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. Of course I had an agenda. I wanted building approval for a house with sizeable darkroom in it. They did said yes to my plans.

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 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 (pounds, not tons) silver tetrathionate and similar compounds which characterise used fixer don't harm sewerage treatment 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 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.

Before my darkroom was approved by my local council I had to calculate the silver concentration in my total household effluent. I'm pretty busy and use a few thousand sheets of film and paper per year but the result came to about 5 parts per billion. By the time this mixes with the output of the other 20 000 households in my area that don't process photographic materials the silver concentration is below any conceivable detection limit down at the sewerage treatment plant.

You can do your own calculations. Just calculate your yearly use of silver from your photographic materials consumption and divide this by your yearly water consumption from the water meter.

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. If you encounter such local regulations and you want to ask permission I guess you have to do what they say.