Thiosulfates and sulfites, the main constituents of fixers, are pretty innocuous. Pouring them down the drain is usually quite acceptable. But in a darkroom that does a lot of work, like a college darkroom, reclaiming the silver dissolved in spent fixer can be very significant. Such places almost always use some means of extracting the silver before discarding the other stuff, or they send their spent fixer to a contract reclamation firm for $.