I looked into this last year when I was doing a project at home (not the same situation I described above). I was trying to restore the inside of a rusted gas tank (I managed to neglect my motorcycle and dispite the beautiful exterior, the inside of the tank rusted). Anyway, I used an over the counter acid solution that was very weak. Someone (on another forum I frequent) cautioned me about the disposal. I called the DEP (Dept of Environmental Protection) in my state (Maine).
To make a long story short, they told me that as a private citizen, there were no disposal rules. I diluted the substance and poured it out on the ground. They also said, if I was a "business", there were many rules to comply with, no matter what the quantity of the substance was. The lady said if a business has any to dispose of it has to be done through at authorized disposal location, no matter how little the quantity.
Using the solution as you intend will leave you with nothing to dispose of in the normal sense. If it was me, I would call the DEP and discuss the matter ahead of time. I would also make sure to keep a copy of the MSDS on file, and write up a short explaination of the use of material. If anyone ever questioned me, I could legitimately say I explored the proper handling and have established a program which results in minimal release of the product (providing the DEP didn't direct me otherwise).
The stuff I was talking about in my last post was being tracked by our safety department. They knew we had it in a locked cabinet. When they discovered how infrequently we used it, they suggested that we dispose of it. That's when I discovered the rules of disposal were more complicated than I thought they would be.
I hate to say it, but common sense use and handling doesn't seem like a good enough approach these days. Someone will find fault no matter how reasonable you try to be. Charles Hall