Ya, it does wonders for your brass fixtures and copper plumbing! But why would anyone throw perfectly good ferric chloride down the drain instead of using it for its intended purpose?!?!?
I wonder, more to the point, what OSHA would say about throwing copper chloride down the drain.
After all, your exhausted etchant is no longer ferric chloride, it's copper chloride! And as others have already admitted -- even to the point of contradicting themselves -- copper salts are good at killing things! (Last time I checked, that's translates to bad for the environment.)
For anyone interested in learning something -- a solution of ferric chloride is nothing but a solution of ferric ions and chloride ions (and obviously some H+, OH- ions). Rust is also made up of ferric ions (and oxide ions), and table salt also comprises chloride ions (and sodium ions). In that very basic way it is very similar to a solution of rust and table salt. In the end, it's relatively harmless to the environment.
When one usese ferric chloride to dissolve copper, the more chemically electronegative copper goes into solution as cupric, the ferric is reduced to ferrous and mostly precipitates out as hydroxide, and the chloride stays in solution (as hydrochloric acid, or more correctly H+ and Cl-).
I'll say it again -- It's not the ferric chloride I'm "ranting" about! It's the copper -- especially when in solution.