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Application of magnetic nanoparticles for UF membrane integrity monitoring at low-pressure operation
Journal of Membrane Science 350 (2010) 172–179
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Application of magnetic nanoparticles for UF membrane integrity monitoring at low-pressure operation H. Guo a , Y. Wyart a , J. Perot b , F. Nauleau b , P. Moulin a,∗ a Université Paul Cézanne Aix Marseille, Laboratoire de Mécanique, Modélisation et Procédés Propres (M2P2 – UMR-CNRS 6181), Europôle de l’Arbois, BP. 80, Bâtiment Laennec, Hall C, 13545 Aix en Provence Cedex 04, France b SAUR, 1 Avenue Eugène Freyssinet, 78064 Saint Quentin en Yvelines Cedex, France
a r t i c l e
i n f o
Article history: Received 4 May 2009 Received in revised form 18 December 2009 Accepted 22 December 2009 Available online 28 December 2009 Keywords: Drinking water Integrity test Magnetic nanoparticles Magnetic susceptibility
a b s t r a c t An alternative ultrafiltration membrane integrity test is presented and evaluated, based on the use of magnetic nanoparticles (Fe3 O4 ) and measurement of magnetic susceptibility. The mean size of nanoparticles used is around 35 nm and they show a good disparity between 20 and 100 nm. A series of integrity tests were carried out on a Norit membrane module containing 100 fibers under a low transmembrane pressure of 0.25 bar. The results showed that no magnetic susceptibility was detected in permeate when the tests were performed on the intact module in both cross-flow and dead-end filtration, indicating the complete nanoparticle retention by the intact module. However, when even one fiber was broken in the module (1% breakage rate), magnetic susceptibility of permeate could be detected instantaneously even at feed concentrations as low as 1.2 ppm with Bartington magnetic susceptibility meter. This detection is valid during all the filtration process. The results also showed that the membrane permeability could be completely recovered after a backwash. This membrane integrity test, with the advantages of simplicity, on-line operation, high detection specificity and sensitivity, quick detection and very low influence on membrane fouling, seems to be suitable for large-scale drinking water plants. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The intact ultrafiltration (UF) membrane can substitute for the disinfection step in drinking water treatment because of its complete removal of pathogens, such as Giardia, Cryptospodidium and viruses. For this purpose, continuous integrity monitoring of the UF membrane system is fundamental during the operation of drinking water treatment plants. As part of the regulatory system, water companies installing membrane filtration have to provide proposals detailing the associated level of integrity monitoring [1]. Different UF membrane integrity monitoring techniques, including direct methods and indirect methods, have been reported [2–4]. Among these methods, the PDT and DAF test are the most frequently used and they are considered to be simple, non-destructive and reliable methods. However, during practical operation, both methods are conducted off-line and they are not sufficient, for operational and economic reasons, to meet removal requirement of viruses (∼20 nm). In contrast, all the indirect methods can monitor UF membrane integrity on-line. However, conventional indirect methods such as particle counter and turbidity monitoring show low detection sensitivity and they cannot guarantee the regulatory
∗ Corresponding author. Tel.: +33 4 42 90 85 01; fax: +33 4 42 90 85 15. E-mail address: [email protected] (P. Moulin). 0376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2009.12.025
requirements for pathogen removal [5]. In addition, as presented by Deluhery and Rajagopalan [6], both particle counter and turbidity monitoring suffer from non-specificity of the particles detected, i.e. they measure all the particles related to a certain size range. Pathogen removal in terms of particle counter or turbidity is thus questionable. Other different membrane integrity methods [7–10] (e.g. acoustic sensor method, liquid–liquid porosimetry, binary gas integrity test and new membrane-based sensor) also have their disadvantages, e.g. off-line operation for liquid–liquid porosimetry and binary gas integrity test. Membrane integrity test frequency needs to be related to a risk assessment as the first priority and operational practicality as the second priority. Given an expected number of fiber breaks per year and a Log Removal Value (LRV) target, the frequency of membrane integrity tests can be calculated based on a simple mode or a statistical model. Membrane integrity tests can be implemented once a day, once 72 h or longer, or after cleaning step. It was summarized that, in most cases, a weekly test is required to ensure a LRV more than 4.5. For Bristol Water, a PDT frequency of once every week to once every other week is recommended to provide a margin of safety for the target of 4 LRV [2]. Therefore, it remains urgent to develop an on-line detection technique for reliable, quick, simple, and relatively inexpensive evaluation of integrity of the UF membrane system in drinking water plants. Recently, surrogate challenge tests for this purpose have attracted great attention [6,11–16].
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As alternative on-line methods, surrogate challenge tests can improve the detection sensitivity of conventional indirect integrity tests and are able to detect nanometric membrane breaches with nanoscale surrogates, although they cannot offer a real-time integrity monitoring. Sakaji [11] reported that maintaining membrane integrity monitoring with challenge tests can achieve a high removal of 5 or 6 LRV. For a surrogate challenge test, a certain concentration of surrogates is spiked into the membrane feed solution to simulate the retention characteristics of pathogens by a membrane. The key point of this test is to search for a suitable surrogate and corresponding measurement method. Multiple criteria have to be considered for the choice of surrogate, such as well defined size, easily detectable, non-destructive, reasonable price, representative of pathogen retention characteristics. The accuracy and the minimum detection level of the measurement method and the surrogate size will directly affect the detection sensitivity. So far, reported surrogate materials include powdered activated carbon (PAC), bacteriophage and oocyst, gold nanoparticles, magnetic particles, etc. [12–16]. For drinking water treatment, the ultimate integrity tests are microbial challenge tests. However, these tests are time-consuming for microbial analysis and may result in additional problems, such as membrane biofouling or penetration of bacteria into permeate in case of compromised integrity. Additionally, microbial monitoring may not correctly characterize the membrane integrity since bacterial re-growth may take place in the permeate piping system. As for other reported surrogate challenge tests, Spiked Integrity Monitoring system-SIMTM [12] offers high detection sensitivity but it is non-specific, which is generally analyzed with particle counters or turbidity monitors. In addition, due to the utilization of high concentration of PAC in this test, the PAC particles may occur in the permeate but it cannot be removed by a backwash due to the module compacity. Gold nanoparticle challenge tests [13] can detect small holes with average diameter of 20 nm when using anodic stripping voltammetry (ASV) as detection method. ASV has a low minimum detection level but it needs a prolonged run time or the addition of some other metal to resolve this problem. To overcome the drawback of non-specificity and thus minimize the background noise, alternative challenge tests using magnetic susceptible particles for membrane integrity monitoring are proposed to improve both specificity and sensitivity [14,15]. The principle of magnetic particle challenge tests is based on the injection of a certain concentration of magnetic particles in the feed and the detection of the material magnetic property in the filtrate. Particles occur in the filtrate and are detected by an appropriate magnetic sensor, indicating the presence of compromised membranes. The size of used particles depends on the required absolute size removal. Generally, magnetic sensors include superconducting quantum interference device (SQUID) magnetometer, giant magneto-resistive (GMR) sensors, measurement of magnetic permeability, etc. [15]. Deluhery and Rajagopalan [6] conducted membrane integrity challenge tests with the magnetic particle size of 1 m diameter. They proposed using a magnetic field to concentrate the particles in the filtrate for measuring transverse relaxation time (T2) with a magnetorelaxometer, which will result in enhancement of the detection limit value or feasibility of smaller and less powerful sensors. However, concentrating particles with an external magnetic field is not so feasible for practical operation, especially for large-scale drinking water treatment plants. Additionally, particles with 1 m diameter are not sufficient to detect virus-scale breaches in the membrane system. As a result, it is vital to improve magnetic sensor and surrogate size for the magnetic particle challenge test. Moulin [14] was the first to develop a new integrity test with magnetic nanoparticles without a concentration step to determine the particle concentration in the permeate side. In addition to simplifying the nanoparticle detection in permeate side, this test also
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improves the size of nanoparticles used in magnetic particle challenge tests. In this paper, this new UF membrane integrity test is presented and evaluated [14], based on the use of magnetic nanoparticles (Fe3 O4 ) and measurement of magnetic susceptibility. This is the first time that it is proposed to measure magnetic susceptibility with a magnetic susceptibility meter for membrane integrity monitoring. The minimum detection level of this detection method was investigated. To evaluate the efficiency of this new membrane integrity test, a series of lab-scale UF experiments were carried out with different concentrations of feed suspension, with different operation modes (cross-flow filtration or dead-end filtration) and with different broken fiber locations. This new surrogate challenge test for UF membrane integrity monitoring is revealed as a plausible solution for large-scale applications with the advantages of simplicity, on-line operation, accurate and quick detection, high specificity, high sensitivity and very low influence on membrane fouling. 2. Materials and methods 2.1. Preparation of magnetic nanoparticles Magnetite (Fe3 O4 ) is preferred for magnetic nanoparticle challenge tests because it is highly magnetic susceptible, non-toxic [17–19] and can be obtained easily. Here, Fe3 O4 magnetic nanoparticles were prepared by the chemical co-precipitation of ferrite (Fe3+ ) and ferrous (Fe2+ ) salts with the molar ratio of 2:1 using ammonium hydroxide (NH4 OH) as precipitating agent [20]. The chemical reaction is 2Fe3+ + Fe2+ + 8OH− ⇔ Fe(OH)2 + 2Fe(OH)3 → Fe3 O4 + 4H2 O To avoid particle aggregation or adhesion, the synthesized Fe3 O4 nanoparticles were stabilized with meso-2,3-dimercapto-succinic acid (DMSA) [21]. After the magnetic nanoparticle suspension was acidified slowly with 0.05 M HCl to pH = 2, the DMSA was added and stirred for 30 min. The ratio of added DMSA to [Fe] is 7%. Then, the pH value of the suspension was adjusted from 2 to 11, and from 11 to 7 by slowly adding 0.05 M NaOH and 0.05 M HCl, respectively. 2.2. Characterization of magnetic nanoparticles The prepared Fe3 O4 nanoparticles were characterized in terms of size distribution, magnetic susceptibility and concentration. The nanoparticle size distribution was determined by a Malvern Zetasizer granulometer (ZEN 1600 Nano S, Malvern Instrument, UK), based on dynamic light scattering measurement. The magnetic susceptibility of Fe3 O4 nanoparticles was measured by means of a Bartington magnetic susceptibility meter equipped with a MS2B sensor (Bartington Instrument, UK) and an AGICO Kappabridge magnetic susceptibility meter (MFK1-FA, AGICO Instrument, Czech Republic). The Fe3 O4 concentration was calculated from Fe concentration, which was measured by 6715 UV–vis spectrophotometer (JENWAY instrument, UK) [22,23] and was determined according to the calibration curve of absorbance versus Fe concentration at a wavelength of 508 nm. 2.3. Preliminary magnetic susceptibility tests To efficiently monitor UF membrane integrity, the method of detecting magnetic nanoparticles should provide advantages of accuracy, simplicity, sensitivity, low detection limit, specificity, quickness, wide analytical range, instrumental availability, affordable price and possibility of on-site detection. In this paper, Fe3 O4 nanoparticles were detected in terms of magnetic susceptibil-
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Fig. 1. Experimental set-up for UF membrane integrity tests using Fe3 O4 nanoparticles: (a) cross-flow filtration and (b) dead-end filtration.
ity. Magnetic susceptibility measurement using the Bartington instrument (portable apparatus) is a non-destructive, simple and cost effective method for determining the presence of magnetic nanoparticles. This instrument can display values from 0.1 to 9999 (e.g. (0.1–9999) × 10−5 SI unit for volume magnetic susceptibility), providing a very wide analytical range for detecting magnetic material. Generally, the response time for measuring a sample is 2 s or 12 s, depending on the selected measurement range. In contrast, AGICO Kappabridge magnetic susceptibility meter have the same advantages of a very wide analytical range and rapid detection. Moreover, it offers more detection sensitivity (the minimum detectable value is 0.001 × 10−6 SI unit). To investigate the specificity and sensitivity of this method, different concentrations of FeCl3 solution were measured. The stabilized Fe3 O4 nanoparticles were diluted with tap water, tap water containing 0.2 g L−1 bentonite, and natural water (lake of Réaltor, Bouches du Rhone, France). All these diluted suspensions were analyzed by the magnetic susceptibility meter to obtain the detection limit value for Fe3 O4 nanoparticles. The nanoparticle magnetic susceptibility was obtained by substracting magnetic susceptibility of water from that of water and nanoparticles.
2.4. UF membrane and integrity tests A Norit membrane module of 100 fibers (area = 0.0628 m2 ), whose nominal cut-off is 150 kDa and fiber diameter is 0.8 mm, was used in this study. The nominal pore size of this membrane is about 25 nm. Generally, Fe3 O4 nanoparticles used for membrane integrity tests should be larger than the membrane pore size so that they can be retained completely by the intact membrane. The Norit membrane module was rinsed with distilled water for 1 h and then was characterized by water permeability measurement prior to membrane integrity tests. Here, membrane integrity tests included magnetic nanoparticles challenge tests with both the intact membrane and the impaired membrane (one broken fiber). All these tests were conducted under a constant low transmembrane pressure (0.25 bar). The magnetic susceptibility in the permeate was measured to investigate whether nanoparticles passed through the membrane. Here, the permeate samples were taken every 2 min at the beginning and then were taken every 10 min. The volume of every sample is about 30 mL (10 s). After each test, the membrane module was well rinsed with distilled water and then was back-washed using distilled water at 2 bars. Finally, the water permeability of the membrane was measured again to determine whether the membrane permeability has been recovered. These lab-scale exper-
iments were carried out to evaluate whether the signal for the module with or without a broken fiber depends on different types of filtration used (dead-end filtration and cross-flow filtration) and the location of the broken fiber in the module. Additionally, the impaired membrane was challenged with different concentrations of nanoparticle suspension to investigate the minimum nanoparticle concentration required to efficiently detect the broken fiber. The experimental set-up is shown in Fig. 1. It should be noted that the materials in the pilot will not interfere with our proposed integrity test because there is no Fe pipe or some magnetic parts in the pilot. The magnetic detector used here only identifies the magnetic nanoparticles used. 3. Results and discussion 3.1. Characterization of magnetic nanoparticles Using chemical co-precipitation proved here to be a simple and economical method for magnetic nanoparticle synthesis. As shown in Fig. 2, these nanoparticles show a good size distribution in the size range of 20–100 nm. The average size of stabilized nanoparticles is around 35 nm, larger than the nominal pore size of the membrane used. Assuming that a UF membrane retains nanoparticles only by a size-exclusion mechanism regardless of surface chemistry and other characteristics, these stabilized nanoparticles can therefore be retained by an intact membrane. Otherwise, these
Fig. 2. Size and undersize distribution by number of stabilized nanoparticles.
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in response to an applied magnetic field. For samples with constant volume, it increases with the magnetic nanoparticle concentration. Results shown in Fig. 3 demonstrate that the magnetic susceptibility can accurately indicate nanoparticle content. As a result, magnetic susceptibility can perform well as an indicator of Fe3 O4 nanoparticle concentration. 3.2. Preliminary magnetic susceptibility tests
Fig. 3. Function of volume magnetic susceptibility as Fe3 O4 nanoparticle concentration.
nanoparticles will pass through the membrane in the presence of nanometric breaches larger than the nanoparticle size. In addition, the number of these nanoparticles is ∼2.7 × 1014 particles g−1 . Calculating from the measured Fe concentration, the Fe3 O4 concentration after stabilization is approximately 1.2 g L−1 in this study. This nanoparticle suspension was diluted by a factor of from 10 to 5000. Plotting of magnetic susceptibility as a function of nanoparticle concentration shows a linear relationship (Fig. 3). Magnetic susceptibility is the degree of magnetization of materials
In this paper, the magnetic susceptibility was measured mainly with the Bartington instrument. Firstly, magnetic susceptibility of FeCl3 solutions was investigated. The aim was to examine whether the presence of FeCl3 , which is generally used as a coagulant in drinking water plants, will affect the detection of Fe3 O4 nanoparticles by magnetic susceptibility measurement. The magnetic susceptibility of different concentrations of FeCl3 solution is shown in Table 1. It can be seen that FeCl3 has a low magnetic susceptibility and cannot be detected by the magnetic susceptibility meter at concentrations as high as 300 ppm. The detection limit of this method for FeCl3 solution is close to 3000 ppm, whose volume magnetic susceptibility is 0.2 × 10−5 SI unit. Based on these results, and given that the residual FeCl3 concentration in drinking water is lower than150 ppm, it can be deduced that the presence of residual FeCl3 in drinking water system will not affect the detection of magnetic nanoparticles using this method. In contrast, the detection limit of magnetic susceptibility for Fe3 O4 nanoparticles is low, as shown in Table 2. It can be seen that the magnetic sus-
Table 1 Magnetic susceptibilities of FeCl3 solutions. Concentration (ppm) 300 3000 30,000 300,000
Volume magnetic susceptibility k (SI unit, containing tap water)
Volume magnetic susceptibility k (SI unit)
−0.9E−05 −0.7E−05 2.5E−05 34.2E−05
0.0E+00 0.2E−05 3.4E−05 35.1E−05
FeCl3 solutions were diluted with tap water, k (tap water) = −0.9E−5. Table 2 Magnetic susceptibilities of Fe3 O4 suspensions (a). Diluted times
10 100 500 1000 2500 4000 5000
Concentration (ppm)
119.6 11.96 2.392 1.196 0.4784 0.299 0.2392
Volume magnetic susceptibility k (SI unit) Diluted with tap water containing 0.2 g L−1 bentonite
Diluted with tap water
k (containing tap water)
k
k (containing tap water)
k
45.8E−05 4.2E−05 0.0E+00 −0.5E−05 −0.7E−05 −0.8E−05 −0.9E−05
46.7E−05 5.1E−05 0.9E−05 0.4E−05 0.2E−05 0.1E−05 0.0E+00
45.8E−05 3.9E−05 −0.1E−05 −0.5E−05 −0.7E−05 −0.8E−05 −0.9E−05
46.7E−05 4.8E−05 0.8E−05 0.4E−05 0.2E−05 0.1E−05 0.0E+00
Initial concentration is 1.196 g L−1 , k (tap water) = −0.9E−5 SI unit, k (tap water containing 0.2 g L−1 bentonite) = −0.9E−5 SI unit. Table 3 Magnetic susceptibilities of Fe3 O4 suspensions (b). Diluted times
Concentration (ppm)
Volume magnetic susceptibility k (SI unit) Diluted with tap water
100 1000 2000 4000 6000 7000 8000
12.69 1.269 0.6345 0.3173 0.2115 0.1813 0.1586
Diluted with canal water
k (containing tap water)
k
k (containing canal water)
k
0.2E−05 −0.4E−05 −0.6E+00 −0.7E−05 −0.7E−05 −0.8E−05 −0.9E−05
1.1E−05 0.5E−05 0.3E−05 0.2E−05 0.2E−05 0.1E−05 0.0E+00
0.3E−05 −0.3E−05 −0.6E−05 −0.7E−05 −0.7E−05 −0.8E−05 −0.9E−05
1.2E−05 0.6E−05 0.3E−05 0.2E−05 0.2E−05 0.1E−05 0.0E+00
Initial concentration is 1.269 g L−1 , k (tap water) = −0.9E−5 SI unit, k (canal water) = −0.9E−5 SI unit.
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Table 4 Magnetic susceptibilities of Fe3 O4 suspensions (c). Diluted times 10 100 1000 10,000 20,000 40,000 60,000 80,000 100,000 200,000 500,000
Concentration (ppm)
Volume magnetic susceptibility (SI unit, containing tap water)
Volume magnetic susceptibility k (SI unit)
9572.7 957.27 95.727 9.5727 4.78635 2.393175 1.59545 1.1965875 0.95727 0.478635 0.191454
2.27E−02 2.16E−03 2.03E−04 1.50E−05 3.00E−06 −3.00E−06 −5.00E−06 −5.00E−06 −6.00E−06 −7.00E−06 −7.00E−06
2.27E−02 2.17E−03 2.12E−04 2.40E−05 1.20E−05 6.00E−06 4.00E−06 4.00E−06 3.00E−06 2.00E−06 2.00E−06
Initial concentration is 95.727 g L−1 , diluted with tap water, k (tap water) = −0.9E−5 SI unit.
ceptibility of Fe3 O4 nanoparticle suspensions is independent of the presence of bentonite. Similar results are also obtained when diluted with natural water (Table 3). This indicates that there is no influence of suspended materials or residual coagulant on the magnetic susceptibility of Fe3 O4 nanoparticles. The minimum volume magnetic susceptibility of 0.1 × 10−5 SI unit is obtained when the Fe3 O4 concentration is about 0.2–0.3 ppm, i.e. the detection limit of this method for Fe3 O4 nanoparticles is about 0.2–0.3 ppm when using the Bartington apparatus. The above results indicate that measuring magnetic susceptibility has both high specificity and high sensitivity for detecting Fe3 O4 nanoparticles. When a higher concentration of nanoparticle suspension (about 95 g L−1 ) is used, the detection was effective for a dilution factor of 500,000 with the portable analyzer (Bartington magnetic susceptimeter with MS2B sensor, see Table 4). With the more sensitive apparatus (Kappabridge MFK1-FA model), the dilution factor can be multiplied by a factor of more than 10. This means that a broken fiber in a module containing 500,000 hollow fibers can be detected with the MS2B sensor. With the Kappabridge MFK1-FA sensor, one broken fiber in a module containing 5,000,000 hollow fibers can be detected (if it is considered that the flow rate of a broken hollow fiber is equal to the permeate flow of one intact hollow fiber).
is continuous during all the filtration process (1 h). Considering the fouling phenomenon, this injection mode is the worst operational condition. In the case of cross-flow filtration, the permeate flux kept constant because the nanoparticle concentration in the lumen of the hollow fiber was almost constant. In contrast, after 1 h of deadend filtration, the nanoparticles in the lumen of the hollow fibers were concentrated by a factor around 700. This increase of nanoparticle concentration on the surface of the membrane explained the decrease of the J/J0 ratio. As a result, for membrane drinking water plants, the integrity test (i.e. the injection of nanoparticles) should be realized at the end of the filtration step (before the backwash) to reduce the membrane fouling.
3.3. Magnetic nanoparticle challenge tests for UF membrane integrity monitoring The number of fibers in the UF module (100) is lower than the detection sensitivity limit (4000) of an MS2B sensor, indicating that it is possible to detect broken fibers with the MS2B sensor. To assess nanoparticle retention performance by the intact membrane, the first set of UF experiments was carried out on the intact membrane in both cross-flow filtration and dead-end filtration. Here, feed nanoparticle concentration was about 12 ppm. The changes of relative flux J/J0 , the ratio of permeate flux of nanoparticle suspension to that of pure water, and magnetic susceptibility in permeate were investigated during the filtration process, as shown in Figs. 4 and 5. No magnetic susceptibility was detected in the permeate whatever the filtration mode (both cross-flow and dead-end filtration), indicating there is no magnetic nanoparticles passing through the intact membrane. This result is consistent with above size analysis that nanoparticles can be retained by the intact membrane due to the size-exclusion mechanism. On the whole, the permeate flux decreased with the operation time for both two filtration types but not to great extent. The overall decline of relative flux J/J0 was very small, about 13% and 22% for cross-flow and dead-end filtration, respectively. For cross-flow filtration, the permeate flux tended to be constant after 10 min of operation. It should be noted here that the SDI15 of tap water used in these tests was 2.0 ± 0.2, which also greatly contributes to these J/J0 decrease. In this study, the nanoparticles were added in the tank (Fig. 1), i.e. the nanoparticle injection
Fig. 4. Variation of relative flux J/J0 and magnetic susceptibility in the permeate with the filtration time: cross-flow filtration on the intact membrane, feed concentration 12 ppm, TMP 0.25 bar, cross-flow velocity 0.1 m s−1 , SDI15,water = 2.0 ± 0.2.
Fig. 5. Variation of relative flux J/J0 and magnetic susceptibility in the permeate with the filtration time: dead-end filtration on the intact membrane, feed concentration 12 ppm, TMP 0.25 bar, SDI15,water = 2.0 ± 0.2.
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Fig. 6. Variation of relative flux J/J0 with the filtration time: cross-flow filtration on the impaired membrane (one broken fiber close to the inlet part of the module), TMP 0.25 bar, cross-flow velocity 0.1 m s−1 , SDI15,water = 2.0 ± 0.2.
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Fig. 8. Variation of relative flux J/J0 with the filtration time: dead-end filtration on the impaired membrane (one broken fiber close to the outlet part of the module), TMP 0.25 bar, SDI15,water = 2.0 ± 0.2.
Then, a series of integrity tests were challenged by different concentrations of nanoparticle suspension in the presence of one broken fiber in the module (1% breakage rate). With one broken fiber in the module, the membrane permeability increases about 4% of that of the intact module. To demonstrate the efficiency of this method for membrane integrity monitoring, magnetic susceptibility in the permeate was measured during the operation as well as permeate flux. Firstly, membrane integrity tests were performed in crossflow mode when the broken fiber was near the inlet part of the module (Figs. 6 and 7). Fig. 7 shows that magnetic susceptibility in the permeate was detected when the impaired membrane module was challenged by 12, 2.4 and 1.2 ppm nanoparticle suspensions, indicating that there were nanoparticles passing through the membrane due to the presence of the broken fiber. Furthermore, the magnetic susceptibility in the permeate increased with the increasing feed concentration. The minimum volume magnetic susceptibility value of 0.1 × 10−5 SI unit was obtained at feed concentrations as low as 1.2 ppm. This means that 1.2 ppm nanoparticle feed concentration is the detection limit of the Bartington apparatus to detect one broken fiber out of 100 fibers. It should be noted from Fig. 7 that nanoparticles passing through the broken membrane can be detected rapidly by means of the magnetic susceptibility measurement, which depends on the dead volume of the
pilot. Within the detection limit range of the magnetic susceptibility meter, they can be detected immediately if nanoparticles pass through the membrane whatever the feed concentration. On the other hand, as shown in Fig. 6, changes of permeate flux during the filtration exhibited the same tendency for these three feed concentrations. The permeate flux decreased in the first 10 min and then tended to be constant. On the whole, the decline of the permeate flux was smooth and small, especially when the feed concentration was 1.2 ppm. The permeate flux decreased with the increasing feed concentration. The higher concentration of nanoparticles injected in the feed side was, the more the permeate flux decreased. When the same tests were implemented just by changing the location of broken fiber close to the outlet part of the module, similar results were obtained. This implied that the sensitivity of this test for UF membrane integrity monitoring is independent of the location of the broken fiber. The magnetic nanoparticle challenge tests were also conducted with dead-end filtration. They offered the same results as crossflow filtration when the impaired membrane was challenged by 2.4 and 1.2 ppm nanoparticle suspensions (Figs. 8 and 9), indicating the sensitivity of this membrane integrity test is independent of membrane operational modes. All above challenge tests exhibited an interesting phenomenon about changes of permeate flux during the filtration. The permeate flux decreased to a small extent and it tended to be constant rapidly. Even when the membrane challenge tests were operated
Fig. 7. Variation of magnetic susceptibility in the permeate with the filtration time: cross-flow filtration on the impaired membrane (one broken fiber close to the inlet part of the module), TMP 0.25 bar, cross-flow velocity 0.1 m s−1 .
Fig. 9. Variation of magnetic susceptibility in the permeate with the filtration time: dead-end filtration on the impaired membrane (one broken fiber close to the outlet part of the module), TMP 0.25 bar.
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Fig. 10. Comparison of membrane water permeability before and after test: deadend filtration on the intact membrane, feed concentration 12 ppm, TMP 0.25 bar.
in dead-end filtration, as shown in Fig. 8, the final relative flux J/J0 still kept more than 85% after 1 h of filtration (overall duration of a filtration step in a drinking water plant). Here, at the end of the dead-end filtration the volume concentration factor in the membrane is about 700, which explains the decrease of the relative flux (15%). However, these low feed concentrations used here (1.2–12 ppm) showed to be sensitive enough to detect the presence of 1% broken fiber in the module. Moreover, no irreversible membrane fouling also can be demonstrated from the measurements of membrane permeability recovery after each test, as shown in Fig. 10 (the worst operational condition). It can be seen that the water permeability of the membrane after a backwash was closed to the initial permeability, indicating that there is no irreversible membrane fouling caused by the nanoparticle filtration. 3.4. Discussion The above-described experiments demonstrate that the magnetic nanoparticle challenge test combined with magnetic susceptibility measurement is sufficiently sensitive for UF membrane integrity monitoring. Fe3 O4 nanoparticles in the size range of 20–100 nm exhibit superparamagnetic properties, i.e. they have a high magnetic susceptibility in the presence of an external magnetic field, thus resulting in low detection limit for a magnetic sensor. Here, Fe3 O4 nanoparticle detection with the magnetic susceptibility meter has the advantages of simplicity, speed and accuracy of the analysis, low cost of the instrumentation, detection specificity and low detection limit (see Sections 3.1 and 3.2). This detection method is helpful to reduce the impact of background noise and thus can improve the detection sensitivity. The high detection sensitivity of the magnetic susceptibility measurement (available to detect 500,000 times or more diluted suspension) is significant, which provides a strong basis for the development of an on-line sensitive membrane integrity test when using magnetic nanoparticles as surrogates. Compared with the magnetically susceptible particle method proposed by Deluhery and Rajagopalan [6], the main advantage of the method proposed here is that the permeate sample is directly analyzed by the magnetic susceptibility meter without using a nanoparticle collector to enhance the detection efficiency. This simplifies the operational procedures, facilitates the applications in large-scale drinking water treatment plants and gives information quickly. The experimental results presented in Section 3.3 showed that the magnetic susceptibility measurement was sensitive sufficiently to detect the presence of 1% fiber breakage rate in the membrane system even without a nanoparticle collector and with
very low nanoparticle concentration. Generally, a high feed concentration is recommended for most on-line indirect membrane integrity tests, such as particle counter, turbidity monitoring and Spiked integrity monitoring system-SIMTM , because the detection sensitivity of these methods increases with the increasing feed concentration. However, the membrane fouling will be more severe with the higher nanoparticle concentration mainly due to the membrane pore blockage, especially for the nanoparticles with smaller sizes [24]. In this paper, as shown in Section 3.3, the magnetic nanoparticle suspension with very low concentration (1.2 ppm) was sensitive enough to detect the presence of one broken fiber in the module (1% breakage rate) even when the membrane system was operated at low transmembrane pressure (0.2–0.3 bar), which contributed to very low reversible membrane fouling. Additionally, compared with other surrogate challenge tests (e.g. microbial and gold nanoparticle challenge tests), this new test shows much quicker detection for impaired membrane. Within the detection limit range of the magnetic susceptibility meter, the response time depends on the dead volume and flux of the pilot. Theoretically, in the event of a impaired membrane, the quantity of particles in the permeate depends on the magnitude of the leakage flow and the transmission ratio of the test particles. So, the nanoparticle quantity in the permeate depends on parameters such as transmembrane pressure, number of membrane breakages, the size ration of nanoparticles to membrane breakage. It maybe also depends on some other system parameters. In this paper, due to the high size ratio of membrane breakage to nanoparticles and high sensitivity of the detection method, nanoparticle quantity in the permeate in terms of magnetic susceptibility is independent of operational mode and location of the broken fiber under the condition of the same number of broken fibers in the same module (see Section 3.3), which is different from the results obtained by Johnson and MacCormick [25]. Based on the experimental results in this paper, this on-line magnetic nanoparticle challenge test for UF membrane integrity monitoring seems to be suitable for large-scale applications. Overmore, this test is proved to be economical for practical operation, e.g. (i) the cost of per gram of nanoparticles is about 3.3 D which is constant whatever the concentration of the nanoparticle suspension, (ii) the cost of nanoparticles needed for per m2 membrane based on an integrity test realized on an industrial plant (35 m2 ) is about 0.12 D and (iii) the analytical apparatus is low cost (about 5000 D for the Bartington apparatus), further work on larger scale testing should be done to validate this method. Additionally, this method can be extended to determine the membrane MWCO if monodisperse nanoparticles can be prepared.
4. Conclusions Alternative magnetic nanoparticle challenge tests for monitoring UF membrane integrity, based on the magnetic susceptibility measurement with the Bartington magnetic susceptibility meter, were evaluated in this paper. Magnetic susceptibility performs as a good indicator for magnetic nanoparticle concentration. No magnetic susceptibility was detected in the permeate when the challenge test was performed on the intact membrane, indicating the complete retention of nanopaticles by the membrane. In contrast, the compromised membrane module with one broken fiber (1% breakage rate) can be successfully detected by means of magnetic susceptibility measurement in the permeate even at feed concentrations as low as 1.2 ppm. The sensitivity of membrane integrity detection using this method is independent of operational mode (dead-end and cross-flow filtration) and location of the broken fiber. Additionally, all these experimental results show that there is very low reversible membrane fouling when
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the UF membrane system is operated with low feed concentrations (1.2–12 ppm) at a low transmembrane pressure (0.2–0.3 bar). This membrane integrity test, with the advantages of on-line operation, high sensibility, detection specificity and very low influence on membrane fouling, seems to be suitable for large scale drinking water treatments. Using the surrogates with nanometric size, this method makes it possible for UF membrane system to better satisfy the regulatory requirements of the drinking water industry. References [1] H. Guo, Y. Wyart, J. Perot, F. Nauleau, P. Moulin, Low-pressure membrane integrity tests for drinking water treatment: a review, Water research 44 (2010) 41–57. [2] A. Bennett, Maintaining the integrity of filtration systems, Filtration & separation 42 (2005) 30–33. [3] W.T. Johnson, Automatic monitoring of membrane integrity in microfiltration systems, Desalination 113 (1997) 303–307. [4] S. Panglisch, U. Deinert, W. Dautzenberg, O. Kiepke, R. Gimbel, Monitoring the integrity of capillary membranes by particle counters, Desalination 119 (1998) 65–72. [5] M.E. Walsh, M.P. Chaulk, G.A. Gagnon, Indirect integrity testing on a pilot-scale UF membrane, Journal of Water Supply: Research and Technology. AQUA 54 (2005) 105–114. [6] J. Deluhery, N. Rajagopalan, Use of paramagnetic particles in membrane integrity testing, Journal of Membrane Science 318 (2008) 176–181. [7] J.M. Laîne, K. Glucina, M. Chamant, P. Simonie, Acoustic sensor: a novel technique for low pressure membrane integrity monitoring, Desalination 119 (1998) 73–77. [8] S. Giglia, M. Krishnan, High sensitivity binary gas integrity test for membrane filters, Journal of Membrane Science 323 (2008) 60–66. [9] A. DiLeo, M. Phillips, Process for evaluating solute retention characteristics of membranes, US Patent 5,457,986 (1995). [10] J. Phattaranawik, A.G. Fane, F.S. Wong, Novel membrane-based sensor for online membrane integrity monitoring, Journal of Membrane Science 323 (2008) 113–124.
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[11] E.R. Sakaji, California surface water treatment: alternative filtration technology, CA Dept of Health Service, Demonstration Report, June 2001. [12] S.C.J.M. Van Hoof, L. Broens, A. Nahrstedt, S. Panglisch, R. Gimbel, G. Hagmeyer, R. Gimbel, Development of a new testing system, in: Membrane Technology Conference, San Antonio, March, 2001. [13] V. Gitis, R.C. Haught, R.M. Clark, J. Gun, O. Lev, Nanoscale probes for the evaluation of the integrity of ultrafiltration membranes, Journal of Membrane Science 276 (2006) 199–207. [14] P. Moulin, Patent Number(s): FR2901607-A1; EP1862791-A2, 2008. [15] N. Rajagopalan, T. Rusk, R. Sanford, Methods and systems for membrane testing, US Patent 7,011,758 (2006). [16] K. Farahbakhsh, S.S. Adham, D.W. Smith, Monitoring the integrity of lowpressure membranes, Journal of American Water Works Association 95 (2003) 95–107. [17] M. Auffan, J. Rose, M.R. Wiesner, J.-Y. Bottero, Chemical stability of metallic nanoparticles: a parameter controlling their potential cellular toxicity in vitro, Environmental Pollution 157 (2009) 1127–1133. [18] M. Auffan, L. Decome, J. Rose, T. Orsiere, M. Meo, V. Briois, C. Chaneac, L. Olivi, J.L. Berge-Lefranc, A. Botta, M.R. Wiesner, J.Y. Bottero, In vitro interactions between DMSA-Coated maghemite nanoparticles and human fibroblasts: a physicochemical and cyto-genotoxical study, Environmental Science Technology 40 (2006) 4367–4373. [19] J. Aaseth, Recent advance in the therapy of metal poisonings with chelating agents, Human & Experimental Toxicology 2 (1983) 257–272. [20] http://mrsec.wisc.edu/edetc/nanolab/ffexp/index.html, Procedure modified from J. Chem. Educ., 76, 943–948 (1999) by Jonathan Breitzer and George. [21] N. Fauconnier, J.N. Pons, J. Roger, A. Bee, Thiolation of maghemite nanoparticles by dimercaptosuccinic acid, Journal of Colloid and Interface Science 194 (1997) 427–433. [22] A.I. Vogel, A Textbook of Quantitative Inorganic Analysis Including Elementary Instrumental Analysis, 3rd ed., Longman, London, 1961. [23] S. Mornet, Synthesis and Chemical Modification of Maghemite Nanoparticles for Medical Application, University of Bordeaux, 2002. [24] P. Lipp, U. Müller, B. Hetzer, T. Wagner, Characterization of particulate fouling and breakthrough during low pressure membrane filtration, in: Membranes in Drinking Water Production and Waste Water Treatment, 2008, Toulouse. [25] W. Johnson, T. MacCormick, Issues of operational integrity in membrane drinking water plants, Water Science and Technology: Water Supply 3 (2003) 73–80.
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Application of magnetic nanoparticles for UF membrane integrity monitoring at low-pressure operation H. Guo a , Y. Wyart a , J. Perot b , F. Nauleau b , P. Moulin a,∗ a Université Paul Cézanne Aix Marseille, Laboratoire de Mécanique, Modélisation et Procédés Propres (M2P2 – UMR-CNRS 6181), Europôle de l’Arbois, BP. 80, Bâtiment Laennec, Hall C, 13545 Aix en Provence Cedex 04, France b SAUR, 1 Avenue Eugène Freyssinet, 78064 Saint Quentin en Yvelines Cedex, France
a r t i c l e
i n f o
Article history: Received 4 May 2009 Received in revised form 18 December 2009 Accepted 22 December 2009 Available online 28 December 2009 Keywords: Drinking water Integrity test Magnetic nanoparticles Magnetic susceptibility
a b s t r a c t An alternative ultrafiltration membrane integrity test is presented and evaluated, based on the use of magnetic nanoparticles (Fe3 O4 ) and measurement of magnetic susceptibility. The mean size of nanoparticles used is around 35 nm and they show a good disparity between 20 and 100 nm. A series of integrity tests were carried out on a Norit membrane module containing 100 fibers under a low transmembrane pressure of 0.25 bar. The results showed that no magnetic susceptibility was detected in permeate when the tests were performed on the intact module in both cross-flow and dead-end filtration, indicating the complete nanoparticle retention by the intact module. However, when even one fiber was broken in the module (1% breakage rate), magnetic susceptibility of permeate could be detected instantaneously even at feed concentrations as low as 1.2 ppm with Bartington magnetic susceptibility meter. This detection is valid during all the filtration process. The results also showed that the membrane permeability could be completely recovered after a backwash. This membrane integrity test, with the advantages of simplicity, on-line operation, high detection specificity and sensitivity, quick detection and very low influence on membrane fouling, seems to be suitable for large-scale drinking water plants. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The intact ultrafiltration (UF) membrane can substitute for the disinfection step in drinking water treatment because of its complete removal of pathogens, such as Giardia, Cryptospodidium and viruses. For this purpose, continuous integrity monitoring of the UF membrane system is fundamental during the operation of drinking water treatment plants. As part of the regulatory system, water companies installing membrane filtration have to provide proposals detailing the associated level of integrity monitoring [1]. Different UF membrane integrity monitoring techniques, including direct methods and indirect methods, have been reported [2–4]. Among these methods, the PDT and DAF test are the most frequently used and they are considered to be simple, non-destructive and reliable methods. However, during practical operation, both methods are conducted off-line and they are not sufficient, for operational and economic reasons, to meet removal requirement of viruses (∼20 nm). In contrast, all the indirect methods can monitor UF membrane integrity on-line. However, conventional indirect methods such as particle counter and turbidity monitoring show low detection sensitivity and they cannot guarantee the regulatory
∗ Corresponding author. Tel.: +33 4 42 90 85 01; fax: +33 4 42 90 85 15. E-mail address: [email protected] (P. Moulin). 0376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2009.12.025
requirements for pathogen removal [5]. In addition, as presented by Deluhery and Rajagopalan [6], both particle counter and turbidity monitoring suffer from non-specificity of the particles detected, i.e. they measure all the particles related to a certain size range. Pathogen removal in terms of particle counter or turbidity is thus questionable. Other different membrane integrity methods [7–10] (e.g. acoustic sensor method, liquid–liquid porosimetry, binary gas integrity test and new membrane-based sensor) also have their disadvantages, e.g. off-line operation for liquid–liquid porosimetry and binary gas integrity test. Membrane integrity test frequency needs to be related to a risk assessment as the first priority and operational practicality as the second priority. Given an expected number of fiber breaks per year and a Log Removal Value (LRV) target, the frequency of membrane integrity tests can be calculated based on a simple mode or a statistical model. Membrane integrity tests can be implemented once a day, once 72 h or longer, or after cleaning step. It was summarized that, in most cases, a weekly test is required to ensure a LRV more than 4.5. For Bristol Water, a PDT frequency of once every week to once every other week is recommended to provide a margin of safety for the target of 4 LRV [2]. Therefore, it remains urgent to develop an on-line detection technique for reliable, quick, simple, and relatively inexpensive evaluation of integrity of the UF membrane system in drinking water plants. Recently, surrogate challenge tests for this purpose have attracted great attention [6,11–16].
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As alternative on-line methods, surrogate challenge tests can improve the detection sensitivity of conventional indirect integrity tests and are able to detect nanometric membrane breaches with nanoscale surrogates, although they cannot offer a real-time integrity monitoring. Sakaji [11] reported that maintaining membrane integrity monitoring with challenge tests can achieve a high removal of 5 or 6 LRV. For a surrogate challenge test, a certain concentration of surrogates is spiked into the membrane feed solution to simulate the retention characteristics of pathogens by a membrane. The key point of this test is to search for a suitable surrogate and corresponding measurement method. Multiple criteria have to be considered for the choice of surrogate, such as well defined size, easily detectable, non-destructive, reasonable price, representative of pathogen retention characteristics. The accuracy and the minimum detection level of the measurement method and the surrogate size will directly affect the detection sensitivity. So far, reported surrogate materials include powdered activated carbon (PAC), bacteriophage and oocyst, gold nanoparticles, magnetic particles, etc. [12–16]. For drinking water treatment, the ultimate integrity tests are microbial challenge tests. However, these tests are time-consuming for microbial analysis and may result in additional problems, such as membrane biofouling or penetration of bacteria into permeate in case of compromised integrity. Additionally, microbial monitoring may not correctly characterize the membrane integrity since bacterial re-growth may take place in the permeate piping system. As for other reported surrogate challenge tests, Spiked Integrity Monitoring system-SIMTM [12] offers high detection sensitivity but it is non-specific, which is generally analyzed with particle counters or turbidity monitors. In addition, due to the utilization of high concentration of PAC in this test, the PAC particles may occur in the permeate but it cannot be removed by a backwash due to the module compacity. Gold nanoparticle challenge tests [13] can detect small holes with average diameter of 20 nm when using anodic stripping voltammetry (ASV) as detection method. ASV has a low minimum detection level but it needs a prolonged run time or the addition of some other metal to resolve this problem. To overcome the drawback of non-specificity and thus minimize the background noise, alternative challenge tests using magnetic susceptible particles for membrane integrity monitoring are proposed to improve both specificity and sensitivity [14,15]. The principle of magnetic particle challenge tests is based on the injection of a certain concentration of magnetic particles in the feed and the detection of the material magnetic property in the filtrate. Particles occur in the filtrate and are detected by an appropriate magnetic sensor, indicating the presence of compromised membranes. The size of used particles depends on the required absolute size removal. Generally, magnetic sensors include superconducting quantum interference device (SQUID) magnetometer, giant magneto-resistive (GMR) sensors, measurement of magnetic permeability, etc. [15]. Deluhery and Rajagopalan [6] conducted membrane integrity challenge tests with the magnetic particle size of 1 m diameter. They proposed using a magnetic field to concentrate the particles in the filtrate for measuring transverse relaxation time (T2) with a magnetorelaxometer, which will result in enhancement of the detection limit value or feasibility of smaller and less powerful sensors. However, concentrating particles with an external magnetic field is not so feasible for practical operation, especially for large-scale drinking water treatment plants. Additionally, particles with 1 m diameter are not sufficient to detect virus-scale breaches in the membrane system. As a result, it is vital to improve magnetic sensor and surrogate size for the magnetic particle challenge test. Moulin [14] was the first to develop a new integrity test with magnetic nanoparticles without a concentration step to determine the particle concentration in the permeate side. In addition to simplifying the nanoparticle detection in permeate side, this test also
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improves the size of nanoparticles used in magnetic particle challenge tests. In this paper, this new UF membrane integrity test is presented and evaluated [14], based on the use of magnetic nanoparticles (Fe3 O4 ) and measurement of magnetic susceptibility. This is the first time that it is proposed to measure magnetic susceptibility with a magnetic susceptibility meter for membrane integrity monitoring. The minimum detection level of this detection method was investigated. To evaluate the efficiency of this new membrane integrity test, a series of lab-scale UF experiments were carried out with different concentrations of feed suspension, with different operation modes (cross-flow filtration or dead-end filtration) and with different broken fiber locations. This new surrogate challenge test for UF membrane integrity monitoring is revealed as a plausible solution for large-scale applications with the advantages of simplicity, on-line operation, accurate and quick detection, high specificity, high sensitivity and very low influence on membrane fouling. 2. Materials and methods 2.1. Preparation of magnetic nanoparticles Magnetite (Fe3 O4 ) is preferred for magnetic nanoparticle challenge tests because it is highly magnetic susceptible, non-toxic [17–19] and can be obtained easily. Here, Fe3 O4 magnetic nanoparticles were prepared by the chemical co-precipitation of ferrite (Fe3+ ) and ferrous (Fe2+ ) salts with the molar ratio of 2:1 using ammonium hydroxide (NH4 OH) as precipitating agent [20]. The chemical reaction is 2Fe3+ + Fe2+ + 8OH− ⇔ Fe(OH)2 + 2Fe(OH)3 → Fe3 O4 + 4H2 O To avoid particle aggregation or adhesion, the synthesized Fe3 O4 nanoparticles were stabilized with meso-2,3-dimercapto-succinic acid (DMSA) [21]. After the magnetic nanoparticle suspension was acidified slowly with 0.05 M HCl to pH = 2, the DMSA was added and stirred for 30 min. The ratio of added DMSA to [Fe] is 7%. Then, the pH value of the suspension was adjusted from 2 to 11, and from 11 to 7 by slowly adding 0.05 M NaOH and 0.05 M HCl, respectively. 2.2. Characterization of magnetic nanoparticles The prepared Fe3 O4 nanoparticles were characterized in terms of size distribution, magnetic susceptibility and concentration. The nanoparticle size distribution was determined by a Malvern Zetasizer granulometer (ZEN 1600 Nano S, Malvern Instrument, UK), based on dynamic light scattering measurement. The magnetic susceptibility of Fe3 O4 nanoparticles was measured by means of a Bartington magnetic susceptibility meter equipped with a MS2B sensor (Bartington Instrument, UK) and an AGICO Kappabridge magnetic susceptibility meter (MFK1-FA, AGICO Instrument, Czech Republic). The Fe3 O4 concentration was calculated from Fe concentration, which was measured by 6715 UV–vis spectrophotometer (JENWAY instrument, UK) [22,23] and was determined according to the calibration curve of absorbance versus Fe concentration at a wavelength of 508 nm. 2.3. Preliminary magnetic susceptibility tests To efficiently monitor UF membrane integrity, the method of detecting magnetic nanoparticles should provide advantages of accuracy, simplicity, sensitivity, low detection limit, specificity, quickness, wide analytical range, instrumental availability, affordable price and possibility of on-site detection. In this paper, Fe3 O4 nanoparticles were detected in terms of magnetic susceptibil-
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Fig. 1. Experimental set-up for UF membrane integrity tests using Fe3 O4 nanoparticles: (a) cross-flow filtration and (b) dead-end filtration.
ity. Magnetic susceptibility measurement using the Bartington instrument (portable apparatus) is a non-destructive, simple and cost effective method for determining the presence of magnetic nanoparticles. This instrument can display values from 0.1 to 9999 (e.g. (0.1–9999) × 10−5 SI unit for volume magnetic susceptibility), providing a very wide analytical range for detecting magnetic material. Generally, the response time for measuring a sample is 2 s or 12 s, depending on the selected measurement range. In contrast, AGICO Kappabridge magnetic susceptibility meter have the same advantages of a very wide analytical range and rapid detection. Moreover, it offers more detection sensitivity (the minimum detectable value is 0.001 × 10−6 SI unit). To investigate the specificity and sensitivity of this method, different concentrations of FeCl3 solution were measured. The stabilized Fe3 O4 nanoparticles were diluted with tap water, tap water containing 0.2 g L−1 bentonite, and natural water (lake of Réaltor, Bouches du Rhone, France). All these diluted suspensions were analyzed by the magnetic susceptibility meter to obtain the detection limit value for Fe3 O4 nanoparticles. The nanoparticle magnetic susceptibility was obtained by substracting magnetic susceptibility of water from that of water and nanoparticles.
2.4. UF membrane and integrity tests A Norit membrane module of 100 fibers (area = 0.0628 m2 ), whose nominal cut-off is 150 kDa and fiber diameter is 0.8 mm, was used in this study. The nominal pore size of this membrane is about 25 nm. Generally, Fe3 O4 nanoparticles used for membrane integrity tests should be larger than the membrane pore size so that they can be retained completely by the intact membrane. The Norit membrane module was rinsed with distilled water for 1 h and then was characterized by water permeability measurement prior to membrane integrity tests. Here, membrane integrity tests included magnetic nanoparticles challenge tests with both the intact membrane and the impaired membrane (one broken fiber). All these tests were conducted under a constant low transmembrane pressure (0.25 bar). The magnetic susceptibility in the permeate was measured to investigate whether nanoparticles passed through the membrane. Here, the permeate samples were taken every 2 min at the beginning and then were taken every 10 min. The volume of every sample is about 30 mL (10 s). After each test, the membrane module was well rinsed with distilled water and then was back-washed using distilled water at 2 bars. Finally, the water permeability of the membrane was measured again to determine whether the membrane permeability has been recovered. These lab-scale exper-
iments were carried out to evaluate whether the signal for the module with or without a broken fiber depends on different types of filtration used (dead-end filtration and cross-flow filtration) and the location of the broken fiber in the module. Additionally, the impaired membrane was challenged with different concentrations of nanoparticle suspension to investigate the minimum nanoparticle concentration required to efficiently detect the broken fiber. The experimental set-up is shown in Fig. 1. It should be noted that the materials in the pilot will not interfere with our proposed integrity test because there is no Fe pipe or some magnetic parts in the pilot. The magnetic detector used here only identifies the magnetic nanoparticles used. 3. Results and discussion 3.1. Characterization of magnetic nanoparticles Using chemical co-precipitation proved here to be a simple and economical method for magnetic nanoparticle synthesis. As shown in Fig. 2, these nanoparticles show a good size distribution in the size range of 20–100 nm. The average size of stabilized nanoparticles is around 35 nm, larger than the nominal pore size of the membrane used. Assuming that a UF membrane retains nanoparticles only by a size-exclusion mechanism regardless of surface chemistry and other characteristics, these stabilized nanoparticles can therefore be retained by an intact membrane. Otherwise, these
Fig. 2. Size and undersize distribution by number of stabilized nanoparticles.
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in response to an applied magnetic field. For samples with constant volume, it increases with the magnetic nanoparticle concentration. Results shown in Fig. 3 demonstrate that the magnetic susceptibility can accurately indicate nanoparticle content. As a result, magnetic susceptibility can perform well as an indicator of Fe3 O4 nanoparticle concentration. 3.2. Preliminary magnetic susceptibility tests
Fig. 3. Function of volume magnetic susceptibility as Fe3 O4 nanoparticle concentration.
nanoparticles will pass through the membrane in the presence of nanometric breaches larger than the nanoparticle size. In addition, the number of these nanoparticles is ∼2.7 × 1014 particles g−1 . Calculating from the measured Fe concentration, the Fe3 O4 concentration after stabilization is approximately 1.2 g L−1 in this study. This nanoparticle suspension was diluted by a factor of from 10 to 5000. Plotting of magnetic susceptibility as a function of nanoparticle concentration shows a linear relationship (Fig. 3). Magnetic susceptibility is the degree of magnetization of materials
In this paper, the magnetic susceptibility was measured mainly with the Bartington instrument. Firstly, magnetic susceptibility of FeCl3 solutions was investigated. The aim was to examine whether the presence of FeCl3 , which is generally used as a coagulant in drinking water plants, will affect the detection of Fe3 O4 nanoparticles by magnetic susceptibility measurement. The magnetic susceptibility of different concentrations of FeCl3 solution is shown in Table 1. It can be seen that FeCl3 has a low magnetic susceptibility and cannot be detected by the magnetic susceptibility meter at concentrations as high as 300 ppm. The detection limit of this method for FeCl3 solution is close to 3000 ppm, whose volume magnetic susceptibility is 0.2 × 10−5 SI unit. Based on these results, and given that the residual FeCl3 concentration in drinking water is lower than150 ppm, it can be deduced that the presence of residual FeCl3 in drinking water system will not affect the detection of magnetic nanoparticles using this method. In contrast, the detection limit of magnetic susceptibility for Fe3 O4 nanoparticles is low, as shown in Table 2. It can be seen that the magnetic sus-
Table 1 Magnetic susceptibilities of FeCl3 solutions. Concentration (ppm) 300 3000 30,000 300,000
Volume magnetic susceptibility k (SI unit, containing tap water)
Volume magnetic susceptibility k (SI unit)
−0.9E−05 −0.7E−05 2.5E−05 34.2E−05
0.0E+00 0.2E−05 3.4E−05 35.1E−05
FeCl3 solutions were diluted with tap water, k (tap water) = −0.9E−5. Table 2 Magnetic susceptibilities of Fe3 O4 suspensions (a). Diluted times
10 100 500 1000 2500 4000 5000
Concentration (ppm)
119.6 11.96 2.392 1.196 0.4784 0.299 0.2392
Volume magnetic susceptibility k (SI unit) Diluted with tap water containing 0.2 g L−1 bentonite
Diluted with tap water
k (containing tap water)
k
k (containing tap water)
k
45.8E−05 4.2E−05 0.0E+00 −0.5E−05 −0.7E−05 −0.8E−05 −0.9E−05
46.7E−05 5.1E−05 0.9E−05 0.4E−05 0.2E−05 0.1E−05 0.0E+00
45.8E−05 3.9E−05 −0.1E−05 −0.5E−05 −0.7E−05 −0.8E−05 −0.9E−05
46.7E−05 4.8E−05 0.8E−05 0.4E−05 0.2E−05 0.1E−05 0.0E+00
Initial concentration is 1.196 g L−1 , k (tap water) = −0.9E−5 SI unit, k (tap water containing 0.2 g L−1 bentonite) = −0.9E−5 SI unit. Table 3 Magnetic susceptibilities of Fe3 O4 suspensions (b). Diluted times
Concentration (ppm)
Volume magnetic susceptibility k (SI unit) Diluted with tap water
100 1000 2000 4000 6000 7000 8000
12.69 1.269 0.6345 0.3173 0.2115 0.1813 0.1586
Diluted with canal water
k (containing tap water)
k
k (containing canal water)
k
0.2E−05 −0.4E−05 −0.6E+00 −0.7E−05 −0.7E−05 −0.8E−05 −0.9E−05
1.1E−05 0.5E−05 0.3E−05 0.2E−05 0.2E−05 0.1E−05 0.0E+00
0.3E−05 −0.3E−05 −0.6E−05 −0.7E−05 −0.7E−05 −0.8E−05 −0.9E−05
1.2E−05 0.6E−05 0.3E−05 0.2E−05 0.2E−05 0.1E−05 0.0E+00
Initial concentration is 1.269 g L−1 , k (tap water) = −0.9E−5 SI unit, k (canal water) = −0.9E−5 SI unit.
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Table 4 Magnetic susceptibilities of Fe3 O4 suspensions (c). Diluted times 10 100 1000 10,000 20,000 40,000 60,000 80,000 100,000 200,000 500,000
Concentration (ppm)
Volume magnetic susceptibility (SI unit, containing tap water)
Volume magnetic susceptibility k (SI unit)
9572.7 957.27 95.727 9.5727 4.78635 2.393175 1.59545 1.1965875 0.95727 0.478635 0.191454
2.27E−02 2.16E−03 2.03E−04 1.50E−05 3.00E−06 −3.00E−06 −5.00E−06 −5.00E−06 −6.00E−06 −7.00E−06 −7.00E−06
2.27E−02 2.17E−03 2.12E−04 2.40E−05 1.20E−05 6.00E−06 4.00E−06 4.00E−06 3.00E−06 2.00E−06 2.00E−06
Initial concentration is 95.727 g L−1 , diluted with tap water, k (tap water) = −0.9E−5 SI unit.
ceptibility of Fe3 O4 nanoparticle suspensions is independent of the presence of bentonite. Similar results are also obtained when diluted with natural water (Table 3). This indicates that there is no influence of suspended materials or residual coagulant on the magnetic susceptibility of Fe3 O4 nanoparticles. The minimum volume magnetic susceptibility of 0.1 × 10−5 SI unit is obtained when the Fe3 O4 concentration is about 0.2–0.3 ppm, i.e. the detection limit of this method for Fe3 O4 nanoparticles is about 0.2–0.3 ppm when using the Bartington apparatus. The above results indicate that measuring magnetic susceptibility has both high specificity and high sensitivity for detecting Fe3 O4 nanoparticles. When a higher concentration of nanoparticle suspension (about 95 g L−1 ) is used, the detection was effective for a dilution factor of 500,000 with the portable analyzer (Bartington magnetic susceptimeter with MS2B sensor, see Table 4). With the more sensitive apparatus (Kappabridge MFK1-FA model), the dilution factor can be multiplied by a factor of more than 10. This means that a broken fiber in a module containing 500,000 hollow fibers can be detected with the MS2B sensor. With the Kappabridge MFK1-FA sensor, one broken fiber in a module containing 5,000,000 hollow fibers can be detected (if it is considered that the flow rate of a broken hollow fiber is equal to the permeate flow of one intact hollow fiber).
is continuous during all the filtration process (1 h). Considering the fouling phenomenon, this injection mode is the worst operational condition. In the case of cross-flow filtration, the permeate flux kept constant because the nanoparticle concentration in the lumen of the hollow fiber was almost constant. In contrast, after 1 h of deadend filtration, the nanoparticles in the lumen of the hollow fibers were concentrated by a factor around 700. This increase of nanoparticle concentration on the surface of the membrane explained the decrease of the J/J0 ratio. As a result, for membrane drinking water plants, the integrity test (i.e. the injection of nanoparticles) should be realized at the end of the filtration step (before the backwash) to reduce the membrane fouling.
3.3. Magnetic nanoparticle challenge tests for UF membrane integrity monitoring The number of fibers in the UF module (100) is lower than the detection sensitivity limit (4000) of an MS2B sensor, indicating that it is possible to detect broken fibers with the MS2B sensor. To assess nanoparticle retention performance by the intact membrane, the first set of UF experiments was carried out on the intact membrane in both cross-flow filtration and dead-end filtration. Here, feed nanoparticle concentration was about 12 ppm. The changes of relative flux J/J0 , the ratio of permeate flux of nanoparticle suspension to that of pure water, and magnetic susceptibility in permeate were investigated during the filtration process, as shown in Figs. 4 and 5. No magnetic susceptibility was detected in the permeate whatever the filtration mode (both cross-flow and dead-end filtration), indicating there is no magnetic nanoparticles passing through the intact membrane. This result is consistent with above size analysis that nanoparticles can be retained by the intact membrane due to the size-exclusion mechanism. On the whole, the permeate flux decreased with the operation time for both two filtration types but not to great extent. The overall decline of relative flux J/J0 was very small, about 13% and 22% for cross-flow and dead-end filtration, respectively. For cross-flow filtration, the permeate flux tended to be constant after 10 min of operation. It should be noted here that the SDI15 of tap water used in these tests was 2.0 ± 0.2, which also greatly contributes to these J/J0 decrease. In this study, the nanoparticles were added in the tank (Fig. 1), i.e. the nanoparticle injection
Fig. 4. Variation of relative flux J/J0 and magnetic susceptibility in the permeate with the filtration time: cross-flow filtration on the intact membrane, feed concentration 12 ppm, TMP 0.25 bar, cross-flow velocity 0.1 m s−1 , SDI15,water = 2.0 ± 0.2.
Fig. 5. Variation of relative flux J/J0 and magnetic susceptibility in the permeate with the filtration time: dead-end filtration on the intact membrane, feed concentration 12 ppm, TMP 0.25 bar, SDI15,water = 2.0 ± 0.2.
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Fig. 6. Variation of relative flux J/J0 with the filtration time: cross-flow filtration on the impaired membrane (one broken fiber close to the inlet part of the module), TMP 0.25 bar, cross-flow velocity 0.1 m s−1 , SDI15,water = 2.0 ± 0.2.
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Fig. 8. Variation of relative flux J/J0 with the filtration time: dead-end filtration on the impaired membrane (one broken fiber close to the outlet part of the module), TMP 0.25 bar, SDI15,water = 2.0 ± 0.2.
Then, a series of integrity tests were challenged by different concentrations of nanoparticle suspension in the presence of one broken fiber in the module (1% breakage rate). With one broken fiber in the module, the membrane permeability increases about 4% of that of the intact module. To demonstrate the efficiency of this method for membrane integrity monitoring, magnetic susceptibility in the permeate was measured during the operation as well as permeate flux. Firstly, membrane integrity tests were performed in crossflow mode when the broken fiber was near the inlet part of the module (Figs. 6 and 7). Fig. 7 shows that magnetic susceptibility in the permeate was detected when the impaired membrane module was challenged by 12, 2.4 and 1.2 ppm nanoparticle suspensions, indicating that there were nanoparticles passing through the membrane due to the presence of the broken fiber. Furthermore, the magnetic susceptibility in the permeate increased with the increasing feed concentration. The minimum volume magnetic susceptibility value of 0.1 × 10−5 SI unit was obtained at feed concentrations as low as 1.2 ppm. This means that 1.2 ppm nanoparticle feed concentration is the detection limit of the Bartington apparatus to detect one broken fiber out of 100 fibers. It should be noted from Fig. 7 that nanoparticles passing through the broken membrane can be detected rapidly by means of the magnetic susceptibility measurement, which depends on the dead volume of the
pilot. Within the detection limit range of the magnetic susceptibility meter, they can be detected immediately if nanoparticles pass through the membrane whatever the feed concentration. On the other hand, as shown in Fig. 6, changes of permeate flux during the filtration exhibited the same tendency for these three feed concentrations. The permeate flux decreased in the first 10 min and then tended to be constant. On the whole, the decline of the permeate flux was smooth and small, especially when the feed concentration was 1.2 ppm. The permeate flux decreased with the increasing feed concentration. The higher concentration of nanoparticles injected in the feed side was, the more the permeate flux decreased. When the same tests were implemented just by changing the location of broken fiber close to the outlet part of the module, similar results were obtained. This implied that the sensitivity of this test for UF membrane integrity monitoring is independent of the location of the broken fiber. The magnetic nanoparticle challenge tests were also conducted with dead-end filtration. They offered the same results as crossflow filtration when the impaired membrane was challenged by 2.4 and 1.2 ppm nanoparticle suspensions (Figs. 8 and 9), indicating the sensitivity of this membrane integrity test is independent of membrane operational modes. All above challenge tests exhibited an interesting phenomenon about changes of permeate flux during the filtration. The permeate flux decreased to a small extent and it tended to be constant rapidly. Even when the membrane challenge tests were operated
Fig. 7. Variation of magnetic susceptibility in the permeate with the filtration time: cross-flow filtration on the impaired membrane (one broken fiber close to the inlet part of the module), TMP 0.25 bar, cross-flow velocity 0.1 m s−1 .
Fig. 9. Variation of magnetic susceptibility in the permeate with the filtration time: dead-end filtration on the impaired membrane (one broken fiber close to the outlet part of the module), TMP 0.25 bar.
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Fig. 10. Comparison of membrane water permeability before and after test: deadend filtration on the intact membrane, feed concentration 12 ppm, TMP 0.25 bar.
in dead-end filtration, as shown in Fig. 8, the final relative flux J/J0 still kept more than 85% after 1 h of filtration (overall duration of a filtration step in a drinking water plant). Here, at the end of the dead-end filtration the volume concentration factor in the membrane is about 700, which explains the decrease of the relative flux (15%). However, these low feed concentrations used here (1.2–12 ppm) showed to be sensitive enough to detect the presence of 1% broken fiber in the module. Moreover, no irreversible membrane fouling also can be demonstrated from the measurements of membrane permeability recovery after each test, as shown in Fig. 10 (the worst operational condition). It can be seen that the water permeability of the membrane after a backwash was closed to the initial permeability, indicating that there is no irreversible membrane fouling caused by the nanoparticle filtration. 3.4. Discussion The above-described experiments demonstrate that the magnetic nanoparticle challenge test combined with magnetic susceptibility measurement is sufficiently sensitive for UF membrane integrity monitoring. Fe3 O4 nanoparticles in the size range of 20–100 nm exhibit superparamagnetic properties, i.e. they have a high magnetic susceptibility in the presence of an external magnetic field, thus resulting in low detection limit for a magnetic sensor. Here, Fe3 O4 nanoparticle detection with the magnetic susceptibility meter has the advantages of simplicity, speed and accuracy of the analysis, low cost of the instrumentation, detection specificity and low detection limit (see Sections 3.1 and 3.2). This detection method is helpful to reduce the impact of background noise and thus can improve the detection sensitivity. The high detection sensitivity of the magnetic susceptibility measurement (available to detect 500,000 times or more diluted suspension) is significant, which provides a strong basis for the development of an on-line sensitive membrane integrity test when using magnetic nanoparticles as surrogates. Compared with the magnetically susceptible particle method proposed by Deluhery and Rajagopalan [6], the main advantage of the method proposed here is that the permeate sample is directly analyzed by the magnetic susceptibility meter without using a nanoparticle collector to enhance the detection efficiency. This simplifies the operational procedures, facilitates the applications in large-scale drinking water treatment plants and gives information quickly. The experimental results presented in Section 3.3 showed that the magnetic susceptibility measurement was sensitive sufficiently to detect the presence of 1% fiber breakage rate in the membrane system even without a nanoparticle collector and with
very low nanoparticle concentration. Generally, a high feed concentration is recommended for most on-line indirect membrane integrity tests, such as particle counter, turbidity monitoring and Spiked integrity monitoring system-SIMTM , because the detection sensitivity of these methods increases with the increasing feed concentration. However, the membrane fouling will be more severe with the higher nanoparticle concentration mainly due to the membrane pore blockage, especially for the nanoparticles with smaller sizes [24]. In this paper, as shown in Section 3.3, the magnetic nanoparticle suspension with very low concentration (1.2 ppm) was sensitive enough to detect the presence of one broken fiber in the module (1% breakage rate) even when the membrane system was operated at low transmembrane pressure (0.2–0.3 bar), which contributed to very low reversible membrane fouling. Additionally, compared with other surrogate challenge tests (e.g. microbial and gold nanoparticle challenge tests), this new test shows much quicker detection for impaired membrane. Within the detection limit range of the magnetic susceptibility meter, the response time depends on the dead volume and flux of the pilot. Theoretically, in the event of a impaired membrane, the quantity of particles in the permeate depends on the magnitude of the leakage flow and the transmission ratio of the test particles. So, the nanoparticle quantity in the permeate depends on parameters such as transmembrane pressure, number of membrane breakages, the size ration of nanoparticles to membrane breakage. It maybe also depends on some other system parameters. In this paper, due to the high size ratio of membrane breakage to nanoparticles and high sensitivity of the detection method, nanoparticle quantity in the permeate in terms of magnetic susceptibility is independent of operational mode and location of the broken fiber under the condition of the same number of broken fibers in the same module (see Section 3.3), which is different from the results obtained by Johnson and MacCormick [25]. Based on the experimental results in this paper, this on-line magnetic nanoparticle challenge test for UF membrane integrity monitoring seems to be suitable for large-scale applications. Overmore, this test is proved to be economical for practical operation, e.g. (i) the cost of per gram of nanoparticles is about 3.3 D which is constant whatever the concentration of the nanoparticle suspension, (ii) the cost of nanoparticles needed for per m2 membrane based on an integrity test realized on an industrial plant (35 m2 ) is about 0.12 D and (iii) the analytical apparatus is low cost (about 5000 D for the Bartington apparatus), further work on larger scale testing should be done to validate this method. Additionally, this method can be extended to determine the membrane MWCO if monodisperse nanoparticles can be prepared.
4. Conclusions Alternative magnetic nanoparticle challenge tests for monitoring UF membrane integrity, based on the magnetic susceptibility measurement with the Bartington magnetic susceptibility meter, were evaluated in this paper. Magnetic susceptibility performs as a good indicator for magnetic nanoparticle concentration. No magnetic susceptibility was detected in the permeate when the challenge test was performed on the intact membrane, indicating the complete retention of nanopaticles by the membrane. In contrast, the compromised membrane module with one broken fiber (1% breakage rate) can be successfully detected by means of magnetic susceptibility measurement in the permeate even at feed concentrations as low as 1.2 ppm. The sensitivity of membrane integrity detection using this method is independent of operational mode (dead-end and cross-flow filtration) and location of the broken fiber. Additionally, all these experimental results show that there is very low reversible membrane fouling when
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the UF membrane system is operated with low feed concentrations (1.2–12 ppm) at a low transmembrane pressure (0.2–0.3 bar). This membrane integrity test, with the advantages of on-line operation, high sensibility, detection specificity and very low influence on membrane fouling, seems to be suitable for large scale drinking water treatments. Using the surrogates with nanometric size, this method makes it possible for UF membrane system to better satisfy the regulatory requirements of the drinking water industry. References [1] H. Guo, Y. Wyart, J. Perot, F. Nauleau, P. Moulin, Low-pressure membrane integrity tests for drinking water treatment: a review, Water research 44 (2010) 41–57. [2] A. Bennett, Maintaining the integrity of filtration systems, Filtration & separation 42 (2005) 30–33. [3] W.T. Johnson, Automatic monitoring of membrane integrity in microfiltration systems, Desalination 113 (1997) 303–307. [4] S. Panglisch, U. Deinert, W. Dautzenberg, O. Kiepke, R. Gimbel, Monitoring the integrity of capillary membranes by particle counters, Desalination 119 (1998) 65–72. [5] M.E. Walsh, M.P. Chaulk, G.A. Gagnon, Indirect integrity testing on a pilot-scale UF membrane, Journal of Water Supply: Research and Technology. AQUA 54 (2005) 105–114. [6] J. Deluhery, N. Rajagopalan, Use of paramagnetic particles in membrane integrity testing, Journal of Membrane Science 318 (2008) 176–181. [7] J.M. Laîne, K. Glucina, M. Chamant, P. Simonie, Acoustic sensor: a novel technique for low pressure membrane integrity monitoring, Desalination 119 (1998) 73–77. [8] S. Giglia, M. Krishnan, High sensitivity binary gas integrity test for membrane filters, Journal of Membrane Science 323 (2008) 60–66. [9] A. DiLeo, M. Phillips, Process for evaluating solute retention characteristics of membranes, US Patent 5,457,986 (1995). [10] J. Phattaranawik, A.G. Fane, F.S. Wong, Novel membrane-based sensor for online membrane integrity monitoring, Journal of Membrane Science 323 (2008) 113–124.
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