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.
I know someone who was discharging fixer onto the ground outside his lab (in the country, no sewage connection). There's a row of pine trees running past the lab and due to silver releases in just 5 years, the trees within 20m of the lab are all about half as high as the others - it's a really noticeable dip in the treeline.
The best thing is to take it to your local lab who will recover the silver and thank you for it. It's not that hard - just carry it in the next time you buy supplies or whatever. For example, I buy spring water (for making up developer) in 10L bag-in-box containers. Once empty, they store old fixer. Every year or so, I take a couple cubes (5 gallons) in and have them reclaimed.
Whingeing about environmental regulations because you resent or don't understand them is the height of self-defeating libertarian stupidity. Go live in China for a while if you think you don't care about pollution; they're a fantastic demonstration of the tragedy of the commons.
Last edited by polyglot; 04-10-2013 at 08:56 PM. Click to view previous post history.