Characterization of nanofiltration membranes for scale prevention in seawater reverse osmosis L. Llenas*, G. Ribera*, A. Yaroshchuk**.***, X. Martínez-Lladó*, M. Rovira*.***, J. De Pablo*.*** * CTM Centre Tecnològic; Av. Bases de Manresa 1, 08242 Manresa, Spain (E-mail: email@example.com, firstname.lastname@example.org) ** Institució Catalana de Recerca i Estudis Avançats (ICREA) ***Department of Chemical Engineering, Polytechnic University of Catalonia; Av. Diagonal 647, 08028 Barcelona,Spain INTRODUCTION One of the major problems in seawater reverse osmosis (SWRO) and a limiting For the parameterization of our experimental data we used the Spiegler – Kedem factor for its proper operation is membrane scaling (Schäfer et al. 2006). Hassan et model (Spiegler and Kedem. 1966). Within its scope the solute rejection. R. is al (Hassan et al. 1998), proposed the use of nanofiltration (NF) as a pretreatment of related to the trans-membrane volume flow in this way: desalination. Feeding of the NF permeate to desalination plants, not only made possible their operation with less chemicals and raising significantly their permeate and distillate recovery ratios, but also allowed for lowering their energy consumption. Several studies have shown that the rejection of scale-forming ions is not the same The fitting parameters obtained with the Spiegler Kedem model are apparent ones, for various membranes (Hassan et al. 2000). The aim of this work is to study the in particular, because they depend on the cross-flow velocity. However, if this is not productivity and the selectivity of eleven different nanofiltration membranes to varied (as it was the case in the present study) the apparent reflection coefficients prevent fouling in SWRO caused by inorganic compounds presents in seawater. All and solute permeabilities can be used for the quantitative comparison of membranes have been characterized in terms of surface roughness by Atomic performance of various membranes as well as for the interpolation of rejection data Force Microscopy. between the available experimental points. METHODOLOGY ION REJECTION RESULTS AND MODEL FITTING All the experiments were carried out in a laboratory scale Figures 2-5 show the scaling forming ions rejections of different NF membranes tested, as well cross-flow test cell in total recirculation mode. See Fig. 1: as the fitting with the Spiegler Kedem model. 100 100 90 NF270 NF270 90 80 NF200 NF200 NF 70 NF 80 % Sulphate rejection % Calcium rejection ESNA 1-LF2 ESNA 1-LF2 60 K-TFCS K-TFCS 70 50 K-SR2 K-SR2 40 K-SR3 60 K-SR3 ALNF99 ALNF99 30 NF99HF 50 NF99HF 20 NF90 NF90 40 10 0 30 0 50 100 150 0 50 100 150 Permeate flow (l·h-1·m-2) Permeate flow (l·h-1·m-2) Fig.1. Flow sheet of experimental system used Fig.2. Calcium rejection and SK fit Fig.3. Sulphate rejection and SK fit 100 100 Atomic Force Microscopy (AFM) has been used to characterize 90 NF270 90 NF270 nanofiltration membranes tested in terms of surface roughness. 80 NF200 80 NF200 % Bicarbonate rejection % Magnesium rejection 70 NF 70 NF ESNA 1-LF2 Analytical methods for the analysis of ion rejections: Ionic 60 K-TFCS 60 ESNA 1-LF2 K-TFCS 50 Chromatography (Dionex ICS-2100); Total Carbon Analyzer. 50 K-SR2 K-SR2 40 40 Shimadzu TOC-5050A. was used to analyze inorganic carbon K-SR3 ALNF99 K-SR3 30 30 ALNF99 and Inductively Coupled Plasma Mass Spectrometry (ICP-MS). NF99HF 20 NF99HF 20 Agilent 9500cx 10 NF90 10 NF90 0 0 0 50 100 150 0 50 100 150 Permeate flow (l·h-1·m-2) Permeate flow (l·h-1·m-2) Fig.4. Magnesium rejection and SK fit Fig.5. Bicarbonate rejection and SK fit ATOMIC FORCE MICROSCOPY RESULTS NF270 NF ESNA 1- K-SR3 NF99 Membrane Rms (nm) Permeate flow (lmh) % Rejection LF2 NF90 103.3 10.7 60.8 K-TFCS 50.17 26.3 52.8 ESNA 1-LF2 49.07 137.9 22.1 NF99HF 12.29 104.3 28.6 NF99 11.87 50.0 31.3 NF 7.43 29.6 NF200 NF90 K-SR2 K-TFCS NF99HF 58.0 NF200 7.39 59.5 23.2 NF270 5.35 108.8 20.5 K-SR3 1.78 49.0 32.8 K-SR2 0.76 127.0 15.7 Table 1. Surface roughness of membranes at a scan size of 5 x 5 µm2 compared with permeate flow and conductivity rejection Fig.6. AFM images for ten membranes characterized CONCLUSIONS REFERENCES• The rejection of divalent ions is good in most of membranes tested and that is • Hassan A.M, et al (1998) A new approach to membrane and thermal seawatervery important for the prevention of scaling. desalination processes using nanofiltration membranes (Part 1). Desalination 118, 35- 51• Roughness have been measured with AFM. Membranes with a higher roughness • Hassan A.M, et al (2000) A demonstration plant based on the new NF-SWROpresent a high conductivity rejection and vice versa. Colloidal fouling in NF process, Desalination 131, 157-171membranes is strongly correlated with membrane surface roughness. Membranes • Schäfer A.I, Fane A.G, Waite T.D (2006) Nanofiltration principles and applications.with high surface roughness are more prone to fouling. Elsevier publications • Spiegler K.S., Kedem O.; Thermodynamics of hyperfiltration (reverse osmosis):• The most suitable NF membranes as pretreatment for scaling prevention are: criteria for efficient membranes (1966) Desalination 1, 311-326NF270 (Dow Chemical), K-SR2 (Koch) and NF99HF (Alfa Laval).This study was financially supported by Sociedad General de Aguas de Barcelona (AGBAR) within the scope of CENIT project “Desarrollos tecnológicos hacia un ciclo del agua urbano auto-sostenible (SOSTAQUA)”.