Plating Line Conversion
Q: We’re a job shop plating operation that performs various barrel and rack acid zinc and alkaline zinc operations. Due to customer demands, we are investigating converting one of our acid zinc lines to a zinc nickel alloy plating line. We have a conventional wastewater treatment system with our discharge permit to the county regulated under Metal Finishing Pretreatment (40CFR433) standards as well as local limits.
Once in a great while our wastewater system experiences a burp and our zinc rises above 3 mg/L. We generate about one, 20 cubic yard roll-off container of wastewater treatment sludge in a month. It is non-hazardous, and it is disposed at a local sanitary landfill. We would like your input as we begin our investigation. What impact would zinc nickel have on our wastewater treatment system? Are there any other environmental issues? B.S.
A: Your question, in various forms, has come up multiple times over the last several years. Based upon our experience, we can give you some guidance regarding zinc nickel plating on air pollution, wastewater treatment, and sludge disposal. While U.S. EPA does not have any existing or proposed national air emission standards for zinc nickel plating, your state may have either specific or general air pollution regulations that could impact your facility. Since nickel compounds are classified as hazardous air pollutants (HAPs), you may need to demonstrate to the state air pollution control agency that your proposed emissions do not exceed an ambient air concentration threshold at your property line through air dispersion modeling. Based on our experience and considering the very low emission rates from the plating tank, we would not expect to be an issue.
However, for any plating operation, we recommend installation of a demister on the plating tank exhaust in order to protect your and neighbors’ properties from the corrosiveness over time of possible misting from the plating tank.
Regarding impact upon your wastewater pretreatment system, you need to obtain information from your plating chemical supplier. We know that complexors are used in the alkaline-based zinc-nickel chemistries, and these can adversely impact wastewater pretreatment by strongly holding on to zinc and nickel, not allowing them to precipitate and clarify. If this is the case, either your plating chemical supplier or your wastewater treatment chemical supplier can help you determine how best to handle this waste stream. Typically, the most cost effective manner is to pretreat the wastewaters from the zinc nickel line to deactivate the complexors before they join your existing waste streams and flow to your wastewater treatment system.
Another possible impact on your wastewater treatment system is the removal of nickel. Removal of nickel as a hydroxide typically requires a higher pH than for zinc; this could be very crucial if your local nickel limit is significantly lower than the metal finishing pretreatment standard (40CFR433) of 3.98 mg/L daily maximum and 2.38 mg/L monthly average.
We recommend that you create some testing wastewater by mixing some zinc nickel plating solution with your wastewater and performing jar tests to determine the narrow pH range that achieves compliance. If you find that this pH range is outside your local pH limit, you will need to install pH neutralization just before sampling and discharge.
Zinc-nickel plating’s greatest impact is upon the regulatory status and cost of your wastewater sludge— or, to use the more environmentally friendly term, filter press cake. Under federal hazardous waste regulations (RCRA 40CFR261.31), the F006 listed RCRA hazardous waste is defined as “wastewater treatment sludges from electroplating operations except from the following processes: sulfuric acid anodizing of aluminum; tin plating on carbon steel; zinc plating (segregated basis) on carbon steel; aluminum or zinc-aluminum plating on carbon steel; cleaning/stripping associated with tin, zinc, and aluminum plating on carbon steel; and chemical etching and milling of aluminum.”
Reviewing the U.S. EPA background documents for the hazardous waste listing of F006, the hazardous constituents evaluated were cadmium, chromium, copper, cyanides, gold, lead, nickel, silver, tin, and zinc. Because nickel was evaluated as a hazardous constituent and is not exempted in the F006 definition, EPA holds that wastewater treatment sludges produced from zinc-nickel plating are F006-listed hazardous waste. And since these sludges mix with the sludges from your zinc plating wastewaters, all of your filter press cake will be classified as RCRA hazardous waste under the EPA’s “mixture rule.”
This will not only cause your disposal costs to dramatically increase, but your regulatory costs will also significantly increase. Based upon your current filter press generation rate, you will be well above the large-quantity hazardous waste generator threshold of 2,200 lb per calendar month.
As a large-quantity generator, you will be required to comply with a host of standards including detailed documentation regarding management of hazardous waste on-site, contingency planning, spill response, recordkeeping, annual or biannual reporting to EPA, initial and annual training and, last but by no means least, periodic inspections by your state EPA.
At this point, it is likely worthwhile to evaluate the costs and savings of installing and operating a separate wastewater pretreatment system for the zinc-nickel line, thereby significantly reducing the amount of RCRA hazardous waste generated and possibly becoming a small-quantity generator (between 220 and 2,200 lb per calendar month).
Small-quantity generators have significantly less regulatory burden than large-quantity generators. If you do pursue this course, make sure the effluent of this small system joins your existing system’s effluent just before your sampling location and discharge to the county.
Whatever course you take, we strongly recommend that you verify these environmental impacts by contacting your local and/or state environmental regulatory agencies; if you desire to do so anonymously, you can engage the services of a professional environmental consultant who can discuss your situation without revealing your identity.
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