume of concentrated liquid and/or solid
Evaporation is the simplest and, in most cases, cost-effective form of total or partial recovery. Recovery rates of more than 90% can be obtained, and some open-loop recovery systems allow discharge of rinse water to public sewers without further treatment.
If natural evaporation is insufficient to provide the desired recovery, evaporators can be installed. Atmospheric evaporators use wet surfaces, forced air and heated solution to cause evaporation. They are used on a variety of process solutions, including cleaners, nickel plating, chromium plating, cyanide plating and zinc plating. Vacuum evaporators have relatively high capital and operating costs, but they have found a niche in evaporating process solutions that have heat sensitive compounds.
Another potential waste treatment technology is ion exchange, which can process a variety of chemicals and even provide selective recovery. In the ion exchange process, columns are packed with resin beads that provide a large surface area for cation and anion sites. Cationic resins exchange hydrogen ions (H+) for positively charged ions such as nickel, copper, sodium and cadmium. Anionic resins exchange hydroxyl ions (OH-) for negatively charged sulfates, chlorides and chromates. Ion exchange columns are regenerated using either acid (for cationic columns) or sodium hydroxide (for anionic columns).
Methods are available for recovering specific metals and process constituents. For example, diffusion dialysis is used for the recovery of mineral acids, while electrowinning has proved to be a cost-effective and technologically sound alternative for the recovery of metals from wastes and rinse water. Electrowinning concentrates the heavy metals to their smallest volume as a metallic solid, which can then be used in the plating tank or sold as scrap metal.
Other recovery techniques include electrodialysis, which uses membranes and direct electrical current to concentrate and separate ionic contaminants in rinse water; electrolytic purification, which uses a semi-permeable membrane surrounding a cathode or anode to purify chromic acid solutions; reverse osmosis, which separates water from larger molecular weight compounds by forcing the waste stream, under high pressure, against a semi-permeable membrane; and ultrafiltration, a process that is similar to RO but uses a larger membrane size and lower feed pressure. A common use for ultrafiltration is removal of oil from process solutions such as alkaline cleaners to extend solution life and reduce chemical costs and sludge generation.