One was the cost of handling based on the pH --- if the pH is to high or to low the cost is significantly higher
This goes back to the different classifications the EPA has for waste. They call them characteristic wastes and the types of characteristic wastes are Ignitability, Corrosivity, Reactivity, and Toxicity. The waste which has a pH less than or equal to 2 or greater than or equal to 12.5 are both considered corrosive. And these are described as D002 type wastes.
Wastes are complicated and whe variations subtle for different industries. AND you, THE GENERATOR are responsible to label the waste correctly. This involves testing. As I mentioned earlier with the TCLP testing and others. The US has a phrase describing its waste handling methods, it's called Cradle to grave. That means for a manifested waste from any business, the waste is picked up and you have a manifest to state what the waste is. You can put down all of the proper codes, define the toxic metals you know to exist above limits and call the rest NOS as Kurt has suggested. And a waste hauler usually will accept your descriptions. And that drum will be traceable back to you from beyond the grave. So you better have labeled it correctly.
But here's the rub. A few years down the line the material you defined as non hazardous waste with all of the right codes but no analytical report to back it up will come back to your business if for example the drum corrodes away and spills the contents. That drum is always traceable to you if it was removed legally on a manifest.
I deal with lots of refiners, they all precipitate the metals out of solution using liquid caustic to form metal hydroxides and most add a compound called CPX which is a proprietary product sold by a company in Florida called ETUS. The sludges are de-watered and annually sampled by TCLP testing and removed as non hazardous waste if they pass the TCLP criterion. Dosing properly with CPX is effective in enough chelation to pass TCLP testing.
Believe it or not, cementation with copper for PM's followed by iron will drop all of the metals in a typical refining process, and a hydroxide chelated for good measure will produce an iron hydroxide which will Pass TCLP testing limits for the RCRA 8 metals from an aqua regia refining operation. A lot of refiners melt this stuff into bars and it is sold as scrap, non hazardous, no manifests, just metal bars. And the iron comes out at a low pH as a gelatinous solid which can be dried as it is just iron and not a hazard.
Just because you are targeting cadmium, as in this thread, do not assume the other metals in the waste stream will not bite you as far as the environmental regs go. You cannot solve the issue for one target metal without affecting everything else in the solution.