Automatic flushing units help save labor, improve water quality and prevent growth of pathogens in a Louisiana parish water distribution system
In 2005, Hurricane Katrina caused devastation to St. Bernard Parish, southeast of New Orleans, and cut its population from 67,000 to 45,000. With spotty redevelopment and more water capacity than is needed for today’s residents, the parish has paid close attention to maintaining drinking water quality.
The Louisiana Department of Health and Hospitals notified the water department that there had been two deaths (one in 2011, another in 2013) related to Naegleria fowleri, colloquially known as the brain-eating amoeba in the parish. This free-living, bacteria-eating amoeba is found in warm, unchlorinated freshwater such as ponds, lakes, rivers and hot springs. If contracted by humans, it can cause a deadly infection in the brain. (In the first case, the amoeba was found at a residence and not in the distribution system.)
Jacob Groby, parish superintendent of quality control, worked with the health department to collect water samples from more than 100 points in the distribution network. While the parish had been vigilant in monitoring water quality, officials determined that the sampling protocols were no longer adequate, as some areas reported lower-than-acceptable levels of chlorine and higher levels of ammonia, which can be conducive to biofilm growth.
To help ensure the safety of its water supply, the parish accelerated its water sampling program and installed flushing stations at strategic points in the distribution system that enable automated flushing based on a water-demand model.
Testing for Naegleria fowleri requires analysis in specialized labs such as the Centers for Disease Control and Prevention and the Biological Consulting Services. In addition, monitoring of other factors that exert pressure on water quality need to be checked and addressed.
For example, the parish does additional testing in low-population sections of the distribution system for total chlorine, monochlorine, free ammonia, nitrite, pH, dissolved oxygen and temperature using a Hach SL1000 portable parallel analyzer. Nitrate levels are monitored with a portable Hach Colorimeter II filter photometer.
These units provide quick in-the-field analysis. In addition, each sample is tested for adenosine triphosphate, or ATP. By tracking trends and not just daily results, water operators can change water conditions before they become a problem.
“Because our problem is unique to a surface water system, we had no prior cases to fall back on for experience,” Groby says. “We had to rethink the age and time of water in our system. The only logical way to do that is to cause the water to move through the system, identifying the point just before we lose the chlorine residual, causing an increase in nitrification that is favorable for bacteria.”
To move water through the system, the parish installed 50 Hydro-Guard TAPS flushing stations, each with a custom 50 mm (2-inch) Singer valve that optimizes flushing for the pipe size, providing the correct velocity to pull freshwater into each zone. This, coupled with a pressure-sustaining feature, scours and cleans the pipes, reducing corrosion and improving water quality.
To mitigate the visual impact of an industrial device in front of a home, a valve box is used to conceal and protect the flushing unit. Each location has a custom-poured slab with the discharge tied directly into the subsurface drainage system, helping to avoid unsightly or hazardous conditions. Each unit includes a water sampling port so that on-site clarity can be observed and samples can be taken easily without disrupting the process.
Due to depopulation of the parish, the distribution system has too much water to routinely turn over in 24 hours, as it did before Katrina. With spotty redevelopment, operators cannot turn off sections of the system. Instead, they need to determine where the water flows and how long it takes to get there, in order to effect a flow-pattern change.
To do this, operators have 2,000 reference points for pressure calculations that measure the mean C-factor of residual pressure loss to determine water usage. This allows operators to reduce flushing and water loss in areas when usage is high.
The parish developed a water-demand model that helps fine-tune flushing times. Daily data is compared with historical data to optimize decision-making. When chlorine (1.0 ppm minimum) begins to fall off, nitrite rises (0.4 to 0.5 ppm maximum) and ATP rises higher than a 10.0 count; a warning point is reached that triggers automatic flushing or heightened testing for the identified section. Once flushing is required, velocity measurement and gallons flushed are recorded for future modeling.
The parish has designed a flush cycle to create a wave action to force-flush 27 miles of the distribution system as needed. “The programming is easy,” Groby says. “The hard part is deciding how to set them so that we can use them in the best manner possible. This is where our water-demand modeling proves valuable.”
With regulated flush cycles and perfected velocity at each installation point, freshwater is effectively drawn into areas where water quality falls below acceptable levels. Flushing of every dead end in the system is vital to ensuring the removal of poor water.
The pressure-sustaining feature ensures that minimum upstream pressure is maintained for system needs and that fire flow is available if needed. The programming allows the parish to activate flushing at any time; the best time is at night when the demand is low.
This prevents cloudy water, low pressure and standing water complaints associated with conventional high-volume daytime flushing. It also enables distribution of the discharged water over several hours, if necessary, by programming the units to activate for shorter multiple durations throughout the day. “By using these flushing units, we have been able to use our greatly reduced personnel in a more efficient manner and save on our limited utility resources,” Groby says.
Groby, an advocate of safe water delivery and a speaker on the topic at industry events, observes, “All southern water systems that experience high temperatures (72 degrees F or higher) should be routinely flushing their systems and using the new testing tools for all water-quality indicators to ensure a consistent level of protection and maintain fresher water.”
While it is common knowledge that routine flushing is important to maintaining freshwater in a distribution system, outdated practices can cause large losses of water and require many labor hours if performed manually.
The Hydro-Guard TAPS automated flushing unit uses less water to get the job done and is easily programmable to minimize labor and do much of the work during low-demand night hours. Groby observes, “Our distribution system has shown a marked improvement on most factors, with improved chlorine residuals,lower total trihalomethane and haloacetic acid values, and reduced consumer complaints.”
About the author
Mark Magda (firstname.lastname@example.org) is global sales manager for Mueller Water Products
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