Quote Originally Posted by polyglot View Post
The problem is not so much the thiosulfate but the silver. It's a biocide and will have a negative effect on the bacterial populations in your sewage processing plant, reducing its capacity to ferment and make safe the raw sewage. The magnitude of the effect depends on how big the plant is - a gallon of fixer might be nothing or it might significantly damage the plant. If you have a septic tank, a gallon of used fixer will be a serious problem for you...
Those small amounts of silver would not pose any problem to any treatment plant for a community bigger than a hamlet (that term actually has a legal definition in New Mexico), unless there was an awful large amount of silver laden effluent. For septic tank users, silver recovery using steel wool is a simple expedient. I once almost (but not quite) shut down a small community waste treatment plant when I disposed of several pounds of outdated antibiotics from hospital supplies. Nobody thought of the effect on the waste treatment system. Fortunately, both the civil engineer and the bioenvironmental engineer for the plant were good friends. The bioenvironmental engineer told me about problems with decreased activity, and I told him about the disposal. We worked things out. For a moderate sized city, that amount of potent antibiotics would not have made any difference at all, but for this small community plant, it did. The point is that it takes a fair amount of antibacterial action to interfere with a waste treatment plant. But what goes down the drain adds up. That's why the waste treatment people put limits, usually based on total sewage volume from your facility, on what you can dump. The problem comes when politicians start making rules for things they do not understand.