Abstract:
The development of Wyoming’s energy resources (coal bed methane [CBM] extraction, hydraulic fracturing) and carbon dioxide (CO2) sequestration sites all invariably result in the production of brackish wastewaters. The treatability of these produced waters varies from relatively simple for CBM water (dissolved solids < 2,000 mg/L) to exceedingly complex for the water that is displaced during CO2 sequestration in deep, porous subsurface formations (dissolved solids > 20,000 mg/L). Reverse osmosis (RO) is a proven desalination process, which requires pressure to pump the brackish water or brine through a semi-permeable membrane. Although RO has been extensively used to treat a variety of source waters, including energy development produced water, managing the concentrate that is produced as a byproduct during RO has persisted as an environmental and economic challenge in maximizing water recovery rate. Here we propose to develop an integrated accelerated precipitation softening (APS)-microfiltration (MF) assembly for reducing the volume of concentrate that must be disposed of when using RO to treat high-salinity, energy activity related waters in Wyoming. The ability of chemical precipitation processes, including APS, to remove scale-forming elements from source waters is well-established. Conventional softening processes are hindered by the production of fine suspensions of mineral precipitates that require relatively long sedimentation times (1.5-3 hrs) and a residual sludge having a low solids content (2 to 30%). These issues generate concerns related to the size of softening facilities, solids carry over to downstream membrane processes, and sludge disposal. All of these concerns hinder the use of APS as a management strategy for RO concentrate. Seeded precipitation processes use calcite crystals to provide a preferential surface area for nucleation and growth to occur, thus accelerating the kinetics of mineral precipitation. As such, the accelerated APS process will allow the removal of CaCO3 as well as other scale forming elements that will be incorporated in the CaCO3 crystals and removed.
Built upon the previous findings in the field of treating challenging source waters, the unique contribution of the proposed work lies in the three folds: 1) incorporating microfiltration (MF) as a polishing step following precipitation softening and prior to secondary RO process; 2) application of calcite seeds to accelerate the softening process and to improve the treatability of the feed water for the secondary RO system; 3) application of an integrated rather than singular approach for maximizing the recovery/reuse potential of highly saline produced waters. The integrated APS-MF assembly for RO concentrate treatment will provide a superior feed water quality to secondary RO systems that will allow for water recovery ratios to approach, or exceed, 90%.