PILOT TESTING CONFIRMS REDUCED Ph AND SCALE INHIBITORS CONTROLS SCALING AT 96-PERCENT RECOVERY

摘要

This paper describes the pilot effort associated with confirming the maximum recovery of thebrine minimization treatment process described within. The treatment approach consists of pHsuppression and scale inhibitors addition to eliminate the lime or caustic soda softening processtypically used to reduce the hardness of concentrate from the low-pressure reverse osmosis(LPRO) system prior to the treatment with a secondary RO (HPRO) system. Eliminating thesoftening process significantly reduces capital and operating costs of brine minimization byeliminating process tanks, filters, chemical storage and residuals handlings. The raw waterquality limitations for operation of the RO systems and deep injection well relative to hardness,silica, total organics and sulfate are discussed in detail in the paper. Bid costs for a 1.2-mgd brineminimization RO system, two, 600-hp high pressure injection pumps and a 600-gpm, 10,000-ftclass I injection well were $6 million, which increases the capital cost of the water systemapproximately $0.60 per gallon of overall treatment capacity. Operational costs are expected toincrease approximately $0.20 per 1,000 gallons of treated water production for acid, electricpower and membrane replacement.The combination of low-pressure RO and brine minimization RO was pilot tested in June andJuly 2011 using a four-stage RO system, treating approximately 44 gpm of well water. The pilotsystem was operated continuously for 6 weeks at recoveries from 91 to 97 percent, and varyingdegrees of pH suppression from 6.8 to 7.2 going into the brine minimization RO system. Thepilot testing confirmed that the overall system could operate at 96-percent recovery at a brineminimization feedwater pH of 7.0, and operate at an LSI of > 2.6, Stiff & Davis > 1.8 and silicaconcentration of 175 mg/L without scaling in the last stage. The operating pressures andpermeate water quality were slightly below the values predicted by the membranemanufacturer’s computer projection software. The supersaturated conditions for silica, calciumand sulfates raises concerns about the potential precipitation of solids after the concentrated brineleave the secondary RO system, and then fouling of injection well. Concentrated brine sampleswere stored in sealed containers for several weeks to determine if precipitates ultimately formed.The samples were analyzed for suspended solids and the concentration was less than thedetection limit. In addition to TSS, the concentrated brine was tested for SDI as it was dischargedfrom the brine minimization unit, and typically ranged from 5 to 10 for a 15-minute SDI. The RO system and deep well injection pumps are scheduled for startup in March 2012, and theinitial full scale performance will be compared to the membrane manufacturer’s computerprojections and the pilot testing data.