Staged reverse osmosis operation: Configurations, energy efficiency, and application potential

• New RO system designs can reduce the energy of desalination.
• Minimum specific energies were formulated for single- and multiple-stage RO systems.
• Staged RO operations are more energy efficient than single-stage RO.
• Multi-stage direct pass RO theoretically consumes less energy than close-circuit RO.
• Closed-circuit RO is a more practical/economical approach for energy reduction.

Reverse osmosis (RO), currently the most energy efficient desalination process, is inherently more energy intensive compared to conventional fresh water treatment technologies, as it is constrained by the thermodynamics of separation of saline solutions. Therefore, pushing the energy consumption towards the thermodynamic limit of separation would lead to significant long-term savings in energy cost. In this work, we quantitatively demonstrate the potential of energy reduction for RO desalination using staged operations with both multi-stage direct pass and closed-circuit configurations. We relate the minimum specific energy of desalination (i.e., the minimum energy required to generate a unit volume of permeate water) to the number of stages in each configuration, and elucidate the fundamental difference between the two configurations. Our analysis suggests that although it is theoretically impossible to reach the thermodynamic minimum energy of separation with closed-circuit RO, this configuration is robust and much more practical to implement than the multi-stage direct pass RO.

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