Backflow is the undesired reverse flow of liquids in a water service line, such as flow from an irrigation line into a building’s domestic drinking water line.

Requirements

Commercial, Industrial, Irrigation, and Multi-family Customers

The California Code of Regulations, Title 17, specifies where backflow prevention devices must be installed to protect the public drinking water systems from contamination. Generally these devices are installed at industrial, commercial, and institutional facilities like hospitals, restaurants, public parks, and auto shops; and at multi-family facilities. Backflow prevention devices also are required on the potable water systems at facilities that use recycled water for irrigation or other non-potable uses.

The water pourveyor may requires that commercial, industrial, irrigation, and multi-family facilities install RP devices on their potable water service lines. Fire service lines, at a minimum, must have a DCDA installed. If chemicals are added to the fire system, an RPDA must be installed.
Residential Customers Requirements
Typically, residential customers are not required to install backflow protection devices on incoming water service lines. However, if any of the devices listed below is connected to a residential plumbing system, a backflow protection device must be installed on the incoming water service line as close as possible to the water meter.

Sewer ejector pump Rainwater collection system Graywater system Groundwater well Recycled water system Septic system Other auxiliary water system