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There are two ways for backflow to occur: backsiphonage and backpressure. Backsiphonage happens when there is a negative pressure in a pipe, causing a vacuum, and water flows in the wrong direction. The vacuum can be caused by high water demand in another part of the system such as from firefighting or a water main break. Backpressure occurs when the pressure at one part of the system is greater than the pressure coming in from the water main. This can happen when a garden hose is connected to a faulty pressure washer, from an overheated water heater or boiler, or from the weight of water on a hill or tall building.
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A cross-connection is any point in the public drinking water system that is actually or potentially connected to a source of contamination or pollution that could enter the system through backflow. Common cross-connections include automatic sprinkler systems, soda fountains, fire suppression systems, and garden hoses among many others. All cross-connections require backflow prevention.
Backflow is water and/or other substances flowing in the opposite direction of the normal and intended course. It is the process by which a cross-connection can introduce contaminants into the public drinking water system.
In order to protect public health, the City of Moscow has implemented a Cross-Connection Control Program as is required by The Idaho Rules for Public Drinking Water Systems (PDF) (IDAPA 58.01.08.552.06) and the Moscow City Code (PDF) (Title 7, Chapter 9).