Backflow

Backflow is a term in plumbing for an unwanted flow of water in the reverse direction.

The precise measures required to prevent backflow depend on the risk of contamination, i.e. the condition of the water in the connected system.

This is categorized into different risk levels:[2] Backflow prevention must be automatic, and manually-operated valves are not usually acceptable.

Category 2 contamination may be prevented by a single check valve, but category 3 requires a double check valve (these are manufactured as a convenient single unit, or even integrated into tap (faucet) fittings).

Standards for these air gaps group them by the amount of separation that they provide and their acceptability for the various risk categories.

Also sewage lifting stations provide comprehensive protection against sewer backflow.