Fouling of reverse osmosis membranes using electrical impedance spectroscopy: Measurements and simulations

Transcription

1 Desalination 236 (2009) Fouling of reverse osmosis membranes using electrical impedance spectroscopy: Measurements and simulations J.M. Kavanagh*, S. Hussain, T.C. Chilcott, H.G.L. Coster School of Chemical and Biomolecular Engineering, The University of Sydney, NSW 2006, Australia Received 30 June 2007; revised accepted 7 October 2007 Abstract The use of modern high flux reverse osmosis membranes with low salinity feeds at high recovery rates, makes the measurement of fouling by flux decline difficult in practice. Early detection of fouling is important, because the onset of fouling occurs at a critical flux which is difficult to predict a priori. An alternative method, measuring the electrical properties of the membrane is proposed here. The fouling of a reverse osmosis membrane was measured with an INPHAZE TM Electrical Impedance Spectroscope from frequencies of 10 1 to 10 5 Hz. Both the conductance and impedance showed dramatic changes when the reverse osmosis membrane was fouled by a small amount of precipitated divalent salts. From these experimental results, electrical properties of the reverse osmosis membrane system were determined. This paper takes electrical properties determined in previous work and models the electrical properties of the first and second stages of a sea water desalination process. The decrease in the electrical conductivity of the membrane skin layer, which would accompany fouling, is significant at frequencies below 100 Hz. Such a method has great potential for online measurement of membrane fouling and more effective operation of membrane filtration systems. Keywords: Electrical impedance spectroscopy; Reverse osmosis; Fouling 1. Introduction Fouling of membranes, resulting in increased power consumption and a reduction in membrane lifespan has been recognised as a major operational and economic issue for the feasibility of *Corresponding author. such plants [1]. Membrane fouling has traditionally been measured by the decline in the permeate at constant operating conditions. Tay and Song [1] have shown through simulation that this method is not satisfactory for the treatment of low salinity feeds using highly permeable membranes, due largely to the flux decreasing along the length of the module. Presented at the International Membrane Science and Technology Conference, IMSTEC 07, 5 9 November 2007, Sydney, Australia /09/$ See front matter # 2009 Published by Elsevier B.V. doi: /j.desal

2 188 J.M. Kavanagh et al. / Desalination 236 (2009) Electrical Impedance Spectroscopy (EIS) has proven to be a valuable tool in analysing the structures of micro and ultrafiltration membranes in a number of studies. The application of EIS to membrane systems can be seen in a number of papers, including Coster et al. [2], who studied the properties of ultrafiltration membranes and determined their permeabilities. Park et al. [3], applied the technique to study the fouling of ion exchange membranes. Chilcott et al. [4], and Gaedt et al. [5], studies of membranes using Electrical Impedance Spectroscopy, both revealed that surface fouling may be more readily detected than fouling within pores. Hence this technique has considerable scope for measuring the fouling of reverse osmosis membranes. In this paper we analyse the results of some initial experiments [6] that characterised the electrical properties of the membrane and a typical foulant. We expand on this work, and use these electrical properties to develop an electrical model of both the first and second stages of sea water desalination, addressing issues such as the effect of solution conductivities and electrode spacing on the detection of fouling. 2. Experimental methods Electrical Impedance Spectroscopy is a powerful and versatile technique for determining structural information that has been used in a wide variety of applications. The method works by the injection of a number of known sinusoidal alternating currents of known frequencies into a system and measuring the voltage (potential difference) across the system including the phase difference between the current and voltage. From this the impedance magnitude and phase angle, and hence the conductance and capacitance can be determined. The variation of these properties with frequency, commonly referred to as dispersion, can then be used to determine the number and properties of layers which compose the system of interest. For impedance measurement of aqueous systems the current and voltage electrodes are commonly separated, to avoid measurements of the impedance of the interface between the metal current-injecting electrodes and the aqueous phases. Ions accumulate at this interface as the current is carried by electrons, not ions, in the metal. This represents a large resistance to electric currents at low frequencies. Using two pairs of electrodes, one pair for injecting current and another separate pair for measuring the voltage developed across the sample is known as a four terminal measurement (see Fig. 1). The current electrodes used in Kavanagh et al. [6], shown side on in Fig. 1, had a large in area to ensure a uniform current distribution across the membrane area, whilst the voltage electrodes were considerably smaller area. This combined with the high input impedance of the INPHAZE TM Electrical Impedance Spectroscope, results in a very small current flowing into the voltage electrodes, thus avoiding the formation of measurement of electrical double layers. Each voltage electrode was positioned ~1.25 cm from the membrane. Initial experiments were conducted using laboratory film as a surrogate membrane. When the laboratory film was coated with a calcium Fig. 1. The osmotically driven cell and the four terminals used in Kavanagh et al. [6].

3 J.M. Kavanagh et al. / Desalination 236 (2009) carbonate solution then dried, the impedance of the membrane system increased by % at frequencies below 100 Hz. This method was not applicable to reverse osmosis membrane as hydration was found to strongly effect the membranes impedance, and the calcium deposit tended to diffuse away from the laboratory film. The membranes used in the experiment were cut from Sterlitech Polyamide RO AK membranes and were soaked in deionised water for 24 h prior to conducting experiments to ensure hydration. The membranes were measured both using vernier callipers and a micrometer and found to have a thickness of 0.16 mm. In order to simulate reverse osmosis processing conditions, where a highly conductive salt solution is in contact with the skin side of the membrane, and a less conductive solution is in contact with subsurface side of the membrane, sucrose was used to balance the osmotic pressure. For fouling experiments, the sucrose had a higher osmotic pressure in order to create a flow of water through the membrane and draw some of the calcium carbonate to the membrane surface. The experiments used to determine the electrical properties of the membrane system in Kavanagh et al. [6], are summarised in Table 1. The conductivities of the sugar solutions varied considerably, due to a mixture of biological activity and variability in the quality of the deionised water used. Whilst this made analysis of the data more difficult, it led to greater insight into the membrane substructure. Table 1 Experiments used to determine membrane electrical parameters in Kavanagh et al. [6] 3. Electrical circuit model The electrical circuit model for the system used to analyse the data in Kavanagh et al. [6], was assumed to comprise of four elements: an element representing the conduction per unit area through both bulk solutions (G sol ) an element representing the membrane skin layer with both conductive and capacitive properties (G skin and C skin ) per unit area an element representing the membrane sub surface layer, which has both conductive and capacitive properties (G sub and C sub ) per unit area an element representing the mass transfer of ions in solution, which is significant at low frequencies and was assumed to be constant in all tests (G dif and C dif ) per unit area The conduction of an element is determined according to Eq. (1): G ¼ x Feed Permeate Higher 0.05 M NaCl 0.1 M Sucrose conductivity Unfouled M NaCl 0.1 M Sucrose Fouled M NaCl þ CaCO M Sucrose ð1þ Where, G is the conductance per unit area, is the conductivity and x is the distance. Whilst the capacitance of an element could is determined by Eq. (2): C ¼ " o" R x ð2þ Fig. 2. Electrical circuit model for reverse osmosis membrane system [6]. Where C is the capacitance per unit area, " o is the permittivity of free space and " r is the relative permittivity.

4 190 J.M. Kavanagh et al. / Desalination 236 (2009) The conductance and capacitance of the diffusion element was determined by fitting to the experimental data and was assumed constant for all experiments and simulations. The specific conductance of the solution was determined by dividing the measured solution conductivities by the distance between the electrode and the membrane. The specific conductances were then added in series to determine the overall solution conductance. The capacitance and conductance of the membrane and the sub layer were determined from experimental measurements. These measurements were then compared to the known physical dimensions of the layers, properties of the materials and solutions. 4. Results and discussion Typical results for the impedance and phase angle of the fouled and unfouled membranes from [6] can be seen in Fig. 3. There was a clear increase in the impedance of the fouled experiment at frequencies from below 100 Hz. Similarly a more negative phase angle is observed for the fouled membrane for frequencies from 10 to 1000 Hz. Analysis of the underlying electrical structure of the membrane from [6], presented in Fig. 4 found that this difference was likely to be due to a reduction in the conductance of the membrane skin layer. The model lines in Fig. 4 are extrapolated to 10 7 Hz as the system conductance approaches that of the solution element at high frequencies. From both fouled and unfouled experiments, the capacitance of the skin layer was found to be approximately 0.4 mf/m 2. This is consistent with a relative permittivity of 9 (between the values for polymers, typically 4 and water 80), and the skin layer thickness of 0.2 mm, which is within the range given by Peterson and Cadotte [7]. After adjustments were made for both the solution conductivity and the subsurface conductivity (assuming that sub-surface conductivity was dominated by the solution and the Fig. 3. Impedance spectra of a reverse osmosis membrane with (~) NaCl salt solution of conductivity of S/m on the feed side and sugar solution conductivity of S/m on the retentate side (g) NaCl and CaCO 3 foulant of conductivity of S/m on the feed side and sugar solution of conductivity of S/m on the retentate side [6]. substructure had a porosity of around 15%) it was also necessary to decrease the conductance of the skin layer from 1.7 to 0.5 S/m 2,toaccurately fit the data. This was evidence of the build-up of calcium carbonate preventing the flow of current through the membrane and that fouling is occurring on the skin of the membrane. 5. Simulation In order to determine the feasibility of this method for measuring the fouling of reverse osmosis membranes for sea water desalination, models were developed for both the first and

5 J.M. Kavanagh et al. / Desalination 236 (2009) Fig. 4. Conductance and capacitance of a reverse osmosis membrane with (~) NaCl salt solution of conductivity of S/m on the feed side and sugar solution conductivity of S/m on the retentate side (g;) NaCl and CaCO 3 foulant of conductivity of S/m on the feed side and sugar solution of conductivity of S/m on the retentate side [6]. second stages of a typical treatment plant. For Stage 1, it was assumed that the feed sea water had a TDS of 35 g/l corresponding to a conductivity of 5 S/m and that the permeate from the first stage had a TDS of 0.35 g/l corresponding to a conductivity of 0.05 S/m. Stage 2 was assumed to use the permeate from the first stage as its feed and the permeate from the second stage was assumed to have a conductivity of 30 ms/m. The conductance of the solution element was calculated as described in the Section 3 of the paper, for industrial application the electrode was assumed to be 1 mm from the membrane. The diffusion elements conductance and capacitance was assumed to be constant. The conductance of the membrane skin layer was assumed to be related to fouling, whilst its capacitance was calculated based on a relative permittivity of 8 and a thickness of 0.2 mm. The conductance of the membrane subsurface layer was determined assuming that it is dominated by the solution in the pores, that the porosity is approximately 50% and that the subsurface layer of the membrane is 0.16 mm thick. The capacitance of the membrane subsurface layer was calculated based on its thickness, the porosity and relative permittivities of 80 for water and 4 for the polymer. The electrical properties for the first stage simulation are shown in Table 2 and for the second stage in Table 3. Tables 2 and 3 also show the characteristic frequency for each element in the electrical model, which was calculated using Eq. (3), giving an indication where the effect of each element is likely to be observed. FðHzÞ ¼ G 2C ð3þ Table 2 Electrical parameters for first stage RO model Electrical property Solution Skin layer Sub layer Diffusion G (S/m 2 ) C (F/m 2 ) F (Hz)

6 192 J.M. Kavanagh et al. / Desalination 236 (2009) Table 3 Electrical parameters for second stage RO model Electrical property Solution Skin layer Sub layer Diffusion G (S/m 2 ) C (F/m 2 ) F (Hz) This characteristic frequency gives an indication of where the effect of the element is likely to be observed. For both the first stage and second stage model reductions of skin layer conductivity to 1.7, 1 and 0.5 S/m 2 were simulated, the effect on impedance is shown in Figs. 5 and 6 respectively. As can be seen in both figures, changes in the membrane skin layer s electrical conductance are readily apparent below 100 Hz. Fig. 5. Impedance simulation for fouling of first stage RO. A number of conclusions can be drawn from the simulation results presented in Figs. 5 and 6. Firstly, the conductivity of the solutions (particularly the permeate) influences the impedance of the system at all frequencies, with the second stage simulation having a higher impedance at all frequencies and most noticeably above 10 khz. Secondly in both cases changes in the membrane skin layer can be detected at frequencies below 100 Hz. Simulations reveal that this frequency is strongly dependent on electrode spacing and the conductivity of the permeate solution. The effect is more pronounced if the electrode spacing is 1 cm, where the second stage membrane is only apparent below 10 Hz. The difficulty in isolating such a system from electrical noise at 50 Hz would make filtering a signal at 100 Hz difficult. Hence a frequency of around 5 Hz would be likely to give better results for detecting the membrane skin layer. Combining this low frequency measurement with a high frequency conductivity or impedance measurement would enable the detection of changes in the electrical properties of the membrane skin layer and hence fouling, in addition to the salt rejection of the membrane. Fig. 6. Impedance simulation for fouling of first stage RO. 6. Conclusions This paper has provided preliminary experimental and simulation work on the fouling of reverse osmosis membranes, using a static osmotic pressure driven system. It has been shown that fouling could potentially be measured by increased impedance at frequencies

7 J.M. Kavanagh et al. / Desalination 236 (2009) below 100 Hz and that this has the potential to be used on industrial systems. The next step in the project will be the measurement of fouled and unfouled membranes, recovered from industrial operation, using the experimental setup detailed in this report. From this data the validity of the proposed electrical model will be determined for industrial membranes. A further step in the project will be the measurement of fouling on a purpose built pressure driven reverse osmosis unit, to determine the relationship between fouling, flux decline and the electrical parameters of the membrane. Acknowledgement The authors would like to acknowledge the Australian Research Council for supporting of the research. References [1] K.G. Tay and L. Song, A more effective method for fouling characterization in a full-scale reverse osmosis process, Desalination, 177 (2005) [2] H.G.L. Coster, K.J. Kim, K. Dahlan, J.R. Smith and C.J.D. Fell, Characterisation of ultrafiltration membranes by impedance spectroscopy. I. Determination of the separate electrical parameters and porosity of the skin and sublayers, J. Membr. Sci., 66 (1992) [3] J.-S. Park, T.C. Chilcott, H.G.L. Coster and S.-H. Moon, Characterization of BSA-fouling of ionexchange membrane systems using a subtraction technique for lumped data, J. Membr. Sci., 246 (2005) [4] T.C. Chilcott, M. Chan, L. Gaedt, T. Nantawisarakul, A.G. Fane and H.G.L. Coster, Electrical impedance spectroscopy characterization of conducting membranes I. Theory, J. Membr. Sci., 195 (2002) [5] L. Gaedt, T.C. Chilcott, M. Chan, T. Nantawisarakul, A.G. Fane and H.G.L Coster, Electrical impedance spectroscopy characterization of conducting membranes II. Experimental, J. Membr. Sci., 195 (2002) [6] J.M. Kavanagh, T.C. Chilcott and H.G.L. Coster, Monitoring fouling of reverse osmosis membranes using electrical impedance spectroscopy, Chemeca, Melbourne, Australia, September [7] R.J. Peterson and J.E. Cadotte, Thin Film composite Resins, In M.C. Porter, Handbook of Industrial Membrane Design, William Andrew Inc., 1990.