Backflow Prevention & Cross-Connection Control
Cross-connections, backflow types, air gaps, RPZ assemblies, testing frequency, and hazard classification.
- Define cross-connection, backflow, and backsiphonage
- Explain the air gap and its minimum dimensions
- Identify when an RPZ assembly is required versus other devices
- State the annual testing requirement for testable backflow devices
Lesson 1
Cross-Connections, Backflow & Backsiphonage
What Is a Cross-Connection?
A cross-connection is any connection between a potable (drinkable) water supply and a non-potable or contaminated source. Cross-connections create the potential for contaminants to enter the drinking water supply if conditions allow reverse flow.
Examples of cross-connections include:
- A garden hose submerged in a swimming pool
- A boiler fill valve without a backflow preventer
- A laboratory faucet with a threaded hose connection
- A lawn irrigation system connected to domestic water
What Is Backflow?
Backflow is the flow of water or other liquids backward into the potable water supply. Backflow occurs through cross-connections and can introduce chemicals, bacteria, and other hazardous substances into drinking water.
Backpressure
Downstream pressure exceeds supply pressure
Cause: Pumps, boilers, elevated tanks on the non-potable side
Example: Boiler pressure exceeds street pressure
Backsiphonage
Negative pressure in the supply system creates suction
Cause: Water main break, fire hydrant use, high demand
Example: Main break creates vacuum that sucks pool water through submerged hose
Backsiphonage
Backsiphonage is caused by negative pressure in the water supply system. When supply pressure drops below atmospheric pressure (creating a partial vacuum), contaminated water can be siphoned backward through cross-connections into the potable supply.
Common causes of backsiphonage:
- Water main breaks
- Fire department pumping from hydrants
- High-demand periods reducing system pressure
- Elevated fixtures draining back through supply lines
Real-world backflow incidents:
Understanding actual backflow events helps contextualize why protection matters. Documented incidents include:
- A home garden hose left submerged in a pesticide mixing bucket: when the water main broke nearby, the chemical was siphoned back into the neighborhood water supply
- A boiler fill valve without a backflow preventer: boiler treatment chemicals (including chromate, a carcinogen) were pushed into the building water supply during a pressure surge
- A car wash soap dispenser: the water main break created negative pressure that pulled soap concentrate backward into multiple homes
These incidents demonstrate that both high-hazard and low-hazard cross-connections pose real threats.
A cross-connection links potable water to a non-potable source. Backflow reverses flow into the potable supply. Backsiphonage is caused by negative pressure in the supply system. Real backflow incidents confirm why even seemingly low-risk cross-connections require proper protection.