1. Find some bushes behind the PD.
2. Spray can until empty.
3. Dispose of can by placing in approved waste container.
4. Make sure to send out e-mail telling everyone where the "no-go" area is located
That is the exact procedure we use, and when we recently changed OC brands, Supply about wore out some buches behind the station. OC in defense sprays is almost always food grade stuff, and there is nothing toxic about it. If you want the official scoop, contact your Environmental safety/OSHA folks and they will likely tell you the same thing.
Unless you are carrying the really big OC projectors, you have no real idea how much OC is left after you have made a deployment, so we always swap for new cans after we have used any from the old can. CAPSTUN used to have a bar on their containers, and you could theoretically float it in a cup of water to get an approximation of how much OC was left, but CAPSTUN on held about 3 shots, and one burst usually used half a can, so the feature was essentially worthless, as any deployment used up too much OC. It really sucked when you ran out of OC and NEEDED more.
I also like the idea of using them for training, and next time I swap cans, I'll practice on a standard pistol target instead of just spraying the flower beds.