One of the pitfalls, here, is that we need to consider the amount of chemicals that are being dumped.
Even when I am in full production mode, I rarely use more than a gallon of stock developer in a week. I often replenish and reuse XTOL until it's exhausted. I recycle fixer until it's used up. Then I get the silver out before turning it in. The only things I am really dumping down the drain are stop bath, hypo clearing agent and Photo-Flo. For printing, the only other thing I use is Dektol. Again, I use it all up before disposing.
Even in busy times, I probably dump less than five gallons of working strength chemical solutions down the drain during a week. Compared to the crap that goes down the drain on a daily basis, that's like spit in the ocean. But for the silver, most well managed municipal sewer treatment plants are well able to destroy most photo chemicals that get flushed down the drain, even at 100 times the quantities that I dispose.
I occasionally develop color film. Except for fixer, I have zero worries about dumping all my used chemistry down the drain at working dilution. (Plus plenty of water to dilute and wash it down.) In the amounts I use, I don't even think it would be noticed, much less cause a problem.
Now, if you are a commercial photo lab dumping 100 gallons of chemicals down the sewer ever week, THAT could be a problem! You had better have a plan to dispose of your chemicals properly!
But, in the quantities we usually dispose of, the average home photographer has nothing to worry about unless there are special circumstances.