You don't tell us where you are...
Most municipalities in the U.S. allow small amounts of photochemicals to be discharged into the municipal sewer system, i.e., hobbyist amounts. Larger users, such as photofinishers are subject to other regulations. Check with your city and see.
Some practical advice. Normal developers and stop baths are relatively benign. You can safely dispose of them into a sewer system (even a septic system in reasonable amounts). Notwithstanding, you should check your local regulations.
Fixer with lots of dissolved silver in it is the real problem (unused fixer is fairly benign and you can dispose of it in the sewer system).
There are a couple of ways to deal with silver-rich fixer:
If there is a photofinisher in your area (they are getting harder to find), you can take your spent fixer to them. They will likely take it for free because they reclaim the silver from it and make a bit on it. That's what I used to do till photofinishers disappeared in my area.
If your volume warrants it, you can get one of John's silver magnets and do your own silver recovery. I do not own one, but have heard good things about them. After silver recovery, fixer can be disposed down the drain.
If your local regulations allow, you may be able to dispose of small amounts of used fixer into the municipal sewer. This is, unfortunately, what I have been doing lately. It's legal, but I'd really like a better solution. But, my U.S. darkroom activity only produces 5-10 gallons of used fixer a year; not that much.
Lastly, you can collect your fixer and take it to the hazmat facility in your area. I used to do this, but became convinced that the fixer would never make it to silver recovery due to the ineptness of the hazmat facility personnel. They simply dumped it into a large drum labeled "photo chemicals" (even though I told them it was fixer and needed to go to silver recovery...). It's likely still sitting in that drum somewhere at a waste depot.
Evaporation and disposing later at a hazmat facility is another option.