They 'flushed' impurities from the system. = They 'flushed' impurities out of the system.
She published her first novel while still 'in the flush of' youth. [=while still young]
The committee succeeded in 'flushing out' [='finding'] several good candidates.
(..) You think of it like a wolf den. You're not gonna fight your way in, so you've got two choices. You trap the wolves inside, or you flush 'em out. If we can't attack straight on, then couldn't we use our hovercraft to strike around it? We'll use the mountains. (..)2015 The Hunger Games: Mockingjay - Part 2