Backflow & Cross Connection

Backflow is the plumbing term for an unwanted flow of water in the reverse direction.Backflow preventer keeps contaminants from flowing back into the public drinking water.
When you connect to the City of Ridgefield water distribution center, the intention is for the water to flow from the system to the customer. However, the flow of water could be reversed from the customer back into the distribution system. If cross-connections exist within the customer's plumbing system when this backflow occurs, then it is possible to contaminate the public water supply.

There are two types of backflow.

Backpressure backflow

Backpressure backflow occurs when the pressure of the nonpotable system exceeds the positive pressure in the water distribution lines; that is, the water pressure within an establishment's plumbing system exceeds that of the water distribution system. For example, the pressure in a hot water boiler system increases to a point that it exceeds the pressure in the water distribution system, a backflow from the boiler to the public water system may occur.

Backpressure can result from an increase in downstream pressure, a reduction in the potable water supply pressure or a combination of both. Reductions in potable water supply pressure occur whenever the amount of water being used exceeds the amount of water being supplied, such as during water line flushing, fire fighting or breaks in water mains.


Backsiphonage occurs when there is a partial vacuum (negative pressure) in a water supply system, which draws the water from a contaminated source into a potable water supply. The effect is similar to siphoning or drinking water through a straw.

For example, during a large fire, high flows of water are pumped out of the distribution system through a fire hydrant. This can result in significantly reduced water pressure around the withdrawal point and create a partial vacuum in the system, causing suction of contaminated water into the potable water system. During such conditions, it is possible for water to be withdrawn from nonpotable sources located near the fire hydrant. Air-conditioning systems, water tanks, boilers, fertilizer tanks and washing machines are possible sources of backsiphonage.

Backflow Prevention Devices

A backflow preventer is a method or mechanical device to prevent backflow and provides a physical barrier to backflow.

The City of Ridgefield's Cross Connection Control Program requires any customer who has a backflow prevention device to test their backflow assemblies:

  • After initial installation
  • After any repair, replacement or relocation
  • Annually by a certified backflow assembly tester

Annual inspection of Backflow prevention devices is mandated by RMC 13.55.040.

As a courtesy, our Public Works Department sends reminder letters when it is time to have your backflow assembly tested.

The city's letter notification cycle is April through September. Annual testing is required per the Department of Health WAC 246-290-490(5)iv. If you recently built a house and have a backflow assembly on your premises, the next test will become due between April and September of the following year.

You can find a current list of City Approved and Certified Backflow Testers at the top of this page. Please note, this list is provided as a convenience only and is not intended to endorse or recommend a particular service.