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Where Did the Water Go?

Posted: May 7, 2012

Imagine leaving a hose running into a tank and returning later to find the tank empty instead of full. At best this would be a mystery at worst a serious problem.
An air gap for a dishwasher drain. The chrome top installs above the counter, the black body goes below.

An air gap for a dishwasher drain. The chrome top installs above the counter, the black body goes below.

The serious problem occurs when the hose is adding drinking water and the tank contains (or did contain) stuff you don’t want to drink. While this is a rare event it has happened. Once on a Pennsylvania farm the tank was a spray tank containing a herbicide intended to control weeds in a corn field. Not only was the well contaminated but the nearby aquifer also. Such a backwards flow is known as a back siphon event and plumbing codes are designed to prevent them. The pasteurized milk ordinance governing dairy production facilities also seeks to prevent these events. To understand how to prevent a back siphon event, first consider the siphon itself.

A tube bent into an upside down “U” shape with one leg longer than the other has the potential to be a siphon. If the short leg is put into water with the long leg open to the air you are very close to having a working siphon. If the tube is suddenly filled with water the tube will draw water up through the short leg and discharge it from the long one. Chances are you make daily use of a siphon. The one I have in mind is the bathroom toilet. By studying the side of the bowl you can see the upside down “U” shape. When the toilet is flushed a large amount of water rapidly enters the bowl filling the short leg of the siphon which begins to flow and draws wastewater out of the bowl until it is emptied and lets air into the short leg which breaks the siphon.

In the home and in the workplace clean water is used and then released to a drain. Obviously no one wants to siphon drainage water back into the clean water supply. What can be done to prevent it from happening?

Submerging the end of a hose while filling a bucket creates a potential for a back siphon event. If the well pump fails, water could flow backwards and the bucket water could end up in the well. This is how the mystery we started with can occur. There is normally a foot valve in the well pump and a check valve at the pressure tank to prevent reverse flow. And they do, as long as they don’t leak. However if the hose end is suspended above the edge of the bucket, the hose can never siphon water out of the bucket. The air gap between the end of the hose and the water surface prevents a siphon from getting started. This air gap is prescribed in plumbing codes and applies to the connection between discharge lines from water softeners and dishwashers to the household drain lines. You will also notice that faucet outlets are above the rim of the sink; this is another application of the air gap rule.

A common violation of the air gap rule is directly connecting a water softener discharge line to the top side of a household sewer line. This counts on the sewer line never running completely full and the treatment system’s check valve working correctly. It is a hazard that should be corrected to protect family health. Back siphons are prevented by making sure air gaps exist at all clean water discharge points and also at all waste water discharge points for water treatment equipment.

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