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Protecting the quality of our drinking water includes protecting the water as it passes through the pipes to all the buildings in the City. Something called a cross connection can threaten the quality of our drinking water in that piping system. A cross connection is where substances other than drinking water may enter the plumbing and come in contact with drinking water. The City of Seattle requires that any cross connection have a backflow preventer and to have it tested yearly.
Some places that can result in cross connections include:
Backflow of contaminants into the water supply can occur due to back-pressure or back-siphonage. Back-siphonage can be caused by high velocities in pipes from maintenance, water main breaks, or fire fighting. Back-pressure occurs when the pressure at the point of use is higher than the supply pressure. Sources of back-pressure include booster pumps, boilers, elevated piping and interconnections with auxiliary systems.