Transverse vibration as novel membrane fouling mitigation strategy in anaerobic membrane bioreactor applications Anusha Kola, Yun Ye, Pierre Le-Clech, Vicki Chen n UNESCO Centre for Membrane Science and Technology, School of Chemical Engineering, The University of New South Wales, 2052 Sydney, NSW, Australia article info Article history: Received 18 July 2013 Received in revised form 19 November 2013 Accepted 30 December 2013 Available online 15 January 2014 Keywords: Anaerobic bioreactor Membrane vibration Membrane fouling Backwash Relaxation abstract While vibratory shear is effective for increasing the shear near the membrane surface, transverse hollow fiber membrane vibration offers additional mass transfer enhancement in terms of generating vortices in the wake of the vibrating surface. In this work, transverse vibration of submerged hollow fibers is explored for enhancing the filtration of anaerobic bioreactor effluents where gas sparging is often undesirable. The critical flux value was increased significantly with the aid of membrane vibration. Even at high mixed liquid suspended solid concentrations, the vibratory system was still able to significantly reduce fouling. In addition to a reduced rate of fouling, fractionation of the fouling layer also showed that a more reversible fouling occurs with vibrational filtration in comparison to traditional fouling limitation method such as gas sparging and crossflow. During the long term constant flux filtration with vibration, a two-stage fouling phenomena was observed, similar with those observed in traditional MBR fouling but with a more extended initial low fouling stage. After the local permeate flux increased above the critical flux, the second rapid fouling stage occurred mainly due to cake formation. By appropriately coupling periodical backwash/relaxation with vibrational filtration, the membrane performance was further improved. At low vibration frequency, filtration with periodical relaxation displayed the best perfor- mance, whereas at high frequencies, coupling with periodic backwash was more beneficial. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Anaerobic membrane bioreactors (AnMBRs) have distinct advantages in treating wastewater, with significantly lower energy consumption than that associated with aerobic MBRs and the potential to generate biogas as an energy source [1–4]. However, in the absence of the gas sparging, AnMBR systems often suffer low shear rates, and fouling mitigation can be a challenge, especially at high solids loadings [2,4]. The biogas produced from anaerobic bioreactor is generally recirculated for natural scouring of the membrane modules [5]. However, such biogas sparging may not be feasible when the biogas production is too low or unstable. Thus AnMBRs offer a potentially attractive application for dynamic shear enhanced membrane systems, which include ultrasound enhanced membrane system [6], rotational disk and vibrational membrane systems [7]. In such systems, local shear rate and turbulence is increased by vibrating or oscillating the membrane surface or fluid [7]. More energy is thus focused on the filtration surface and less dissipated into circulation of the bulk solution. The vibratory shear enhancement process (VSEP) was the first commercially available dynamic membrane system intro- duced by Armando and Culkin for flat sheet membranes [8]. Due to high energy requirements and cost of scale up, applications of vibratory systems have been limited to difficult feeds such as those in the pulp and paper mills or high solids loading [7,9,10]. The principle of membrane vibration has also been applied to hollow fibers at lab scale by moving the membrane in a parallel or transverse direction to the axis of the fiber [11–14]. Comparing with VSEP system, relatively low vibration frequency is applied in vibrational hollow fiber systems, which significantly reduces the energy consumption. With the aid of vertical vibrations parallel to the fiber axis, at a frequency of 30 Hz and displacement of 0.2–1.175 mm, the critical flux was improved 2–5 times during the filtration of yeast solution [11]. In our previous study, detailed investigation on transverse vibration was carried out [15]. Trans- verse vibration could potentially generate higher shear rates and turbulence, due to the added benefit of the secondary flows induced as a result of the interactions between the vortices from the vibrating hollow fibers. In our previous study with model solutions such as yeast and alginate, it was shown that transverse vibration can mitigate cake formation; however, its effect on reducing pore blocking was found to be limited [15]. Similarly, Beier and Jonsson [16] have also reported that the macromolecular concentration had more significant influence on the fouling resistance than yeast particles during filtration with vertical Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/memsci Journal of Membrane Science 0376-7388/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.12.078 n Corresponding author. Tel.: þ61 293 854 813; fax: þ61 293 855 966. E-mail address: v.chen@unsw.edu.au (V. Chen). Journal of Membrane Science 455 (2014) 320–329