What causes back-siphonage in a water system?

Back-siphonage occurs primarily due to the creation of negative pressure or a partial vacuum within a water system, which allows contaminated water from a lower pressure area to be drawn into the clean water supply. When there is a significant drop in pressure in the distribution system, it creates a condition where the normal flow of water can be reversed. This typically happens when there is a break in the main line or when there are sudden demands on the water system, such as when a fire hydrant is opened.

In essence, the establishment of this negative pressure is what leads to the siphoning effect, pulling potential contaminants from less pressurized areas into the drinking water supply. Understanding this mechanism is crucial for water operators to prevent contamination and ensure safe drinking water.