Cost optimization of osmotically assisted reverse osmosis.

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.