MPWD's Backflow Prevention & Cross-Connection Control (NSF)
Non-Single-Family Customers
Mid-Peninsula Water District’s Cross-Connection Control Program (PDF) sets requirements for cross-connection control for all MPWD customers, ensuring the water system is protected and safe from potential backflow hazards. Due to more complex and potentially hazardous plumbing associated with commercial, industrial, and similar users, MPWD is requiring all non-single-family service customers to install a reduced-pressure principle backflow prevention assembly (RP) within 90-days of notification. Notices will be mailed beginning in Spring 2026.
Non-single-family service connections include all of the following:
- Commercial
- Fire Protection
- Industrial
- Institutional
- Irrigation
- Multi-Family Residential
There are many hazards associated with these types of service connections that could result in contamination of the water system if left unprotected. MPWD’s full list of high-hazard features and systems can be found HERE (PDF). Some common hazardous cross-connections are:
- Boiler water makeup lines
- Carbonators (soft drink dispensers)
- Chemical storage or manufacturing
- Cooling towers
- Fire sprinkler systems
What to Expect
MPWD is committed to clear communication throughout this process.
- First, an initial letter will be mailed, containing educational links, information on when official 90-day notices will be mailed, and the required RP installation date.
- Then, an official notice will be mailed, requiring the installation of a RP within 90 days.
Once an RP is installed, the pipe connecting the water meter to the backflow prevention assembly must be exposed and inspected by an MPWD employee prior to being backfilled.
MPWD is here as a resource to help you understand the new requirements. We also understand that scheduling constraints exist, and the installation requirements may take longer than the 90 days provided. Requested extensions will be considered on a case-by-case basis.
To schedule an on-site inspection or request an installation extension, please email backflow@midpeninsulawater.org or call (650) 222-1187.
To avoid water service disruptions, it is important to begin the process as early as possible. Reminders and cross-connection control program updates will be mailed throughout the process. Reminders will be continually mailed until a pre-backfill inspection is scheduled.
Installation, Maintenance & Testing
MPWD’s backflow prevention assembly installation, maintenance and testing requirements can be found HERE (PDF).
- Assemblies must be inspected and tested annually by a tester authorized by San Mateo County (PDF)
- Test tracking and documentation is currently managed through San Mateo County Environmental Health Services
Accidents Happen!
Backflow occurrences do happen and have real world health effects! Below are summaries of a few recorded commercial and industrial backflow occurrences. These examples and more can be found at The American Backflow Prevention Association Website.
- Bakersfield, California: A "do not use the water" notice was issued after a local oil producer hooked its equipment to an unprotected hose bib, allowing crude oil to backflow into the drinking water system.
- Corpus Christi, Texas: A do not use the water notice was issued after it was discovered 24 gallons of the chemical INDULIN-AA-86, an asphalt emulsifier, was pumped into a city water main. The chemical plant had no backflow preventer on their incoming water line, allowing this cross-connection to backpressure into the drinking water system.
- Lenexa, Kansas: Two dozen people became ill with copper poisoning after a soda machine (carbonator) experienced backflow, sending carbonated water into the copper pipes.
Questions or More Information?
For any questions relating to backflow, backflow prevention assemblies, cross-connections or MPWD’s Cross-Connection Control Program please email Backflow@midpeninsulawater.org
We can also be reached by phone, Monday through Thursday 7:30am-4:00pm at (650) 222-1187
Frequently Asked Questions (FAQs)
What is backflow, and why is it a concern?
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.
Why Are Non-Single-Family Connections considered more hazardous?
Generally, commercial properties have a much higher likelihood of containing high-hazard cross-connections. These hazards range from in-line chemical pumps, chemical storage, boiler water makeup lines and cooling towers, to, mop sinks, urinals, and tankless toilets. Commercial properties are also much more likely to be multi-story, which presents hydraulic risks. Added to these risks and hazards, commercial properties generally have more complex plumbing, including process piping, that makes accurately surveying and assessing their hazard level more difficult. For MPWD’s full list of high hazard features, CLICK HERE (PDF).
How can I tell if I already have backflow protection installed?
Backflow protection takes many forms. However, 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
What is an RP?
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. More broadly speaking, an RP is a testable, mechanical assembly designed to prevent backflow. It provides the greatest level of protection offered by a backflow prevention assembly. RPs vary in make, model and size; an example of what an RP could look like is pictured below.
What if I already have an RP?
Good work, you are ahead of the curve! Thanks for proactively protecting the public drinking water system. Pease email Backflow@midpeninsulawater.org and let us know that your water service connection is already protected. The following must be included in your email:
- MPWD account number
- Backflow prevention assembly serial number
- San Mateo County Environmental Health tag number
If you are unsure how to find this information, please call us at (650) 222-1187. Make sure to have your device 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.
Why are backflow prevention assemblies tested annually?
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.
Who is responsible for the backflow prevention assembly?
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.
Who can I contact if I have questions about my specific property?
Customers are encouraged to contact MPWD’s cross-connection control program coordinator at Backflow@midpeninsulawater.org or (650) 222-1187 for property-specific questions or general information.