What does the saying 'separating the toilet from the tap' mean?

A plumber could elaborate..... but toilet water (being dirtiest) and tap water (being cleanest) must each flow separately within different pipes in the house, for obvious reasons. So basically it's a colorful way to make a difference, put things at order, divide in two. Jesus said he would one day separate the wheat from the chaff, goats from the sheep, or a season my arrive in which some of the participants will be "separated "the men" from "the boys." Some may see these as using parables: which is simply " A comparison, or a parallel, by which one thing is used to illustrate another."