Hi Jens,
I was not aware that a water wash could clean-up used motor oil. (and carry away the acids).
Sounds like a good idea. Don't have a wash system yet built yet.
Have you tried this ?
PS: where would this "acidic, heavy metal, wash water" go once the wash is done 
Sounds like a disposal problem to me.
Veggie
personally, I thought about that, but the solids get filtered out, the acid gets nutralized and what little water I use for testing gets thrown back in the tank and evaporated. The few solids I get go in a barrel with discolored oily water that settles in drums, sludge at the bottom of drums etc. I take that stuff to one of the local waste oil dumping places like oil change places or walmart.
large Amounts of clear water at the very bottom of barrels that is not discolored, I test for acid and nutralize if necessary and water one perticular section of grass with. That section of grass is just as green as the rest of the yard.
Strangely, few people even use those oil waste dumping facilities. At walmart you have to sign a sheet each time you dump something. Over a 6 month period, I was the only one to sign the sheet.
