New insights on early stages of RO membranes fouling during tertiary wastewater desalination Wang Ying a , Nadine Siebdrath b , Wolfgang Uhl b , Vitaly Gitis c , Moshe Herzebrg a,n a Ben Gurion University of the Negev, Zuckerberg Institute for Water Research, Jacob Blaustein Institutes for Desert Research Sede-Boqer Campus, Israel b Technische Universität Dresden, Institute of Urban Water Management, Chair of Water Supply Engineering Dresden, Germany c Ben Gurion University of the Negev, Unit of Environmental Engineering, Faculty of Engineering Beer-Sheba, Israel article info Article history: Received 23 December 2013 Received in revised form 15 April 2014 Accepted 16 April 2014 Available online 25 April 2014 Keywords: Biofouling Organic fouling Reverse osmosis EfOM EPS Membrane swelling abstract In tertiary wastewater desalination, effective minimization and cleaning of membrane fouling requires an understanding of the physicochemical interactions between conditioning film–membrane surface, conditioning film–bacteria, bacteria–EPS and within the EPS. In this study, the effect of conditioning film on the attachment of bacteria and EPS (isolated from RO membrane biofouling layers) to the surface was analyzed using quartz crystal microbalance with dissipation (QCM-D) equipped with polyamide-coated sensors. A conditioning film originated from membrane bioreactor (MBR) ultra-filtration permeate (UFP), mainly consisting of effluent organic matter (EfOM). This conditioning film was shown to enhance bacterial attachment but had no effect on the EPS adsorption. Furthermore, the UFP and EPS were characterized and their effects on the swelling behavior of the model polyamide surface were delineated using QCM-D by altering pH values from 6 to 8 and back to 6. In parallel, fouling experiments with UFP and EPS were conducted in a parallel RO plate and frame unit. Reversible changes in the RO permeate flux were observed as pH values were altered (from 6 to 8 and back to 6) with pristine and fouled membrane with EPS. In contrast, irreversible flux decline was observed for the UFP fouled membrane: pH variation, back to 6, promoted further interaction between the UFP organic matter and the membrane, leading to a further reduced membrane permeability. These results are likely related to the swelling phenomenon and its inhibition by the UFP layers observed in the QCM-D. More studies need to be carried out in order to analyze the effects of humic substances, abundant in the UFP fouling layer, that irreversibly interact with the polyamide surface. Reduced membrane swelling capability, which is important for water transport, by different foulants is proposed as additional RO membrane fouling mechanism. & 2014 Elsevier B.V. All rights reserved. 1. Introduction With increasing water demand for human consumption, indus- try and agriculture, and the local deterioration of the quality of raw water available, reclamation of tertiary effluents is of increasing importance. This is especially essential for arid regions of the world. As desalination with reverse osmosis (RO) or nanofiltration (NF) can both ensure the removal of microorgan- isms and most dissolved substances, these membrane processes have become an important technology for the treatment of tertiary effluents. However, membrane performance can dramatically decrease as a consequence of fouling, which includes mineral-, colloidal-, organic- and bio-fouling. The latter, which results from the deposition of microorganisms (mainly bacteria) and the formation and deposition of extracellular polymeric substances (EPS), is considered of special importance in the reclamation of tertiary effluents. Numerous studies have proposed that biofouling could be considered to be a combination of the following mechan- isms: (1) passive hydrodynamic and diffusive transport of both nutrients and bacteria to the membrane surface, (2) adsorption of nutrients and organic matter to the surface resulting in the formation of a so-called conditioning film, (3) reversible or irreversible adhesion of microbial cells on the conditioned surface and (4) proliferation of the attached microbial cells on the conditioned surface resulting in colonization, the development of sessile micro-colonies and eventually a biofilm [1,2]. Generally, the significant decrease of membrane filtration performance asso- ciated with biofouling, as well as the challenges for membrane cleaning, were shown to arise from the presence of EPS rather than from the microorganisms themselves [3]. Thus, in water Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/memsci Journal of Membrane Science http://dx.doi.org/10.1016/j.memsci.2014.04.027 0376-7388/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ972 8 6563520; fax: þ972 8 6563503. E-mail address: herzberg@bgu.ac.il (M. Herzebrg). Journal of Membrane Science 466 (2014) 26–35