Journal of Membrane Science 377 (2011) 206–213 Contents lists available at ScienceDirect Journal of Membrane Science j ourna l ho me pag e: www.elsevier.com/locate/memsci Understanding the oxidative cleaning of UF membranes Inna Levitsky a , Aviv Duek a , Elizabeth Arkhangelsky a , Diana Pinchev a , Tali Kadoshian a , Hila Shetrit a , Ronen Naim a , Vitaly Gitis a,b,∗ a Unit of Environmental Engineering, Ben-Gurion University of the Negev, PO Box 653, Beer-Sheva 84105, Israel b van’t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098XH Amsterdam, The Netherlands a r t i c l e i n f o Article history: Received 21 January 2011 Received in revised form 16 March 2011 Accepted 24 April 2011 Available online 30 April 2011 Keywords: PES PVDF PEG Free chlorine BSA a b s t r a c t Increased protein fouling of polyether sulphone membranes after NaOCl cleaning was previously reported but not explained. Here we show that the cleaning increases the hydrophilicity, and the degree of increase linearly correlates with the amount of adsorbed protein. The high initial flux through the cleaned mem- brane is a result of the hydrophilization of the membrane surface and a promise for the enhanced fouling. We propose that the proper oxidative cleaning should target the restoration of the initial flux and not its increase over initial values. The previously reported pore size changes are subjective as higher hydrophilicity of the membrane surface increases water permeability and adsorption of size test solutes. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Chemical separations are a major cost component of most pharmaceutical and biotechnological industrial applications. Low- pressure polymer nanoporous membranes with a low cost/area ratio, good flux, low energy consumption, range of pore sizes, rich surface chemistry, simple up-scaling and continuous separa- tion ability [1] are the attractive separation technology. The most persistent problem associated with the low-pressure micro- and ultra-filtration (MF/UF) membranes is the problem of organic foul- ing. The problem has been of interest of more than 1200 papers [2]. The solution to the fouling problem, the periodical chemical clean- ing applied to relieve “foulants” [3,4], has received significantly less attention. Only 50 papers had been published, and all of the papers point on the immediate and long-term effects of cleaning on membrane performance. The cleaning is a sequence of 4–6 steps that include trans- port of the cleaning agents through fouling layers and membrane surface reactions to detach the foulants from the membrane surface. To overcome mass transfer barrier and to maintain rea- sonable reaction rate the cleaning is usually performed with concentrated solutions. One of the most popular cleaning agents, sodium hypochlorite [5], is very successful in restoration and some- times even growth of the permeate flux [6]. A more detailed ∗ Corresponding author. Tel.: +972 86479397. E-mail address: gitis@bgu.ac.il (V. Gitis). investigation revealed that the increase comes on the expenses of the polymer chain breakage [7–12]. The breakage is held responsible for the expansion of the membrane pore size [7,13], changes in membrane hydrophilicity [13], increased streaming potential [14], and deteriorated mechanical strength [15,16]. The oxidative cleaning also results in profound fouling [17] and enhanced protein retention [17,18] of post-cleaned membrane, the two phenomena that were previously observed but not explained. Our study confirms the previous reports on chlorine-induced changes in hydrophilicity, initial flux, degree of fouling and protein retention. However, the pore size changes were minor and did not influence post-cleaned performance. Experiments with intensity of chlorine cleaning of polyethersulphone (PES) and polyvinyldifluo- ride (PVDF) membranes proved that the key change that affects the membrane performance is the increased hydrophilicity. The hydrophilization of the membrane surface improves adsorption of polyethylene glycols (PEGs) [19] and dextrans [20] convention- ally used for pore size estimation. The higher water permeability of post-cleaned membrane [6] is related to higher hydrophilic- ity and increased surface charge [7]. The boosting protein fouling [17] of intensively cleaned membranes is governed by hydrophilic nature of proteins. On the operational level, the NaOCl cleaning with aggregate free chlorine doses of 5 g/(l h) and higher increases surface charge [21,22] and affects the membrane hydrophilicity. An increase in the permeate flux over the initial values can be viewed as a worrying sign that points on changes in membrane structure and increased fouling potential. 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.04.046