Improved ultrasonic cleaning of membranes with tandem frequency excitation Silvestre Roberto Gonzalez-Avila a,b , Firdaus Prabowo a , Anshuman Kumar a,b , Claus-Dieter Ohl a,n a Physics and Applied Physics, Nanyang Technological University, 21 Nanyang Link, Singapore 637371, Singapore b Civil and Environmental Engineering, Nanyang Technological University, N1-01a-22, 50 Nanyang Avenue, Singapore 639798, Singapore article info Article history: Received 13 February 2012 Received in revised form 29 May 2012 Accepted 29 May 2012 Available online 5 June 2012 Keywords: Ultrasound-cleaning Membrane-fouling High-speed video imaging abstract In water purification membrane fouling is the most common problem that decresases efficiency in the purification process. In this work we propose a simple method to remove the particles deposited on the membrane surface by applying two different ultrasound frequencies in tandem. First a high ultrasound frequency, here 220 kHz, is used to create microscopic bubbles that are immediately after excited with a lower ultrasound frequency, e.g., 28 kHz. High speed video of the bare membrane shows the higher number of bubbles after the application of the high ultrasound frequency, as opposed to applying only a low ultrasound frequency. The method is then evaluated by recording the flux delivered and the transmembrane pressure (TMP) in this small scale experiment. The results show that after the application of the tandem frequency the transmembrane pressure is restored to the value of the non fouled membrane. & 2012 Elsevier B.V. All rights reserved. 1. Introduction Membrane separation process is a widely used technology in areas such as food and dairy products processing, and water purification. Membrane fouling, one of the major problems encountered in membrane filtration, occurs by irreversible deposition of retained particles, colloids, macromolecules, salts, etc. Consequently, fouling causes significant decrease in permeate flux [1]. Currently, the most common methods used to clean membranes include backflushing/backwashing and chemical cleaning of membranes, but these methods have some drawbacks and limitations [2]. Though backflushing is used in both hollow fiber [3] as well as flat sheet membranes[4,5], it results in degradation of maximum flux after repeated backflushes. Chemi- cal cleaning may damage the membrane and cause secondary pollution [6]. Ultrasound (US) is used for the cleaning of contaminated surfaces [7,8]. Various studies report the enhancement of perme- ate flux using US [9–11]. This enhancement is attributed to phenomena related to bubble oscillations, acoustic streaming, and heating [12–14] which increase flux by affecting the concen- tration polarization at the membrane’s surface [15]. The frequency of the US driving is an important parameter. For example the effect of permeate flux with US irradiation at three different frequencies of 28 kHz, 45 kHz and 100 kHz had been investigated by Kobayashi et al. [16]. They found that lower frequencies (here 28 kHz) was more effective in cleaning fouled membranes. Similar results were obtained by Lamminen et al. [14]. Maskooki et al. [17] tested three ultrasound frequencies as well as a combination of these both with and without a chemical cleaning agent (ethylenediaminetetraaceticacid, EDTA). Interestingly, they found the best cleaning performance when the frequency was alternated between 28 kHz, to 45 kHz, and 100 kHz. Kim et al. [18] have demonstrated that high frequency can be applied to remove particles from silicon wafers. By means of high speed images they show that the bubble oscillating close to their resonant size induce fluid motion that causes the detach- ment of the solid particles attached to the silicon wafer. More recently, Thiemann et al. [19] studied the bubble structure present in a 230 kHz ultrasonic field; their work is relevant because their studied ultrasound frequency is similar to the high frequency of this work. They captured high speed video and performed sono- luninescence and surface cleaning tests; two groups of bubbles were investigated, bubbles larger than the linear resonant size and bubble near or slightly below the linear resonant size. They concluded that both bubble populations showed cleaning poten- tial, although they acknowledged that the mechanisms associated with particle removal may be different. It is commonly accepted that the large amplitude bubble oscillations from so-called cavitation bubbles are responsible for 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.2012.05.069 n Correspondence to: School of Physical and Mathematical Sciences Division of Physics and Applied Physics Nanyang Technological University, Singapore 637371, Singapore. Tel.: þ65 65138039; fax: þ65 6515 9663. E-mail address: cdohl@ntu.edu.sg (C.-D. Ohl). Journal of Membrane Science 415–416 (2012) 776–783