Colloids and Surfaces B: Biointerfaces 109 (2013) 90–96 Contents lists available at SciVerse ScienceDirect Colloids and Surfaces B: Biointerfaces jou rn al hom epage: www.elsevier.com/locate/colsurfb Fouling mitigation in membrane bioreactors using multivalent cations Mohammad Reza Mehrnia ∗ , Hamed Azami, Mohammad Hossein Sarrafzadeh Biotechnology Group, School of Chemical Engineering, College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran a r t i c l e i n f o Article history: Received 12 June 2012 Received in revised form 22 February 2013 Accepted 3 March 2013 Available online 6 April 2013 Keywords: Submerged membrane bioreactor (sMBR) Fouling Cationic coagulants Bio-flocculation Dewaterability Settleability a b s t r a c t Several cations have been used to study the effect of mineral coagulants on activated sludge proper- ties and membrane fouling in submerged membrane bioreactors (MBRs). The flocculability and settling properties of activated sludge were studied in various concentrations of sodium, potassium, magnesium, calcium, ferrum (in Fe 3+ form), and aluminum. Significant effect of cations and their concentrations on different parameters have been analyzed by 2-way ANOVA. Results showed that multivalent cations induce flocculation in bioreactors while monovalent cations have a detrimental effect on flocculability of activated sludge. The ratio of tight bound extracellular polymeric substances (TB-EPSs) to loosely bound (LB-EPSs) increased with concentration of multivalent cations that was in accordance with enhance- ment of biosorption in high concentration of multivalent cations. Moreover, these cations improved the settleability and dewatering properties of activated sludge. They also mitigated membrane fouling in the MBRs so that calcium and aluminum reduced membrane fouling to nearly 30%. Simple but useful correlations were developed for description of activated sludge properties based on easy measurable parameters that could be used also for estimation of membrane fouling. It was found that the ratio of TB-EPS to LB-EPS and also flocs size distribution are the main parameters affecting membrane fouling. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Removal of the soluble organic substances is the main purpose of biological wastewater treatment plants. It is also necessary to separate biomass and suspended solids from treated wastewater to meet the environmental standards and obtain a high quality effluent. Separation of suspended materials is achieved by sec- ondary sedimentation tank in activated sludge process and they are removed from wastewater through the membrane in membrane bioreactors (MBRs) [1]. In order to improve separation process in conventional activated sludge and MBRs, it is recommended to have flocs as large and dense as possible. Larger and denser flocs sediment faster and also have less hydraulic resistance in MBRs [2]. One of the most important properties of activated sludge is its bioflocculability which depends to the flocs size and density [3]. Study of flocs composition and structure can help to optimize the properties of activated sludge and also to improve the bioflocculation process. The flocs in activated sludge consist of cellular aggregates, filamentous microorganisms, organic and mineral particles and extracellular polymeric substances (EPSs) [4]. The filamentous microorganisms induce bioflocculation process, however they are ∗ Corresponding author. Tel.: +98 2161112184; fax: +98 2166954041. E-mail addresses: mmehrnia@ut.ac.ir, jtd mrm@yahoo.com (M.R. Mehrnia). not considered as the main effective factor on flocculability in acti- vated sludge [5]. Many investigations have been carried out on the role of EPSs in bioflocculation and settleability of activated sludge [6–13]. Bruus et al. [14] have shown that the flocs are made through the Brown- ian motions of suspended solids and EPSs binding. EPSs either in soluble or bound/colloidal form are the major cause of membrane fouling in MBRs [15]. It is believed that they act as glue and bind cells together. They have a remarkable impact on the flocculability and settleability of activated sludge [16]. EPSs are excreted during cellular metabolism, cells’ growth and death. Although EPSs are essential for formation of flocs in activated sludge culture, exces- sive EPSs in the form of loosely bound EPSs (LB-EPS) could weaken either cells attachment or flocs structure [17]. Some authors have shown that cations in activated sludge induce the adsorption of excessive soluble EPSs on cellular sur- face which results in higher bioflocculation propensity in high cations concentration [18,19]. Presence of cations in activated sludge results in growth of flocs and improves the sedimentation properties and dewaterability of activated sludge. An investiga- tion on wastewater treatment plants revealed that the settling and dewatering properties of activated sludge depends on concentra- tions of monovalent and divalent cations in influent [20]. For better understanding the effects of mineral cations on bioflocculation, six cations from three initial groups of periodic table of elements were considered in this work and their impacts on 0927-7765/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfb.2013.03.009