backsiphonage

Backsiphonage is a form of backflow in which water or other liquids are drawn back into a potable water supply due to a drop in pressure within the supply piping, reversing the normal flow direction. It occurs when the pressure in the drinking-water system falls below the pressure in connected fixtures or vessels, creating negative pressure that sucks water back through any cross-connections.

Causes of backsiphonage include main waterline breaks, hydraulic shocks, sudden large water demands, pump failures, and improper venting of drainage systems. It can occur in residential plumbing, irrigation systems, laboratories, and industrial facilities, particularly where cross-connections exist and negative pressure can develop.

The primary concern with backsiphonage is the potential contamination of the potable supply with substances from the connected systems, such as fertilizers, pesticides, wastewater, or chemicals, if cross-connections are present.

Prevention relies on preventing negative pressure events or isolating cross-connections with backflow prevention devices. Common protections include air gaps, atmospheric vacuum breakers, and backflow prevention assemblies such as double-check valve assemblies and reduced pressure zone (RPZ) devices. Proper installation, regular maintenance, and testing of backflow prevention devices are important, and many jurisdictions require such protection at irrigation connections, boilers, and other high-risk points.

Distinctions: backsiphonage is caused by negative pressure leading to backflow, while backpressure backflow occurs when downstream pressure exceeds the supply pressure, forcing flow backward. Both are forms of undesirable backflow that backflow prevention aims to mitigate.