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What are backflow prevention assemblies for?

Backflow prevention assemblies are utilized on water supply piping to protect public water supplies from backflow hazards. 

The assemblies protect against  hazards caused by backpressure or

back-siphonage conditions that could cause pollutants or contaminants to backflow into the public water supply or the water user's system.

Source http://www.portlandoregon.gov/water/29782

For a detailed description of why 

backflow assemblies are needed see