Ultrasonic cleaning of polytetrafluoroethylene membrane fouled by natural organic matter Chi-Chuan Kan a , Divine Angela D. Genuino b , Kim Katrina P. Rivera c , Mark Daniel G. de Luna c,n a Institute of Hot Spring Industry, Chia-Nan University of Pharmacy and Science, Tainan 71710, Taiwan b Environmental Engineering Program, National Graduate School of Engineering, University of the Philippines,1101 Diliman, Quezon City, Philippines c Department of Chemical Engineering, University of the Philippines,1101 Diliman, Quezon City, Philippines article info Article history: Received 24 July 2014 Received in revised form 29 June 2015 Accepted 12 August 2015 Available online 3 September 2015 Keywords: Membrane fouling Natural organic matter PTFE membrane Coagulation Ultrasonic cleaning abstract Chemical cleaning of membranes fouled by natural organic matter (NOM) generates secondary pollution. In this study, the effect of ultrasonic cleaning of polytetrafluoroethylene (PTFE) membrane fouled by pre- coagulated humic acid-bentonite mixture was investigated. Results show that chemical coagulation prior to microfiltration improved turbidity and TSS removals by 9.5% and 11.4%, respectively. Experimental data fitted to constant pressure filtration models determined the sequence of dominant fouling me- chanism as follows: (i) membrane resistance-limited, (ii) pore blocking resistance-limited, and (iii) cake formation resistance-limited. Relative membrane permeability of 53 and flux recovery of 45% were achieved when continuous ultrasonic cleaning was done at a 2.0 cm probe distance, 25 min total cleaning time, 15 mg/L coagulant dose, and 15 W ultrasonic power. Ultrasonic cleaning was found to be more effective than hydraulic cleaning in terms of flux recovery. Compared to chemical cleaning, it is a competitive and safer alternative in mitigating NOM-induced fouling. & 2015 Elsevier B.V. All rights reserved. 1. Introduction Natural organic matter is a product of microbiological, chemi- cal, and photochemical transformations of plant and animal re- sidues. In surface waters, the concentration of NOM ranges from 0.1–20 mg/L, the dominant fraction of which comprise humic substances such as humin, fulvic acid, and humic acid [1–3]. Hu- mic acid (HA) accounts for almost 50–90% of the total organic matter in surface water and is one of the most difficult NOM fraction to remove [4, 5]. The deterioration of the quality of water resources partly due to NOM contamination has led to stricter water quality regulations and a growing need for effective water treatment technologies [6]. One such technology is membrane filtration, a pressure-driven, size-exclusion based separation process classified according to minimum allowable particle size (i.e. microfiltration (MF),  0.1 μm) [7]. It may completely remove hardness, particu- late and microbial contaminants, and disinfection by-product precursors (i.e. NOM) from aqueous media [8,9]. Based on cost, polymeric membranes such as PTFE are preferred to ceramic membranes [10]. Modern membrane filtration systems boast of compact in- stallation, high efficiency, and low capital and operation cost re- quirements but the technology remains limited by fouling [11–13]. Fouling occurs when particles adhere onto the membrane surface causing a decline in the effective filtration area of the membrane and reduction in liquid throughput, membrane productivity and lifetime [14–19]. It is usually affected by feed water chemistry, membrane characteristics, filtration mode, and hydrodynamic conditions [19]. The fundamental mechanisms of fouling such as pore constriction, complete and intermediate pore blocking, and cake filtration have been described by various mathematical models [12,20]. Several strategies have been implemented to minimize mem- brane fouling and to maintain the economic viability of the fil- tration operation: (i) pretreatment of the feed solution [21–24], (ii) modification of the properties of the membrane material [25–28], (iii) membrane cleaning [29–32], and (iv) improvement of oper- ating conditions during filtration [25,26]. The pretreatment of NOM-contaminated water prior to MF by coagulation causes par- ticle aggregation which improves NOM filterability [4,33,34]. The formation of insoluble aggregates during the coagulation–floccu- lation of colloidal organic matter suspensions is attributed to a combination of charge neutralization, adsorption, complexation and colloid entrapment [35]. While aluminum- and iron-based Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/memsci Journal of Membrane Science http://dx.doi.org/10.1016/j.memsci.2015.08.031 0376-7388/& 2015 Elsevier B.V. All rights reserved. n Corresponding author. Fax: þ632 929 6640. E-mail addresses: mgdeluna@up.edu.ph, mgdeluna@gmail.com (M.D.G. de Luna). Journal of Membrane Science 497 (2016) 450–457