Do you clean carpets with dirty water from other jobs you completed?

Every once in a while we get a customer that asks us if we are cleaning their carpet with the same water extracted from the carpets from previous jobs that day(meaning dirty).  We really can’t fathom this because #1 we have a freshwater tank, and separate waste tank.  On top of that we would get no cleaning results cleaning with dirty water, and it would actually clog up our whole 30000 dollar system.  I honestly am not even sure how someone would get the dirty water back into the freshwater tank.