Hi Roger & Papa Smurf,
Unless your town is very small, its sewage treatment facility very inadequate, and there are a large number of photographers rolling their own, I doubt very much that the amount of water soluble silver is significantly high to cause problems with the eco-system.
The problem doesn't occur in the ecosystem, it's in your sewage treatment facility. Sewage treatment works using a milieu of bacteria ("activated sludge") to clean the water. Silver is hazardous to those bacteria. By pouring stuff down the drain you're damaging your local sewage plant more than the environment. You don't have to be a tree-hugger to consider this a bad idea.
Yes, but the amounts of silver involved are pretty trival,
Actually they're not. A liter of spent fixer can contain several grams of silver - up to 4-5 g/l if you use one-bath fixing, which is what you see from the scale on fixing bath testing sticks, up to 10-15 g/l if you use two-bath fixing. For comparison: commercially available antibacterial tablets for cleaning drinking water (Micropur MC10T) contain about 0.1
mg/l for cleaning ten liters of drinking water. In other words, one liter of spent fixer from the first step of a two-bath fixing cascade contains enough silver for the antibacterial treatment of about 100.000 liters of water. While this obciously doesn't translate to 100 cubic meters of microbial sludge in your sewage plant, it still is
quite bad.
which is why most sane and scientifically literate local authorities don't worry about the tiny amounts that are poured down the drain by amateur photographers.
Given that your local sewage plant processes quite a bit more than those 100 cubic meters, the amount of damage is rather controlled; I guess that is what you mean. ("I don't know how many fishponds there are all over the country, but if you dump stuff in just one of them it doesn't doo too much damage.") The fact that it was bad twenty years ago, that people were irresponsible back then, and the often-heard (not from you in this case) that other people behave irresponsible too so why shouldn't you, doesn't change that it's still irresponsible today.
I don't know about the US or southern France, but over here handing in used photochemicals for treatment is free for a reason. Given that there's practically no extra work involved in behaving responsibly instead of irresponsibly, the same sane and scientifically literate local authorities would probably tell you that small quantity is no excuse for laziness. Even where it isn't free, or if it's too much work to drive over to wherever you can hand in your chemicals every few weeks, you can get the silver out at home. It is no work at all to pour your spent fixer into a jug, put in a swab of iron wool or a tablespoon of sodium dithionite (at $20 the kilogram) and leave it in your back yard for two weeks before you pour it down the drain. There's really no reason not to do so.
On a different note, if you're chemically minded and use your darkroom regularly, you can smelt the silver out of the residue from the sodium dithionite and have your local dental technician cast it into a ring for the significant other once a year or so. Nobody said there's no secondary benefits to darkroom work.
😉
Philipp