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If I neutralize the acid bath after use, it too should contain dirt, oil, water and bits of metal right? So shouldn't this qualify as acceptable to dump in the used oil dump at the parts store?



No. The metals/metal oxides and dirt is not the problem. (It would also contain salts, but again they would not be a problem either.) The reason why is because it only contains a small amount of oil (hydrocarbons) and a lot of water in the solution. As we all learned in grade school science classes, oil and water will not mix, they separate into layers given enough time (unless acted upon by detergents, which interacts with both layers). Where the layers sit depends upon the densities of the materials. Recycling companies do not want to mess with used oil that has been contaminated with large quantities of water and not properly separated. If you separated (ie. decanted off) the oil layers from the cleaning process and disposed of them in the used oil recycling container, that is fine. But, you cannot just dump all of the aqueous solution in there as well. You will become liable for the disposal costs if you do so. Open your wallet wide if that happens....