Non-invasive measurement of membrane scaling and cleaning in spiral-wound reverse osmosis modules by ultrasonic time-domain reflectometry with sound intensity calculation

This study describes the ultrasonic time-domain reflectometry (UTDR) with a sound intensity calculation for monitoring membrane scaling and cleaning in a 4-inch spiral-wound reverse osmosis module. Three focused transducers with the frequency of 2.25 MHz were located equidistantly along the feed flow direction. The scaling experiments were carried out with 2.0 g/L calcium sulfate for 90 h fouling and the cleaning processes consisted of three phases: pure water flushing, immersion cleaning and acid-washing. According to this study, the ultrasonic response signals and the corresponding sound intensity gradually decreased to minimum with the fouling and deposition of the foulants on the membrane and then increased with the formation of fouling layer. The initial induction time in the change of sound intensity became shorter and the sound intensity became lower along the fluid direction during the fouling process. It was found that the CaSO4 scaling first precipitated near the downstream owing to the increased concentration polarization. Furthermore, the permeate flux and the sound intensity all recovered step by step with the cleaning processes. The results obtained by independent methods such as SEM and weight measurements were in good agreement with the observations by ultrasonic technology. Overall, this study confirms that the UTDR technique with a sound intensity calculation and modeling is capable to evaluate the membrane scaling and cleaning in spiral-wound modules.

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► The UTDR technique with the sound intensity calculation has been developed to detect membrane scaling and cleaning in spiral-wound RO membrane modules.
► The initial induction time in the changes of the relative sound intensity became shorter and the sound intensity became lower along the fluid direction during the scaling experiment.
► The CaSO4 scaling first precipitated near the down stream.
► Both the permeate flux and the sound intensity recovered step by step with the cleaning processes.