Integrated electrolytic sensors for monitoring of concentration polarization during nanofiltration

Potable water for municipal and agricultural use is produced in many regions of the world by membrane-based desalination processes. Brackish water is driven by a pump through pressurized modules where the water passes through a semi-permeable membrane, while rejected salt ions are retained. During pressure-driven desalination, the rejected salt ions accumulate by convective mass transport near the membrane surface, resulting in elevated concentration levels. Concentration polarization (CP) of the dissolved salts can lead to precipitation and crystal growth on the membrane, known as scaling. Over time, scaling can decrease process efficiency because water can no longer permeate through the affected areas. Scalant removal can incur significant chemical costs, result in system downtime, and reduce membrane lifetime. Several techniques have been reported in the literature for real-time monitoring of scaling, but little work has been conducted on real-time characterization of the concentration polarization that leads to scaling. This work describes an electrolytic sensor system that can be used in monitoring solute concentration within the thin concentration polarization boundary layer (CPBL) that forms near the membrane surface. All experiments were performed in a bench-scale flat-sheet cross-flow system using aqueous calcium sulfate feed solutions. This design has improved upon previously reported work by applying a more robust sensor system in a challenging flow-channel environment that more closely resembles industrial desalination units.