Reverse Osmosis Pressure Vessel with Bypass Flows: Optimized Flow Rate and Location for Reduced Energy Consumption

Reverse osmosis (RO) remains energy-intensive due to frictional pressure losses and concentration polarization (CP) within multielement pressure vessels. This study investigates diverting feedwater around selected elements as a bypass strategy to mitigate these intrinsic irreversibilities. A combined experimental–computational approach using pilot-scale testing and numerical optimization evaluated how bypass flow affects performance across seawater, brackish water, and wastewater reuse conditions with varying stage configurations. Results show that optimized bypass configurations can reduce specific energy consumption (SEC) by 16–19% in brackish and wastewater systems by lowering feed pressure requirements, with minor permeate quality trade-offs. Multiobjective optimization revealed that bypass allocation must emphasize end-element injection at low feed salinities (friction control) and front-element injection at high salinities (CP control). In contrast, bypass flows were less effective for seawater applications, where CP penalties outweigh frictional savings. These findings demonstrate that bypass flow optimization is a practical approach for improving RO energy efficiency, particularly in low-to-medium salinity multistage systems. The results provide quantitative design guidance for implementing bypass flow vessels and highlight opportunities to combine bypass strategies with adaptive control and spacer design optimization for further gains.