Preventing Backflow

Backflow happens when liquids such as harvested rainwater flow back into the drinking water system, which can pose serious potential health and safety risks.  Marin Municipal Water District is requiring all rainwater harvesting systems which involve tanks or barrels to install a backflow prevention device at the water main. This applies even if your rainwater harvesting system is not directly hooked up to any irrigation lines connected to MMWD lines. This is a safety measure intended to protect Marin residents from any contamination of drinking water supplies.

Backflow Prevention devices must be installed by a licensed professional, and require annual inspection and testing. 

For guidance and more information in this area, please visit Marin Municipal Water District's Backflow Prevention website page or contact MMWD at (415) 945-1571 or (415) 945-1559 or

Note: Before installing cisterns or rain barrels, please see our Primer on Safety and Legal Considerations for Rainwater Harvesting in the MMWD Service Area.
Backflow Preventer