I've worked for a couple different studios and labs over the years and each one that did even a modest amount of work (ie at least a dozen or so rolls of film a day plus the prints to accompany them) had a silver recovery unit. For small personal darkrooms and labs it's not really worth it, but most labs will have one- so the idea of taking the used chems back is the best. The labs I've worked for have all had contracted companies take care of the silver recovery units and from what the techs have told me it's the silver that is really the most harmful part of the chems. After the recovery unit did it cycle on the chems they were in turn routed down the drains.

Now a word on that. The two larger companies that I have worked for are large national companies that have gone through the epa and the right channels to find out the correct way to do things (or so they claim) and they both had the same practice when it came to chems. Plus- the fact that they were both in malls and the massive amount of water being put through the sewage system all the time probably diluted the chems at the slow rate that they were introduced.

So, all in all taking the chems back seems like the most logical way of taking care of them. Besides if it something they are supposed to get rid of it wouldn't screw up anything on the business related end of it. You'd just get to be a middle man!