Backwater valve

A backwater valve is a backflow prevention device used to prevent outbound water through a dwelling's drain pipes from re-entering -- "back flowing"—into a home. The valve contains a flap that allows water to exit the home, but closes to prevent the back flow into the home.^[1]

Backwater valves commonly activate when a city's sewer lines are unable to handle a large amount of falling precipitation; this puts homes that are tied into the storm lines at risk of having water back flow into them.^[2]

There is a big difference between backwater valves and backflow preventers:

• A backwater valve prevents raw sewage from backing up into your home through your toilets, showers, etc.
• A backflow preventer deals with protecting your potable water source from being contaminated by a reverse flow of foul water (ex: isolating your toilet's flush cistern and water supply from the toilet bowl water itself).

^[3]

References

1. "Backwater Valve". www.london.ca. Retrieved 2016-07-19.
2. "What Is A Backwater Valve?". Absolute Draining & Plumbing. Retrieved 6 November 2016.