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.”
