Pressure retarded osmosis enhanced by micro-nano bubbles: Coupling effects of operation conditions and mass transfer resistance analyses

The concentration polarization is one of the bottlenecks of pressure retarded osmosis (PRO) for industrial-scale application. Micro-nano bubbles (MNBs) could form a strong repulsion and shear force on the membrane surface, thus, the degree of concentration polarization is reduced, and hence, both the water flux and power density could be increased. In this paper, the synergistic effects of MNBs with concentration, temperature, water and air flow rates, pressure, and spacers were investigated through membrane cell scale experiments. A numerical model with trans-membrane heat flux and MNBs effects is built and validated by the experiment results. The mechanism of strengthening effect of MNBs could be explained. A maximum promotion of 6.28 % was achieved for concentration combination of 0.171–1 mol/L. However, the promotion ratio of water flux was deceased significantly with higher temperature (<1 % for 45 °C) and higher pressure (nearly zero for 7 bar). The light MNBs would be pushed away from membrane under larger water flux with larger osmotic pressure difference. The variations of mass transfer resistances, and the thicknesses of boundary layers were discussed. Moreover, the key variables that have significant impacts on the metrics of PRO process were identified through partial least squares analysis.