Backflow Prevention

Backflow is the unwanted reversal of the flow of water or other substances travelling through a water line.

When water backflow occurs, contaminants can be introduced into potable (drinking) water, making it unsafe for consumption.

When sanitary backflow occurs, wastewater from public sewer lines flow backwards into a building’s plumbing, commonly in the basement.

Backflow can occur in two different ways: back siphonage or backpressure.

Back siphonage

Back siphonage happens when contaminated water is sucked into the potable water system.

Back siphonage can occur when:

• Pressure in the main distribution pipes significantly drops 
• A supply valve is open 
• There is no protection from back siphonage, or an existing device is malfunctioning

Backpressure

Backpressure happens when contaminated water is pushed back into the potable water system.

Backpressure can occur when:

• The water supply is connected to another system that is operating at a higher pressure (e.g. a customer’s water pressure is stronger than the pressure in a City’s distribution system)
•  Excessive thermal expansion causes backpressure when the water is heated (e.g. through malfunctioning water heaters or boilers)

The City of Thorold implements backflow prevention programs that include the installation of specialized devices such as reduced pressure zone (RPZ) valves, double check valve assemblies (DCVA), and vacuum breakers. These devices are typically installed at service connections where there is a higher risk of contamination, such as commercial, industrial, or institutional properties.

Certified testers must inspect these devices annually (or more frequently depending on risk level) to ensure proper operation.

In sewer systems, backflow prevention focuses on protecting buildings from sewage backups, which can occur during heavy rainfall or system surcharging. Devices such as backwater valves are installed on private sewer laterals to prevent wastewater from reversing direction and entering basements or lower levels of buildings. The City of Thorold requires these devices in new construction.