High Recovery Inland Desalination: A Technical and Economic Performance Evaluation of Zero Discharge Desalination and Other Technologies

Increasing population and diminishing freshwater resources are leading to increased competition between users of freshwater, both in terms of surface water and groundwater. Desalination can augment the water supply, but conventional approaches like reverse osmosis are usually limited to 70-80% recovery. Inland brine disposal options are often limited, presenting significant technical and economic obstacles to implementing desalination technology. In comparison to conventional desalination technologies, high recovery desalination methods such as Zero Discharge Desalination (ZDD) offer substantially higher potable water yield (i.e., 95-98%) from brackish water. ZDD is a hybrid process that combines reverse osmosis (RO) or nanofiltration (NF) with electrodialysis metathesis (EDM). The goal of this research was to increase the sustainability of inland brackish desalination by improving the technical and economic feasibility of high recovery inland brackish desalination. The research objectives were to evaluate: 1) ZDD performance using different feed water chemistry to develop designs optimized for variations in water quality; 2) the performance tradeoffs in ZDD design on salinity removal, specific energy consumption, and hydraulic recovery; and, 3) the lifetime cost impacts of high recovery processes, as compared with conventional (low-recovery) treatment systems. This research includes three distinct chapters, each of which is intended to be a standalone document that was, or will be, submitted for publication in a journal article. Chapter 1 presents the results of an article entitled, “Improving Desalination Recovery Using Zero Discharge Desalination (ZDD): A Process Model for Evaluating Technical Feasibility,” which was published in Industrial & Engineering Chemistry Research [1]. A semi-quantitative model that incorporated pilot testing results, theoretical calculations, and membrane and antiscalant manufacturer models, was developed in order to evaluate the tradeoffs (i.e., specific energy consumption vs. product water quality) involved in ZDD design. The model shows that 97% system recovery is theoretically possible for moderate feedwater salinity, relatively high multivalent ion concentrations, and less than 40 mg/L silica. Chapter 2 evaluates the economic performance of ZDD and also updated the ZDD model to include potential for salt recovery, namely calcium sulfate, magnesium hydroxide, and sodium chloride. The chapter investigates the tradeoffs associated with capital and operating costs by way of a sensitivity analysis that compares the relative impacts of RO, EDM, and evaporation pond capital costs, and quantified the impact to ZDD costs from varied power costs. The updated model and associated economic analysis indicate that an optimized ZDD design involves choosing the maximum RO/NF recovery, minimum EDM ion removal, and balancing the salt rejection and silica passage of the RO or NF membrane in order to achieve the water quality desired while maximizing the ZDD system recovery. The incorporation of salt recovery processes led to a decreased unit cost, even when revenue from the sale of byproducts is excluded. Chapter 3 compares ZDD performance with other commercially-available technologies that show potential for high recovery, namely UTEP’s Concentrate Enhanced Recovery Reverse Osmosis (CERRO) and Desalitech’s Closed Circuit Reverse Osmosis (CCRO). CERRO was developed at The University of Texas at El Paso by Dr. Anthony Tarquin and has been evaluated at the Kay Bailey Hutchison Desalination Plant (KBH) in El Paso, Texas, and other locations. CCRO is a technology that was pilot tested in 2013 at the KBH. Both CERRO and CCRO take advantage of the relatively slow scale formation of silica and other ions, but differ in their design. Pilot or full scale operational data from ZDD, CERRO, and CCRO were used for this research. While there are many caveats with the assumptions made, my results indicate that semi-batch processes like CERRO and CCRO may be better-designed for medium and high recovery desalination of brackish groundwaters with relatively low concentrations of divalent ions. While ZDD is able to achieve comparable recovery, the energy and cost requirements, and associated complexity, it may be less optimal relative to CERRO and CCRO. However, when the concentration of divalent ions is high (e.g., greater than 60% of TDS), and there is a definite need for high recovery, it seems that ZDD is well-suited for desalination. Additional research is needed to fine tune the economic assessment of these technologies and models should be built to further evaluate the design choices