Water Protection Program fact sheet
Division of Environmental Quality Director: Ed Galbraith

Backflow Prevention
In 1997, the Missouri Department of Natural Resources revised a drinking water regulation entitled “Backflow Prevention.” This revised regulation places certain responsibilities on water suppliers to ensure that customer facilities identified as actual or potential backflow hazards provide the necessary protection to prevent contaminants from entering the public water system.

What is Backflow?
Backflow is defined as the unwanted reversal of flow in a water distribution system. Due to changes in the hydraulic pressure in a water distribution system or a piping system inside a customer’s premises, backflow occurs on a regular basis.

The polluting substance, usually a liquid, tends to enter the potable water supply if the net force acting upon the liquid acts in the direction of the water supply. Therefore, two factors are essential for backflow to occur. First, the normal direction of flow in the distribution system must be interrupted. Second, there must be a link or connection between the potable system and the source of contamination.

Backflow only becomes a serious problem when there are cross connections within the water distributions system.

Public Health Significance of Cross Connections
A cross connection is a physical link between a source of pollution or contamination with a potable water supply.

Public health specialists have long been aware of the threat to public health posed by cross connections. Education is the most important factor in cross connection control.

Various court decisions have held water suppliers responsible for the delivery of safe water to consumers. But the safety of our drinking water supply can be jeopardized at any location, at any time because of the frequency of plumbing defects and cross connections. Due to frequent changes in piping systems, an effective cross connection control program, including continued surveillance of the public water system, is necessary to prevent backflow incidents.

Components of an Effective Cross Connection Control Program
The first step in preventing backflow incidents is enacting local rules that grant the water supplier the authority to enforce the cross connection control program. For the water supplier to comply with the state backflow prevention regulation, the local rules should include the following provisions:

Another responsibility of the water supplier is to notify customers, where backflow hazards exist, that they must comply with the local rule. Once these customers have been notified, the supplier must maintain records of inspections, exemptions, or installation of assemblies.

A local program may not be less stringent than state regulations. Local plumbing codes may require additional backflow prevention devices.

Methods of Backflow Prevention
The department’s Public Drinking Water Branch maintains a list of backflow prevention assemblies approved by the Foundation for Cross Connection Control and Hydraulic Research at the University of Southern California, The following methods of backflow prevention meet the requirements of the state backflow prevention rule.

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 pipe. For example, a two-inch separation is required for a one-inch water supply pipe.

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 designed for low hazard protection only. The double check valve assembly has two spring valves that act independently to provide protection from back pressure and back-siphonage. 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 allows backflow to occur.

Testing/Inspection Requirements
The function of all backflow prevention devices must be reviewed annually. Air-gaps may be inspected by the water supplier. A state-certified backflow prevention assembly tester must perform the specific testing procedures required to verify the proper function of reduced pressure principle assemblies and double check valve assemblies.

The Public Drinking Water Branch maintains a list of certified backflow prevention assembly testers.

For Additional Assistance
Please contact your local regional office for more information.
Public Drinking Water Branch 573-751-5331
Northeast Region Office 660-385-8000
Southwest Region Office 417-891-4300
Southeast Region Office 573-840-9750
Kansas City Region Office 816-251-0700
St. Louis Region Office 314-416-2960