Backflow is a form of plumbing contamination in which water for drinking mixes with non-potable water, creating a significant health hazard. This occurs most commonly when a system design error or device failure causes a municipal water stream to cross-connect with a private water supply in a commercial property. Other related terms you may hear include “back siphonage,” a form of backflow caused by negative pressure, and “backpressure backflow,” which occurs as a result of downstream pressure. Both are serious problems that must be corrected.