Optimizing Recovery and End Use of Gypsum in Super-Saturated RO Concentrate at PRWC Southeast Water Production Facility

Polk Regional Water Cooperative (PRWC), a collective of 16 member utilities operating in Polk County, FL, formed for the sole purpose of developing reliable, sustainable, and cost-effective alternative water supplies (AWS) to meet future demands in central Florida. As population grows in the region, so does the need to diversify away from the heavily relied-upon Upper Floridan Aquifer and towards AWS projects that are innovative and sustainable. With these unique AWS projects also comes additional treatment and disposal challenges - or opportunities - like what to do with the residual gypsum produced as a byproduct at PRWC's first treatment facility. Design is currently underway for PRWC's first AWS project, the Southeast Lower Floridan Aquifer (LFA) Wellfield and Water Production Facility, a 7.5 million gallon per day (Phase 1) reverse osmosis (RO) water treatment plant (WTP) in Lake Wales, FL. During early stages of the design, Carollo and its team of subconsultants identified that the RO concentrate stream will contain calcium sulfate levels four times saturation and will need to be pretreated prior to disposal via deep injection well (DIW) due to concerns of irreversibly plugging the calcium sulfate-saturated injection zone. Without pretreatment, the calcium sulfate precipitate will plug the well and irreversibly reduce its permeability and therefore injection capacity. Based on early discussions with equipment manufacturers and bench scale proof-of-concept evaluations, a process was tested to precipitate, thicken, and dewater calcium sulfate dihydrate (gypsum) into an ultra-high-purity end product, adapting treatment technologies typical in flue gas desulfurization (FGD) at power plants. The process will reduce the CaSO4 concentration in the supersaturated concentrate stream by approximately 75% (back down to, or below, normal saturation levels). Horizontal vacuum belt filters (HVBFs) were selected for dewatering due to their ability to dewater mineral slurries to achieve up to 90% solids in cases of high loading rates. HVBFs are not commonly found at municipal WTPs, yet they are an industry standard for dewatering FGD gypsum at power plants nationwide and are regularly used in the phosphate, mining/aggregate, and metals processing industries. To the engineer's knowledge, this is the first installation for this type of equipment and application at a municipal WTP in North America. It's estimated that the plant will produce approximately 23,000 dry tons of gypsum per year in Phase 1 and double that at buildout. This equates to more than 75 tons of dewatered gypsum daily. This natural gypsum has advantages over synthetic gypsum potentially containing high radionuclides and heavy metals generated from making phosphate fertilizers (phosphogypsum) and heavily contaminated FGD gypsum produced as a byproduct at coal-fired power plants. Rather, the gypsum produced at SE WPF will be an ultra-high-purity gypsum product useful in various industries. What options does the utility have to dispose of this gypsum? Avoiding landfills by giving it away would save this young cooperative $2.5 million dollars per year in Phase 1 alone in hauling costs. Better yet, can this beneficial resource be refined, marketed, and sold to other industries to offset operational costs for PRWC in a period of rising construction and operation costs worldwide? While the majority of gypsum produced in the US is used in manufacturing wallboard or plaster, it is also used in concrete mixes, as a soil conditioner, in food processing (e.g. tofu), in toothpaste, or even brewing beer and winemaking. The degree of final treatment for the WTP is dependent on the desired (and available or practical) end use. This presentation will highlight the design, operation, and disposal considerations associated with various end uses for the gypsum produced at the SE WPF, and one utility's challenge to turn an otherwise costly problem into a beneficial, recoverable product and source of meaningful revenue. This will include: - An overview of the SE WPF gypsum desaturation process (Figure 1), with focus on dewatering and solids handling. - Market analysis and survey of the end use/disposal options for other similar gypsum residuals in Florida and their individual quality requirements. - Design modifications to meet those requirements. - Comparison of life cycle costs for each treatment and end use scenario.