First to the OP's original question: Fixer, even acidic rapid fixer, has a high enough pH so that it is OK to mix ammonium and sodium residues. Some SO2 may be evolved, but not a significant amount.
Originally Posted by jnanian
Now to the quote. Limited amounts of most water soluble things, including cyanide, can be disposed down the drain. Disposal of machine shop solvents, most petrochemical derivatives, and many other organic things is a definite no-no. Petrochemicals and a lot of other things are not water soluble and actually interfere with waste processing. As has been noted above, do not put chemicals down a storm drain. They will re-enter the environment quickly and in an uncontrolled manner that could cause local harm.
While small quantities of photographic chemicals can be disposed of down the drain safely, industrial amounts are another matter. The waste treatment people worry about two thing here. One is toxic contamination. This is usually not a problem, since the sewage environment and the treatment usually takes care of it. But large quantities may cause special problems, particularly if the come in a short period of time. The other problem is maintaining the sewage treatment system. Sewage treatment is a fermentation process, and anything that would kill the bacteria used in the treatment is obviously detrimental. There are some other concerns, but these are the biggest ones. Most sewage treatment authorities have a schedule of what can be disposed of without concern, and the limits for uncontrolled discharge of things like chromium, thiosulfate, cyanide, silver ion, hydroquinone, and the like. For greater quantities, up to a limit, you usually need a permit, need to keep records, and need to adhere to a strict schedule. A few things, like petrochemicals, are usually forbidden. But there are only a few. The rules vary a lot depending on the size and location of the waste disposal facility. Your local sewage treatment agency can give you details.
Thiosulfate is a pretty innocuous chemical, as are the other chemicals in spent fixer. Any government that strictly forbids small quantities of them in sewage is just plain stupid and is no doubt reacting to political pressure or panic.