When you're trying to quickly get rid of an unwelcome item, like some remnants of new paint or expired medication, your first instinct might be to rinse it down the sink. In the end, sewers are like wondrous portals, flushing all our filth into some watered underworld, never to be noticed again. Or so we like to think. Still, you'd be impressed that whatever we put down our drains ends up in estua...