Several of us use a very nicely laid out darkroom at our local Art Gallery. There are two large sinks, and all chemicals eventually get dumped down them. Each sink is equipped with a recovery unit of some sort. The units are plastic holding tanks, about 20 gal. each. I'm told there are rocks or stones of some sort in each tank. To my knowledge they haven't been cleaned out in 4 or 5 years. Can anyone offer advice on how to go about doing this? Where would one dispose of the sludge that I imagine is sitting in each tank? I assume there is some silver involved, does it make sense to try to recover it? What else should we be aware of? Thanks.