Water can behave in strange ways. While it generally will travel downhill unless it is being pumped in the opposite direction, there are situations in which the water draining out of your sewers could end up turning around and head back the way it came. Usually, this will be in connection with a major pressure change somewhere near your home. It could come from a fire truck using large amounts of water or a water main springing a leak. This phenomenon is called backflow, and it could be very harmful, causing contaminated water to enter your drinking water where it can spread disease.
To prevent this from happening, a pipe called an atmospheric vacuum breaker is used. These pipes go above the ground and prevent the vacuum that forms from allowing sewer water to be sucked back up into your home. If you find one of these pipes near your home, you should make sure not to mess with it, since it plays a vital role in the safety of your drinking water.
