A sewage backflow preventer does exactly what the name implies: it prevents sewage from backing up and entering your basement, garage, or home.  It performs this task using some sort of valve, or a series of valves, which sense when the flow has reversed and automatically shuts down that flow until normalcy is restored. To

A reduced pressure zone device, otherwise known as a RPZ or RPZD, is a type of backflow preventer used to guard against the polluting effects of backflow in your sewer, irrigation and water system.  Annual RPZ testing is required for most major metropolitan areas, so make sure you turn to professional, certified and licensed RPZ

The Watts Backflow Preventer 009 is a reduced pressure zone assembly designed to prevent the reverse flow of polluted water from entering the potable water supply. This is caused, in most cases, by backflow, back-pressure, or back-siphonage, and the only thing that can prevent this polluted water from entering the water supply is a backflow