I post this one a few times a year but it is still relevant.
I've had the luck to enjoy a career in scientific research and analytical chemistry before taking up photography full time. One of my challenges was teaching chemists at the local water supply and sewerage department about photographic chemicals in the effluent they had to treat. The "no fixer down the drain" anxiety comes up about a hundred times a year and has been doing so for at least half a century.
The following does not apply to industrial scale photo materials manufacturing or a major processing lab, only households connected to a sewer line or a proper septic system:
Developers are mild reducing agents that oxidise rapidly to inert components. The BOD (Biological Oxygen Demand) challenge offered by a home darkroom is much (much!) smaller than the BOD from a dishwasher, in-sink garbage disposal unit, or a toilet.
Stop bath is a very mild acid that has no measurable effects on highly buffered systems like septic tanks or sewerage treatment plants.
In moderate quantities (ounces, not tons) silver tetrathionate and similar compounds which characterise used fixer don't harm sewerage treatment systems or septic systems. The silver very quickly gets converted to silver sulphide in the presence of the free sulphide ion (smells like rotten eggs!). Silver sulphide is geologically stable and biologically inert and has one of the lowest solubility products known in chemistry. The stability and inertness of silver sulphide is the key to the remarkable archival properties of sepia toned photographs.
Do your own calculations. Just estimate your yearly use of silver from your photographic materials consumption, allow 1 milligram per square inch, and divide this by your yearly water consumption from the water meter. I bet it's in the parts per billion range where no conceivable biological effect can be credibly imagined. If you are discharging into a sewer system your used fixer contribution will be diluted by thousands of household that don't do photographic processing; that's just about everybody. Down at the treatment plant your speck of silver won't be detectable by any known analytical technique. People washing silverware in their dishwashers will send down more silver than you will ever do.
In my professional career I have inspected home septic systems that have been "ruined" by people doing photographic processing. In every case it has been the fault of extravagant archival washing at the end of the processing sequence. Sending maybe two or three hundred extra litres of water a day into a system for days on end dilutes the activated sludge and slows the biological reactions that process and neutralise the usual septic stream. The extra water can also overwhelm the soakage pit or trench that lies at the end of the septic system and deliver a squelchy smelly mess.
The world being what it is many local effluent standards are written by lawyers and/or accountants who don't know a dot of chemistry but know about culpability and lawsuits. Even Kodak publication J-300 which is the de facto last word on "fixer down the drain" is more about avoiding potential disputes and less about the niceties of chemistry.
Here's a typical calculation: A home darkroom processes, say, 2000 sheets of 8x10 black and white photographic paper a year. If half the silver goes down the drain that's 3 ounces. If, instead of disposal, much of that is recovered and sold the net return after refining costs could be up to $30 per annum on today's silver market. A lot of people would say its not worth the trouble. Opinions differ.
If you choose check with local authorities, ask their permission for what you intend to do, and they say no, I guess you have to comply.