Backflow or backsiphoning occurs when a negative
pressure develops in the water supply line, causing
water that has been contaminated to be drawn back
into the supply lines. The National Plumbing Code,
which has been adopted in most states, requires
that backflow preventers be installed on any
supply fixture when the outlet may be submerged.
Examples of this are a hose that fills a spray tank or
barrel, a fertilizer injector, or an equipment wash
tub