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Environmental Quality  |  206 W. Church St., 2nd Floor |  P.O. Box 534045  |  Grand Prairie, TX  75053
Phone 972-237-8055 |  Fax 972-237-8228

What is backflow?

The City’s drinking water flows into your house through a series of pipes called the water distribution system. The flow of water is pressure driven, meaning that when you open a faucet the pressure at that faucet drops to zero and the water flows towards the lowest pressure, causing the water to flow out of the faucet. That’s why you may get a drop in your water pressure at your house if there is break in the City’s line, the water is flowing towards the lowest pressure, the break in the water line. There are different ways the City keeps pressure in the drinking water distribution system, the most important being the elevated storage tanks. The elevated storage tanks keep the system pressurized so that water can flow to your taps when they are turned on.

Backflow 1

Since the water distribution system works on pressure, there can be a problem with backward flow of the water when an unintended break or pressure drop occurs in the system. An example is using the hose to fill your backyard pool. Say you drop your hose into the pool to fill it. The opening of the hose is under water. Say there is a fire near your neighborhood and the fire department taps into the fire hydrant near your house, causing the pressure to drop at that point. The water in the area will flow towards the largest drop in pressure (i.e. the fire hydrant, not your hose, simply because of the size of the lines involved), causing the water flow to reverse and flow backwards at your water hose. Since the water hose is under water, the hose starts draining the pool back into your house and the City’s distribution system. This situation is called a backflow event. The hose being submerged into the pool created a cross connection between the pool and the City’s water distribution system. The drop in pressure on the City side of the system caused the backward flow of water, causing the pool to be drained into the City’s drinking water system.

Backflow 2a Backflow 2b

What can we do to prevent such occurrences? The main thing is public education about possible cross connections at your home and how to avoid them. Many people just are not aware of the potential problems such situations can cause. The City regulates all commercial and industrial facilities to prevent cross connections and backflow events, but residents must be reached through public education venues such as this website.

Potential Cross Connections at Your Home

Backflow 3The example of using a hose to fill your pool and dropping the end of the hose into the pool while filling is a very common example of a residential cross connection. The simplest solution is to keep the end of the hose out of the pool when filling. Another potential cross connection involving your outdoor hose is using a hose sprayer when applying pesticides or fertilizers. Some of the hose sprayers set ups come with a built-in backflow preventer and some do not. Another situation is if the end of the hose is left in a puddle of water.
The best way to prevent backflow situations with your outdoor hose is to install a Hose Bibb Vacuum Breaker on all outdoor faucets. This is a small device that screws onto the end of your faucet that will stop the water flow when it detects a backflow of water from your hose. These devices can be purchased at most hardware stores for a nominal price.

Irrigation systems can also present opportunities for cross connections. If you have the kind of sprinkler heads that pop up when in use and then drop down when not in use, dirty water can collect Backflow 4around those sprinkler heads and when a pressure drop occurs, that dirty water can be sucked back into the distribution system through those sprinkler heads. This is why the City has required that for all new in-ground irrigation systems, a backflow preventer is required to be installed on the water line.
A backflow preventer is a device that has two check valves that will shut if the device detects a drop in pressure on the upside line or a backward flow of water.
The type of device required on all in-ground irrigation systems is called a double check valve and it has two of these check valves in place to prevent the flow of water backward into the distribution system.

Backflow 5There are potential cross connections inside your house as well. Most faucets on your sinks are at least one inch higher than the rim of the sink or tub. This is called an air gap and it totally prevents any cross connection because there is no way the end of the faucet can get below the rim of the sink. An exception to this is your kitchen vegetable sprayer. It is essentially a hose extension of the water faucet.
If you leave it in the sink there is the potential for a cross connection if the sink fills up with the sprayer head under water. Should a drop in pressure occur in the City line, that dirty water could flow backward, through the sprayer, into the water distribution system. The solution is to be aware and be careful when using the vegetable sprayer in your kitchen.

Backflow 6Your toilet may also present a potential cross connection in your home. Many older toilets use a submerged fill valve in the tank. These submerged fill valves are still on the market today. You can tell which ones they are, because when they are put in, the fill valve sits under the water level of the tank. When repairing your toilet, always buy the type of flush mechanism where the fill valve sits above the water level in the toilet tank.

There is a classic cross connection situation where the resident calls the City complaining of blue water coming from their faucets. Upon investigation, the home owner uses a toilet tank cleaner product that makes the water blue and has the under water type of fill valve in their toilet tank. When a big enough drop in pressure occurs outside the house, or even in another area of the house, that water in the toilet tank starts draining back into the house distribution system, coming out at the water faucets.
The City wants to keep your drinking water supply clean and drinkable. You can do your part by preventing potential cross connections in your home!

Backflow Testing

Information on policies for backflow testing can be found HERE.

Last updated: 12/21/2011 8:29:18 AM