Not sure exactly what you may have with rocks and stones unless it's some sort of simple sediment trapping arrangement. Typical silver recovery buckets have a steel wool type of packing which... (do a thread search on that, very long explanations)... but the bottom line is that if it is that type it can be recovered, there lots of buyers out there for it. If it is just sediment trapping, pick a day the local dump accepts "hazmat" stuff - grab a copy of something showing why you have so much of it and your non-profit status (hey, better to have it with you than try to explain 40 gallons of potentially hazardous sludge in the back of the station wagon) and take ti them for disposal.