See if there are any "Care Farms" in your area, this is one of them in the So OR area Sanctuary One: People, animals, and the earth: better together.
and try contacting them for ideas. I have seen alot about, as previously mentioned, doggie septic systems. They seem like a good idea but likely only if your outside of city limits, and not near a water table, check your local codes.
We just bag and trash ours, we pick up according to the trash day, no one wants a weeks worth of doodie sweltering in the can