Id be surprised if no one else has done this already, but ive been taking the used abrasive, drying it on sheet pans, sifting it, and then using it as a blasting medium. Works much better than the aluminum oxide or any of the other mediums that i have tried in the past. If you dont do any sandblasting, you may want to consider donating it to a local machine or auto body shop. Reduce, reuse, and recycle.
Hello @Shelson, thanks for joining the Forum and Welcome!
I have heard stories of people using the used abrasive in their gardening. Additionally, you can supply your garnet to concrete/construction companies. Garnet is a very useful additive to concrete which gives it strength, so check with your local companies to see if anyone would want to take your used abrasive.
As a final note, it is important to remember that whatever material you are cutting will remain and be found in the used abrasive. If you are cutting something hazardous, that means your drain water and used abrasive also becomes hazardous.