Donald is correct in saying that if you get the used chems from a lab, they should take them back as they were going to dispose of them anyway.
I just flush my C41 and E6 down the toilet. I rang the local EPA and they told me small amounts (like the 500ml I mix up to develop 10 films) will not make any difference at all in the grand scheme of their chemical treatment plants. It'll just be part of the millions of litres of gunk they process each day to turn back into safe stuff.
The same EPA guy mentioned I should use the toilet rather than a basin, as the dilution will help to prevent harmful effects on any sewer pipe lining.