Ask the Expert Question-and-Answer Archive (Wastewater Treatment)

by Mike McGinness, EcoShield Environmental Systems, Inc.
March, 2002

Black Oxide

Q. Does black oxide process qualify as a "conversion coating" under EPAs catagorical discharge regs? If so, what are your thoughts on treatment prior to sewer discharge?

A. EPA rules for Metal Finishers under CFR 40 Part 433 capture facilities with any of 40 Metal Finishing processes only if they also first meet the definition of an Electroplating Point Source Category (see additional exceptions below). Black oxide coating meets the coating definition "chromating, phosphating and coloring" as coloring. Many non-electrical plating processes such as electroless Nickel and Chrome conversion coating were added to the electroplating category as Chromating, Phosphating and Coloring since they contain many of the same pollutants. The way the rule reads at this point it would even capture Black oxide coating.

The only formula I am familiar with is a high caustic, high nitrate formula that is solid at room temperature and the drag out rate (since the bath runs about 6 lbs per gallon) is very high in the first rinse. There are other black oxide (or black coatings) coatings that use other formulas so I would need to know more about the specific formula used to answer your second question. If they are using the old caustic - nitrate formula I would suggest a dead DI rinse (or two) to capture and recycle the salts to the black oxide bath. The rinses could be used to replace water lost to evaporation in the black oxide bath. The final rinse would need to be pH adjusted and then, depending on the type of discharge (direct or to a POTW), local discharge regulations (POTW), the nitrate final rinse flow rate and concentration of nitrate in the discharge, you may be able to discharge to a POTW with out further treatement under a permit (unless there are other metal finishing processes that have waste water that would need treatment). POTWs can handle small amounts of nitrates. The bacteria under anaerobic conditions will convert nitrates into nitrogen and oxygen and use the oxygen themselves. It is also possible to build a biological system to convert nitrates to nitrogen before discharge but such a system would also require a source of carbon, sulfur, phosphorous and several other trace elements to support the anaerobic bacteria.

Here is a condensed version from 433.10:

433.10 Applicability; description of the metal finishing point source category.

(a) Except as noted in paragraphs (b) and (c), of this section, the provisions of this subpart apply to plants which perform any of the following six metal finishing operations on any basis material: Electroplating, Electroless Plating, Anodizing, Coating (chromating, phosphating, and coloring), Chemical Etching and Milling, and Printed Circuit Board Manufacture. If any of those six operations are present, then this part applies to discharges from those operations and also to discharges from any of the following 40 process operations: Cleaning, Machining, Grinding, Polishing, Tumbling, Burnishing, Impact Deformation, Pressure Deformation, Shearing, Heat Treating, Thermal Cutting, Welding, Brazing, Soldering, Flame Spraying, Sand Blasting, Other Abrasive Jet Machining, Electric Discharge Machining, Electrochemical Machining, Electron Beam Machining, Laser Beam Machining, Plasma Arc Machining, Ultrasonic Machining, Sintering, Laminating, Hot Dip Coating, Sputtering, Vapor Plating, Thermal Infusion, Salt Bath Descaling, Solvent Degreasing, Paint Stripping, Painting, Electrostatic Painting, Electropainting, Vacuum Metalizing, Assembly, Calibration, Testing, and Mechanical Plating.

(b) In some cases effluent limitations and standards for the following industrial categories may be effective and applicable to wastewater discharges from the metal finishing operations listed above. In such cases these part 433 limits shall not apply and the following regulations shall apply:

Nonferrous metal smelting and refining (40 CFR part 421)
Coil coating (40 CFR part 465)
Porcelain enameling (40 CFR part 466)
Battery manufacturing (40 CFR part 461)
Iron and steel (40 CFR part 420)
Metal casting foundries (40 CFR part 464)
Aluminum forming (40 CFR part 467)
Copper forming (40 CFR part 468)
Plastic molding and forming (40 CFR part 463)
Nonferrous forming (40 CFR part 471)
Electrical and electronic components (40 CFR part 469)

(c) This part does not apply to:

(1) Metallic platemaking and gravure cylinder preparation conducted within or for printing and publishing facilities; and
(2) Existing indirect discharging job shops and independent printed circuit board manufacturers which are covered by 40 CFR part 413.)

Hope this answers your question.