Ultrasonic sensor control of flow reversal in RO desalination. Part 2: Mitigation of calcium carbonate scaling

The use of ultrasonic time-domain reflectometry (UTDR) in combination with statistically based analysis has been utilized to provide timely warning of the initial onset of local calcium carbonate scaling. The methodology was able to detect the onset of scaling in real time before any significant decrease in permeate flux was observed. Ultrasonic detection of scaling was confirmed via post-mortem examination of the membrane that showed the presence of aragonite crystals that were as small as 20 μm and covered less than 10% of the membrane surface. The ultrasonic methodology was then employed to trigger a change in flow direction (reversal of the concentrate and feed ends of the flow channel) when scaling was detected. When in forward flow, the upstream, midstream and downstream sections of the membranes had aragonite supersaturation values of SI∼2.0, ∼5.3 and∼7.65, respectively; when the flow direction was changed, the SI values at the ends were switched but the SI at the midstream remained at SI∼5.3. Results indicated that flow reversal effectively mitigated scaling in downstream sections of the membrane even though they were periodically exposed to the highest supersaturation conditions. It was demonstrated that prompt intervention after an ultrasonic scaling signal improved the efficacy of flow reversal. The results reflect a significant advance in the use of UTDR in which the methodology is applied for active control of scaling miti ation rather than limited to passive detection of scale formation.

► UTDR identified onset of CaCO3 scaling with crystallites <20 μm in diameter.
► Ultrasonic amplitude is a reliable metric for determination of scaling onset.
► UTDR triggering of flow reversal mitigated CaCO3 scaling of RO membranes.
► A timely response to a UTDR scaling-onset signal is the key to the efficacy of FR.