Ultrasound, gravimetric, and SEM studies of inorganic fouling in spiral-wound membrane modules

Ultrasonic time-domain reflectometry (UTDR) is combined with permeate-flux, gravimetric, and scanning electron microscopy (SEM) measurements for studying calcium-sulfate dihydrate fouling in a commercial spiral-wound reverse osmosis membrane module. The UTDR amplitude response is shown to be more sensitive than permeate-flux decline for detecting the early stages of fouling because it is a local rather than integral measurement. Hence, appropriately located acoustic transducers can be used to detect the onset of fouling well before it impairs module performance so that appropriate remediation can be undertaken. The UTDR arrival-time response in principle can be used to determine the fouling layer thickness. However, this study indicates that arrival-time measurements in a spiral-wound module can be compromised by movement of the membrane envelope layers as is the case for the Koch 2521 spiral-wound module used in this study. The estimated module expansion of 210 µm inferred from the UTDR measurements exceeds the thickness of the fouling layer deposits estimated from the gravimetric measurements. The gravimetric studies reveal a 6 to 10-fold increase in the thickness of the fouling layer deposits in the feed-flow direction owing to concentration polarization. A particularly interesting result of the gravimetric studies is a 16 to 27-fold increase in the thickness of the fouling layer deposits in the permeate-flow direction presumably owing to expansion of the outer relative to the inner feed channels. The SEM studies indicate that fouling is initiated along the feed-spacer mesh presumably due to dead-flow regions behind mesh elements transverse to the flow. Overall, the present study demonstrates that when UTDR is combined with other independent measurement techniques, it can provide significant insights regarding fundamental fouling and membrane module behavior.