Grand Canyon Groover Dump
Want to arrive home with groover boxes already emptied out? If you are coming off the Grand with boxes full of, um, "product," and if your route home takes you through Flagstaff, you may not empty them at the KOA, but you may empty them at the town water treatment plant. It is called "Wildcat Hill," on the NE side of town. $1 per box. It's a good setup, nice people too. Google it for details such as the hours during which they are open. Bring gloves and a cell phone - you encounter a locked gate with a phone number on it, which you call and they buzz you in.
Also consider renting your cans from a local outfitter there - could be more convenient. Or, there is a service there called "River Cans Cleaned" who will do the dirty work for you, although I cannot offer specifics as to what they charge or how they might schedule the deed with a traveling private party. I learned this because I met the guy from River Cans Cleaned, while I was at Wildcat. He was cleaning out five or six dozen groovers from the University and the local commercial companies.