What is the 'right' way to dispose of spent ammonium persulfate etchant? I have a septic system, so I really don't think I want to pour it down my drain! Right now, I have a gallon or two in my shop, stored in plastic jugs that originally contained PEG. The nearest 'hazardous waste disposal facility' is about 40 mile away, so trying to avoid that, especially during this global pendemic.

I have done a number of things in the past with my used ammonium persulfate solution, but wondering what options are really acceptable? The easiest for me is to let it evaporate about half way, then pour it in a lined garbage can which has mostly used paper towels, Other things I've thought of are:
1) Using it as weed killer.
2) Pouring it down the floor drain (followed by a good water rinse!) at work where they have municipal sewer.
2) Waiting for it to completely evaporate and throwing the crystals in the garbage.
3) Storing for a year (or more?) in a container with a piece of copper pipe, then pouring the whole thing out in the woods.
4) Calling a local hair salon and asking what they do with the run off solution from perms. I thought I read ammonium persulfate is part of the chemical mix used for that.

Sorry in advance for opening a can of worms by inviting wild conjecture and opinions, but Googling "ammonium persulfate disposal" is pretty much useless.

 

From the Fischer Scientific MDS
Clean Air Act:
This material does not contain any hazardous air pollutants. This material does not contain any Class 1 Ozone depletors. This material does not contain any Class 2 Ozone depletors.
Clean Water Act:
None of the chemicals in this product are listed as Hazardous Substances under the CWA. None of the chemicals in this product are listed as Priority Pollutants under the CWA. None of the chemicals in this product are listed as Toxic Pollutants under the CWA.
OSHA:
None of the chemicals in this product are considered highly hazardous by OSHA.

Dilute it and...

 

If you want to be detailed, your persulfate solution contains copper salts, this discusses them: http://www.sciencemadness.org/smwiki/index.php/Proper_disposal_of_chemicals

Where I live, some people dump up to 25# of copper sulfate in their septic systems per year to control roots.

I suspect an ounce of copper isn't going to cause all animal and plant life to disappear.

 

The main waste product you are disposing is Copper sulphate.
This is what the health and safety datasheet for Copper Sulphate says:

Environmental hazards Acute aquatic toxicity Category 1 (H400) Chronic aquatic toxicity Category 1 (H410)

6.2. Environmental precautions Do not flush into surface water or sanitary sewer system. Do not allow material to contaminate ground water system. Prevent product from entering drains. Local authorities should be advised if significant spillages cannot be contained. Should not be released into the environment.

How about plating it out and weighing in the copper?
Or add some lime and call it Bordeaux Mixture and spray it on your tomatoes. That's allowed - it's even "organic" because Copper Sulphate is a "naturally occurring substance".