Backflow Prevention & Cross-Connection Control Plan

Backflow is the unintended reversal of water flow that can allow contaminants from private plumbing systems to enter the public drinking water system. It can occur when water pressure drops due to events such as a water main break or firefighting activity. Preventing backflow helps protect public health and maintain safe drinking water for the entire community.

Real world threats to the public water supply are more common than you would think. While not every home is a high-hazard environment, many homes do contain high-hazard cross-connections. Simple everyday items like irrigation systems, fountains, and pools can pose threats to our water if left unprotected. Without a hazard assessment, it is impossible to accurately assess the threat a service connection poses to the public drinking water system.

To ensure our public drinking water system is adequately protected, MPWD is requiring an RP downstream of the water meter. If you already have an RP installed, it will be located between the water meter and the building shutoff valve. View the section titled "What is an RP?" For further assistance with identifying an RP. If you are still unsure if you have an RP installed on your water line, please contact San Mateo County either by phone at (650) 372-6250 or by email at envhealth@smcgov.org.

A backflow prevention assembly consisting of a relief valve, located between two check valves that are located between two shutoff valves, with four properly placed test cocks. Broadly speaking, an RP is a testable, mechanical assembly designed to prevent backflow. It provides the greatest level of protection offered by backflow prevention assemblies. RPs vary in make, model and size; an example of what an RP could look like is pictured below.

Good work, you are ahead of the curve! Thanks for proactively protecting the public drinking water system. Please indicate that you already have an RP installed when we send out the Hazard Assessment Questionnaire. Ensure your device is tested annually by a certified backflow prevention assembly device tester. Reach out to San Mateo County, either by phone at (650) 372-6250 or by email at envhealth@smcgov.org, if you have any questions regarding backflow prevention assembly testing.

Backflow prevention assemblies are mechanical assemblies that can wear out, become damaged, or fail over time due to freezing, debris, or plumbing changes. Annual testing by a certified tester helps ensure the assembly will function properly if a backflow event occurs and continues to protect the drinking water system.

No. Not every property requires a backflow prevention assembly. Requirements are based on a hazard assessment that considers property type, plumbing configuration, and how water is used. Many single-family homes are considered low risk, while commercial, industrial, and multi-family systems require additional protection.

MPWD uses a hazard-based evaluation process that may include record reviews, questionnaires, and, in some cases, on-site inspections conducted by certified professionals. Properties are categorized as having no hazard, low hazard, or high hazard. Required backflow protection is based on this evaluation.

When a backflow prevention assembly is required, installation, testing, and maintenance are typically the responsibility of the property owner, similar to other private plumbing components. MPWD is committed to clear communication, reasonable timelines, and coordination with certified professionals when requirements apply.

Customers are encouraged to contact MPWD’s cross-connection control program coordinator at backflow@midpeninsulawater.org or (650) 591-8941 for property-specific questions or general information.