Regarding EPA, I can tell you how that works. There is an EPA regulation called the Clean Water Act (CWA). Its purpose is to keep as much pollution as possible out of "Waters of the U.S." which is rather broadly defined. The main activity regulated is wastewater treatment plants (WWTPs), as they discharge mainly into "Waters of the U.S." i.e., rivers, streams, oceans lakes, etc. As a part of the CWA, Municipal (city, county, etc.) wastewater treatment plants (and even some private WWTPs) are required to permit industrial wastewater discharges into WWTPs. They are technically not EPA permits, but it amounts to the same thing. To do this, the CWA identifies several dozen industrial activities that require WWTP permits in order to discharge their industrial wastewaters. Those are called "Categorical Activities". One of the Categorical Activities is "Metal Finishing" which includes a large number of different metal finishing operations, one of which is oxide treatment (metal bluing). The consequence of any industry performing categorical activities is that they must obtain a pretreatment permit from the WWTP before they can discharge industrial wastewaters to it. And in order to obtain such a permit, the industrial facility must generally perform some type of industrial wastewater treatment to reduce or eliminate wastewater toxicity, which, depending upon the specific wastewater, can be simple or very complicated, according to the requirements of the WWTP pretreatment discharge permit. It's actually somewhat more complicated than that, but that is the general requirement . In brief, if an industry generates a categorical wastewater, it must be pre-treated to some specified purity level prior to discharging it under WWTP pretreatment permit into a Municipal WWTP. And that can be expensive if you generate a lot of bluing-related wastewater.