Hydraulics characteristics of forward osmosis membrane module boundary based on FBG sensing technology: Hydraulic properties and operating condition optimization

Chemosphere 226 (2019) 553e564

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Hydraulics characteristics of forward osmosis membrane module boundary based on FBG sensing technology: Hydraulic properties and operating condition optimization Ruzhen Bai a, b, Jie Wang a, b, *, Hui Jia a, b, **, Cheng Zhang a, c, Fei Gao a, d, Zhao Cui a, d, Guang Yang a, d, Hongwei Zhang a, b a

State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin Polytechnic University, Tianjin 300387, China School of Environmental and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, China School of Electronics and Information Engineering, Tianjin Polytechnic University, Tianjin, 300387, China d School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China b c

h i g h l i g h t s
A FBG sense method for measuring membrane stress distribution was proposed.
A nonuniform spatial variation of shear-force distribution exists along the FO channel.
Stress distribution was affected by increasing feed solution flow rate.
The impact of flow rate on the shear-force distribution in the FO was evaluated.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 January 2019 Received in revised form 21 March 2019 Accepted 25 March 2019 Available online 28 March 2019

To obtain more information on the hydraulic properties of membrane interface, the fiber Bragg grating (FBG) sensing technology was imported to investigate the effect of feed solution (FS) flow rate, draw solution (DS) flow rate and cross-flow direction on the membrane flux and membrane shear-force distribution of forward osmosis (FO) process. Results from experimental work demonstrated that a nonuniform spatial variation of the shear-force distribution exists along the membrane, and higher shear force is distributed in the middle position which resulted in higher diffusion load on the particular location of the membrane rind. Besides, increasing the inlet flow simply to a certain value didn't result in a higher shear force and lower the effect of concentration polarization (CP). Compared to co-current mode, counter-current mode showed the better hydraulic characteristics of higher shear-force, faster scouring frequency and consistent shear-force distribution, which will enhance the utilization of membrane and exhibit higher flux by increasing the inlet flow. Moreover, with the increase of FS and DS flow, the stress distribution showed more uniformed. Higher FS flow is more beneficial to FO process which will reduce ECP and improve flux in comparison to increasing DS flow which will produce adverse influence on ICP and diminish flux. © 2019 Elsevier Ltd. All rights reserved.

Handling Editor: Y Yeomin Yoon Keywords: Forward osmosis FBG sensing Hydraulic properties Membrane interface Shear-force distribution

1. Introduction As an emerging technology, forward osmosis (FO) has shown

* Corresponding author. School of Environmental and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, China. ** Corresponding author. School of Environmental and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, China. E-mail addresses: [email protected] (J. Wang), [email protected] (H. Jia). https://doi.org/10.1016/j.chemosphere.2019.03.155 0045-6535/© 2019 Elsevier Ltd. All rights reserved.

great promise in both water supply and energy production, and even in applications such as wastewater treatment, desalination, power generation and food processing (Wang et al., 2016a, b; Zhao et al., 2012). FO is a natural osmotic process which depends on the chemical potential between a high concentration draw solution (DS) and a low concentration feed solution (FS) separated by a semi-permeable membrane (Cath et al., 2006; Shaffer et al., 2015). Due to the very low hydraulic pressure required, FO delivers many potential advantages such as less energy input, lower fouling

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tendency and higher water recovery. However, there are some problems of membrane fouling and concentration polarization (CP), resulting in a decrease in flux in the FO (Chanukya et al., 2013; Gao et al., 2018). In general, CP is regarded as a main shortcoming for reducing membrane flux, and it could be further classified as external concentration polarization (ECP) on the membrane active layer and internal concentration polarization (ICP) in the membrane support layer (Gao et al., 2017; Wang et al., 2016a, b). It's possible to slow the CP and control the membrane fouling by changing the operating conditions (flow direction, flow rate, pressure, etc.), and it's necessary to investigate the hydrodynamic characteristics of membrane surface (Cui et al., 2018; Tian et al., 2018; Zhang et al., 2014). If hydrodynamic shear force and pressure at the membrane interface can be measured, it will efficiently understand the hydraulic properties of membrane interface to control the membrane fouling, slow down the CP, command the hydraulic pressure and energy consumption scientifically, and guide the optimization of the module structure (Phuntsho et al., 2013; Wang et al., 2014a, b). Therefore, the means to directly measure the stress on the membrane interface are needed. Usually, the analysis of the membrane interface during separation process is mostly conducted by numerical simulation and computational fluid dynamics (CFD) that can acquire the flow rate, pressure and concentration analysis at different locations on membrane interface (Wang et al., 2014a, b). The influence of the membrane orientation and flow direction on the performance of FO permeation was simulated by Jung et al. (2011). They found that the concentration distribution exist a spatial variation along the channel, which resulted in a change in the membrane flux. Furthermore, for preventing a local area of the membrane from being seriously fouled, the flow direction might need to be changed intermittently. Gruber et al. used CFD to study the effect of cross-flow velocity on the water flux in the FO process. It was obvious that the ECP could be decreased by enhancing the tangential flow along the FO channel, which directly led to the enhancement of membrane flux (Gruber et al., 2011). Moreover, Hawaii et al. investigated the impact of FS flow at active layer facing-draw solution (DS-AL) mode using 1.2 L/min DS flow whereas the FS increased from 1.2 to 3.2 L/min. This suggests that increasing FS flow maybe lead to a slight enhancement of the hydraulic pressure on the FS which perhaps caused the advance of water flux (Hawari et al., 2016). These studies showed the relationship between hydraulic configuration and permeation efficiency, but they did not combine the hydraulic properties of membrane interface with membrane flux to analysis in the experimental case. Neither the membrane pore size nor foulants and particles depositing on the membrane surface are in very small size, which are often in micrometers or even below microscales. Therefore, it is not possible to analyze the hydraulics behavior of foulants on the membrane surface through the conventional hydraulics analysis equipment, and the stress distribution on the membrane can hardly be measured in real time. When the fiber Bragg grating (FBG) is bonded onto a structure, the local axial strain of the structure can be measured (Liu et al., 2002; Rao, 1997). FBG has been utilized for sensing different measurands, such as pressure, temperature, and strain (Liu et al., 2014; Ren et al., 2014; Shu et al., 2007). FBG sensors are particular in many aspects, which make them ideal for flow-induced vibration measurement. It's simple in structure, small in size (the diameter could be as small as 80 mm), light in weight and corrosion resistance. Besides, it has little influence on the measured object and can be used to monitor the change of the physical quantity in a narrow space. Moreover, the FBG sensors have potential to be used for simultaneous multi-point measurement on a single structure by using wavelength division multiplexing (WDM). When light is

guided by a flexible fiber, the FBG sensors can be used to measure the fluctuating strain at any point on a structure, which are placed within an array. These may be of significance in measuring the information directly on the membrane surface because it can be dimensionally closer to membrane microfluidics behavior in shape (Dai et al., 2014; Ling et al., 2007; Pei et al., 2014). At present, the FBG sensor had be used to measure the flow-induced vibrations on a circular cylinder in a cross-flow by Jin et al. It is expected that the FBG sensor plays an important role in the study of fluidestructure interactions because of its physical uniqueness (Jin et al., 2000). Tjin et al. studied shear-force sensor using FBG as the sensing element. It was found that the shift in the FBG wavelength Dl shows a linear variation with the applied shear force (Tjin et al., 2004). Therefore, WDM method was used based on FBG sensing technology to investigate the hydraulic properties of membrane interface. Multiple FBG sensors in series closed to membrane surface are inserted on the FO module to achieve a spatially distributed measurement of stress on the membrane. In this study, the impact of the cross-flow direction, FS and DS flow rates on the water flux and stress distribution was discussed, and the membrane interface behavior and flow field characteristics under different flow rates was analyzed in the FO process.

2. Theoretical analysis 2.1. FBG sensing principle The FBG wavelength lB (nm) is given by:

lB ¼ 2neff L

(1)

where L (nm) is the period of the grating, and neff is the effective refractive index of the fiber core (Jin et al., 2000; Rao, 1997). When the FBG is strained, the lB changes due to the shift in the L and/or neff induced by photoelastic effect. As shown in Fig. 1(a), the FBG wavelength shift (DlB) with strain can be expressed as:

DlB ¼ 1  PC lB $Dε

(2)

PC ¼ 0:5n2eff fP12  mðP11 þ P12 Þ g

(3)

Where m is the Poisson's ratio of the fiber material, PC is the photoelastic coefficient of fiber, P11 and P12 are the changes of longitudinal and transverse refractive index respectively, and Dε is the axial strain in the fiber. An imposed axial strain Dε would cause a shift DlB in FBG. For a standard single mode silica fiber, PC z 0.22, so the relationship between Dε and DlB is given by:

Dε ¼

DlB

0:78lB

(4)

The relationship between axial stress s and DlB is given by:

s ¼ E$Dε ¼

E DlB 0:78lB

(5)

where E (Gpa) is the elastic modulus of the fiber, it's determined by the inherent nature of the material as a fixed value. Let a constant K ¼ E/0.78lB, a theoretical model is developed to establish an explicit linear relationship between the axial stress and the DlB. The wavelength shift DlB could be detected by some techniques, and an optical sensing demodulator was used to get it in this work.

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Fig. 1. Principle analysis of membrane surface stress measurement by FBG sensors: (a) Fiber Bragg grating schematic; (b) FBG sensors feedback with different scouring force; (c) Force analysis of single FBG.

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s ¼ K DlB

(6)

2.2. Principle of stress distribution measurement by FBG sensing system The water flow is introduced into the module after FBG sensor is fixed at both ends by UV glue on the membrane module, as shown in Fig. 2(b). Then FBG sensors closing to the membrane surface generate axial stress s due to the scouring force F of the flow. The

axial stress signal sensed by the FBG is converted into a wavelength signal by the optical sensing demodulator, so that we can get DlB of FBG which is used to indicate the stress on the membrane surface. Fig. 1(b) shows the DlB of FBG signals with different scouring force F, which can be used to measure real-time stress change on the membrane surface. For different applied scouring force F, the magnitude of s is different, that is, the measured DlB is different, where DlB1 < DlB2 corresponding to F1 < F2. Therefore, the DlB of FBG can be used to express the stress on the membrane. A preliminary CFD simulation was carried out to understand the flow field distribution on the membrane (Durst et al., 1974; Kaushik et al., 2012; Park and Kim, 2013; Shakaib et al., 2009). As shown in

Fig. 2. FO experimental rig: (a) Schematic diagram for FO experimental setup; (b) Distribution and position of FBG sensing system on the membrane.

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Fig. S13, different locations on the membrane surface are subject to different degrees scour, resulting in different DlB of the FBG at this location. The integration of DlB obtained during a certain time can be calculated by Eq. (7)

ðb S¼

f ðxÞdx ¼

f ðti Þðxiþ1  xi Þða  n  bÞ

(7)

i¼0

a

Dl ¼

n1 X

S n

(8)

where l (nm) is the average wavelength change at the position of the film surface during a certain time, S is the integral area, and n is the amount of data collection in the whole experimental time. Multiple FBG sensors can measure the stress distribution on the membrane when the local ‾l of FBG is obtained. The larger is DlB of FBG, the greater is stress suffered on the film surface.

2.3. Force analysis of FBG on the membrane surface FO filtration is a natural osmosis process which depends on the chemical potential between two solutions with different concentrations. But the velocity of fluid permeation through the membrane is much lower than the cross-flow velocity in the FO process. And from the test results in Supplementary (Fig. S2), this effect doesn't significantly affect the wavelength variation of FBG. Therefore, in the force analysis, only the force generated by the water flow washing is considered. Furthermore, calculation results from Reynolds number (Re < 2100) indicate that the hydraulic regimes are laminar at all flow rates in the experiment. Hence, the force generated by water flow can be approximately divided into the shear-force parallel to the membrane surface and the pressure perpendicular to the membrane surface. In Fig. 1(c), the forces on the FS side are PF⊥ and FFk, and the forces on the DS side are PD⊥ and FDk. For the case of FS and DS inlet flows VFS¼VDS, PF⊥ ¼ PD⊥, the membrane is only subjected to the role of shear force in countercurrent mode or co-current mode. At this time, the shear force causes the FBG to generate axial stress sFk or sDk, and the FBG can be used to measure shear-forces on both sides of the membrane. For the case of VFS > VDS, PF⊥>PD⊥, the membrane is subjected to the shear-force parallel to the membrane surface and the pressure perpendicular to the membrane surface. At this time, the shear force causes the FBG to generate axial stress sFk or sF⊥, and the FBG was used to measure the effect of FS resultant stress on the membrane. For the case of VFS < VDS, PF⊥
557

3.2. Experimental rig A schematic diagram of the laboratory-scale FO setup with FBG sensing system which could be used to measure the stress or shearforce distribution on the membrane interface is shown in Fig. 2(a). The FO membrane with an efficient filtration area of 1400 mm2 was made of a commercially available asymmetric cellulose triacetate (CTA) membrane provided by Hydration Technology Innovations (HTI, USA). Two peristaltic pumps (BT300-2J, Baoding Longer, China) were used to control and circulate the FS and DS the flow rates. Besides, two flow meters (LZB-6, Shanghai Instruments, China) were installed in the FS and DS respectively to supervise the flow in the experiment. The temperature of both solutions was kept constant at 26 ± 2  C that would not affect the measurement of FBG sensing system during the entire experiment. The membrane flux and reverse salt flux (RSF) were calculated through the detected data of the balance (ES-1002, D&T, China) and the electrical conductivity meters (DDS-307A, Shengbang, China). Optical sensing demodulator and matched software (SM130, Micron Optics, USA) were used to measure the wavelength change (DlB) of the FBGs at different positions, which corresponded to the change of the stress on the membrane surface. The optical specifications of SM130 optical sensing interrogator are shown in Table S1. Data acquisition frequency was set to collect 1e1000 data per second. Every experiment was run for 60 min, and then the whole system should be rinsed with de-ionized water for 10 min. 3.3. FBG sensing system In Fig. 2(b), the experiments were carried out using a custommade flat organic glass cell with symmetric channels circulating the FS and DS independently. The dimension of each channel is 70 mm long, 20 mm wide and 5 mm deep, and the inlet diameter is designed by 4 mm. The membrane was fixed between two organic glass plate using silicon gasket and six stainless steel screws. Before placing the FO membrane in the module, six FBG sensors (Comay Instruments, China) connected in series by single-mode fiber were placed in close contact with the membrane. From the simulation results (Fig. S4), the placement range of the FBG needs to be 0e1 mm. However, in order to better reflect the hydraulic characteristics of the membrane surface, the FBG was in close contact with the membrane in this experiment, and the distance between them was 0.5 mm. FBGs fixed on the module by UV glue (Qianxin, China) were positioned within an array at equal intervals of 1 cm. All FBG sensors were distributed along the direction of water flow in which the module worked and within the channel area. The parameters of FBG's optical properties are shown in Table S2. According to the distance from the module inlet, the center wavelength and position of FBGs are listed in Table 1. The center wavelength of each FBG sensor doesn't overlap with each other. Furthermore, one end of the FBG sensors in series is a free end, and the other end is connected with an input end on the optical sensing demodulator through a single-mode fiber. Then, the output of the optical sensing demodulator was connected to a computer which was equipped with the

3. Materials and methods 3.1. Draw and feed solutions Feed and draw solution was obtained by dissolving a certain amount of sodium chloride (NaCl, 99.5%, AR) into deionized water (DI, 18.2 MU/cm). When operated with the same inlet flow on both side of the membrane, the concentration of DS was 1.0 M whereas the FS was DI. When operated with the different inlet flow on both side of the membrane, FS was 0.1 M whereas the DS was 1.5 M.

Table 1 Central wavelength and position of FBGs. FBGs

Center wavelength

Distance from inlet

FBG1 FBG2 FBG3 FBG4 FBG5 FBG6

1556.110 1550.251 1547.943 1537.880 1539.885 1542.096

10 mm 20 mm 30 mm 40 mm 50 mm 60 mm

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software matched with it to get the date. These constituted an FBG sensing system, which can monitor the stress distribution on the membrane surface in real time. 4. Results and discussion 4.1. Membrane surface shear-force distribution and FO performance at high flow rates Eight different cross-flows 0.08, 0.16, 0.24, 0.32, 0.40, 0.48, 0.56 and 0.64 m/s were used to study the shear-force distribution on the membrane surface at higher FS and DS cross-flow rates in the FO process. The experiments were run with the active layer facing-feed solution (AL-FS) and counter-current mode. In this case, the flow rate on both sides of the membrane remained equal, so FBG's wavelength change (DlB) showed the shear force on the membrane surface. Data acquisition frequency of optical sensing demodulator was set to collect 1000 data per second because it could better respond to the shear-force distribution at different positions on the membrane surface. 4.1.1. Hydraulic performance in the FO process Based on FBG sensing principle, the variation of FBG's DlB indicates the shear-force tendency on the membrane surface. In Fig. 3(a), at the same flow rate, the DlB at different positions has the same periodicity and trend. This suggests that the shear forces change over time and present identical periodic. Nevertheless, the shear-force fluctuation range always varies at different positions. Furthermore, with enhancing the flow rates of DS and FS, the shear force increased on the membrane surface. The higher is the inlet flow, the larger are the shear-force value and variation frequency on the membrane interface. However, when the inlet flow rate increased to an optimal operation range, the shear-force magnitude could be no longer improved, even though the shear-force fluctuating frequency still increases. Analyzing the variation of DlB within 1s at different flow rates, when the inlet flow rates are 0.08, 0.16, 0.24, 0.32, 0.40, 0.48, 0.56 and 0.64 m/s, the period can be calculated as 1000, 500, 333, 250, 200, 167, 145 and 125 ms respectively. So the relationship between the inlet flow V and variation period T of DlB (shear force) could be established as shown in Fig. 3(b). With the increase of the inlet flow, the shearforce periodicity decreased and gradually tended to be stable. The results showed that the shear-force periodicity didn't always decline indefinitely and shear-force fluctuant frequency, which is inversely proportional to the variation period, was always increasing. As is known to all, the working characteristics of the peristaltic pump will cause the water flow to regularly wash the membrane surface. So these results suggested that FBG can reflect the characteristic of shear-force variation period at various locations on the membrane due to different operating frequency of the peristaltic pump. Analyze the ‾l at different positions under different flow rates in Fig. 3(c). The ‾l was used for showing the mean value of shear force which increased with the enhancement of DS and FS inlet flow. In general, the ‾l increased with increasing inlet flow at all locations on the membrane. But, it was found to be smaller at 1 cm and 6 cm from the entrance while larger at 2 cme5 cm in the middle position. These results thus confirm that the shear-force distribution exist a nonuniform spatial variation along the membrane, which is followed by a variation in the membrane flux, and higher diffusion load exists on the specific location of the membrane surface. Moreover, increasing the inlet flow rate could effectively improve the shear force at all locations, but it was more distinct to improve the middle position on the membrane. This phenomenon might be due to the following analysis. From

the perspective of fluid motion, in the FO module, besides the movement tangential flow along the FO channel, there is a tendency for the fluid to move perpendicular to the membrane surface. The velocity is relatively larger closing to the entrance, and the tendency of perpendicular motion to the membrane surface is larger, which makes the shear force smaller at this place. When the distance is far away from the entrance, the shear force paralleled to the membrane surface reduces with the velocity decreasing. In addition, from the analyze of fluid mechanics, the results of CFD simulation show that the maximum velocity of the membrane surface is not at the inlet, and the backflow zone is formed due to the complex flow field characteristics of the module, which causes a large change in the shear force at different locations on the film surface. The inlet flow rates and shear-force distributions will directly make the change in flux. The research had shown that the DS concentration distribution exist a spatial variation along the channel (Jung et al., 2011). The closer to the entrance, the higher DS concentration, while the farther away from the entrance, DS concentration gradually decreased. The shear-force generated by water flow can produce different concentration distribution on the membrane, thus influencing the water flux. As a result, different inlet flow rate will form a different shear-force distribution and scour frequency, which result in different membrane surface concentration distribution, thus changing the membrane flux. 4.1.2. Effect of shear force distribution on the FO performance Shear-force variations are closely related to the change in water flux and a similar trend of membrane flux was observed in Fig. 4. The membrane flux increased with improving DS and FS inlet flow rates, but beyond the critical inlet flow such as 0.48 m/s in this study, the influence of crossflow was insignificant and this was observed in other studies as well (Phuntsho et al., 2013; Suh and Lee, 2013; Xu et al., 2010). According to the shear-forces distributions and changes, the shift in flux was mainly due to the following reasons: When the flow rate increased from 0.08 m/s to 0.24 m/s, 27.70% improvement of shear force caused a linear increase in flux of 28.99%. Shear-force advance obviously relieved the ECP on the FS membrane, which greatly increased the water flux. Increasing FS and DS shear forces simultaneously accelerated the solute transfer from boundary layer to the bulk solution. The solute accumulated on the FS membrane gradually decreased to relieve ECP, and the diluted solution on the DS membrane was effectively mixed with the bulk solution, which increased the osmotic pressure at DS membrane (Phuntsho et al., 2013). These resulted in improving effective osmotic pressure difference between FS and DS, and better water flux was obtained. Besides, the increase of shear-force fluctuant frequency accelerated the scour frequency of the flow on the membrane (Fig. 3(a)). Based on the film theory, high frequency scouring action caused the boundary layer to be thinner, which improved the mass transfer rate and further increased the water flux. At this time, the increase in shear force on both FS and DS sides was conducive to improve water flux. When the flow rate increased from 0.24 m/s to 0.48 m/s, 55.20% advance of shear force only increased 18.56% in water flux. This is mainly due to that higher FS and DS shear forces enhance turbulent extent which accelerates solute transfer rate at the bulk solutionmembrane interface and reduces the negative effect of ECP on the FS. Meanwhile, the decreased ECP allows more draw solute diffusing into the support layer which will aggravate the ICP in the support layer (Wang et al., 2016a, b; Suh and Lee, 2013). As a result, a significant reduction in the negative influence of ECP but an aggravation in ICP made the increase in water flux is not obvious even though at higher membrane shear forces. At this time, increased FS shear force promotes DS solute diffusing into support

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Fig. 3. Effect of increasing both FS and DS flow rates on: (a) FBG's wavelength change (DlB); (b) Shear-force variation period; (c) Membrane shear-force distribution.

layer and the effect of ECP decrease is offset by ICP increase. When the flow rate increased from 0.48 m/s to 0.64 m/s, even if the shear-forces variation frequency increased, the shear-force magnitude hardly changed and the water flux reached a

maximum. Therefore, simply increasing the inlet flow to a certain value wouldn't result in a higher shear force and lower concentration polarization. The effect of increasing shear-force fluctuant frequency on solute transport no longer affected the shift in water

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higher shear force and shear-force variation frequency caused relatively lager and more uniform osmotic pressure difference between both sides of the membrane. It was contributed to significantly increase driving force and improve the use efficiency of the membrane separation surface. And it was also more conducive to slow down the solute accumulation on the membrane surface and help to alleviate CP. Furthermore, in terms of film theory, altering the shear force changes the thickness of the mass transfer boundary layer on the membrane surface. At counter-current mode, the boundary layer was thinner due to higher shear force, which led to higher mass transfer rate and increases membrane flux. In summary, it is better to choose counter-current mode because it showed the better hydraulic characteristics by increasing the crossflow. 4.3. Influence of DS flow or FS flow on FO

Fig. 4. Effect of membrane shear force on the flux at different flow rates: (a) Membrane flux at different flow rates; (b) Membrane shear-force at different flow rates.

flux. As a result, the design of operating conditions requires coordination of ECP and ICP on both sides of the membrane. There is an optimal operation range 0.24e0.32 m/s of FS and DS to get better hydraulic performance which will effectively control the CP on both sides of the membrane. 4.2. Influence of cross-flow directions on the performance of FO process The cross-flow directions can be kept either in co-current mode that the FS and DS flow in the same direction or in counter-current mode that the FS and DS flow in opposite directions. The impact of cross-flow directions on the performance of FO process was evaluated according to membrane flux and shear-force distribution on the membrane. To better compare the difference of shear forces in the two flow patterns, data acquisition frequency of optical sensing demodulator was set to collect 1 data per second for a total of 1 h. Compared to co-current mode, counter-current mode had a larger shear-force value and variation frequency, which produced better shear-force distribution and was more beneficial to the FO process (Fig. 5(a) and (b)). The ‾l at different positions under various flow rates in two modes are presented in Fig. 5(c). It indicates that nonuniform spatial variation of the shear-force distributions exist along the membrane in both modes, which directly cause difference in the water flux. For co-current mode, the shear force was larger at 2e4 cm on the membrane, and increasing the inlet flow rate could effectively improve the shear force at the middle locations (from 2 cm to 5 cm). However, as the inlet flow rate increased, the shear forces didn't change at 1 cm and 6 cm until slight changes occurred when the flow rate increased to 0.4 m/s. For counter-current mode, the shear force was larger at 3e5 cm, and it was distinctly improved at all locations on the membrane by increasing the inlet flow. Furthermore, the flux and shear force were greatly increase at 0.32 m/s, and this mode showed the relatively consistent shearforce distribution in higher inlet flow. Different shear-force distributions altered the concentration distribution on the film surface, resulted in different water flux. The water flux enhanced with the increase of the FS and DS shear force in both operating mode, but it is higher in counter-current mode especially at high flow rates (Fig. 5(d)). In counter-current mode,

The impact of increasing DS (or FS) flow from 0.16 m/s to 0.56 m/ s on the performance of FO process was investigated using a 0.16 m/ s FS (or DS) at AL-FS and counter-current mode. In this case, the flow rate on both sides of the membrane was not equal, so FBG's wavelength shifts showed the stress changes on the membrane surface. Data acquisition frequency of optical sensing demodulator was set to collect 1 data per second for a total of 1 h. 4.3.1. Draw solution (DS) In usual, the advance of stress on the membrane with increasing the DS inlet flow (Fig. 6(a)). However, the stress distribution on the membrane surface got to change when the FS flow is less than the DS. In the case of VFS¼VDS, there is no hydraulic pressure difference on both sides of the membrane. The ‾l is larger at the position 3, 4 and 5 cm away from the entrance, where the maximum is 4 cm. Whereas for VDS > VFS, different stress distribution was observed due to higher inlet flow and the hydraulic pressure difference. The larger ‾l is located at 2, 3 and 4 cm from the inlet, where 4 cm is also the largest. Altering stress distribution directly led in the shift of flux. With the increase of the stress on the DS side, the membrane flux and RSF reduced by 10% and 19.19% respectively (Fig. 6(b)). The primary reason for flux decrease was because of the higher DS shear-force improved solute diffusion into the support layer interior from membrane, indirectly aggravated the ICP and decreased effective osmotic pressure of DS (Hancock and Cath, 2009). RSF decrease also confirmed that more solute didn't diffuse into FS but accumulated inside the support layer. Besides, increasing the DS flow rate may increase the hydraulic pressure on the DS and slightly change the stress distribution which directly resulted in the variation of concentration distribution on the membrane surface. It is also adverse for the penetration of water flux. The reduce of RSF with increasing DS hydraulics pressure in the same direction as reverse salt diffusion suggested that the existence of pressure don't produce distinct effect on RSF. Hence, it can be concluded that DS shear-force may indirectly affected ICP, and using high DS flow rate was not conducive to the FO process though could produce a smaller RSF. 4.3.2. Feed solution (FS) In Fig. 6(c), for the inlet flow VFS¼VDS, there was only shear force existence on both sides of the membrane, and the maximum ‾l was at 4 cm. While for VFS > VDS, a slightly higher pressure was obtained resulting in a variation in stress distribution by increasing the difference between VFS and VDS, and the ‾l was larger at 3 cm. The stress distribution on the membrane surface was relatively different in both cases which directly led to changes in concentration distribution on the membrane surface and affected the water flux. The water flux and the RSF increased with FS stress

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Fig. 5. Effect of cross-flow directions on the FO performance: (a) FBG's wavelength change at counter-current mode; (b) FBG's wavelength change at co-current mode; (c) Membrane shear-force distribution at both modes; (d) Membrane flux at both modes.

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Fig. 6. Effect of DS or FS inlet flow on the FO performance: (a) Membrane stress distribution at different DS inlet flow; (b) Membrane flux and RSF at different DS inlet flow; (c) Membrane stress distribution at different FS inlet flow; (d) Membrane flux and RSF at different FS inlet flow.

(Fig. 6(d)) increased. The flux and RSF were about 19.18% and 15.69% increase respectively. In fact, the increase of flux could be achieved by changing the stress on the membrane including the shear force parallel to the film surface and the pressure perpendicular membrane. On the one hand, increasing FS flow rate can increase the shear force on the FS side and accelerate the solute transport on the membrane interface, thereby reducing the solution concentration near the membrane surface, slowing the ECP and increasing the water flux. On the other hand, in the case of VFS > VDS, there will be a positive hydraulic pressure in the same direction as the osmotic pressure and contrary direction to reverse salt diffusion, which is developing on the FS and conducive to permeation. At this point, membrane surface stress distribution got changes, so that the concentration distribution on the membrane was obviously changed. The increase of RSF with enhancing FS hydraulics pressure on the contrary direction to reverse salt diffusion suggested that the existence of pressure don't produce obvious prevention on RSF. For the FS-AL operation mode, it is possible to slow down the ECP and increase the water flux by increasing the shear force and hydraulics pressure produced by increasing FS flow rate. In this case, the energy consumption of the pump would be reduced. It was clear that the stress on membrane and flux were increased with the inlet flow, however the stress and flux were increased smaller when 0.40e0.56 m/s FS was applied compared to that at 0.32 m/s FS. Therefore, the 0.32 m/s FS and 0.16 m/s DS flow rates showing preferable membrane stress distribution, which lead to better membrane surface concentration distribution and increase permeability in the FO process, were confirmed as the better operation conditions. The flow rate on FS side must be kept higher than DS side because it is more conducive to the permeate process.

5. Conclusion WDM method based on FBG sensing technology was used to investigate the hydraulic properties of membrane interface in the FO module. Different operating conditions on the membrane flux were experimentally investigated combining with the stress distribution and shear-force distribution on the membrane surface. Based on this study, a number of key conclusions could be provided as following: (1) FBG results showed that a nonuniform spatial variation of the shear-force distribution existed along the FO membrane surface, which was followed by a variation in the membrane permeate. Besides, higher shear force is distributed in the middle position which resulted in higher diffusion load on the particular location of the membrane rind. (2) The shear-force variation presented identical periodicity due to different operating frequency of the peristaltic pump. With the increase of the inlet flow, the shear-force magnitude and variation frequency increased, which directly led to alter CP and improve FO flux. However, increasing the shear force simply to a certain value didn't result in a higher shear force and lower concentration polarization. There is an optimal operation range of inlet flow rates to get better hydraulic properties which will effectively control the ECP on FS and ICP on DS. (3) The shear force could be effectively improved at all locations on the membrane in counter-current mode while it was only efficiently improved at the middle locations in co-current mode by increasing the inlet flow rate. Therefore, compared to co-current mode, counter-current mode

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showed the better hydraulic characteristics with higher shear-force value, faster scouring frequency and consistent shear-force diffusion which will enhance the utilization of membrane and get higher flux. (4) For increasing DS inlet flow alone, the stress distribution on the membrane surface only acquired slightly change, and the ICP was aggravated owing to increase the stress on the DS side. On the contrast, increasing FS flow rate alone could change the position of the maximum stress on the membrane, and the ECP was alleviated by increasing the stress on the FS side. Therefore, higher FS flow rate is more beneficial to FO process in comparison to increase DS flow rate. RSF is directly correlated to water flux, whereas the hydraulic pressure does not have a significant effect on it. Results revealed that FBG sensing technology could be used to monitor membrane boundary hydraulic properties which provide more information on filtration process. Moreover, it make possible to test on line in narrow spaces because of small size which will play a promising role in membrane module design.

Acknowledgments This study was financially supported by the National Natural Science Foundation of China (No.51578375, No.51638011, No.61307094), China Postdoctoral Science Foundation (2017M621081), and Program for Changjiang Scholars and Innovative Research Team in University of Ministry of Education of China (Grand No. IRT-17R80). The authors would thank for the supports of China Scholarship Council (No. 201709345009, No.201606250111).

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.chemosphere.2019.03.155.

Nomenclature

lB

L neff

DlB m

PC Pij Dε E

s F T

l S n VFS VDS FF⊥ FFk

sF⊥ sFk

Bragg wavelength (nm) grating period effective refractive index of the fiber core Bragg wavelength shift (nm) Poisson's ratio of the fiber material photoelastic coefficient of fiber strain-optic coefficients axial strain in the fiber elastic modulus of the fiber axial stress scouring force variation period of FBG (ms) average wavelength (nm) integral area experimental time (s) feed solution inlet flow (m/s) draw solution inlet flow (m/s) pressure perpendicular to the membrane surface shear-force parallel to the membrane surface axial stress produced by pressure perpendicular to the membrane surface axial stress produced by shear-force parallel to the membrane surface

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