Quantifying and reducing concentration polarization in reverse osmosis systems

Reverse osmosis (RO) is an efficient water purification technique because of its high selectivity and permeability. During RO operation, rejection of solutes at the membrane surface leads to generation of a concentration gradient of the impermeable solute on the membrane surface, referred to as concentration polarization (CP). CP is a challenge as it reduces the driving force for transport and leads to a lower overall solute rejection. Compared to membrane fouling, which is a gradual process, CP is more challenging to monitor because it forms from the inception of filtration and decreases rapidly when filtration stops. Concentration profile monitoring devices are generally not available within a high-pressure crossflow framework. However, recent developments have integrated detection devices into filtration cells to allow for CP visualization. Measurements of concentration gradients enable development of active methods to alleviate or even eliminate CP during filtration. Active membranes aim to eliminate CP by increasing turbulence. This review illustrates the development of CP visualization tools that have been applied to or have the potential to be applied in RO. It also includes a discussion of innovative active membranes for CP reduction that could be key to adoption of next-generation ultra-permeable RO membranes.