Have you ever seen a city of Grand Prairie truck sitting in front of a fire hydrant that was wide open allowing water to flow down the street? When you passed that truck, did you ask yourself “Why are they wasting water?”
Well, there are many reasons why we are flushing these hydrants and all of them have to do with protecting public health.
The City of Grand Prairie purchases its water from the cities of Dallas and Fort Worth (about 90% from Dallas and 10% from Fort Worth). The treated water from these cities enters Grand Prairie at 3 locations and is pumped through 700+ miles of pipe to service residential and commercial needs. This water is treated with a chemical compound called Chloramine (a combination of Chlorine and Ammonia). Chloramine is the active disinfectant used to prevent bacterial growth in our water system. Over time, the level of chloramines in the system may dissipate due to: age, temperature or demand.
The minimum disinfectant (chloramine) residual for our system is 0.5 mg/L. Studies show that this is the lowest level that chloramines can effectively prevent bacterial growth. The state’s rules also require us to keep enough water in storage to provide adequate pressure for use and fire fighting activities. Finally, the City is required to periodically flush “dead ends.”
So, why is the water running???
Dead End Flushing
All cities in the state of Texas are required to flush areas of their water system called Dead Ends. Dead Ends in the water system are like dead ends on the street; they are points of no return. They often occur where streets terminate. So, why do we have to flush these areas? As water ages, the level of chlorine residual decreases and the chance of bacterial growth increases. When water gets to a dead end, it stays there until it is used, which may be several days depending on the size of the water line and the usage. So, to keep fresh water flowing in these dead in areas, the City will open a fire hydrant or install a pipe known as an automatic flush site. Water in these areas is flushed at least once a month to maintain disinfectant residual and palatable water.
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In 2007, the City of Grand Prairie was rated as the 5th fasted growing city in the nation! With that growth comes a need for new and more water lines. Before these new lines are put into service, they must be disinfected by adding a high concentration of chlorine. New water lines are required to be disinfected with 50 mg/L of chlorine solution. The maximum amount of chlorine (or chloramine) that can be delivered to customers is 5 mg/L. Since the disinfection dosage is so high, that water in the pipe must be “dumped” and replaced with potable water, i.e. water with less disinfectant. The “super-chlorinated” water to be dumped is first de-chlorinated then released to the streets and storm drainage system.
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Super-Chlorinating Just as you have to clean your car or bath tub, the water system has to be occasionally cleaned. This cleaning is accomplished by adding high doses of chlorine to the pipes, a process called Super-chlorination. Super-chlorinated water cannot be consumed by the public, so it must be “dumped.” To ensure that no one consumes super-chlorinated water, the water meters along the section of pipe being treated are disconnected. After allowing the high chlorine dose to do its work, usually overnight, a fire hydrant is opened to release the non-potable water.
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Occasionally, citizens will contact city offices with concerns about the taste, odor or appearance of their drinking water. We classify these concerns as water quality complaints. In addition to checking our water system and collecting analytical samples, city crews will often open fire hydrants to “dump” water of poor quality and “pull” water of favorable quality to the area of concern. How does the water become “poor”? Several factors can contribute to water tasting different. Some of the most common factors are low usage, temperature, and changes in source water. Low water usage can lead to “old” water which may develop unpalatable tastes. Coupled with low water usage, temperature can affect the taste and odor of water by causing pH fluctuations. As the pH changes, the active disinfectant changes its configuration and develops an odor. Although this change does not pose any health threats, it does make the water difficult to drink. This problem is easily solved by opening a fire hydrant to release the old, warm water and draw fresher water to the area of concern.
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Unidirectional Flushing Program
The Unidirectional Flushing Program, also known as UDF Program, is a method of cleaning the water mainlines through a network of flushing sequences with the water being discharged from a fire hydrant. A UDF program involves closing valves in a specific sequence to create water movement in one direction while opening specific hydrants at the end of that sequence. Maintaining the flushing sequence is important so that the water used in the flushing sequence remains clean. The UDF technique allows higher water flow velocities by isolating certain sections of water mains. This higher water velocity allows for better scouring of pipes. The flushing of the pipes will dislodge and remove mineral deposits, sediments and biological deposits that accumulate in the water mains.
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Servicing Groundwater Wells