As someone who worked in precious metal recovery, until Jan 05, and was heavily involved with photo-chemistry disposal, I had to be fully aware of EU legislation on disposal of photo chemistry, EU legislation mirrors US legislation quite accurately.
As long as the silver is removed from the Fix or Blix then with permission chemicals can be diluted and diluted and disposed of via the foul / waste drain. They should be neutralised first and mixing desilvered fix with dev, bleac etc is a good way.
Water boards are really only interested in the load at sewage treatment plants. It's important to realise the potential load even with non de-silvered fix or blix from an normal home darkroom is miniscule, and undetectible at the works.
Septic tanks could be affected if the volume/quantity of dissolved silver in solution was excessive, but average home darkroom use is unlikely to get anywhere near that threshold.
What must be rembered is that in commercial disposal, after de-silvering, dev, fix, blix etc is mixed - the pH balanced then diluted to drain.
If you want a quick, cheap, eco friendly, way to recover your silver stick some wire wool in the spent solution. Save it up for your old age then send it for smelting
Ian