Whenever something like this gets going in a forum you get all kinds of crazy edicts, misinformation, and off the cuff comments. Most of it is out of context or wrong. Things that need to be kept in mind, but usually aren't are these:
What is toxic? In what form? At what exposure? What kind of toxicity? How can exposure occur? What does the substance in question bond to or break down in to? For instance in this case the depleted selenium mixture mostly isn't selenium. It's depleted. So what is it now? At what concentration or dilution? If it goes down the drain what will it do? What will it be or what form will occur if a plant or animal takes it up? Which plants or animals? Everything is toxic at some level or form of exposure including light and water. Pyrogallol for instance is pretty "toxic". It comes from acorns and other seed pods. Used as a developer, it rapidly oxidizes into something else. So when you dispose of it, you aren't disposing of Pyrogallol, or at least what's left won't be very soon, so what exactly are you dumping down the drain in real terms? What I'm trying to say is that there is more to it than just yelling "Poison!" or "Toxic heavy metal!" or as a counterpoint, dismissing safe and responsible practice.
What is the proper method of disposal? "Dispose of Properly" is a statement to limit liability, nothing more. It can mean anything from pouring it down the drain to taking it to a hazardous waste disposal facility. If you aren't sure, find out what is proper for a particular compound for your locality.
It's important to have an understanding of the things you are working with. It doesn't take a chemistry degree to understand, but it does take some effort. For instance, here we already had selenium described as a "toxic heavy metal". While photographic selenium solution requires handling procedure, and shouldn't be dried out and inhaled, etc. describing it that way is incorrect. It is a common ingredient in fertilizer, dandruff shampoos, and quite a few other things, and can even be taken as a supplement. Again, quantity, kind, and exposure. Toxic is a broad term. Context is key. How much ETDA is in two pounds of spinach, and do I need to be afraid?