I did wash the front of a house down once that was absolutely filthy with thick black dust on a main traffic route prior to it being repainted. I did'nt use my main tank though, I plumbed the pump up to a seperate 25 ltr barrel and used just ordinary water and a bit of bleach to help shift it and then just made sure I flushed my pump through with pure water after. Not sure why you are asking but maybe for a similar reason?
And before anybody says it, it was a couple of years back and I did'nt know about traffic film remover (tfr) then ok, but it seemed to work anyhow.