By Jim McMahon
By 2012, EPA regulations will require a higher level of compliance for disinfection byproducts (DBPs) in drinking water. This has prompted a number of water districts to install tank covers to reduce algae growth in process basins, launders and tube settlers. By blocking sunlight that the algae requires, the need for chlorine, chloramines and hypochlorite is reduced, saving money for chemicals and cutting DBP production.
One of the more versatile cover designs is the retractable, structurally supported geomembrane cover system, which provides not only sunlight blockage to reduce algae growth, but facilitates easier access to water treatment processes for maintenance and repairs – a critical factor in systems like GAC contactors that require frequent maintenance.
One of these drinking water producers is the Palmdale Water District in California. Palmdale is the fastest growing city in Los Angeles County and the largest desert city in California. The city has grown by 30 percent since 2000, with a current population exceeding 150,000 residents. This rapid growth has put continual necessity on its water district to evolve and expand its drinking water treatment processes. The district maintains a drinking water treatment plant producing 28 mgd, over 345 miles of pipeline, multiple well sites, booster pumping stations, and water storage tanks with a total capacity of over 52 million gallons.
Palmdale receives its water from a combination of surface and groundwater sources. The surface water represents 60 percent of the district’s water, and primarily comes from the Sacramento River delta, some 400 miles away, via the California Aqueduct. Additional surface water comes from the local San Gabriel Mountains. Surface water is filtered and disinfected at Palmdale’s treatment plant. The district’s groundwater is pumped and treated with chlorine before being released into its distribution system. This well water makes up 40 percent of the district’s annual production, and is sourced from three separate aquifers.
|A critical point in Palmdale’s selection of covers was their ability to withstand extreme environmental conditions. This area is a desert, with wind speeds of up to 70 mph.|
Because of the district’s split surface-water/well-water sourcing, using chloramination to meet the EPA’s 2012 guidelines for DBPs was not an ideal option.
“We went in a different direction from most of the water industry to comply with EPA 2012 guidelines,” said Peter Thompson, Plant Superintendent for the Palmdale Water District. “We chose not to go with chloramination in our system because we have multiple wells and pump stations. We would have to very carefully monitor the chlorine to ammonia ratios throughout the system with additions along the way, and that appeared to be a very difficult task. If the ammonia to chlorine ratio is not carefully monitored, nitrogen bacteria can form in the water distribution system which will cause taste and odor problems.”
“Also, we had concerns that with chloramination an additional chemical, ammonia, is being added to the process,” Thompson said. “Nothing is being removed from the water, it is just changing the chemical process. We had issues with that based on maintaining water quality. When ammonia is added to the water along with chlorine, it inhibits the formation of the regulated disinfection byproducts. But the nitrogen molecules from the ammonia in chloramination can create additional byproducts within the disinfection process that are as of yet unregulated. There is the potential, however, for these to be researched and regulated by the EPA at some point in the future, which would require a system change for many water producers using chloramination.”
For these reasons, Palmdale opted to build eight GAC contactors to act as secondary polishing filters, following primary filtration and hypochlorite disinfection, to not only comply with 2012 DBP guidelines but to provide a strong barrier for EPA compliance well into the future. Also, should the district have any problems with water received from the California Aqueduct, such as with contamination from pesticides or pharmaceuticals, additional technology to remove them would not be needed. This water polishing is excellent for sustaining water quality – the GAC contactors can strip out a wide variety of contaminants, dissolved organics, and taste and odor compounds from the water.
The water district selected a cover system for these large polishing filters, as its finished water was exposed. A major concern was the need for access to each of the eight contactors as often as every month for backwashing, servicing, cleaning of debris and replacement of the GAC media. Carollo Engineers, an environmental engineering firm specializing in the planning, design and construction of water and wastewater facilities, recommended the use of retractable, structurally supported covers manufactured by Geomembrane Technologies Inc.
“Being experienced with other cover designs, I was concerned that when we needed to access the contactors for backwashing it was going to be a difficult process to open the covers,” Thompson said. “But, it turns out to be a fairly quick and easy two-man job. Five to ten minutes and it is done. Of all the cover systems we looked at, these were the easiest for opening and closing. The beauty of these retractable covers is that they are a very simple system to operate.”
|Installing geomembrane covers at Palmdale Water District. The covers are a simple system to operate and easy to open and close.|
A critical point in Palmdale’s selection of the covers was the ability of these retractable, structurally-supported geomembrane systems to withstand extreme environmental conditions that exist in Palmdale. This area is a desert, extremely dry and receiving only seven inches of rain annually. Temperatures can range from more than 115 degrees F in the summer to less than 10 degrees F in the winter. Gusty winds blow over Palmdale almost every day of the year, so consistently that wind turbines are used to generate electricity, and peak wind speeds of 70 mph are not uncommon.
“At first, the idea of having covers with a fabric material didn’t seem like they would hold up very well,” Thompson said. “But, these covers have held up exceptionally well despite the harsh environment that they are in. After seeing their performance for almost two years now, with wind and debris hitting them, the extreme temperatures and 300 days a year of relentless sunshine, I have absolutely no concerns over their durability.”
The retractable, structurally supported geomembrane covers are an attractive option for municipal drinking water facilities functioning under almost any operational and climatic conditions. UV stabilized with NSF 61 certification, GTI’s covers are designed to last for 15 to 20 years in the field. The first structurally supported geomembrane cover system applications, installed in 1999 in Louisiana, are still in place and functioning as designed with full structural integrity. Since then, these retractable cover systems have been installed in many water and wastewater plants throughout the world.
About the Author: Jim McMahon writes on water and wastewater systems. His stories have been carried in hundreds of trade publications worldwide.
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