I m a graduate chemical engineer with some knowledge of hazardous materials. You only have my word that the only truly potentially dangerous material in B&W processing is the fixer. As stated above, there can be a small silver content in the form of complex silver ions. To get rid of this, one can place a steel wool (not s.s. wool) in a container with your exhausted fixer. The silver will displace the iron and coat the steel wool in a manner similar to copper coating of a steel nail placed in a solution of copper sulfate (high school chemistry). An overnight treatment with occasional agitation will convert sufficient silver to render the problem mute. Disposal of the iron halide solution should pose no problem. I am told there are establishments which will accept the steel pads for recycling the silver. I wouldn’t try it myself as it involves a complex process with truly hazardous materials.
Also I would advise one to disregard those do-it-yourself electroplating kits I have seen. Electroplating is even more of a black art than photography. You will end up with a goddawefull mess of hydrolyzed silver jell precipitate which will be more difficult to dispose of than fixer.
The bottom line is that there is so little silver in most darkroom effluent that it can be safely disposed of down the drain. I used a septic system for years with absolutely no ill effects. Living next to the Chesapeake Bay makes me the target of every environmental testing agency in the Government and they have never found the silver even though I told them about it. (They did find mercury but never deduced the source. Not from my darkroom! Someone probably broke a thermometer 50 years ago – you know – one of those “Coke–a-Cola” thermometers?)