A backwater valve is also known as a backflow valve.  The purpose of these valves is to guard against the sudden reversal of water flow due to a loss in water pressure within the pipes. For individual homeowners this loss of water pressure could result in contaminated sewer water backing up into the house, or

A backflow valve is a prime example of a backflow preventer, the purpose of which is to prevent contaminated water from flowing back into a clean water supply.  The backflow assembly is generally installed at exit points in a basement or a sewer pipe.  The valve closes immediately when sewage flows or when the pressure