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.

Background information

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 600+ 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. 

Regulatory Requirements

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”.

When you see a fire hydrant flowing water, it is typically for one of the reasons listed below.  To find out more about each, click on the topic that interests you.

• Dead End Flushing Program
• New Construction
• Super-chlorinating
• Water Quality
• Unidirectional Flushing Program
• Servicing Groundwater Wells
• Fire Tests
• Wash down after Sanitary Sewer Overflow

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.

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.

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. 

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.

During most summers, the amount of water used by Grand Prairie residents exceeds the amount of water purchased from Dallas and Ft Worth.  When we reach these periods of peak demand, the City may turn on any of its emergency supply wells.  These ground water wells need to be flushed before they can be put into the water system.  Flushing of these wells may take 1 – 3 days depending on flow rate and location.

Annually, the Fire Department is required to have the water system tested to make sure adequate pressure exists to fight fires.  This system check may include opening fire hydrants in various areas of town.