Article pubs.acs.org/est
Novel Approach To Characterizing the Growth of a Fouling Layer during Membrane Filtration via Optical Coherence Tomography Yiben Gao,†,‡ Sanna Haavisto,§ Weiyi Li,*,†,∥ Chuyang Y. Tang,*,⊥ Juha Salmela,*,§ and Anthony G. Fane†,‡ †
Singapore Membrane Technology Centre ‡School of Civil and Environmental Engineering, and ∥DHI-NTU Centre, Nanyang Technological University, Singapore 637141 § VTT Technical Research Centre of Finland, FI-02044 Jyväskylä, Finland ⊥ Department of Civil Engineering, The University of Hong Kong, Hong Kong SAR, China S Supporting Information *
ABSTRACT: Fouling control is one of the critical issues in membrane filtration and plays a very important role in water/ wastewater treatment. Better understanding of the underlying fouling mechanisms entails novel characterization techniques that can realize a real-time noninvasive observation and provide high resolution images recording the formation of a fouling layer. This work presents a characterization method based on optical coherence tomography (OCT), which is able to detect the internal structures and motions by analyzing the interference signals. An OCT system was incorporated with a laboratory-scale membrane filtration system, and the growth of the fouling layer was observed by using the structural imaging. Taking advantage of the Doppler effects, the OCT-based characterization also provided the velocity profiles of the fluid field, which are of great value in analyzing the formation of the cake layer. The characterization results clearly reveal for the first time the evolution of the morphology of the cake layer under different microhydrodynamic environments. This study demonstrates that OCT-based characterization is a powerful tool for investigating the dynamic processes during membrane fouling.
1. INTRODUCTION Membrane technology has been applied in numerous fields such as water/wastewater treatment and seawater desalination due to its capability of producing a vast amount of water with high quality.1,2 However, membrane filtration is inevitably associated with membrane fouling. The water production and membrane life spans are adversely affected by the fouling layer formed on the membrane surface or inside the membrane structure.3−7 Therefore, the investigation of the fouling mechanisms is of primary importance for improving membrane filtration performance. Membrane fouling is a dynamic process. Formation of a fouling layer is a function of filtration time. Fouling processes are characterized by complex interactions between the species in the fluid channel, e.g., foulant−foulant and foulant− membrane interactions.8 Conventionally, membrane fouling is assessed by performing ex situ membrane autopsy studies under stationary conditions, such as electron microscopy.9,10 These characterization methods usually require a cessation of the filtration process so that the membrane samples can be prepared for the microscopic examinations. Therefore, it is difficult to reveal the dynamic characteristics associated with the fouling process. In addition, sample preparation for these conventional approaches is usually destructive. The foulant © 2014 American Chemical Society
layer could be somehow damaged by certain preparation procedures (e.g., membrane coating), thereby losing some original characteristics of the cake layer. Several sensor-based techniques have been developed to realtime monitor the fouling behavior in a noninvasive manner, for example, fouling characterization via electrical impedance microscopy (EIS)11,12 or ultrasonic time domain reflectometry (UTDR).13,14 EIS measures the variation of electrical impedance caused by the growth of a cake layer on the membrane surface; UTDR detects the fouling by analyzing the reflected acoustic signals from different interfaces between the fluid, cake layer, and membrane. However, it is difficult for these sensor-based approaches to realize a so-called “direct observation”, i.e., a series of high resolution images of a fouling process. The direct observation through membrane (DOTM) is a type of technique that applies a direct microscopic observation to a membrane filtration process. Common optical microscopes have been employed in DOTM systems to continuously and in situ record the deposition of foulant Received: Revised: Accepted: Published: 14273
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Figure 1. Schematic of the OCT-based characterization system for visualizing the growth of a cake layer during FO filtration. The OCT system (TELESTO) consists of an OCT engine (a superluminescent diode light source with a central wavelength of 1325 nm and a linear InGaAs arraybased spectrometer), a detector (a video camera integrated in the scanning probe for live video imaging), and a data acquisition unit (TELESTO software version 4.0). The top plate of the filtration cell is replaced by a thin glass window. The feed solution (1 mM NaCl) is circulating through the upper channel, while the draw solution (2 M NaCl) is circulating through the lower channel. The filtration flux is measured by weighing the tank that contains the draw solution.
particles on the membrane surface.15−17 More advanced microscopic instruments, such as confocal laser scanning microscopy (CLSM),18,19 are also explored to provide a direct observation with higher quality. Although these DOTM techniques are able to obtain clear images showing the inplane distribution of the foulant, cross-sectional profiles of the cake layer are not available. Optical coherence tomography (OCT) is a noninvasive optical technique that has been extensively applied in medical and biological fields.20,21 OCT is capable of acquiring crosssectional images with microlevel resolutions (∼10 μm).22,23 Compared to some other tomography techniques (e.g., X-ray computed tomography),24 OCT can perform the crosssectional imaging at a relatively fast rate, i.e., tens of kilohertz.22,23 Visualization of the sample structures via OCT is based on the optical interference between the reference light and the light reflected from the sample. Taking advantage of the Doppler effects, not only structural images but also velocity profiles are obtained in the OCT characterization. There are limited studies that employ OCT techniques in membrane applications. For example, Derlon et al. employed an OCT system to perform offline autopsies of membrane coupons fouled by bacterial species.18,25 However, the variation of the fouling layer during the filtration was not recorded since the OCT system was not integrated into the membrane filtration system in their study. In our recent study, it was the first time that an OCT system was used to characterize the fluid dynamics in a spacer-filled channel by exploiting the Doppler imaging technique.26
Therefore, we were motivated to further modify this system so that the growth of a cake layer during membrane filtration could be investigated using OCT structural imaging. This study was aimed at exploring the feasibility of applying OCT techniques to visualize and quantify the growth of a cake layer during membrane filtration. Specifically, it was intended to incorporate the OCT facility with a membrane filtration system so as to implement real-time scans to the foulant layer that is being developed during a filtration process. A series of structural OCT images was obtained as a function of filtration time under different flow conditions (including filtration with and without a spacer in the fluid channel). To the best of the authors’ knowledge, this is the first study of adopting OCT techniques to characterize the dynamic processes of the fouling during membrane filtration.
2. EXPERIMENTAL PROCEDURES 2.1. Structural Imaging by OCT. A spectral domain OCT (SD-OCT) facility (TELESTO 1325 nm OCT System, Thorlabs, Inc., Newton, NJ, USA) was adopted in this study. The principle of an OCT system involves complicated optical and mathematical knowledge and has been fully discussed in various references.27,28 The basic idea for detecting sample structures via OCT is to utilize the phenomenon of light interference. In an SD-OCT system an incident light beam is emitted from a broadband and low coherent light source (a superluminescent diode) and divided into the sample and reference arms. The light penetrating the sample is partially reflected whenever the optical property of the sample structure, 14274
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Albany, OR, USA) was used in this work; the active layer was exposed to the feed solution (AL-FS mode) during the filtration. All filtration experiments were carried out at a temperature of approximately 20 °C. Bentonite microparticles with a size range of 1−10 μm were chosen as the model foulant. The foulant particles were added into the feed solution to give a concentration of 2 g/L. A foulant layer was gradually formed on the membrane surface facing the OCT probe, and the process of the growth of the cake layer was recorded by the OCT system. In each fouling experiment OCT scans were carried out as the fouling occurred. The scan interval was gradually increased with an initial scan interval 1 min that was increased to 2 min after a half-hour (eventually, the interval was 5 min after 50 min). In addition, the variation of the permeate flux (i.e., the water flux) was monitored by weighing the tank that contained the draw solution during the filtration. The membrane was fouled for 1 h in all of the fouling experiments. As shown in our prior work,26 the most attractive function of OCT is the Doppler imaging that visualizes the velocity profiles of a fluid field. One of the major tasks of this work was to demonstrate the ability of OCT to correlate the fluid dynamics with the fouling process. Therefore, the Doppler imaging was also implemented to characterize the flow patterns in the fluid channel with two different modes. In the first mode there was no spacer in the fluid channel whereas a commercial spacer (GE Osmonics, SEPA) was placed between the glass window and the FO membrane in the second mode. The detailed specifications of the spacer were described in our previous study.26 In particular, the diamond configuration of the spacer was adopted in this work since it is most commonly used in practical applications.31,32 In this configuration all of the filaments were inclined to the bulk flow direction at an angle of 45°. It was expected that a different fouling layer would be formed in each channel mode due to the different hydrodynamic environment. In this study the bentonite microparticles not only were used as the foulant but also played the role of tracers for the Doppler imaging. The detection light scattered back from the tracer particles interfered with the reference light to generate the interferograms that carried the information about the phase shift caused by the particle motion, i.e., the Doppler effect. The current Doppler imaging was only able to detect the motion in the direction parallel to the detection light beam. Therefore, the observation cell was intentionally inclined at a small angle to increase the sensitivity of the Doppler imaging. More details about the fluid dynamics characterization via OCT can be found in the literature.26,33,34 The basic mode of characterizing the growth of a fouling layer is to continuously record the structural images focusing on one or more cross-sections of interest. Although scanning in multiple cross-sections is able to provide a three-dimensional view, the scanning duration required for obtaining such a view could be too long to reflect the instantaneous morphology of the cake layer. As the first step to establish the characterization of fouling via an OCT system, the scanning was focused on a single cross-section during the fouling process and resolved the fouling layer in a two-dimensional fashion. The thickness of the cake layer was quantitatively analyzed by postprocessing the structural images. The original structural images were first cropped so as to focus on the region mainly showing the cake layer. Image subtraction then was applied to each cropped image with the initial one (for the clean membrane) as the subtrahend. Higher values of the pixels
e.g., the refractive index, is changed in the axial direction. The light reflected from the different depths then interferes with the reference light, which travels a different light path, to generate the interferograms as a function of frequency. Subsequently, these frequency-dependent interferograms are Fourier-transformed to generate the intensity signals as a function of the sample depths, that is, the sample structure profile at a point.22 The process of obtaining a depth profile at a certain point is commonly known as an A-scan.29 For an SD-OCT system the rate of an A-scan is mainly controlled by the speed of the digital camera in the spectrometer. In this study the A-scan rate was set at 28 kHz. The axial resolution is determined by the center wavelength and the bandwidth of the light source. The used light source had a center wavelength of 1325 nm, and the bandwidth was fixed at 100 dB. As a result, a depth profile with 450 pixels for a penetration depth of approximately 1.67 mm was achieved for each A-scan. A series of scans at different points can be continuously carried out in a straight line on the sample surface. Such a collection of sequential A-scans is called a B-scan.29 Compared to the axial resolution of an A-scan, the lateral resolution (i.e., the spatial density of the sequential Ascans in one B-scan) depends on the focusing performance of the probe adopted in the OCT system. In this work each Bscan was composed of 1364 A-scans covering a line distance of about 8.00 mm. As a result, the resultant OCT structural images were 1364 pixels × 450 pixels (width × depth) for a physical profile of approximately 8.00 mm × 1.67 mm; the spatial resolution was of the order of 10 μm. All of the OCT images were acquired and processed by the ThorImage OCT software (Version 4.0, Thorlabs). 2.2. Fouling Characterization. As discussed in the Introduction, one of the main objectives in this study was to realize real-time imaging of fouling processes via an OCT system. The key point for doing this characterization is to introduce the detection light beam into the channel of the filtration module. Such a combination of the OCT and membrane filtration systems has been accomplished in our previous study by replacing the top channel wall with a transparent window.26 In order to guarantee the optical transparency of this observation window, a very thin glass plate (2 mm) was adopted. Therefore, a membrane process with low pressures is preferable so as to avoid potential failure of the glass window. In this study forward osmosis (FO) was selected as the model membrane process since FO processes are driven by the osmotic pressure difference across the membrane.30 The general principles developed in this work can be easily adapted to other membrane processes, such as microfiltration (MF) and reverse osmosis (RO). The OCT-FO system is schematically shown in Figure 1. The observation cell used in this work has the same size (3.8 cm × 9.2 cm) as the one for the fluid dynamics characterization. The glass window (3.2 cm × 6.4 cm) was glued onto the acrylic plate. In order to accommodate the FO filtration, the lower compartment of the observation cell was modified such that the draw solution, i.e., the solution with a higher salinity (2 M NaCl), could be circulated through the channel beneath the membrane. When the feed solution, i.e., the solution with a lower salinity (1 mM NaCl), was flowing through the upper channel, a water flow was osmotically driven from the feed solution side to the draw solution side. The cross-flow velocity for each channel was controlled at about 3 cm/s in all of the filtration experiments. For demonstrative purposes, a thin film composite (TFC) FO membrane (Hydration Technology Inc., 14275
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were rendered in the regions having more significant changes, that is, the region showing the cake layer. With the aid of a welldesigned pixel filter, the cake layer was digitally separated from the background in the subtracted images. The separated pixels with a value indicating the gray scale were polarized to generate a binary image, in which the white pixels denote the cake layer whereas the background is covered by the black pixels. All these posttreatments of the images were automatically performed using MATLAB codes developed in-house.
3. RESULTS AND DISCUSSION 3.1. Characterization of the Growth of a Fouling Layer in a Channel without a Spacer. The fouling characterization was first carried out by using the filtration cell without a spacer in the channels. It was expected that the simplest flow pattern would be generated since a very low value of Reynolds number (about 30) was used in the filtration, which was evaluated based on the hydraulic diameter of the channel cross-section. The schematic in the upper left panel of Figure 2 demonstrates the spatial domain characterized by the OCT, and the scanned cross-section is denoted by the surface with red color. This primary cross-section is parallel to the direction of the bulk flow, and a Doppler image was first obtained to verify the flow pattern in the channel as shown in Figure 2a. A dashed−dotted curve is added into the OCT images to help distinguish the membrane surface. It shows that the membrane surface is inclined at a small angle. This is because the inlet of the filtration cell was slightly raised so as to increase the intensity of the velocity components parallel to the light beam, which can be detected by the Doppler OCT.23 As discussed in previous work,26 the single color band in Figure 2a indicates a laminar flow completely guided by the channel, and the velocity profile evaluated from the color gradients (Supporting Information Figure S-1) are consistent with the parabolic distribution of the velocity predicted by the classical fluid hydrodynamic model. The Doppler imaging is dependent on the variation of the phase angle resulting from the motions of the sample particles, whereas the intensity (modulus) of the interference signals is used to resolve structural images showing the cake layer at a certain instant. In comparison to the color bands in Doppler images, structural images are composed of gray-scaled pixels, of which values reflect the variation of the refractive index of the material in the scanned section. Therefore, the quality of visualizing the cake layer mainly depends on the differences of the optical properties between the fouling particles and the ambient fluid. During the fouling a series of structural images were continuously recorded in the primary cross-section. In particular, the structural images at the beginning (t = 0 min) and end (t = 60 min) of the fouling are shown in Figure 2b,c, respectively. When compared with the image for the clean membrane in Figure 2b, the profile of the fouling layer can be easily identified in Figure 2c. In addition, an additional OCT scan in the cross-section perpendicular to the flow direction was implemented at the end of the filtration. The structural image in this transverse plane is displayed in Figure 2d, showing the cake layer profile from a different perspective. Both the axial and transverse profiles indicate that a uniform cake layer was deposited on the membrane surface after the 1 h filtration. In the structural images it is intriguing to note that there are some discernible, though not very clear, changes of the gray scale from the center of the channel to both the membrane
Figure 2. OCT images of the fouling (2 g/L bentonite microparticles) in the channel without a spacer. The primary cross-section scanned by the OCT was parallel to the direction of the bulk flow as indicated by the red area in the upper left schematic. The images obtained in the primary cross-section include the following: (a) the Doppler image of the velocity field in the channel at t = 0 min, (b) the structural image in the channel at t = 0 min, and (c) the structural image in the channel at t = 60 min. The secondary cross-section scanned by the OCT was perpendicular to the direction of the bulk flow as indicated by the red area in the lower left schematic. The image obtained in the secondary cross-section is (d) the structural image in the channel at t = 60 min. The direction of the bulk flow is denoted by the arrow. The membrane surface is indicated by the dashed−dotted curve.
surface and the upper channel wall (i.e., the observation window). The variation of the gray scale indicates the change of the scattering characteristic of the fluid. In this study the only reason responsible for this change is the variation of the concentration of the foulant particles (i.e., the bentonite microparticles). Therefore, the distribution of the particles can be evaluated from the structural image (an example is given in Supporting Information Figure S-2). Similar observations were obtained in the work by Saarinen et al. studying the rheology of suspensions via OCT, and the concentration gradient near the wall is attributed to the shear-induced diffusion.35 In terms of the literature,36,37 flows of suspended particles could be induced by the gradient of the shear rate, and the direction of the particle migration is from the regions of a higher shear rate to the regions of a lower shear rate. At the steady state this shear14276
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induced particle flux is balanced by the counterflux driven by the concentration gradient. During membrane filtration a thicker concentration boundary layer could be generated near the membrane surface due to the net convective transfer of the particles toward the membrane surface. The presented results suggest that the OCT characterization can be potentially applied to investigate concentration polarization (CP) phenomena in membrane filtration, though further systematic studies are needed. As introduced in section 2.2, by using the image subtraction and pixel polarization, all of the structural images were digitally converted into binary images showing the cake layer isolated from the background. These binary images are of higher visualization quality that is more suitable for a quantitative analysis compared to the original structural images. Some characteristic binary images at different moments are displayed in Figure 3. Especially, the images of the cake layer (the white
hydrodynamic environment in which the flow is constantly laminated in the direction of the bulk flow. At each point of the membrane surface the fouling particles were deposited with the same probability, thereby giving rise to a uniform cake layer at all times. When combining all the binary images (more than 40 frames) in the order of the filtration time, a video clip was generated to demonstrate the dynamic fouling process in a more continuous fashion (Support Information Figure S-3). This characterization for visualizing a dynamic process is not available for most conventional characterization approaches. Although the video cannot be shown in the main part of this work due to the format limitation, the continuous variation of the cake layer can be quantitatively described by the relationship between the cake layer thickness and the filtration time. With the aid of the binary images, the thickness of the cake layer can be easily evaluated by counting the number of the white pixels in a line perpendicular to the membrane surface. In particular, the small fluctuations at different locations are accounted for by averaging the thickness of the cake layer over the entire cross-section. The average thickness of the cake layer is plotted as a function of the filtration time in Figure 4a. The variation of the permeate flux is shown in Figure 4b for a comparative study.
Figure 3. Binary images of the foulant layer in the channel without a spacer at different filtration times. The cake layer is denoted by the white pixels whereas the background is shown by the black pixels. The original structural images were scanned in the primary cross-section, and the posttreatment was implemented by using the self-developed Matlab codes.
Figure 4. Characteristic curves of the fouling in the channel without a spacer: (a) the average thickness of the cake layer as a function of the filtration time and (b) the water flux as a function of the filtration time. Experimental errors are reported as the standard deviation of at least two repeated measurements.
A simple way to describe the formation of a foulant layer is the classical cake filtration model stating that the rate of the particle deposition is approximately proportional to the flux normal to the membrane surface.38 In spite of the coupled effects of the CP and fouling during the FO filtration, a typical fouling process of cake filtration is indicated by both plots in Figure 4. During the initial filtration the permeate flux was
pixels) were partially truncated at both ends for convenience of observation; image rotation was also applied so as to reduce the effects of the inclination angle on the quantitative analysis. The images in Figure 3 evidently illustrate the uniform growth of the cake layer along the membrane surface. This is consistent with the result of the Doppler imaging indicating a 14277
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relatively high, thereby resulting in higher growth rates of the cake layer (i.e., the slope of the thickness−time curve in Figure 4a). As the cake layer was getting thicker, the hydraulic resistance of the cake layer increased. Therefore, a gradual reduction of the permeate flux is observed in Figure 4b, which in turn decreases the growth rate of the cake layer in the longterm filtration as indicated by Figure 4a. 3.2. Characterization of the Growth of a Fouling Layer in a Spacer-Filled Channel. In a realistic process of membrane filtration the channels of the membrane module are usually filled with netlike spacers, which could give rise to some secondary flows in the interstices. The formation of the cake layer in the spacer-filled channels could be significantly affected by the complex hydrodynamic environment. Therefore, it is of great interest to verify the ability of OCT to explore these coupled phenomena during membrane fouling. The flow patterns can be changed by varying the orientation of the spacer as revealed by previous studies.26,32 Instead of a full spectrum investigation, the diamond configuration of the spacer was employed in this work considering the fact that this spacer configuration has been extensively used in membrane modules.31,32 Specifically, all filaments of the spacer were inclined at an angle of 45° with respect to the direction of the bulk flow. The primary area scanned by the OCT was parallel to the bulk flow and sections two consecutive spacer knots as illustrated by the schematic in the upper left panel of Figure 5. The Doppler image scanned during the initial filtration is shown in Figure 5a. The profiles of the sectioned knots are indicated by the dashed curves. The membrane surface is slightly curved due to the uneven support of the filament mesh. As discussed in our prior work,26 only the velocity components parallel to the detection light beam can be measured by the OCT. For a complex flow pattern, the magnitudes of the velocity components cannot be evaluated due to limitations of the current OCT software. Instead, the relative values of these components (with respect to the maximum value in a scan) were obtained and indicated by the variation of the color intensity. The color map defined in our previous work assigned blue-colored elements to the regions with motions primarily away from the membrane (i.e., toward the detector) while redcolored elements for the regions with motions in the opposite direction (i.e., toward the membrane).26 In terms of this color map, it clearly shows that there were two pairs of eddies with opposite circulations attached to the upstream and downstream sides of the knots, respectively. The lower eddies created some regions with relatively high shear rates covering the membrane surface near the filaments. Nevertheless, there is a dark region between the upstream and downstream eddies indicating that the fluid motions were minimized in this region owing to the merging of the opposite circulations. During the fouling structural images were continuously scanned in the primary cross-section, and the initial and last images are displayed in Figure 5b,c, respectively. Similar gray gradients are found near the wall regions indicating the particle migrations induced by the gradients of shear rate. In contrast, more significant gray patterns are observed in the regions corresponding to the eddy-dominated areas in the Doppler image. It indicates that the eddies tended to move the particles into the noneddy regions. This is consistent with the particle migration induced by the shear difference; that is, the particles could be removed from the eddy regions in which the shear rate was relatively high. In addition, the centrifugal caused by the density difference between the particles (∼2400 kg/m3) and
Figure 5. OCT images of the fouling (2 g/L bentonite microparticles) in the channel with a spacer. All filaments of the spacer were 45° to the bulk flow (diamond configuration). The primary cross-section scanned by the OCT is parallel to the direction of the bulk flow as indicated by the red area in the upper left schematic. The images obtained in the primary cross-section include the following: (a) the Doppler image of the velocity field in the channel at t = 0 min, (b) the structural image in the channel at t = 0 min, and (c) the structural image in the channel at t = 60 min. The secondary cross-section scanned by the OCT was perpendicular to the direction of the bulk flow as indicated by the red area in the lower left schematic. The image obtained in the secondary cross-section is (d) the structural image in the channel at t = 60 min. The direction of the bulk flow is denoted by the arrow. The point of observation for the secondary cross-section is on the upstream side. The membrane surface is indicated by the dashed−dotted curve.
fluid might contribute to the removal of the foulant particles away from the eddy core.39 These effects synergistically created a highly varied concentration field of the particles in the channel, thereby giving rise to a nonuniform fouling, as can be seen in Figure 5c. The thickness of the fouling layer attains a maximum in the area where the region with a relatively high particle concentration coincides with the region with a relatively low shear rate. The distribution of the foulant particles in the transverse direction can be examined in the structural image in Figure 5d, which was scanned in the cross-section normal to the direction of the bulk flow. It clearly shows that the deposition is concentrated around the axial area with a bias to the right side. 14278
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this corner area was dominated by the lower eddy attached to the downstream filament, which generated a shear flow toward the membrane surface (i.e., the red band denotes the fluid motions away from the detector). As a result, more particles were carried into the dead corner by this stream, thereby giving rise to a higher growth rate in this region. While similar foulant entrapment effects by a feed spacer have been reported previously,31,40 the ability of OCT to provide both Doppler and structural images makes it possible to directly relate the foulant deposition distribution to the localized microhydrodynamic environment, such fine-scale information could assist in the design of improved spacers. All of these OCT images reveal that the thickness of the cake layer formed in the spacer-filled channel is highly varied in all directions. Therefore, the plot of the average thickness versus the filtration time in Figure 7a does not fully reflect the
In particular, this structural image is demonstrated in a way that the transverse cross-section is viewed from a point on the upstream side. Therefore, it is justified that the asymmetric distribution of the cake layer could result from the asymmetric configurations of the filaments, which are schematically illustrated in the lower left panel of Figure 5. Specifically, the fluid was accelerated when passing through the gap (on the left side) between the filament and the membrane surface, whereas the shear layer was kept away from the membrane surface by the filament touching the membrane surface (on the right side). More details about the dynamic process of the cake growth are given by the binary images in Figure 6. During the initial
Figure 7. Characteristic curves of the fouling in the channel with a spacer (diamond configuration): (a) the average thickness of the cake layer as a function of the filtration time and (b) the water flux as a function of the filtration time. The averaging of the cake layer is in the primary cross-section, thus not accounting for the variation of the thickness in the transverse directions. Experimental errors are reported as the standard deviation of at least two repeated measurements.
Figure 6. Binary images of the foulant layer in the channel with a spacer (diamond configuration) at different filtration times. The cake layer is denoted by the white pixels whereas the background is shown by the black pixels. The original structural images were scanned in the primary cross-section, and the posttreatment was implemented by using the self-developed Matlab codes.
variation of the total particle depositions on the membrane surface, since the averaging of the cake thickness does not account for the particle distributions in the transverse directions (i.e., the averaging is based on the binary images for the primary axial cross-section). Similar uneven and patch depositions have been observed in spacer-filled channels by a DOTM system.41 In comparison to the results for the case without a spacer, the highly nonuniform growth of the cake layer gave rise to higher filtration fluxes (steady state values of approximately 15 L/m2h versus 10 L/m2h without a spacer) during the fouling as indicated by Figure 7b, which in turn increased the growth rate of the cake layer especially during the initial fouling.
fouling the growth of the cake layer was relatively uniform as indicated by the binary image at t = 1 min. As the filtration time was increased, the deposition rate of the particles was varied through this cross-section. Most of the particles were deposited in the central region, and the cake layer was gradually enlarged. During the long-term fouling the deposition became more concentrated in the location slightly biased toward the upstream end. It is interesting to observe that preferential depositions also occurred at the downstream corner between the filament and the membrane surface, though this is not very clear in the structural images. As shown in the Doppler image, 14279
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4. IMPLICATIONS This study demonstrates that the growth of a fouling layer during membrane filtration can be observed noninvasively and in real time via the structural imaging of an OCT system. The depth profiles of the cake layer can be resolved in the structural images. When the process of the cake growth is recorded, the evolution of the morphology of the cake layer can be clearly demonstrated by a series of binary images. It is difficult for conventional approaches to achieve this real-time observation of the depth profiles. Moreover, the concentration field of the suspension particles can also be visualized by the structural imaging, which has great value in understanding the underlying fouling mechanisms. The ability to measure velocity profiles gives the OCT system extra benefit for characterizing membrane fouling. Without the need for additional independent measurements, Doppler images can be obtained simultaneously from the same interference signals. Therefore, the dependence of the fouling process on the microhydrodynamic environment can be analyzed by comparing the structural images with the Doppler images. This dual-function characterization brings a new perspective to the study of membrane fouling, and more possibilities can be explored in other applications concerning water technologies. Although this work demonstrates a successful example of characterizing a fouling process during membrane filtration via OCT, there must be some limitations of this advanced optical technique that were not investigated in the current study. For example, the response of OCT to a turbulent flow still remains unknown. This is mainly because the filtration experiments could not be implemented with higher cross-flow rates owing to the current observation cell. Studies to explore the limitations of the OCT-based characterization for membrane processes are of critical importance for adopting this novel technique in more applications.
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Environment and Water Industry Development Council of Singapore (EWI) and Nanyang Technological University.
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ASSOCIATED CONTENT
* Supporting Information S
Figure S-1 showing velocity profile in the channel without a spacer at t = 0 min (the values are normalized by the maximum), Figure S-2 showing a concentration profile in the channel without a spacer at t = 0 min (the values are normalized by the maximum), and Figure S-3 showing a video clip of a foulant layer in a channel without a spacer. This material is available free of charge via the Internet at http:// pubs.acs.org.
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REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*(W.L.) Tel.: +65 6592 7726; email:
[email protected]. *(C.Y.T.) email:
[email protected]. *(J.S.) email: juha.salmela@vtt.fi. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the Singapore Ministry of Education (Grant No. MOE2011-T2-2-035, ARC 3/12) for the financial support of the work. We are also thankful for the financial support from Tekesthe Finnish Funding Agency for Technology and Innovation through the FrontWater Programme. The Singapore Membrane Technology Centre is supported by both the 14280
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