Brine concentration allows for increased recovery ratios in water treatment systems, reduction of waste volumes, and the production of minerals from saline brines. Existing methods of brine concentration, while robust, are often very energy intensive. Better efficiency may be possible using Counterflow Reverse Osmosis (CFRO), a membranebased, pressure-driven brine concentration technology. The present work develops a model for CFRO. Using this model, a single CFRO module is simulated and its performance characterized. Exergy destruction within a single-stage system is analyzed, which provides insights for configuring and optimizing multistaged systems. Additionally, a parametric analysis of membrane parameters provides direction for the development of CFRO-specific membranes. Two existing configurations of CFRO are discussed, and compared with a new third configuration, split-feed CFRO, which is presented here for the first time. Split-feed CFRO systems are simulated and optimized to provide guidance for system design. A variety of multistage systems operating at a range of recovery ratios are simulated, and the results compared are with existing desalination and brine concentration technologies, showing the potential for improved recovery ratios and reduced energy consumption.Keywords: brine concentration, desalination, energy efficiency, counterflow reverse osmosis, zero liquid discharge

The primary energy consumption of a spectrum of desalination systems is assessed using operating information and technical bids for real plants congured with coproduction of electricity. The energy eciency of desalination plants is often rated on a stand-alone basis using metrics such as specic energy consumption, gained output ratio, and second law eciency, which can lead to inconsistent conclusions because the heat and electrical work inputs to the plant have very dierent exergies and costs, which must be taken into account. When both the heat and work inputs are drawn from a common primary energy source, such as the fuel provided to electricity-water coproduction systems, these inputs can be compared and combined if they are traced back to primary energy use. In the present study, we compare 48 dierent congurations of electricity production and desalination on the basis of primary energy use, including cases with pretreatment and hybridized systems, using performance gures from real and quoted desalination systems operating in the GCC region. The results show that, while reverse osmosis is still the most energy ecient desalination technology, the gap between work and thermally driven desalination technologies is reduced when considered on the basis of primary energy. The results also show that pretreatment with nanoltration or hybridization of multiple desalination systems can help to reduce energy requirements. Additionally, the specic type of power plant in the coproduction scheme and its operating parameters can have a signicant impact on the performance of desalination technologies relative to one other.

Membranes offer a scalable and cost-effective approach to ion separations for lithium recovery. In the case of salt-lake brines, however, the high feed salinity and low pH of the post-treated feed have an uncertain impact on nanofiltration's selectivity. Here, we adopt experimental and computational approaches to analyze the effect of pH and feed salinity and elucidate key selectivity mechanisms. Our data set comprises over 750 original ion rejection measurements, spanning five salinities and two pH levels, collected using brine solutions that model three saltlake compositions. Our results demonstrate that the Li + /Mg 2+ selectivity of polyamide membranes can be enhanced by 13 times with acid-pretreated feed solutions. This selectivity enhancement is attributed to the amplified Donnan potential from the ionization of carboxyl and amino moieties under low solution pH. As feed salinities increase from 10 to 250 g L −1 , the Li + /Mg 2+ selectivity decreases by ∼43%, a consequence of weakening exclusion mechanisms. Further, our analysis accentuates the importance of measuring separation factors using representative solution compositions to replicate the ion-transport behaviors with salt-lake brine. Consequently, our results reveal that predictions of ion rejection and Li + /Mg 2+ separation factors can be improved by up to 80% when feed solutions with the appropriate Cl − /SO 4 2− molar ratios are used.

Energy cost contributes a large portion of the overall cost of desalinated water. Improving the energy efficiency of desalination plants is therefore a primary design goal. However, accurately evaluating and comparing the energy consumption of desalination plants that use different forms and grades of energy is difficult, especially for power-water coproduction systems in which primary energy consumption leads to both salable electricity and potable water. The power plant converts primary energy into grades of thermal energy and electricity usable by the desalination plant. To fully capture the thermodynamic and economic cost of energy, and to fairly compare desalination systems that use different grades of input energy, we must compare energy consumption not at the point where energy enters the desalination plant itself, but as primary energy consumption entering the power plant. This paper investigates a variety of metrics for comparing the energy and exergy consumption attributable to desalination in coproduction plants. Previous results have shown that reverse osmosis (RO) is approximately twice as efficient as multiple effect distillation (MED) on a primary energy basis. We then compare the primary energy consumption of MED and RO from a thermoeconomic perspective. The entropy generation at the RO membrane and in the MED effects are derived in similar terms, which enables a comparison of the overall heat transfer coefficient in an MED system to the permeability of an RO membrane. RO outperforms MED in energy efficiency because of a balance of material costs, transport coefficients, and cost of energy.

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