Backflow is the reverse flow of water due to hydraulic forces on the downstream side of the connection that would cause water to reverse and move back into the city water. A backflow device is designed to prevent any sort of water from moving back into the city main line. Any time that you do anything with the piping system that influences the city main – that’s what we’ve done here. To connect to the irrigation system, we’ve tapped into the piping system. So, we’re making a cross-connection.
There are two ways that backflow can occur. One is backpressure and the other is backsiphonage. For example, if there was a city main line break, it could interfere with how fast the water is moving through the pipes due to the break. If the irrigation system was turned on, and there is a change in pressure, the water could move back into the city water and possibly contaminate it. Backsiphonage is similar to drinking water out of a straw. That’s how it works. Backpressure is a similar hydraulic force.
The city main is downstream of the irrigation system, and with an abrupt change in pressure downstream it could draw the water from the irrigation system back into the city drinking water. Contaminates such as fertilizers and pesticides could get into the drinking water, and we don’t want that to happen.
When we think about backflow, a garden hose is a big culprit. Just using a garden hose won’t cause a problem. But if you had a garden hose connected to your water faucet at home, and the end of the garden hose was in a tub with chemicals or contaminated fluids, and the hydraulic forces are just right, the water can move from the tub of contaminated fluids or pesticide back into the plumbing system of the home.
When you look at your outdoor water faucets, there should be a little device on it. And, that small, round piece is a backflow device. It’s on there to prevent backflow from happening. On older homes, you may not see the device. If you check your outdoor water faucets and discover that they only have a handle to turn on and off, and there’s no round device on top, then that indicates that you don’t have a backflow device on the faucet. Any plumbing contractor can install a new one for you with a backflow device.
In Manhattan, we use a double check valve backflow device for irrigation systems. It goes in the ground where you don’t see it. But, a pressure vacuum breaker device has to be at a specified height above ground. So, if it’s sitting above ground at the highest point on an irrigation system it will be highly visible, prone to vandalism, and prone to freezing. The last device is called a reduced pressure assembly – to prevent high hazard situations. This one also has to be installed above ground at a certain height. Again, it’s exposed to the elements, vandalism, etc. On any device that’s above ground, and has brass fittings, it must be winterized. When winterizing, the water has to be removed from the system before the potential of freezing.
This feature story prepared with Cathie Lavis, Kansas State University Research and Extension Professor of Landscape Management. For more information, visit your local county extension office or visit our website at KansasGreenYards.org.
Note: Backpressure can occur when a pressure higher than the supply pressure is generated within the pipes due to elevation differences, a pump, or any other pressure-producing device. This can occur during water line flushing, fire fighting or breaks in water mains.
Where as backsiphonage can occur if there is a partial vacuum (negative pressure) in a water supply system, which draws the water from a contaminated source into a potable water supply. The water pressure within the distribution system falls below that of the plumbing system it is supplying. The effect is similar to siphoning or drinking water through a straw. For example, during a large fire, a pump is connected to a hydrant. High flows pumped out of the distribution system can result in significantly reduced water pressure around the withdrawal point. A partial vacuum has been created in the system, causing suction of contaminated water into the potable water system. During such conditions, it is possible for water to be withdrawn from non potable sources located near the fire -- for example, air-conditioning systems, water tanks, boilers, fertilizer tanks and washing machines -- into buildings located near a fire. The same conditions can be caused by a water main break. Garden hoses, toilets or similar devices create most household cross-connections. Under certain conditions, the flow in household water lines can reverse and siphon contaminates into the water supply. A toilet installed incorrectly without a "plumbing-code approved" toilet ballcock (air gap) will allow contaminated water to backflow to other water outlets in your house, including the kitchen sink.