I'm not Maris, but I'll risk hijacking the thread and take this on since it is one of my pet topics.
Originally Posted by DAK
I never dispose of selenium toner. I replenish it, filter it and reuse it -- indefinitely. I have two gallon jugs of selenium toner of different strengths that have been going for about 10 years (maybe longer). No problems with toning activity or print longevity. I routinely do residual hypo and residual silver tests on my prints and they all pass with flying colors.
My technique is as follows: When toning times become too long, I simply add a small amount of the selenium toner stock to the working solution to increase its activity. You'll have to determine what amounts work for you, but start with, say, 30ml/liter. If that doesn't step up the activity enough, add more; if it ends up being to active, simply dilute your toner and use less next time.
Toner used in this way precipitates out a black residue that is primarily silver/selenium. This is harmless to the prints and easily filtered out using coffee filters or the like.
There are those who use selenium toner "mechanically," e.g., at a particular dilution (say 1+19) for a particular time (say 2 minutes). This is done for "archival processing" reasons or "to increase D-max." Toning like this without monitoring the visual effect of the toner is ineffective. Toner activity begin to change after the first print is toned and decreases steadily with each print toned. Not extending the time simply results in successive prints receiving less toning. Additionally, if there is no visible effect either a change in image tone or a deepening of the lower print values, you are getting neither an "archival" effect nor an increase in D-max. Tone using your eyes, a good light source that approximates the display lighting you prefer and an untoned, wet print next to the toning tray as a reference. FWIW, only fully-toned prints are "archivally" protected; partial toning (which is what we most often do) only partially protects the prints. Careful fixing and washing maybe plus a stabilizer (Sistan or one of its successors) is the best way to ensure print permanence if you don't want to tone to completion.
If you must dispose of selenium toner, use it to exhaustion and then toss a few scrap prints into the solution and let them sit overnight to scavenge as much selenium from the solution as possible before discarding.
--- Down from the soapbox ---
And back on topic: Do a quick search for Kodak Technical Publication J-300 on disposing of chemistry. It may not take local regulations into consideration, but contains sound advice.
For the home darkroom and low-volume darkrooms, developer and stop-bath disposal should be of no concern. Simply dilute and dump into the sewer or septic system (I used a darkroom with a septic system for years and had zero problems doing this). Fixer with its silver content is the main concern for me. I used to take my spent fixer to a local photo lab, which gladly took it for silver recovery. They disappeared, so I began taking my fixer to the hazmat collection site. I soon learned that they didn't send the fixer for silver recovery; they simply mixed it with lots of other stuff and sent it for incineration. I don't think it is particularly eco-friendly to use a lot of energy incinerating relatively benign chemistry instead of recycling, so I stopped doing that. (Besides, the hazmat personnel I dealt with were badly trained, had no idea of photochemistry, silver recovery or the like and treated everything, including my five-gallon jug of spent fixer as if it were a mixture of sarin gas and nuclear waste... really overkill and way ineffective for my tastes).
Lately, I've been dumping small amounts of spent fixer down the drain into the municipal sewer, as Maris and Kodak suggest. When my volume increases after my darkroom remodel, I'll likely get a small silver recovery unit from jnanian here and simply dump the fixer down the drain after silver recovery.