Having been in the trade - precious metal recovery and photographic effluent disposal - I totally agree with Doremus,

We were licenced to carry and dispose of Developer, fixer, Bleach, bleach fix etc, on an industrial scale the silver's removed then the dev & fix etc mixed as that neutralises the alkali dev with the slightly acid fixer then the whole lot discharged to the sewer. The water board (UK) stipulated the discharge volumes/rate so that they knew their sewage treatment works could cope.

We also fitted silver revery units and the water boards actually preferred smal scale licensed disposal from labs straight to the sewer as that meant far greater dilution. They weren't interested in small (home) darkrooms although technically they were covered by the same regulations.