Sodium Persulfate disposal (see "Direct dilution" section for summary). This guy just takes his to a hazardous materials center (where batteries and paint cans go).
Sources
General
All of the digikey stuff below is from Injectorall, and lucky us, we can buy a wider variety for less directly.
locally available generally at real hobby shops (eg. not hobby lobby... find the smaller stores where only spooky 25 year olds hang out)
locally available at Royal Hobby Shop, one block east of the Rockford DMV (packs of 2 are around $2.50, though supplies are limited (but only because they have one of about 30 different sizes)).
