Journal of Membrane Science 367 (2011) 45–54 Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci Evaluation of fouling mechanisms in the nanofiltration of solutions with high anionic and nonionic surfactant contents using a resistance-in-series model Yasemin Kaya ∗ , Hulusi Barlas, Semiha Arayici Istanbul University, Faculty of Engineering, Department of Environmental Engineering, Avcilar, Istanbul 34320, Turkey article info Article history: Received 9 June 2010 Received in revised form 24 September 2010 Accepted 19 October 2010 Available online 25 October 2010 Keywords: Resistance-in-series model Surfactant Gel layer Concentration polarisation Internal pore blocking abstract The effects of feed concentration and transmembrane pressure (TMP) on membrane fouling in the treatment of cleaning-in-place (CIP) wastewater originated from the production of liquid dishwashing detergent with a nanofiltration (NF) membrane were investigated. The resistance-in-series model was used to evaluate the flux decline caused by a gel layer, a concentration polarisation layer, and internal pore blocking in the NF membrane for CIP solutions of varying concentration termed CIP 5, CIP 10 and CIP 20. With an increase in feed concentration and TMP, it was observed that resistance of the gel layer (R g ) played a more important role in the flux decline than that of the concentration polarisation layer or internal pore fouling (R cp + in ). Considering the membrane resistance (R m ) values, the CIP solutions containing high concentrations of anionic and nonionic surfactants and low dye and salt concentrations did not cause serious fouling of the NF membrane. Characterisation of the membrane surface by atomic force microscopy (AFM) and contact angle measurements also showed that the deposition of surfactant aggregates on the NF membrane surface likely played an important role in the gel layer fouling. The NF membrane showed a rejection efficiency of over 98% for anionic surfactants, nonionic surfactant and dye in all CIP solutions. Salt rejection was not achieved in the CIP 5 solution because of the Donnan effect, whereas salt rejections were around 11–34% and 28–54% in the CIP 10 and CIP 20 solutions, respectively. The resistance-in-series model was successfully tested to evaluate the flux decline for the CIP solutions containing anionic and nonionic surfactants, dye and salt at various TMPs. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Fouling is the most important problem in membrane filtra- tion, causing significant flux decline, increased operating costs and decreased membrane lifetime. Flux decline in membrane processes is mainly caused by the adsorption of solids onto the membrane, pore blocking, concentration polarisation, deposition on the inside membrane surface forming a gel layer and compression of the gel layer [1,2]. During the transport of solutes from the feed solution to the membrane surface, solute concentrations at the membrane surface are higher than in the feed solution, leading to concen- tration polarisation. In some cases, solute concentrations at the membrane surface can reach a limiting value. In these conditions, a gel layer begins to accumulate, leading to surface and/or inter- nal pore fouling [3]. Surface fouling is generally readily removable from the membrane by physical washing: it is thus often classi- fied as reversible fouling. Internal fouling causing pore blocking is ∗ Corresponding author at: Istanbul University, Faculty of Engineering, Depart- ment of Environmental Engineering, Avcilar Campus, Avcilar, Istanbul 34320, Turkey. Tel.: +90 212 4737070; fax: +90 212 4737180. E-mail address: y kaya@istanbul.edu.tr (Y. Kaya). considered irreversible fouling and is generally removed by chemi- cal cleaning. However, gel layer formation on a membrane surface, although ideally reversible, is in practice most often irreversible because it forms a cake layer. Some kinds of fouling by pore block- ing and adsorption may be partially reversible depending on the strength of foulant adhesion and the efficiency of the physical washing process [4]. The fouling mechanisms, including concentration polarisation, gel layer formation and pore blocking, introduce additional resis- tances to transport across the membrane due to the increased osmotic pressure on the feed side. A resistance-in-series model developed by Tu et al. [3], containing membrane resistance, gel layer resistance, concentration polarisation resistance, and inter- nal pore resistance, was applied to describe these mechanisms. These authors studied the ultrafiltration (UF) and nanofiltration (NF) of groundwater containing natural organic matter (NOM) to evaluate flux decline using the resistance-in-series model. They reported that the model proved useful in evaluating the resistances in membrane filtration. This model has also been confirmed in sev- eral later studies investigating the effects of membrane fouling on flux decline. Purkait et al. [5] investigated flux decline during the micellar-enhanced ultrafiltration (MEUF) of an acidic dye with a cationic surfactant using a resistance-in-series model. They found 0376-7388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2010.10.037