Problem: Chemical water softeners used in potable water treatment created high salt content in reuse water at a resort.
Solution: Switching to electrodialysis reversal and vibratory shear membrane filtration systems for water softening reduced salt content.
As owner of the Cache Creek Casino Resort (CCCR; Brooks, Calif.), the Yocha Dehe Wintun Nation needed to overcome hurdles to water treatment. The resort, located in the Capay Valley approximately 113 km (70 mi) north of San Francisco, uses onsite systems to produce potable water and treat wastewater for reuse. To adhere to its sustainability initiatives, the self-governed Native American nation began installing systems at the resort to recycle water in 2004, expanding them through 2006. Recycled water is used to irrigate the resort’s golf course, flush toilets, control dust, and supply decorative water features. The resort’s groundwater is hard — that is, high in dissolved minerals, in this case more than 300 ppm — and high in total dissolved solids.
Attempt at Water Softening Backfires
In 2005, the resort began using ion-exchange water softeners to treat its drinking water. Using these softeners has unintended consequences for reuse, requiring the use of additional dilution water, which reduced the efficiency of using recycled water in the first place. The softeners added sodium and chlorides to the water, significantly increasing the salt concentration in the recycled water. A significant volume of raw creek water was used to dilute the recycled water before irrigation. Using the softeners also required dilution of the recycled water to meet discharge compliance standards for discharge to land. Also, additional potable water was needed to dilute the water used to flush toilets, because the higher salinity was corroding flushing-valve seals at an exacerbated rate. And adding more potable water only served to add more salinity to later reuse water.
In early 2007, the resort installed equipment to separate the backwash from the ion-exchange system and have the concentrated brine hauled offsite for disposal. The ion-exchange water softeners were the primary contributor to increased salt in the recycled water, so addressing those units as a point source was the most logical strategy. The new strategy was successful but provided only a partial, short-term solution until a full-scale solution could be implemented.
Salt concentrations did not drop sufficiently to maximize reuse applications, because not all of the chlorides could be separated out for disposal during the softener backwash process, and sodium was still added to the potable water during the ion-exchange process.
Finding the Right Approach
Since the resort’s owner maintained control over its water and wastewater systems, a unique approach was implemented to desalinate potable water for the direct benefit of recycled water quality. Due to the handling and cost to dispose of the brine, an innovative two-stage approach employing two separate membrane desalination technologies was selected.
In April 2009, the CCCR Water Desalination Treatment Facility was commissioned with an electrodialysis reversal (EDR) system as the first stage of treatment, and a vibratory shear enhanced process (VSEP) system as the second stage. The EDR system, provided by General Electric (Fairfield, Conn.), and the VSEP system, provided by New Logic Research (Emeryville, Calif.), were incorporated into the water treatment system by HydroScience Engineers (Sacramento, Calif.). The new systems enabled CCCR to decommission the ion-exchange water softeners that were exacerbating the salt content in the recycled water. To maximize water efficiency, permeate from the VSEP system is blended with the EDR-produced water for potable use.
Operation of the Systems
The EDR system softens and desalinates the groundwater for potable-water distribution throughout the resort. The potable water delivered for distribution is softened to the resort’s hardness standard of 3 grains, from a hardness of more than 17 grains. But the water is not as soft as it was when delivered directly from the ion-exchange water softeners, which was around zero grains. Less salinity leads to reduced corrosion potential and lower subsequent repair costs. The EDR membranes include a 10-year warranty. Reject generated by the system is concentrated through the VSEP process and hauled away for disposal The VSEP system is an enhanced form of reverse osmosis that utilizes a shearing force to minimize fouling on the vertical membrane plates.
The VSEP filter packs are rated for 3 to 5 years of operational life. Both systems require daily checks and monthly and quarterly bolt-tightening checks.
Successes of the Solution
The EDR and VSEP systems both are running at 85% recovery. When including the combined operations of all the discharged cleaning-in-place solutions, the desalination treatment facility is operating at 97% recovery, with only 3% of the brine being hauled away for disposal, said Jason M. Brabec, the HydroScience Engineers design engineer for the project. In addition, the recycled-water quality has improved dramatically and requires much less dilution for direct irrigation purposes, he added. “The recycled-water quality reduces maintenance costs for the toilet flushing valves that were previously malfunctioning due to corrosion via the high salt content,” he said.
“The project effectively addresses the salt concentrations of the effluent discharge and significantly reduces brine off-haul costs through the two-stage brine concentration step,” Brabec said. “When a holistic approach to similar salinity challenges is feasible, this is a very efficient strategy versus treating at the effluent end of the process.”
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