A cross-connection is any physical connection between a potable (drinkable) water supply and non-potable (non-drinking quality) water, or other substance of unknown or unsafe quality.

Cross-connections can allow contamination or pollution of the public water supply as a result of backflow. Backflow is an unwanted hydraulic condition that may develop in the distribution system under certain conditions.

Northglenn strives to keep the water flowing in the right direction at all times. However, this is not always possible. Therefore, Northglenn maintains a Cross-Connection Control Program. This program seeks to identify all existing cross-connections within the city's water distribution system. Once a cross-connection has been identified, the city and property owner then protect the public water system from potential pollutants through the use of backflow prevention devices, assemblies and practices.

Common cross-connections in homes and commercial properties:
• Submerged hoses
• Lawn sprinkler system
• Fire protection system
• Toilet tank
• Tank with submerged inlet
• Solar heating system
• Boiler
• Cooling tower
• Swimming pool
• Fountain
• Photography equipment

Backflow

There are two types of backflow: backpressure and backsiphonage. Think of backpressure as “pushing” and backsiphonage as “pulling” water through a system.
• Backpressure is an increase of pressure within a plumbing system that becomes greater than the pressure of the incoming water supply. This pressure difference may allow certain systems to "push" harmful substances back into the water main. Backpressure can be caused by boilers (thermal expansion), pumps, elevated tanks or piping or solar heating systems.

• Preventing backflow by backpressure involves installation of spring-loaded "one-way" check valves in the water line. Most backflow prevention assemblies consist of one or more check valves which close when water flows backward through them.

• Backsiphonage is caused by negative or sub-atmospheric pressure in the supply piping. This pressure difference allows water and other substances to be siphoned, or "pulled" back into the water main. This is the same principle that is involved when you drink through a straw.

• Backsiphonage is created by a loss or severe reduction of pressure in the water distribution system. During events such as a water main break or firefighting, a high volume of water exits the drinking water system at one or two points, thereby creating zones of reduced or sub-atmospheric pressure. During these events substances may be backsiphoned from inside a home and end up in a water main.

• Backsiphonage can be prevented by the installation of backflow prevention assemblies, some of which “break' the vacuum by allowing air to enter the water line. Others use check valves to stop the reversal of flow during a backsiphonage event.