My local Environmental Health Officer's recommendation is to dump it down the drain as the volume is inconsequential. Were I running a minilab then that would be a very different matter.
Silver is not mercury, nor is it lead. It is simply not accurate to state that all "heavy metals" (I use the quote marks as such a term has no scientific meaning) are dangerous and accumulate in the drinking water and the human body. Iron is a "heavy metal". Try living without ingesting iron and see how long you survive - amongst other things, without iron you would not be able to produce the red blood cells that carry oxygen around your body... Likewise half a dozen other "heavy metals".
Silver in particular is not very toxic at all to humans - your kidneys will happily process it. It does not generally accumulate in the body and cause problems except in exceptional cases of massive ingestion such as Andy's "blue man" - and even then it just pigments and does no damage....
The problem from large quantities of silver comes from its toxicity to micro-organisms: it can kill the bacteria that are used to process our sewage. However, the numbers killed by what remains from that litre of fixer that was dumped down the drain has no detectable effect: bacteria reproduce at quite a rate.
Indeed, that's probably one good reason for not ingesting a lot of silver: around 10% of your body weight is bacteria and a lot of those are very helpful in maintaining digestion and other, often unknown and unresearched, body functions (some are believed to assist the immune response for example). Silver compounds are another matter and I believe just about all are poisonous...