Commercially you need a license to do this but yes that's the usual method of disposal, the fixer/bleach dix can also go downn the drain provided it's de-silvered to less than 1 ppm silver.
When I worked for a Bullion dealers I regularly worked alongside Photolabs advising on chemical disposal, we sold silver recovery units or offered a collection service. (Either way we recycled the Silver). Part of my job was liaising with the licensing authorities and I worked alongside the senior chemists etc they said there was normally no impact at all on water/sewage treatment plants from even quite large commercial labs, provided the silver was removed. They would be concerned if the water/sewage treatment plant was very small, it might not be able to cope with the effluent from a large lab, but this hadn't happened anywhere.
We had a license to dispose over 1000 gallons of treated fixer, bleach-fix, developer etc a day provided it was diluted and between certain pH limits.
In practice most UK minilabs produced a volume of effluent that fell well below the threshold for requiring a full license to discharge, and just needed a letter of consent to discharge, but that varied depending on the water authority.
There's a lot of ambiguity about disposal of chemicals from a home darkroom, but in general most authorities assume & accept that people discharge spent chemistry to the drain, the volumes are so minuscule and there are far worse effluents discharge regulary from every household.
Ian
