Hi John,
I've just joined the APUG, saw your post. I have one of those tanks for my B&W darkroom. Local council regs state that a permanent darkroom must have one- (I rescued mine in a throwout at the local college). The stones are marble, analagous to limestone, and they supposedly neutralise acidity, making their effluent ph neutral and therefore better for the environment. Works, apparently. We never clean them. Having said that, you should be keeping your exhausted fixer in the containers it came in, not just pouring it down the sink, as this is where the silver is. Try and find someone who recovers silver in your state or area, they will take the fixer, get the silver, and give you half the money. I haven't done this for a long time, am just starting my first darkroom in 25 years, so I'll let you know how it goes.
Re. the silver, apparently as the other reply said, steel wool or old scrap steel is chucked into big plastic tubs, the fixer is poured in. After a certain amount of time, don't know how long, but days, not hours, - silver binds to the steel. Liquid is poured off, steel is put in a furnace, silver melts out somehow, is poured out, thereby recovered. I've never done this myself, but once knew a bloke who did it for a living. Ask an industrial chemist.
Either way, throwing fixer down the drain is throwing away silver. All other (B&W) chemicals should be OK to pour down. Naturally vestiges of silver will end up there, but if films/prints are properly fixed, the silver will stay in the fixer. You won't get any silver out of the marble tank. And don't throw out the rocks!
Hope this answers your question, and I'm curious too, about what it does to the other typical chemistry which goes down the drain. Have fun and a good new year,

Jan Dallas.

Quote Originally Posted by thebanana View Post
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.