Cleaning water is potable water used alone or with detergents. After use, that water may contain large amounts of metals, impurities and contaminants. At the very least, mineral accumulations can bind with organic soil to create a more complex hardened soil that resists general cleaning. This hardened soil creates a binding surface for microbes and allows the development of biofilms and slime (a thick biofilm).

In addition, cleaning water splashes everywhere, often ejecting soil and biofilm while transporting these to new locations, often hidden inside processing equipment. The biofilm supports bacteria, yeast and mold. It can also become thick and moist enough to support several species of very small fungus-eating flies. Look for hidden places contaminated by ejected soil.