I have been trying to figure out what to do with used fixer chemical other than pouring it down the drain.
Some web research showed that the following is the proposed, most environmentally acceptable (silver thiosulfate is toxic in minute concentrations) method of disposal:
Chemically, the key to the process is Na2O4S2 Sodium dithionite (aka sodium hydrosulfite or sodium hydrosulphite), a white crystalline powder with a weak sulfurous odor. This substance will fall out the silver and separate it from the rest of the components, creating a benign liquid that can be poured into sewage.
This is how it is done: put the fixer into a canister and add the sodium dithionite, maybe around two table spoons per litre. Don’t close the canister. Put it in a well-ventilated location at around room temperature, perhaps out on the balcony, as it will produce some sulphur dioxide which happens to smell (similar to Japanese onsen – enjoy!). The silver will fall out as a black sludge of colloid silver and silver sulfide to the bottom and some to the wall of the canister. After a week or so, pour off the excessive liquid and filter the rest through a coffee filter. The black stuff that remains in the filter and the canister is silver; dry the cake and collect it for further processing or disposal. The liquid can go into sewage without any trouble.
In theory, you can collect the resulting silver and later take it to your next-door dental technician – a liter of exhausted fixer contains some three to six grams of silver – who then can smelt it down into a ring for your loved one every year or two if you have enough throughput. You can also use nitric acid to dissolve it and coat your own photographic plates.
Sodium dithionite must be stored dry, otherwise it will decompose and corrode the container it is stored in. Don’t inhale the dust (mask is recommended) and wear gloves, i.e. take the usual lab precautions. Obviously don’t ingest the stuff either. Wash hands after handling. One kilogram of sodium dithionite should suffice for 80 liters of fixer.
You may have also heard of the steel wool method, which also works.
The big question is where to buy sodium dithionite in Japan? I don’t have the answer to that one yet. One would have to find sellers of chemical substances. In Europe/Germany, Mikon Mineralienkontor, a dealer in minerals, sells sodium dithionite for EUR 8.81 per kg, excluding shipping. This Japanese site (Naitoh Shouten Co.) lists 450g of ハイドロサルファイトソーダ （Sodium hydrosulfite） for JPY 1260 and 15kg for JPY 11655, but I am not sure if it can be bought by consumers.