Does water ever flow backwards?

Drinking water normally flows in one direction, from the public side to the customer side, however, under certain circumstances, it can flow in the opposite direction. This is known as “backflow”. A significant drop in public side water pressure possibly caused by a water main break or extreme high usage on the water system, or due to a pump or thermal expansion on the property side are backflow conditions. Under these conditions, a connection not protected could allow pollutants to be siphoned back into the public water system.