Brine Minimization Using Closed Circuit Reverse Osmosis (CCRO)

Marine Corps Air Ground Combat Center (MCAGCC) Twentynine Palms is in the Mojave Desert in California and is one of the largest military training areas in the nation. MCAGCC relies on groundwater for its drinking water supply. Due to concerns about depletion of the existing aquifer and the presence of hexavalent chromium in some of the wells, MCAGCC intends to construct two new groundwater wells in an adjacent aquifer and a new water treatment plant (WTP) to treat and blend water from the existing and new wells. MCAGCC has eleven existing wells in the Surprise Spring Aquifer. Maximum water demand is about 3 mgd. Groundwater levels in the aquifer have declined more than 190 feet, prompting exploration of a neighboring aquifer with preliminary investigations indicating the neighboring aquifer can be used as a long-term drinking water source. An initial test well showed concentrations of arsenic and fluoride above their maximum contaminant levels (MCLs), as well as total dissolved solids (TDS) and boron above their secondary MCLs. Fluoride and TDS appear to be a concern throughout the aquifer, with arsenic and boron a potential concern. Water from the new aquifer will be conveyed to a new WTP where it can be blended with water from the existing wells and treated to reduce all contaminants to below the MCLs. The new 3 mgd WTP will utilize reverse osmosis (RO) as the primary treatment process. The RO system will be a two-stage system operating at 75 percent recovery. Brine minimization will be accomplished using closed circuit reverse osmosis (CCRO), operating at approximately 80 percent recovery. This will allow the WTP to achieve a minimum overall recovery of 94 percent. The final brine from the CCRO will be discharged to evaporation ponds equipped with mechanical evaporators, essentially resulting in a zero liquid discharge facility. CCRO is a single-stage RO process that operates in a batch mode by recirculating brine back through the RO membranes to increase recovery. Once the desired amount of water has been produced, based on the recovery setpoint, a valve opens and directs the brine to the concentrate pipeline for disposal to the evaporation ponds. The brine recirculation increases cross-flow velocities in the system which reduces fouling. At lower flows, the plant is designed to bypass the RO process and operate the CCRO at 94 percent recovery, maintaining target recover over the full range of plant flows. The new wells and WTP will allow MCGCC to meet its goals of producing high quality drinking water while maximizing their limited water resources.

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