Causes

WHAT IS BACKFLOW?
Backflow is the undesireable reversal of flow of nonpotable water or other substances through a cross-connection and into the piping of a public water system or consumer's potable water syetem.

There are two types of backflow - - they are called backpressure backflow and backsiphonage.
What is backpressure backflow?
It is backflow caused by a downstream pressure that is greater than the upstream or supply pressure in a public water system or consumer's potable ater system. Backpressure (i.e. downstream pressure that is greater than the potable water supply pressure) can result from an increase in downstream pressure, a reduction in the potable water supply pressure, or a combination of both. Increases in downstream pressure can be created by pumps, temperature increases in boilers, etc. Reductions in potable water supply pressure occur whenever the amount of water being used exceeds the amount of water being supplied, such as during water line flushing, fire fighting, or breaks in water mains.

What is backsiphonage?
Backsiphonage is backflow caused by a negative pressure (i.e., a vacuum or partial vacuum effect) in a public water system or consumer's potable water system. This effect is similar to drinking water through a straw. Backsiphonage can occur when there is a stoppage of water supply due to a nearby firefighting effort or a break in a water main, etc