Chemical Engineering Journal 188 (2012) 30–39 Contents lists available at SciVerse ScienceDirect Chemical Engineering Journal jo ur n al homep age: www.elsevier.com/locate/cej Effect of flow velocity, substrate concentration and hydraulic cleaning on biofouling of reverse osmosis feed channels A.I. Radu a,b,∗ , J.S. Vrouwenvelder a,b,c , M.C.M. van Loosdrecht a , C. Picioreanu a a Department of Biotechnology, Faculty of Applied Sciences, Delft University of Technology, Julianalaan 67, 2628 BC Delft, The Netherlands b Wetsus, Centre of Excellence for Sustainable Water Technology, Agora 1, P.O. Box 1113, 8900 CC Leeuwarden, The Netherlands c King Abdullah University of Science and Technology, Water Desalination and Reuse Center, Thuwal, Saudi Arabia a r t i c l e i n f o Article history: Received 16 September 2011 Received in revised form 27 January 2012 Accepted 30 January 2012 Keywords: Biofouling Biofilm Model Reverse osmosis Fluid dynamics Feed spacer a b s t r a c t A two-dimensional mathematical model coupling fluid dynamics, salt and substrate transport and biofilm development in time was used to investigate the effects of cross-flow velocity and substrate availability on biofouling in reverse osmosis (RO)/nanofiltration (NF) feed channels. Simulations performed in chan- nels with or without spacer filaments describe how higher liquid velocities lead to less overall biomass amount in the channel by increasing the shear stress. In all studied cases at constant feed flow rate, biomass accumulation in the channel reached a steady state. Replicate simulation runs prove that the stochastic biomass attachment model does not affect the stationary biomass level achieved and has only a slight influence on the dynamics of biomass accumulation. Biofilm removal strategies based on velocity variations are evaluated. Numerical results indicate that sudden velocity increase could lead to biomass sloughing, followed however by biomass re-growth when returning to initial operating conditions. Sim- ulations show particularities of substrate availability in membrane devices used for water treatment, e.g., the accumulation of rejected substrates at the membrane surface due to concentration polarization. Interestingly, with an increased biofilm thickness, the overall substrate consumption rate dominates over accumulation due to substrate concentration polarization, eventually leading to decreased substrate concentrations in the biofilm compared to bulk liquid. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Desalination of brackish water and seawater using reverse osmosis (RO) technology has become increasingly important in recent years due to severe water shortages experienced in several regions in the world [1]. Despite recent innovative pretreatment strategies for the feed water of the RO module, prevention of bio- fouling remains a problem. Biofilms develop in all RO membrane plants from the start of operation [2], but what actually has an impact on the system performance are the amount and the place where the biofilm forms. Biofouling, i.e. the substantial decline of membrane performance due to biofilm development, only occurs when a threshold value of biomass is attained in the system [3]. Better understanding of biofilm formation in membrane systems is important so that the operational problems caused by the biofilm can be reduced. Computational approaches can complement ∗ Corresponding author at: Department of Biotechnology, Faculty of Applied Sci- ences, Delft University of Technology, Julianalaan 67, 2628 BC Delft, The Netherlands. Tel.: +31 15 2781482; fax: +31 15 2782355. E-mail addresses: a.i.radu@tudelft.nl (A.I. Radu), J.S.Vrouwenvelder@tudelft.nl (J.S. Vrouwenvelder), M.C.M.vanLoosdrecht@tudelft.nl (M.C.M. van Loosdrecht), C.Picioreanu@tudelft.nl (C. Picioreanu). experimental studies if one could evaluate by numerical means the effect of different operational conditions on biofouling. In general, models for membrane processes account for the liquid flow pat- tern and the associated mass transfer of solutes (e.g., salts, organic substrates). Besides these essential membrane-related processes, a numerical study of biofouling must include also a model of biofilm development and of its effect on membrane processes. Many studies reported a complex relationship between liquid flow and biofilm structure [4–7]. Mass transport and fluid shear stress are both dependent on the hydrodynamic conditions, thus the flow pattern will significantly influence biofilm processes (as described in models [8–11]). One would expect that the higher the fluid velocity, the larger the external mass transfer rate of nutrients to the biofilm and therefore the biofilm growth is faster. On the other hand, the net biofilm accumulation is determined by the bal- ance between biomass growth and detachment rates. High velocity also implies larger shear forces at the biofilm surface. If the biofilm had constant mechanical properties in time, larger shear could lead to more biomass detachment. However, on the long term, denser, stronger and more resilient biofilms develop at large shear rates. Clearly, due to its multiple roles fluid flow has a crucial importance in determining the biofilm amount in a system and its structure [12–15]. 1385-8947/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2012.01.133