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.
(..) What about these bruises here? I haven't figured that out yet. Gun pressed against his head? Pressed hard enough. Fuck, yeah. On the front, flush with the muzzle. Ladies and gentlemen, we have ourselves a homicide. Killer put a bucket beneath him, kept on serving. Took his time too. (..)1995 Se7en