National Drinking Water Clearinghouse
West Virginia University
PO Box 6893
Morgantown, WV

What's the word on cross connections, backflow, and biofilms?

by Kathy Jesperson
On Tap Managing Editor

When it comes to drinking water treatment, almost everything is regulated. So it comes as no surprise that the U.S. Environmental Protection Agency (EPA) is considering drafting regulations for distribution systems.

The agency announced it was considering distribution regulations in 1994 when it asked for comments on whether EPA should require states and/or systems to have a cross-connection control program. But that wasn’t the first time the integrity of drinking water in the distribution system was questioned.

According to the University of Southern California (USC) Founda-tion for Cross-Connection Control and Hydraulic Research’s Web site, when a dysentery outbreak occurred in Chicago in 1933, it was the first time there was a general move toward correcting possible plumbing hazards. By 1974, the Safe Drinking Water Act was enacted and required residual disinfection in distribution lines.

Then in 1982, the National Research Council, which is part of the National Academy of Sciences, prepared a report that identified the biological and chemical contaminants associated with adverse health effects that are attributed to the quality of drinking water within distribution lines.

According to the report, "Contam-inated water may enter a potable supply either through the system or through a defect in the user’s plumbing system. Cross connections, together with backflow or back-siphonage, are the most critical factors in protecting a distribution system from contamination.

"Cross-connection contamination can provide another opportunity for large amounts of biological material to enter the distribution system. These events generally result in noticeable change in water quality, including turbidity, increased content of solids, and undesirable tastes and odors.

"In many cases, cross connections are not obvious and the resulting changes in water quality are not detected by the consumer. Often, small intermittent flows through cross connections can back-siphon and be responsible for outbreaks of disease."

Besides this report, the Centers for Disease Control and Prevention (CDC) notes that distribution system deficiencies cause a significant portion of waterborne disease outbreaks.

The CDC defines distribution system deficiencies as cross connections, water mains contaminated during construction or repair, and contaminated storage facilities. Further, the CDC states that between 1971­1994, approximately 53 waterborne disease outbreaks were associated with cross connections or back-siphonage. Fifty-six outbreaks were associated with other distribution system deficiencies. Water main breaks or repairs caused some outbreaks.

EPA initially planned to include cross-connection control regulations in the Interim Enhanced Surface Water Treatment Rule. However, the agency has since stated that it will include regulations as a part of future microbial regulations and in the context of a broad range of issues related to distribution systems.

What is a cross connection?
According to USC Foundation’s Web site, a cross connection is any link—whether actual or potential—through which contaminated material may enter a potable water supply. Bypass arrangements, jumper connections, removable sections, swivel or changeover arrangements, or other "temporary" arrangements through which backflow can occur are considered cross connections.

There are two basic types of cross connections: direct and indirect. A direct connection can be affected by backpressure. An indirect connection cannot. An example of a direct connection is a make-up line feeding a recirculating system. An example of an indirect connection is a hose connected to a supply line and then left in a bucket filled with some hazardous substance. (See figures below.)

Direct and Indirect Connections


What is backflow?
Backflow is the reversal of water flow in water lines caused by a negative pressure (vacuum) in the supply line. Typically, backflow happens when a water main breaks or where the mains are pumped dry during fire fighting.

A number of specific conditions are responsible for backflow. But usually the reverse pressure gradient may be caused by either a loss of pressure in the supply main known as backsiphonage, or by the flow from a customer’s pressurized system through an unprotected cross connection, which is called backpressure.

A flow reversal in a distribution main—or in the customer’s system—can be created by any change of system pressure wherein the pressure at the supply point becomes lower than the pressure at the point of use. When this happens in an unprotected situation, the water at the point of use will be siphoned back into the system, potentially polluting or contaminating the remainder of the customer’s system.

It is also possible that the contaminated or polluted water could continue to backflow into the public distribution system. The point at which it is possible for a non-potable substance to come in contact with the potable drinking water system is called a cross-connection. To prevent backflow from occurring at the point of a cross-connection, a backflow prevention assembly must be installed.

What is biofilm?
Biofilm may appear as a patchy mass in a section of pipe or as a uniform layer along the inner walls of a storage tank, notes Edwin E. Geldreich in Microbial Quality of Water Supply in Distribution Systems. Biofilms are initially formed when organisms enter the distribution system and become entrapped in some slow-flow area, line obstruction, or dead-end section.

Biofilms may be a single-layer or as thick as 10 to 40 millimeters. Biofilms this thick are usually found in reservoir bottom sediment or in raw water intake structures.

Biofilms appear to be complex structures that consist of microcolonies of various organisms embedded in an organic material, which then adhere to moist surfaces. Water mains, storage reservoirs, standpipes, joint connections, fire hydrant connections, valves, service lines, and metering devices have the potential to be excellent sites for microbial habitation.

The reason that EPA has concerns about biofilms is that coliform bacteria may colonize in them. Also, biofilms may interfere with coliform detection, and they may cause taste and odor problems.

How can problems be avoided?
EPA’s Cross-Connection Control Manual suggests that education is the primary way to avoid backflow problems.

Plumbing installers need to be made aware that hydraulic and pollution factors may combine to produce a health hazard if a cross connection exists. They must also be made aware that simple, reliable backflow prevention devices exist and may be substituted for dangerous direct connections. The EPA manual states that it is imperative for installers to understand that hazards resulting from direct connections outweigh any convenience gained.

To control biofilm growth, Geldreich notes that distribution lines must be continually maintained. Adequate water pressure of at least 20 pounds per square inch must be sustained throughout the entire pipeline. And chlorine residuals must be provided in all sections of the pipeline to protect against contaminants that may leak into the lines.

For more information about cross-connection control, see the Spring 1997 and Fall 1993 issues of On Tap. EPA’s Cross-Connection Control Manual is available by calling the agency at (800) 490-9188 or (800) 426-4791. Request the document EPA/570/9-89/007. Also see "An Introduction to Cross-Connection Control," on the University of Southern California’s Web site at

CCC Devices Protect Water Supplies

Six basic types of devices can be used for cross­connection control (CCC). They are:

Air Gaps—An air gap is an actual physical separation between the supply pipe (faucet) and the receiving vessel (bucket, etc.). This separation must be at least twice the pipe diameter, but never less than one inch.

Atmospheric Vacuum Breaker (AVB)—When the inlet valve of an AVB is closed, water flows in the normal direction. But, as water stops flowing, the air inlet valve opens, and interrupts any backspiphonage effect.

Atmospheric Vacuum Breaker. Flow Condition on the right and Non Flow Condition on the left.

Double Check Valve—This device consists of two single check valves in one piece of equipment and is equipped with test cocks and two tightly closing gate valves. Double check valves protect against hazards, such as backsiphonage from a food processing plant.

Double Check Valve

Double Check with Intermediate Atmospheric Vent—This device has a double check valve with the extra protection of an atmospheric vent between the two checks.

Residential Dual Check—This device furnishes reliable and inexpensive backsiphonage and backpressure protection for individual residences. Plastic check modules and elimination of test cocks and gate valves keep the cost reasonable while providing, good dependable protection.

Reduced Pressure Principle Backflow Preventer—This device provides maximum protection against backsiphonage and backpressure conditions. It is essentially a modified double check valve with an atmospheric vent. It is designed so that the area between the checks is always at least two pounds less than the supply pressure.

Atmospheric Vacuum Breaker

Source: U.S. Environmental Protection Agency’s Cross Connection Control Manual