Types of Cross-Connections

There are two types of cross-connections, direct and indirect.

A direct cross-connection is a physical connection of a piping system permanently connected between the potable plumbing water system and the non-potable piping system. A direct cross-connection is prone to both backsiphonage and backpressure.

An indirect cross-connection is a connection between the potable water system and non-potable system but may or may not be actually or permanently between the potable water system and the non-potables piping system. An indirect cross-connection is subject to backsiphonage only.

It is important to understand the types of cross connections because by knowing the cross-connection type one can properly prescribe and or install the appropriate backflow prevention device for the protection of the potable water system.

An example of a direct cross-connection would be a water storage tank physically connected by piping to the public water system. If the water in the storage tank is used for firefighting purposes and is treated with chemicals, the water in the tank would be considered non-potable. Therefore, a cross-connection exists between the potable water system and the water in the tank. This is a direct connection because the tank water is subjected to the reversal of flow into the potable water system by both backpressure and backsiphonage.

The head pressure of the water in the tank would create backpressure. If there were no check valves or backflow prevention devices installed on the water line feeding the tank and the pressure in the public water system distribution system was to decrease to a pressure lower than the head pressure in the tank, a backpressure condition would exist.

Backsiphonage would be created if the public water system were to become sub-atmospheric. The negative pressure within the public water system would create a siphon effect and pull the non-potable tank water out of the tank into the public water system.

Since the tank head pressure in both cases is greater than the public water system pressure, non-potable tank water is entering the pubic water system causing a cross-connection incident and creating a public health threat.

Example of a Direct Cross-Connection

This photo shows a water tank used for fighting fire purposes. The tank connected to the public water system is equipped with a backflow prevention device. The fire pump connection can also be seen in the front left of the photo. The public water distribution system is subject both backpressure because of the head pressure in the tank and backsiphonage due to loss of pressure within the distribution system.

Examples of Indirect Cross-Connections

An example of an indirect cross-connection would be a hose in a non-potable source of water. As the photo illustrated a hose with no backflow protection is submerged into a barrel of non-potable water. This is an indirect cross-connection because it is only subjected to backsiphonage. Backpressure is not an issue because the water is exposed to the atmosphere. Backsiphonage would occur if the pressure in the domestic water system were to become sub-atmospheric. The domestic water piping system could become sub-atmospheric when an event in the public water system reduces the pressure in the domestic water system to below atmospheric. If the public water system pressure were negative, a reversal of flow would occur in the domestic water piping system causing the water the barrel to be siphoned into the potable water system. Individuals consuming water when this was occurring would be consuming non-potable water from the barrel.  A backflow prevention device such as a hose bibb vacuum breaker installed on the spigot would prevent the non-potable barrel water from entering the potable water system. A backflow prevention device installed on the service line would NOT prevent this incident from occurring.  If negative pressure were present in the public water system, the domestic system would be in an atmospheric state.  If someone opened a tap on a lower level of the domestic piping system, the non-potable water barrel water would enter the potable domestic water system creating a public health threat within the building but the service line backflow prevention device would protect the public water system.

Above are examples of Unprotected Indirect Cross-Connections. All of the examples are submerged inlets and unprotected. Submerged inlets are subject backsiphonage situations therefore submerged inlets are all indirect cross-connections.  The example on the left is a submerged inlet that is below the flood lever rim. The piping is large for a backflow prevention device so the best alternative to controlling the cross-connection would be to install a proper air gap. The illustration in the upper right corner is a submerged hose in a utility sink. There is no backflow prevention device install on the water supply valves. The cross-connection is between the potable water system and the sewer system.  If the hose is necessary fixture, an air gap is not the proper cross-connection control method.  To control this cross-connection a hose bibb vacuum breaker or an atmospheric breaker shall be installed on each valve. If there is a hot water line, a hot water backflow prevention device shall be utilized.  The bottom right illustration is a submerged inlet in an ornamental fountain. To control this cross-connection, a proper air gap should be installed. A pressure vacuum breaker can be installed however, this would become maintenance issue especially if the piping is subjected to cold weather and freezing.