I don't know the chemistry, but precipitating the silver out of fixer is moderately easy, I think. It's possible that the result is more toxic than what you started with, but I don't think so.
Most b&w chemistry (except the silver loaded fixer, as noted) is moderately eco friendly to start with. You don't want to drink it, but it's pretty far down on the list of nasty stuff we dump into the environment.
Good respond. With septic systems however there is maybe a question of influence of the chemicals on bacteria cultures in the system.
There is a plenty of info regarding silver recovery on the net. Like this:
Silver Recovery
Exactly. The silver in a septic tank kills the bacteria that's supposed to be down there and it can wreak havoc. The other chemicals are probably in a low enough concentration that they can be safely diluted, but the silver is a problem. I do NOT want the expense of septic system repair/replacement.
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 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 centimetre, 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.
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 archival washing that ends most processing sequences. 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 dodging potential disputes and less about the facts of chemistry. If you check with local authorities, ask their permission, and they say no, I guess you have to do what they say.
