Water Protection Program fact sheet
Division of Environmental Quality Director: Kyra Moore
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The Missouri Department of Natural Resources Backflow Prevention Rule establishes requirements for protection of public water systems from the introduction of contaminants by backflow. This rule applies to all community water systems and allows the supplier of water to require customer facilities identified as actual or potential backflow hazards to provide the necessary protection to prevent contaminants from entering the public water system through an unprotected cross-connection.

What is a cross-connection?

A cross-connection is an actual or potential link connecting a source of pollution or contamination with a potable water supply. A cross-connection can occur at a residential, commercial, industrial or institutional facility. A cross-connection is a fixture with a direct connection or submerged inlet. Common examples of fixtures with direct connections include boiler systems, dishwashers, fire sprinkler systems, laboratory equipment and swimming pools. Common examples of fixtures with submerged inlets include lawn sprinklers, utility sinks, drinking fountains and floor drains. 

What is backflow?

Backflow is the unwanted reversal of flow in a water distribution system. Backflow occurs due to backpressure or backsiphonage and allows non-potable substances to enter a public water system through an unprotected cross-connection.

Backpressure occurs when the pressure in a building’s plumbing exceeds the pressure in the water distribution system.

Backsiphonage occurs when there is a partial vacuum (negative pressure) in a water supply system, drawing liquid from the non-potable source into the water distribution system.

What is a backflow hazard classification?

A Class I backflow hazard is an actual or potential health hazard to customers of the public water system should backflow occur. Class I backflow hazards require the installation of a department approved air-gap separation or installation of a reduced pressure principle backflow prevention assembly on the customer service line.

A Class II backflow hazard threatens to degrade the water quality of the public water system should backflow occur. Class II backflow hazards require, as a minimum, a department approved double check valve assembly on the customer service line.

What is a backflow prevention assembly?

A backflow prevention assembly is a means or mechanism to prevent backflow. The department recognizes three types of backflow prevention assemblies: air gaps, reduced pressure principle assemblies (RPZ) and double check valve assemblies.

Air-gap: An air-gap is the most positive method of backflow protection. It is a physical separation between the water supply and the customer’s internal piping system. The distance for an air-gap must be at least two times the diameter of the supplying pipe. For example, a two-inch separation is required for a one-inch water supply pipe. The water supplier may inspect air-gaps.

Reduced Pressure Principle Assembly: A reduced pressure principle assembly is the highest level of mechanical backflow protection. It has a hydraulically operated relief port located between two spring loaded check valves. A drop in pressure from the supply or an increase in back pressure from the customer’s facility will cause the check valves to close and the relief port to open, creating an air-gap within the assembly. If either check valve becomes fouled by debris, the relief port will also open. The drawback to using a reduced pressure principle assembly is that it will lower the pressure available to the customer’s premises.

Double Check Valve Assembly: The double check valve assembly is for low hazard protection only. The double check valve assembly has two spring valves that act independently to provide protection from back pressure and backsiphonage. The drawback to double check valve assemblies is that both check valves are susceptible to fouling by debris in the water system, which hinders their function and can allow backflow to occur.

The department requires backflow prevention assemblies installed at community water systems be approved by the Foundation for Cross-Connection Control and Hydraulic Research at the University of Southern California or the American Society of Sanitary Engineering. Information about approved assemblies is available on the department’s Backflow Prevention webpage along with a list of certified backflow prevention testers.

Are community water systems required to have backflow prevention?

Yes. Community water systems must have backflow prevention in accordance with 10 CSR     60-11.010. The supplier of water or local governmental agency (if one exists) shall:

  • Require backflow prevention assemblies be installed to protect the community water system from backflow hazards that present actual or potential health hazards to customers of the public water system or threatens to degrade the water quality of the public water system should backflow occur.
  • Require backflow prevention assemblies be inspected and tested by a certified backflow prevention assembly tester.
  • Record the date of the initial inspection or test of backflow prevention assemblies.
  • Establish an annual anniversary date for backflow prevention assembly inspections and tests. If an inspection or test report is not received within 60 days following the anniversary date, the supplier of water shall promptly notify the customer, the local government agency (if one exists) and the department.
  • Retain records of reports of inspections, tests and repairs on backflow prevention assemblies for a period of five years.

The supplier of water shall remove the water meter or otherwise sever the public water system from the customer service line serving a facility when the supplier of water:

  • Has knowledge that the customer is causing or maintaining an unprotected cross-connection.
  • Has knowledge that the customer is failing or refusing to proceed without delay to correct any violation of the provisions of this rule after having been notified to do so.
  • Is ordered by the appropriate local governmental authority (if one exists) or the department.

The supplier of water shall notify the department within 48 hours whenever a cross-connection problem has occurred which results in contamination of the public water system.

Can community water systems develop written procedures for backflow prevention?

The supplier of water may develop written procedures to implement the provisions of 10 CSR 60-11.010. A local program may not be less stringent than state regulations but can be more stringent. For example, many public water systems classify all in-ground lawn irrigation systems as a Class I backflow hazard. In addition, local plumbing codes may require more stringent backflow prevention devices.

For additional assistance

Please contact your local regional office for more information. A map and list of the department’s regional offices is available online or by calling 800-361-4827.

Nothing in this document may be used to implement any enforcement action or levy any penalty unless promulgated by rule under chapter 536 or authorized by statute.

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