With one exception, the solutions used in a home BW darkroom are pretty benign.
Please note, I am not referring to big commercial operations where thousands of gallons of everything need disposal; I refer only to modest home operations where annual film use is in tens or hundreds of rolls. Larger scale processing needs to get a thorough environmental overview.
Nor am I referring to non-standard processes. I'm talking about your basic develop, stop, fix, wash aid, wash, and modest volume toning. If you use comparatively exotic items, you will need to research them one by one. Also, if you live in a community where chemical phobic ninnies run the show, they will tell you that everything photographic is a terrible danger, and that all solutions need to be dealt with as grave hazards. You will simply need to comply in those communities. Funny, the sodium hydroxide in some developers is an environmental disaster, but the same used as drain cleaner is fine. Hydroquinone in hair products is fine, but Hydroquinone in developer is dangerous... It goes on and on in that vein.
But, in saner localities, developer is not an issue. Stop bath and wash aid are fine too. Selenium toner in moderation too.
The only solution that is always at issue is used fixer because it contains silver that has been dissolved out of the film and paper. Silver is toxic enough in small amounts that it is used widely as a anti-microbial. It should be kept out of the waste stream even in hobby / household amounts.
There are a number of threads that deal extensively with removal of silver from used fix so you can easily get up to speed with a search. Use APUG's Google search feature. It works way better.
I'm not a chemist so what follows on use of selenium toning comes from
www.heylloyd.com/technicl/acid_free.htm and
www.heylloyd.com/technicl/seuse.htm. There are many threads discussing selenium toner usage. Unfortunately much of the widely available information on selenium toning appears to to encourage wasteful usage and premature disposal. If selenium toner is always used in an alkaline environment, it doesn't crud up, and stays clean, and can be used for a very long time. Don't go direct from an acid fix to selenium toner. Make the paper alkaline by an sulfite bath (wash aid). Don't mix selenium toner with the wash aid because wash aid dies far earlier. It will take you a few minutes longer, but your toner should last almost indefinitely.