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 contaminated house water flowing back into the main city water supply.

Federal law requires that backwater valves and other backflow devices be inspected yearly and proper maintenance and repair be done immediately, and it is the homeowners responsibility that this be done.