A generalized framework for seawater reverse osmosis design beyond energy minimization through brine and cost trade-offs: Design atlas for two-stage SWRO systems.

Seawater reverse osmosis (SWRO) design is commonly optimized around specific energy consumption, although practical deployment also depends on thermodynamic efficiency, brine burden, pressure feasibility, membrane selectivity, and water cost. Here, a staged dimensionless SWRO formulation is applied to a two-stage configuration to construct a design atlas across feed salinities of 32000-45000 ppm and system feed pressures of 50-80 bar, while accounting for membrane-selectivity effects. The model is coupled with mixed-integer multi-objective optimization to resolve trade-offs among system specific energy consumption, practical second-law efficiency, brine-volume burden, and cost-related design functions. Increasing feed salinity progressively contracts the attainable domain, with the minimum retained pressure rising from 50 to 60 bar over the examined salinity range. When each salinity case is optimized independently, the best attainable system specific energy consumption still increases from 4.21 to 6.09 kWh/m3, and the minimum brine-volume burden rises from 0.994 to 1.618. Minimum-energy, maximum-efficiency, minimum-brine, and minimum-cost designs remain distinct, confirming that energy minimization alone is insufficient for two-stage SWRO design screening. The dominant architectural response is hydraulic: the inter-stage pressure ratio varies considerably with salinity and pressure, whereas membrane-area scale and vessel partition remain comparatively stable. The resulting atlas provides a compact screening tool for identifying feasible two-stage SWRO designs that balance energy, thermodynamic, brine, and cost objectives before detailed vendor-specific design.