We develop a nonlinear optimization model to identify minimum cost designs for osmotically assisted reverse osmosis (OARO), a multi-staged membrane-based process for desalinating high salinity brines. The optimization model enables comprehensive evaluation of a complex process configuration and operational decision space that includes nonlinear process performance and implicit relationships between membrane stages, saline sweep cycles, and make-up, purge, and recycle streams. The objective function minimizes cost, rather than energy or capital expenditures, to accurately account for the tradeoffs in capital and operational expenses inherent in multi-staged membrane processes. Generally, we find that cost-optimal OARO processes minimize the number of stages, eliminate the use of saline make-up streams, purge from the first sweep cycle, and successively decrease stage membrane area and sweep flowrates. The optimal OARO configuration for treating feed salinities of 50-125 g/L total dissolved solids to a water recovery of 30-70% results in process costs less than or equal to $6 per m3 of product water. Sensitivity analysis suggests that future research to minimize OARO costs should focus on minimizing the membrane structural parameter while maximizing the membrane burst pressure and reducing the membrane unit cost.
Although the energy efficiency of brackish water capacitive deionization (CDI) and reverse osmosis (RO) have been extensively compared, their relative costs remain poorly defined. We develop a parametric model to estimate the levelized cost of water (LCOW) of three CDI configurations (CDI, membrane CDI, and flow electrode CDI) and compare it with the LCOW of brackish water RO calculated using a process-based optimization model. We find significant deviations between costoptimal and energy-optimal RO design and operation, highlighting the importance of LCOW in comparative evaluations of desalination technologies. Our results suggest that material (including electrode and ion exchange membrane) costs are the largest cost component for CDI processes. As such, the economic viability of CDI critically depends on the component lifespan, with lifespans longer than 1 year (10 5 cycles for 5 min cycle duration) required to reduce brackish water desalination costs relative to RO. Finally, sensitivity analyses indicate that CDI processes are unlikely to be cost-competitive against RO for feedwater concentrations greater than 2 g/L. Future research to enhance the economic feasibility of CDI processes should focus on developing more durable electrodes, increasing cost-normalized electrode capacitance, and developing low-cost ion exchange membranes and coatings.
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