and also:Kodak does not recommend the use of septic systems for disposal of photographic processing chemicals because the disposal of photographic processing solutions may affect the proper operation of the septic system. Other disposal options are available for septic system users, including household waste collection facilities, discharge to a nearby municipal treatment plant, or a licensed hazardous waste hauler.
When I was researching this, I also referred to the Kodak material, and found a similar statement in a different document about photo waste disposal. I can probably dig up a reference, but it said much the same.Septic tank systems are designed and used for the disposal of domestic waste, primarily in areas where municipal sewers are unavailable. They operate with anaerobic (without oxygen) biological action to treat the wastes discharged. This also includes leach fields and cesspools. Septic systems do not have the ability to properly treat photographic processing solution waste.
Honestly, I suspect that small quantities of developer and stop and wash water ( with whatever amount of silver and emulsion residue they contain ) would not hurt a septic system. But I'm trying to err on the side of caution both with regard to the functioning of my septic and the environment. I admit I may be going overboard, but that same publication says that municipal waste treatment facilities are able to handle developer and stop, and really it's not hard to save them up so they can be processed in a treatment plant. I can imagine it would be more of a problem if you lived very far from a municipality or a hazardous waste disposal site. I guess if I was in that situation, I'd be looking for a way to dispose without involving the septic system, maybe via evaporation, not sure.
It's okay with me if people think I'm being overly cautious. I need to do what I think is safest and best and I'm not an expert by any means.