Journal of Membrane Science 327 (2009) 87–95 Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci Combined fouling of nanofiltration membranes: Mechanisms and effect of organic matter Alison E. Contreras a , Albert Kim b , Qilin Li a,∗ a Department of Civil and Environmental Engineering, Rice University, Houston, TX, 77005, United States b Department of Civil and Environmental Engineering, University of Hawaii at Manoa, Honolulu, HI 96822, United States article info Article history: Received 3 September 2008 Received in revised form 7 November 2008 Accepted 8 November 2008 Available online 27 November 2008 Keywords: Nanofiltration Combined fouling Foulant interactions Colloids Organic foulant abstract The accurate prediction of nanofiltration membrane performance in industrial applications is dependent upon understanding the fouling behavior of representative feed solutions. However, most membrane studies focus on fouling of a single type of foulant, which is not a good predictor for realistic feed solutions that contain multiple foulant types. In this study, combined fouling by organic and inorganic colloidal foulants is studied. Through the use of model foulants, three hypothesized mechanisms responsible for the enhanced membrane flux decline in the presence of multiple foulant types are examined: increased hydraulic resistance of the mixed cake layer structure, hindered foulant diffusion due to interactions between solute concentration polarization layers, and changes in colloid surface properties due to organic adsorption. All three mechanisms were found to play a role in combined fouling to various degrees. Organic adsorption was shown to cause the greatest synergistic effect. The synergistic effect caused by increased resistance of a heterogeneous fouling layer indicates that current fouling layer models need to be reexamined to include the suggested mechanisms. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Nanofiltration (NF) is an attractive technology for producing clean water from non-traditional sources, i.e. brackish water and wastewater, since it can provide high multivalent ion and organic contaminant rejection at a much lower operating pressure than reverse osmosis. Unfortunately, as with all membrane filtration processes, an inherent problem of NF is decreased productivity due to fouling of the membrane by colloidal materials, dissolved organics, inorganic precipitates, and microorganisms. While fouling can be controlled by using low fouling membrane materials [1–4], pre-treatment of the feed stream [5–8], and optimizing the sys- tem configuration and operation [9,10], proper use of these control strategies still requires a deeper understanding of the responsible fouling mechanisms. Most previous studies on membrane fouling have focused only on a single, well characterized foulant of homogenous physico- chemical properties (referred to in this paper as individual fouling). One marked limitation in applying the theoretical and experimen- tal results obtained from these studies to water and wastewater filtration systems is that fouling in these systems is almost always caused by more than one type of foulant with various particle sizes ∗ Corresponding author. Tel.: +1 713 348 2046; fax: +1 713 348 2026. E-mail addresses: Alison.Contreras@rice.edu (A.E. Contreras), Albert.S.Kim@hawaii.edu (A. Kim), qilin.li@rice.edu (Q. Li). and surface characteristics, most commonly both colloidal mate- rials and dissolved organic macromolecules, e.g., natural organic matter (NOM) and soluble microbial products. Several studies have identified that poly-dispersed suspensions form cake layer struc- tures with resistances different than mono-dispersed solutions and that interactions between foulants can be correlated to flux decline behavior [11–14]. A limited number of studies on combined fouling (i.e., fouling with multiple types of foulants) by both inorganic colloids and dis- solved organic matter have shown that fouling behavior differs under varying solution conditions and with different membrane types [15–17]. Li and Elimelech [16] performed combined foul- ing experiments with a low salt rejection NF membrane in the presence of silica colloids and NOM. Flux measurements revealed significantly faster membrane fouling in combined fouling experi- ments than what might be predicted by summing the contributions from each foulant based on the individual fouling experiments. The aggravated membrane fouling or enhanced flux decline, referred to as a synergistic effect, was attributed to the hindered back dif- fusion of each foulant. In another study performed on a high salt rejection NF membrane, Lee et al. [15] found that flux decline dur- ing filtration of a mixture of NOM and silica colloids was initially greater than the sum of the flux declines caused by each foulant individually, but was reduced in the latter filtration stages. It was hypothesized that an “active salt rejecting layer” formed during combined fouling negated the effect of cake-enhanced concentra- tion polarization (CECP), which increases the salt concentration and 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.11.030