Ultrasonic sensor control of flow reversal in RO desalination—Part 1: Mitigation of calcium sulfate scaling Xiaoyun Lu a , Elmira Kujundzic a , Guy Mizrahi b , Jay Wang a , Keith Cobry a , Michael Peterson c , Jack Gilron b , Alan R. Greenberg a,n a Department of Mechanical Engineering, Membrane Science, Engineering and Technology (MAST) Center, University of Colorado Boulder, 427 UCB, Boulder, CO 80309-0427, USA b Zuckerberg Institute for Water Research, Ben Gurion University of the Negev, POB 653, Beer Sheva 84105, Israel c Department of Mechanical Engineering, University of Maine, 5711 Boardman Hall, Orono, ME 04469-5711, USA article info Article history: Received 4 January 2012 Received in revised form 4 May 2012 Accepted 7 May 2012 Available online 30 May 2012 Keywords: Reverse osmosis Membrane scaling Ultrasonic time-domain reflectometry Sensor-controlled flow reversal abstract Reverse osmosis (RO) desalination is a well-established process that removes salt from sea water or brackish water sources. However, scaling by sparingly soluble salts on the membrane surface has been shown to significantly degrade membrane performance. We report the results from a comprehensive study evaluating the effectiveness of flow reversal (FR) controlled by ultrasonic sensors in mitigating scaling during RO desalination. The work utilized a sophisticated multi-port bench-scale flat-sheet cross-flow module with the ability to operate in FR mode with externally mounted ultrasonic sensors. A novel signal-analysis methodology was developed to utilize the ultrasonic waveforms to control a switch in flow direction at the onset of local scaling. Experiments were conducted under controlled conditions with repeated forward-flow (FF) and reverse-flow (RF) cycles. Data from the experiments confirmed that FR controlled by ultrasonic sensors can effectively mitigate scaling on the membrane surface and avoid the expected level of permeate-flow decrease. These results were validated by post- mortem membrane analysis including scalant gravimetric and membrane area-coverage measure- ments. Overall, the work demonstrates the successful adaptation of ultrasonic sensors for active process control in which the timing of the change in flow direction is critical. & 2012 Elsevier B.V. All rights reserved. 1. Introduction In recent years, rapid human population growth and pollution of water sources have greatly increased the need for clean water. Reverse osmosis (RO) desalination is a well-established and widely utilized process that produces pure water by removing salt from seawater or other brackish water sources [1–4]. How- ever, a major challenge in utilizing RO desalination is membrane scaling, which is the result of the precipitation of sparingly soluble salts on the surface of the membrane [5–6]. Scaling is of great practical importance since it significantly degrades mem- brane performance and/or water quality, and hence increases the cost of desalination. Processes to enhance water recovery by adding antiscalants, base softening, or adjusting the pH involve relatively high chemical costs and/or increase the complexity of the overall desalination process [7–9]. Antiscalants can also provide nutrients for biofilm growth on membrane surfaces, and thus their use can be problematic. During RO, the accumulation of rejected salt ions at the mem- brane surface results in a higher solute concentration boundary layer at the membrane wall, which is termed concentration polar- ization [10–12]. In addition, higher water-recovery operation is accompanied by higher solute concentration at the exit end (down- stream) than the entrance end (upstream) since permeate pene- trates the membrane along the longitudinal flow direction in the module. The higher concentration encourages surface nucleation and crystal growth (i.e. scaling) at downstream end first. This phenomenon underlies the concept of the flow reversal (FR) in which scaling is mitigated by periodically reversing the flow direction so that upstream and downstream directions can be switched [13,14]. Recently, FR was linked to induction time, which is the time required for salt nucleii to grow into embryos of a critical size, above which they will continue to grow as scale deposits on the membrane surface [15–17]. FR has been successfully applied to flat- sheet and spiral-wound modules (SWM) [15,16]. A critical issue for the application of FR is the timing of the switch in flow direction. Here, prior knowledge of the induction time or real-time information regarding early-stage scaling is essential. Permeate flux decline has been used to monitor early-stage scaling, Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/memsci Journal of Membrane Science 0376-7388/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2012.05.027 n Corresponding author. Tel.: þ1 303 492 6613; fax: þ1 303 492 4637. E-mail addresses: xiaoyun.lu@colorado.edu (X. Lu), elmira.kujundzic@colorado.edu (E. Kujundzic), guymiz@bgu.ac.il (G. Mizrahi), jay.wang@colorado.edu (J. Wang), keith.cobry@colorado.edu (K. Cobry), michael.peterson@maine.edu (M. Peterson), jgilron@bgu.ac.il (J. Gilron), alan.greenberg@colorado.edu (A.R. Greenberg). Journal of Membrane Science 419–420 (2012) 20–32