Effect of operation parameters on the mass transfer and fouling in submerged vacuum membrane distillation crystallization (VMDC) for inland brine water treatment

Author’s Accepted Manuscript Effect of operation parameters on the mass transfer and fouling in submerged vacuum membrane distillation crystallization (VMDC) for inland brine water treatment Helen Julian, Suwan Meng, Hongyu Li, Yun Ye, Vicki Chen www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(16)31363-1 http://dx.doi.org/10.1016/j.memsci.2016.08.032 MEMSCI14682

To appear in: Journal of Membrane Science Received date: 4 May 2016 Revised date: 26 July 2016 Accepted date: 19 August 2016 Cite this article as: Helen Julian, Suwan Meng, Hongyu Li, Yun Ye and Vicki Chen, Effect of operation parameters on the mass transfer and fouling in submerged vacuum membrane distillation crystallization (VMDC) for inland brine water treatment, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2016.08.032 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effect of operation parameters on the mass transfer and fouling in submerged vacuum membrane distillation crystallization (VMDC) for inland brine water treatment Helen Julian, Suwan Meng, Hongyu Li, Yun Ye and Vicki Chen* UNESCO Centre for Membrane Science and Technology, School of Chemical Engineering University of New South Wales, Sydney NSW 2052, Australia * corresponding author: [email protected]

Abstract Membrane distillation-crystallization process has great potential to recover high quality water and valuable precipitates from inland brine water. Incorporation of submerging membrane in a feed tank with agitation provides opportunity to reduce temperature and concentration polarization and energy consumption. This study evaluated membrane transverse vibration and feed aeration on the mass transfer of water vapour, crystallisation and fouling in a submerged vacuum membrane distillation and crystallization for inland brine water. At selected vibration frequency, more than 700 hours of high desalination performance was achieved. However, at elevated initial feed conductivity, the operation time was significantly reduced due to severe fouling. With accelerated tests with extreme feed concentration, both transverse vibration and aeration were able to increasethe initial flux by reducing boundary layer thickness on the membrane surface. However, with transverse vibration, rapid reduction of flux occurred earlier due to the enhanced CaCO3precipitation on the membrane surface. The aeration of feed improved productivity with longer period of high flux with the increased nucleation in the bulk feed solution. Thermal water softening procedure prolonged the operation time by increased calcium precipitation in the feed solution and slower CaCO3 nucleation on the membrane surface.

Keywords: membrane distillation crystallization, concentrated inland brine, transverse vibration, aeration, thermal softening

1. Introduction Membrane distillation-crystallization (MDC) process has important potential in brine management applications for recovery of high quality water and valuable precipitates. The advantages of this process include no requirement of hydraulic pressure and is possible for high non-volatile solute rejection [1] and flexibility in dealing with fluctuations in water compositions as opposed to the complexity in changes of energy input in RO process. The possibility of utilising geothermal energy [2] and other low grade heat as heat source in MD process could also significantly reduce the energy demand. Recent studies on membrane 1

distillation of highly saline solution have demonstrated promising results, with up to 89% water recovery and high salt rejections [3-7], while zero liquid discharge when coupled with a crystallizer [8, 9] have also been reported. As a thermally driven separation process, membrane distillation employs hydrophobic membrane as barrier between feed and permeate stream, as water vapour at the feed side of the membrane diffuses through the membrane pores to the permeate side, to be condensed and collected outside of the membrane [10, 11].Temperature polarisation on both the feed and the permeate side of the membrane can lead to reduction of actual driving force for water vapour transport [12, 13], while concentration polarization can reduce the mass transfer of water towards (in the feed side) and away from the membrane (in the permeate side) as well as promoting fouling and scaling. Conventional MDC is operated with pumping of the heated feed solution from the feed tank to the membrane module for distillation process, and the retentate recycled back to the feed tank for reheating. The distillation process is carried out in cycle until the feed solution is in supersaturation condition followed by cooling of feed to induce crystallization [5]. The retentate can also be pumped directly to a separate crystallizer for cooling, induction of nucleation and crystallization before reheating and recycling back to the feed tank [14]. As feed reheating and heat loss due to the circulation dominate the energy requirements for conventional MD process, a novel MDC configuration is utilised in this study, in which the membrane module is submerged in a unit operation which serves as both the feed tank and the crystallizer; thereby eliminating the need for feed reheating and avoiding heat loss due to recirculation. The heating element is placed on the top section of the container to create sufficient temperature gradient for the MD process, while the relatively cooler section at the lower part of the container was designed for the crystallization process. In the combined MDC process, crystallisation due to the homogeneous nucleation can occur in the bulk solution [15]. In addition, the presence of membrane and the localised concentration polarization near the membrane surface could lower the energy barrier for heterogeneous nucleation [16]. The homogeneous nucleation in the bulk solution and sedimentation of crystals is desirable as it leads to reduced salt content and feed concentration thus reduced effect of concentration polarisation. The heterogeneous nucleation including the precipitation of salt ions and formation of crystals on the membrane surface, however, is undesirable as they adversely affect the mass and heat transfer process. The crystals on the membrane surface can block the membrane pores for vapour transport and alter the hydrophobicity of the membrane surface, therefore initiate the penetration of feed solution into the membrane pores that not only reduces the available pores for vapour diffusion, but also the passage of solution [17-21] also known as membrane wetting. Among the four possible MD process configuration designs based on the permeate removal method, namely direct contact (DCMD), air gap, sweeping gas and vacuum MD (VMD), VMD configuration has a greater potential in energy saving as the direct extraction of water 2

vapour in permeate side reduces the temperature polarization in the permeate side with improved energy efficiency and higher flux [22, 23]. In a comparison of cross flow and submerged DCMD and VMD operating mode, higher mass transfer in the submerged VMD was observed [24], although more severe membrane wetting could occur in the submerged VMD system due to the lack of agitation to mitigate salt crystal deposition on the membrane. In the studies that attempted to improve feed agitation near the feed side of the membrane, strategies such as the introduction of baffles and spacers [25-33], vibration [24] and aeration [34], have been investigated to overcome temperature polarization, concentration polarization and fouling in membrane distillation. While baffles and spacers can lead to reduction in temperature and concentration polarizations the pressure drop of the circulated fluid can be significant. On the other hand, aeration in the feed container and vibration of the membrane in the submerged system has shown to increase the shear rate at the membrane surface and increase the flux [24, 34]. However, a gap of those studies is the influence on the crystallization process in the combined MDC process and the nucleation and precipitation of crystals in both the feed bulk and on the membrane surface. Scaling/fouling and possibly membrane wetting under these circumstances can be influenced by crystal growth rate, crystal type, size, shape and population. In particular, studies on VMDC process in applications for particularly difficult feed solutions, such as inland brine are very limited. As those difficult feed solutions normally contain relatively high concentration of low solubility ions such as Ca2+, Mg2+, HCO3-/CO32and SO42-, the formation of crystals in the feed and scaling of the membrane surface is real possibility. Understanding the effect of scaling to the performance of VMDC at various feed agitation strategies and understanding of the agitation method on the mass transfer, crystal growth in the feed solution and on the membrane surface are essential for better design of operation protocol for sustainable performance in the ever pressing demand to the final stage of many water treatment processes. This study investigated the feasibility of submerged membrane distillation process for treatment of inland brine water at various modes of feed agitation and operation conditions in a submerged VMDC configuration. The effect of feed concentration on the permeate production and salt transmission were evaluated together with characterisation of crystals formed in the feed solution as well as on the membrane surface. The systematic evaluation at various operation conditions was intended to ascertain the influence of individual factors as well as their combined effect. Using some extreme operating conditions, such as highly concentrated feed concentration, which often found towards the end of prolonged MD process, this study probes the response of VMD process to complex feed solutions and feed history. Apart from the inorganic ions modelled in this study, real inland brine may also contain organic matter depending on the water sources. However, the focus of this study is in the influences of membrane fouling caused by the crystal precipitation, and the influence of organic matter on the crystal precipitation and fouling is currently under investigation.

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2. Experimental 2.1 Materials and methods: Membranes used for the submerged VMDC tests were commercially available hydrophobic polypropylene (PP) hollow fibre membranes (Accurel PP S6/2, purchased from Membrana GmbH). The pore size, wall thickness, inner diameter and porosity of the membrane were 0.2 µm, 450 µm, 1800 µm and 73% respectively (manufacturer data). The modelled feed solution simulating the inland brine with the concentration and the compositions shown in Table 1 was determined on the average or the maximum values found in several case studies[35-38]. Maximum values were used for some components to increase the likelihood of crystallization. Similar solution was used in our previous study [24]. The actual inland brine water contains organic matter, which value is depend on feed water source [4, 39]. While the presence of organic matter in inland brine water is important, this paper focussed on the crystal formation, therefore organic matter does not present in the modelled feed solution. Table 1 Modelled feed composition Content (g/L)

Modelled Inland brine (MIB)

Sodium Chloride (NaCl)

5.84

Magnesium Chloride hexahydrate (MgCl.6H2O)

3.25

Calcium Chloride (CaCl2)

0.35

Sodium Sulphate (Na2SO4)

0.76

Sodium Hydrocarbonate (NaHCO3)

0.73

Sodium azide (NaN3)

0.20

TDS

11.13

Conductivity (mS/cm)

17.9

Calcium chloride (93-100.5%), sodium bicarbonate (99.1-100.3%) and sodium chloride (99.9%) were purchased from Ajax Finechem while Sodium azide (99.5%) was purchased from Sigma Aldrich. Sodium sulphate (99%) was purchased from Chem Supply. Magnesium chloride hexahydrate (99.0-102.0%) was purchased from Sigma-Aldrich. 2.2 Submerged Vacuum Membrane Distillation and Crystallization

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The experimental set-up of the submerged VMDC test rig used in this study is illustrated in Figure 1. The submerged membrane module was made of three hollow fibres with the length of 6.5 - 7.5 cm potted into a rectangular aluminium frame. Due to the potting procedure for the relatively short hollow fibre membrane, control of actual length was difficult even when the same length of fibre were used for the potting. However, the real length for the transport can be estimated with some certainty for membrane area calculation. The actual membrane area was varied from 16.53 to 19.08 cm2. In the experimental process, the vacuum pressure of the permeate was applied to the tube side of the membrane with a Masterflex®, Cole Palmerperistaltic pump with the maximum pressure of -95 kPa. The vacuum pressure at the permeate side of the membrane and the weight of the permeate over the operating time were recorded by use of a data acquisition software (Labview) at 20 second interval. The temperature of the feed solution was set at 70oC throughout the tests. Prior to each experiment with the modelled inland brine feed, the pure water flux of the membrane was measured to screen the integrity of the membrane with MilliQ water as feed at the operating temperature of 70oC over one hour. There were two pure water tests (MilliQ water as feed) conducted for each membrane module, first was conducted without agitation (all membrane modules experienced this test) and second was a pure water test with particular agitation (for example: membrane modules that will be used in transverse vibration experiment had pure water test with transverse vibration). The feed was heated by an immersed heating element, placed at the upper section of the feed tank. With a temperature controller connected to the heating element, a constant feed temperature 70±0.5 oC could be maintained without the need of insulation. When the thermal water softening procedure was applied, the feed solution was heated at 70oC for 24 hours before immersion of the membrane module for commencement of MD process. Thermal water softening was a strategy to precondition the feed solution in order to reduce the nucleation rate of calcium carbonate (CaCO3) on the membrane surface in the submerged VMDC operation. After the thermal water softening and prior to submerged VMDC operation, a sample of the feed solution was subjected to the Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) to characterize the concentration of calcium and magnesium in the feed solution. During the submerged VMDC operation, the salt transmission through the membrane was monitored by measuring the conductivity of the condensed permeate using a water proof conductivity meter (TPS, WP81). A sample of 20 ml condensed permeate was taken at regular frequency from the permeate tank for analysis. The permeate sample conductivity was measured in every 2 hours during the day and once during the night. Transverse vibration of membrane was introduced by connecting the membrane module to a purpose built motor. The vibration displacement was selected as 0.6 cm while the vibration frequency was set at 9.75 Hz. The vibration parameter was selected as a result of preliminary screening tests on a range of vibration conditions. In the experimental tests with aeration in the feed, the diffuser for air bubble was positioned away from the membrane module in order to avoid heterogeneous nucleation on the membrane surface. The bubble induced turbulence of the feed solution could provide shear rate at the membrane surface. The air pressure for the 5

aeration was set at 20 kPa and the diffuser was made of a flexible tube with 2 mm diameter holes and 20 mm distance between holes. The duration of the experiment time was dictated with the permeate flux reduction, normally 30 minutes after the flux was reduced to near zero. At the termination of the MD process, the feed solution was cooled to the room temperature by natural cooling, followed by harvesting of the crystals produced in the feed by use of a filter paper. The collected crystals were then dried in a 100 °C oven for 24 hours before estimation of the yield by weighting. At the termination of the MD process, the membrane was taken out of the feed solution and rinsed with MilliQ water followed by air drying at the room temperature for 24 hours. The crystal deposition on the membrane surface, cross-section and the inside surface were then evaluated by use of Scanning Electron Microscopy (SEM) as well as Energy Dispersive XRay Spectroscopy (EDS) to identify the type and morphology of the crystals attached on the membrane. Samples for SEM/EDS were prepared by carbon coating and the characterizations were performed with a Hitachi S3400.

Figure 1 Experimental set-up for the VMDC tests with inland brine with variable feed agitation modes. The characterisation of the morphology of the crystals collected in the bulk feed solution was conducted by use of X-Ray Diffraction (XRD) with PANalytical Xpert Pro MPD. XRD operating parameters were set to 45 kV and 40 mA, scanning from a 2θ of 10° to 100o with 0.026 step size. 3. Results and discussions 3.1 Effect of transverse vibration on the submerged VMDC performance

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The inland brine (as opposed to MIB in Table 1) with TDS of 14.2 g/L and conductivity of 22.9 mS/cm was used as feed at the feed temperature of 70 °C to study the performance of the submerged VMDC and the effect of transverse vibration. Periodic top up using feed solution was performed to compensate the water vapour lost to the permeate side as well as water vapour lost due to evaporation to the air. The volumetric rate of the periodic top up was in the range of 60-120 ml/h, which was very small compared to the volume of the feed tank of 40 litres. Therefore the feed solution was assumed to be homogeneous and the homogeneity of the ions was not affected by the periodic top up. The normalised flux (actual flux normalized to the tests with MillliQ water as the feed at identical agitation condition with the related submerged VMDC tests), the feed conductivity and the permeate conductivity throughout the experiments were presented in Figure 2.

(a)

(b)

7

(c)

Figure 2 (a) Permeate flux, (b) feed conductivity and (c) permeate conductivity profile of submerged VMDC with concentrated inland brine water feed. The pure water flux for test without agitation and with transverse vibration was 8.2 L/m2h and 8.6 L/m2h, respectively.

As shown in Figure 2 (a), the submerged VMDC with no agitation lasted for 200 hours. The initial flux was 7.8 L/m2h and the value was stable in the first 50 hours. Significantly better performance was achieved in the submerged VMDC with application of transverse vibration of membrane for more than 700 hours operation. While the permeate flux was reduced from the initial value of 8.2 L/m2h to 4 L/m2h, a more stabilised flux was maintained after 400 hours of operation. While the permeate recovery ratio was not measured for the experiments, the clear results in the normalized flux in Figure 2(a) would suggested a higher permeate recovery ratio of the experiment with transverse vibration than the experiment with no agitation. With the permeation of water, the feed conductivity increased at the similar rate in both tests as expected (Figure 2 (b)). With the increase in feed concentration with time, the concentration polarization at the membrane surface was also expected to increase, resulting in higher resistance for mass transfer on the feed side of membrane and reduced flux. In addition to that, severe concentration polarization could lead to substantially higher salt concentration near the membrane surface. When the local concentration near the membrane exceeds the critical solubility concentration, precipitation of salts on the membrane could occur followed by crystals growth on the membrane surface, especially when combined with the heterogeneous nucleation on the membrane surface. The conductivity of the permeate in both tests were stable at below20 µS/cm at the operation time below 400 hours. For submerged VMDC with transverse vibration, the permeate conductivity fluctuated with instantaneous values of above 1000 µS/cm after 400 hours as shown in Figure 2 (c). Although the overall quality of the permeate would have been acceptable as irrigation water, at less than 2 mS/cm for most horticultural crops[40]. The permeate quality during the first 400 hours, which was less than 900 µS/cm, could satisfy the Australian Drinking Water Guidelines [41]. The instantaneous high permeate conductivity in submerged VMDC with transverse vibration after 400 hours was due to the passage of feed solution, in addition to the expected vapour, 8

caused by partial wetting of some membrane pores. The return of the conductivity to normal low value at the next measurement was likely due to the blockage of those wetted membrane pores by the precipitated salt crystals formed inside the membrane pores or at the permeate side of the membrane surface, as the result of the water vapour evaporation at vacuum pressure [42]. Visual observation of the membranes at the end of each tests confirmed the precipitated crystals at different locations of the membrane as shown in SEM/EDS images in Figure 3. In the test with transverse vibration, aggregates of crystals were observed inside the crosssection and on the permeate side of the membrane surface in Figure 3 (c) and Figure 3 (d). However, the precipitation on the feed side did not form continuous crystal layer, thus the majority of the membrane pores were still exposed for water vapour penetration that corresponds to the stable flux over long period of time. On the contrary, the crystals formed a continuous layer in the test without agitation, leading to the blockage of the membrane pores and severe reduction in water vapour transport (Figure 3 (b)).

Figure 3 SEM images at the end of submerged VMDC tests with concentrated inland brine water feed (a) feed side (outer surface of hollow fibre) with transverse vibration, (b) feed side surface no agitation, (c) permeate side (inner surface of hollow fibre) cross section with transverse vibration and (d) permeate side surface with transverse vibration. The blue circles in Figure (c) and (d) indicate the crystals at the permeate side of the membrane surface. 3.2 Effect of feed concentration on the performance of submerged VMDC 9

To evaluate the submerged VMDC performance at even higher feed concentration, tests were conducted with the concentrated model inland brine feeds of 30.7 mS/cm and 44.9 mS/cm conductivity, (while maintaining the composition ratio of MIB shown in Table 1)at the same feed temperature of 70oC. In those tests, the feed solution was topped up at hourly frequency using the MilliQ water in order to maintain the constant feed concentration as well as volume. The comparison of the normalized fluxes at varied feed concentration at the same transverse vibration of the membrane is presented in Figure 4.In the tests with both the higher concentration brine feeds, the onset of the flux decline was much quicker than that with the modelled inland brine feed, while relatively stable fluxes were observed prior to rapid flux reduction. The rapid flux reduction occurred around 12 hours of operation in feed with conductivity of 30.7 mS/cm and the test was ended at 67.5 hours. For the feed with the highest conductivity of 44.9 mS/cm, rapid flux reduction occurred around 7.5 hours and the test was ended at 23.5 hours.

Figure 4 Normalized flux profile of inland brine water with transverse vibration (0.6cm, 9,75 Hz) at various concentrations. Initial flux for initial feed conductivity of 22.9 mS/cm, 30.7 mS/cm and 44.9 mS/cm was 8.3 L/m2h, 9.57 L/m2hand 9.86 L/m2h, respectively. As membrane performance differs from batch to batch, the initial flux of membrane (purchased in an earlier date) used in the 22.9 mS/cm feed conductivity was lower than the ones in the other two tests. However, the similar normalized flux between all three experiments in Figure 4 indicated that the older membrane (with a lower initial water flux) has comparable permeation property and the comparison in Figure 4 was valid. Between the 30.7 mS/cm and 44.9 mS/cm feed conductivity experiments, the small difference in the normalised flux at the first 30 minutes of tests was observed, however, the 10

concentration polarization effect became more prominent after that and the lower normalised flux was observed in the test with the high concentration feed (44.9 mS/cm feed conductivity) due to the influence of concentration polarization. The permeate conductivity was below 20 µS/cm throughout the tests with both the more concentrated feed, suggesting negligible membrane wetting in both case. As the feed concentration was constant during those tests, sudden crystallisation event in the bulk feed could be excluded, thus the onset of the rapid flux decrease could only be caused by the rapid precipitation and growth of crystals on the feed side of the membrane surface. In the test with concentrated feed (Figure 4 and 6) the crystal formation at the membrane surface was rapid with the overall membrane covered with dense structure of CaCO3 which resulted in zero flux after relatively short period of time. Had the experiment continued (rather than terminated 30 minutes after the flux reached zero), the continuous vacuum pressure in the permeate side of the membrane could lead to evaporation of water vapour through the membrane pores and penetration of feed was possible. In the test with normal feed (Figure 2), slow formation and growth of the crystal on the membrane surface only partially covered the membrane surface over time. With the exposed membrane pores, the deposited crystals not only reduced the number of membrane pores for vapour transport through the membrane, but also reduced the membrane surface hydrophobicity, that could lead to transmission of feed solution, i.e. membrane wetting [19] The total crystals harvested in the feed at the end of each tests shown in Table 2 was compared with the theoretical maximum crystal production, calculated according to the reaction stoichiometry of CaCO3 crystal in the feed solution. For the initial feed conductivity of 30.7 mS/cm and 44.9 mS/cm, the actual crystal production in the feed solution were around 40% of the theoretical maximum. However, for the initial feed conductivity of 22.9 mS/cm, crystal produced in the feed solution was negligible. The reason of this noteworthy difference is under further investigation. Table 2 Operation time and crystal production in submerged VMDC tests with transverse vibration at various inland brine water concentration Initial Feed conductivity (mS/cm)

Induction time for severe fouling (hour)

22.9 30.7 44.9

>760 12 7

Time taken for Crystal the flux produced in reduced to zero feed (g) (hour) Not observed insignificant 67.5 10 23.5 16.14

Theoretical maximum crystal production (g) 18.4 24.7 35.9

Stark difference in the flux profile and the harvested crystal from the feed tank was observed with different initial feed concentration. In the experiment results shown in Figure 2, the 11

normalized flux decreased as the feed conductivity increase to 30 mS/cm, yet relatively high value of 0.8 was maintained. After the feed conductivity reached 30 mS/cm, the submerged VMDC with transverse vibration continued for another 200 hours before the normalized flux was reduced to 0.5 and occurrence of spontaneous wetting. However, in the test with initial feed conductivity of 30.7 mS/cm, the submerged VMDC with transverse vibration only operated for 20 hours before the normalized flux was decreased to 0.5. Similar trend was observed in the test with feed of 44.9 mS/cm initial conductivity. This suggests that the crystal formation history in the feed solution has an important role in determine the overall operation time and flux profile. In addition to that, the composition ratio of the salts may not have been maintained as the feed conductivity increased in the test with the low initial feed concentration. In the test with 22.9 mS/cm initial feed conductivity, the salts concentration increased gradually with time. At the low salt concentration at the beginning, although the crystals nucleation on the membrane surface was still possible; the shear force provided by the transverse vibration could limit the crystals deposition and growth on the membrane surface. In the contrast, the heterogeneous nucleation in the concentrated feed could occur almost instantaneously and the salt crystallization in the feed was more rapid; the crystal deposition on the membrane surface was also faster thus the effect of transverse vibration was limited. Comparison of the SEM images of membrane surface at feed side at different operating time with different feed concentration shown in Figure 5 indicated the differences in both crystal size and membrane surface coverage. With the inland brine feed of 44.9 mS/cm conductivity, the crystal was smaller in size (8 µm) with about 50% coverage of membrane surface at the first hour (Figure 5(a)). After 2 hours of operation, the crystals size was increased to about 15 µm while the membrane surface coverage was near completion, although the crystals were scattered (Figure 5(b)). This is in contrast with the images of membranes in the operation of lower concentration (22.9 mS/cm feed conductivity), with significantly less crystal precipitation observed after 24 hours of observation (Figure 5(c)) while most the membrane pores still visible after 48 hours (Figure 5(d)).

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Figure 5 SEM images of membrane surface at (a) 1 hour, (b) 2 hours in 44.9 mS/cm feed conductivity and (c) 24 hours, (d) 48 hours in 22.9 mS/cm feed conductivity.

3.3 The effect of different agitation under rapid crystallization condition As the transverse vibration of the membrane module resulted in more rapid flux decline in tests with high feed concentration than with low feed concentration, aeration in the feed was utilized as an additional agitation method. To accelerate the experimental tests, the submerged VMDC tests were conducted with the concentrated feed (44.9 mS/cm conductivity)at temperature of 70oC, using MilliQ water to topped up feed hourly. As shown in Figure 6, relatively stable fluxes between 0.8 – 0.9 were achieved for all of the tests before rapid flux reduction occurred at different times for different agitation mode. The difference in the stable flux between the different agitation modes could be attributed to the differences in concentration polarization. In the test with no agitation, the normalised flux was stable over the first 9 hours, while in the test with transverse vibration, the flux declined earlier with only 7 hours of stable flux period, but higher amount of permeate was produced, as shown in Table 3.It is worth noting that a longer period of stable flux (14.5 hours) was maintained in the test with aeration. Furthermore, superior performance with 19 hours of stable flux period was achieved in the test with the combined aeration and transverse vibration agitation mode, with no indication of pore wetting as the permeate conductivity was below 20 µS/cm.

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Figure 6 Normalized flux profile of 44.9 mS/cm feed conductivity tests at various agitation mode. Initial flux of no agitation=5.53 L/m21, transverse vibration=9.86 L/m2h, aeration=9.09 L/m2h, transverse vibration+aeration=9.63 L/m2h While the homogenous nucleation in the feed solution was difficult due to high energy barrier, the heterogeneous nucleation on the membrane surface was relatively easier due to the higher salt concentration in the concentration polarization layer. The heterogeneous nucleation sites on the membrane surface could induce continued precipitation of salt ions

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Table 3 Experiment time, permeate and crystal production of extreme concentration inland brine water feed at various agitation mode

Total experi ment time (hour)

Inductio n time for severe fouling( hour)

Total Perme ate produc ed (L/m2)

27.5

9

98.65

23.5

7

120.79

20

14.5

153.63

29

19

231.41

24.5

8

87.97

23.5

5.5

88.83

14.9

CaCO

20

12

140.1

30.22

CaCO 3+ MgC O3

No agitation Transverse vibration

Aeration

Transverse vibration + aeration 7 hours delayed vibration 2 hours delayed vibration

Aeration + 12 hours delayed vibration

Cryst Flux for Amount al MilliQ of type water Crystal on test produce memb without d in rane agitatio feed (g) surfac n e (L/m2h) CaCO 6.93 13.31 3 CaCO 8.49 16.14 3 CaCO 8.59 3+ MgC 27.19 O3 CaCO 7.78 3+ MgC 23.8 O3 16.17 CaCO 8.03

Flux for MilliQ water test tested with agitation( L/m2h) 10.19 10.31

10.89

9.63

3

8.96

10.75

8.49

10.21

3

and growth of crystals as the operation progresses. The rapid flux reduction in the concentrated feed test was the consequence of the profound fouling. The shorter period of stable flux in test with transverse vibration compared to the test with no agitation was resulted from the reduced temperature polarization and the concentration polarization, with higher temperature and lower salt concentration on the membrane surface. The higher temperature on the membrane surface not only could lead to higher driving force for water vapour transportation through the membrane, but also increased tendency of CaCO3 precipitation on the membrane surface, because the temperature-solubility correlation of Ca2+ and CO32- ions in the solution was negative (i.e. lower solubility at higher temperature).

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However, more total permeate was produced in the transverse vibration test due to the higher flux than in the test without agitation as shown in Table 3. The test with transverse vibration in the 44.9 mS/cm feed conductivity was inconsistent with the tests in the 22.9 mS/cm feed conductivity, with reduced overall operation time compared to the test with no agitation. Transverse vibration was utilized to reduce the temperature and concentration polarization through increased shear near the membrane. Compared to the tests of the same feed without agitation, the feed temperature on the membrane surface should be higher at the reduced temperature polarization, and the salt concentration on the membrane surface should be lower due to reduced concentration polarization. In the test with 44.9 mS/cm feed conductivity, the concentration of Ca2+ and CO32- at membrane surface was much higher than in the test with 22.9 mS/cm. The higher concentration induced crystal deposition in this case was much higher than the limiting effect by the transverse vibration. Aeration, on the other hand, provided not only the agitation to produce shear rate at the membrane surface for reduced concentration polarization, but also opportunity for heterogeneous nucleation of crystals in the bulk feed. The additional nucleation in the bulk solution by aeration could lead to rapid crystal growth in the feed and their settlement, which lead to reduced salt concentration in the feed, and reduced likelihood of salts precipitation on the membrane surface. As shown in Table 3, the operation with aeration in the feed led to production of the highest amount of crystals at the end of tests at 27.19 g, while the vibration combined with aeration produced 23.8 g. These values were significantly higher than the amount of crystal produced in the tests without aeration, supporting the hypothesis of the additional heterogeneous nucleation by aeration in the feed solution. XRD conducted for the crystal collected in the feed solution in all of the tests indicated that CaCO3 was the main form of the crystals. The SEM of membrane cross section, shown in Figure 7, also indicate thicker crystal layer on the membrane on the test with transverse vibration than on test with aeration, confirming lower salt precipitation on the membrane in the tests with aeration of the feed. Similar to the test with the transverse vibration, the test with aeration and the combination of transverse vibration and aeration also resulted on higher flux of submerged VMDC.

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Figure 7 SEM of membrane cross section at the end of 44.9 mS/cm feed conductivity tests with (a) vibration and (b) aeration The influence of the transverse vibration and aeration on the enhancement of the submerged VMDC flux can be calculated by observing the relationship between the flux (Jm) and the driving force (Pfm – P), as follow:

Jm =

where

(Pf,m– P)

Equation 1

is the membrane permeability, related to the membrane resistance and

determined by the membrane geometrical property. As indicated earlier, all membrane modules were attested for pure water flux without agitation (second last row in Table 3) at the feed temperature of 70 C. At the same test condition, the driving force ((Pf,m – P) in Equation 1) for transport is the same. According to Equation 1, the difference of in pure water flux was then caused by the membrane permeability. The normalized membrane permeability was calculated by the ratio of pure water flux data in the second last column in Table 3 and shown in the first column in Table 4. In this process, the unknown value of vapour pressure at membrane surface was avoided. As the membrane permeability is an intrinsic property of a membrane and does not change with the experimental condition, in the test with the concentrated feed, the normalized membrane permeability values were constant before the occurrence of membrane fouling. Taking account the difference in membranes permeability shown in the first column in Table 4, the relative driving force (normalised against the test without agitation) was calculated with the stable flux measured at the first few hours (before the rapid flux decline) to avoid the complication caused by fouling. The normalized driving forces were shown in the second column of Table 4. While the concentration and temperature polarization were unavoidable in all tests and affected real driving force, the degree and severity of the concentration and temperature polarization differ from one test to another due to the agitation conditions as reflected in results shown in Table 4. 17

Table 4 Normalized membrane permeability and driving force of the tests under rapid crystallization condition Condition No agitation (base of comparison) Transverse vibration Aeration Transverse vibration + aeration

Comparison of Driving force relative to the no membrane permeability agitation test in brine feed 1

1

1.2 1.2

1.3 1.3

1.1

1.5

The driving force in the test with transverse vibration was 1.3 times higher than that in the test with no agitation. Similar driving force enhancement was also obtained when the aeration was introduced to the test. When transverse vibration and aeration were conducted simultaneously, the driving force was further enhancement compared to the transverse vibration or aeration alone. The EDS surface elemental analysis on the membrane in tests without aeration displayed crystals of cuboid structure, associated with Ca as shown in Figure 8. Meanwhile, the crystals on the membrane in tests with aeration displayed cuboid structure, which is associated with calcium and sharp floret structures, associated with Mg. As the crystals collected from the feed solution were CaCO3, the total amount of CaCO3 in the feed solution can be estimated (35.9 g). The crystals collected in the feed in tests with aeration (at 27.19 and 23.8 g CaCO3) were 60-75% of the available CaCO3in the feed. As the CaCO3 crystal growth in the feed was much faster and settled to the bottom of the feed tank, the Ca2+ concentration in the bulk feed in those tests was much reduced thus Mg2+ could compete with Ca2+ to bind with CO32- ions and precipitate on the membrane surface.

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Figure 8 SEM/EDS and mapping of membrane surface of44.9 mS/cm feed conductivity tests with (a) no agitation (b) transverse vibration, (c) aeration and (d) transverse vibration+aeration 3.4

Effect of delayed agitation on submerged VMDCperformance under rapid crystallization condition

Transverse vibration of the membrane module was expected to lessen the negative effect of concentration and temperature polarization by reducing the boundary layer thickness. However, earlier flux reduction was observed in test with 44.9 mS/cm feed conductivity with the transverse vibration, due to severe concentration polarization and crystallization of 19

CaCO3 at the membrane surface. To prolong the operation time, the application of transverse vibration was delayed after initial period of operation without agitation. Introduction of transverse vibration was initially selected at the time when the flux reduction was observed at 7 hours with the intention to improve the driving force. As shown in Figure 9, the flux of submerged VMDC continued to drop with the introduction of transverse vibration at the 7th hour. This seems to suggest that the crystals might have already deposited at the membrane surface when the vibration was applied and the membrane pore were already blocked. The reduction of boundary layer thickness by transverse vibration would only result in the increase of crystal layer surface temperature which could encourage more precipitation. The overall operation time in the 7 hours delayed vibration test at 24.5 hours was almost identicalwith that in the test applying transverse vibration from the beginning. The test with 7 hours delayed vibration had lower flux than the test with transverse vibration from the beginning due to the lack of polarization mitigation. In addition, compared to the test with no agitation, the period of stable high flux in the test with 7 hours delayed vibration was shorter. Therefore, the permeate produced in this experiment was also less than that in the tests with no agitation and the test with transverse vibration throughout, as shown in Table 3. With a reduced delay time of 2 hours, the normalized flux after the transverse vibration was introduced was slightly higher than that without agitation. However, the onset of flux decline was observed more rapidly.

Figure 9 Normalized flux profile of44.9 mS/cm feed conductivity tests with delayed vibration. Initial flux of vibration=9.86 L/m2h, delayed 7 hours =6.69 L/m2h, delayed 2 hours =7.06 L/m2h. Difference of the initial flux for delayed 7 hours and 2 hours tests was due to the slight difference of membrane permeability.

As the aeration of feed could enhance not only the shear rate at membrane surface, but also create competitive nucleation site in the bulk solution, and the test with the combination of aeration and transverse vibration resulted on the best performance. However, it is unlikely to 20

be feasible to operate with both those agitation modes throughout the experiment. Therefore, test with the aeration prior to transverse vibration was conducted. Application of aeration at the earlier stage was intended to induce crystallization of CaCO3 in the bulk solution and to reduce the amount of CaCO3 in the feed, while introduction of transverse vibration was intended to maintain the flux by creating the shear rate at membrane surface. The introduction of transverse vibration was selected at the time when the flux reduction was observed at 12 hours.

Figure 10 Normalized flux profile of44.9 mS/cm feed conductivity tests with aeration + delayed vibration. Initial flux of aeration= 9.09 L/m2h, delayed vibration 12 hours =9.10 L/m2h As shown in Figure 10, the flux reduction continued after vibration was introduced. As discussed earlier, at the time the flux started to decrease, the crystal deposition on the membrane may have occurred with pore blockage. Improved shear rate on the membrane and reduced temperature polarization layer thickness could exasperate crystal precipitation at high surface temperature.

21

Figure 11 SEM/EDX of membrane surface at the end of VMDC tests with extreme concentrated inland brine water feed (a) delayed vibration 7 h (b) delayed vibration 2 h, (c) aeration delayed vibration 2 h.

The crystal collected in the feed after VMDC tests in the aeration + delayed vibration experiment was also comparable to the crystal produced in the aeration experiment, as shown in Table 3. SEM/EDS images of the membrane surface shown in Figure 11 indicate cuboid structure of CaCO3in tests without aeration in Figure 11(a) and 11(b). The crystals structure in tests with feed aeration shown in Figure 11(c) was the combination of cuboid structure and sharp florets, associated with magnesium.

22

3.5

Effect of thermal water softening on submerged VMDC performance

As the crystal formation history in the feed solution showed a significant influence on the submerged VMDC operation time and flux profile, feed thermal water softening was conducted prior to submerged VMDC test with no agitation and with transverse vibration, due to the rapid flux decline in those experiments. As shown in Figure 12, with no agitation, the addition of thermal water softening resulted in more than 30 hours of stable and high flux. Meanwhile, with the transverse vibration, 20 hours of stable and high flux was achieved. Significant improvement in productivity was achieved by the inclusion of the pre-treatment procedure in both cases. Those results suggested the benefit of allowing nucleation and crystallization processes took place during the thermal water softening before the membrane was inserted to the feed solution. As the CaCO3precipitatedin the feed tank beforehand, the membrane was inserted to a less saturated feed solution. When the feed solution had a lower saturation index, the tendency of scale formation on the membrane surface was lower, therefore enabling the membrane to operate longer. However, the rapid flux decline was still observed for all tests with and without transverse vibration, which showed that the inevitable crystals formation at the membrane surface.

(a)

(b)

23

Figure 12 Effect of thermal water softening on submerged VMDC performance at 44.9 mS/cm feed conductivity tests with (a) no agitation and (b) transverse vibration.

The calcium and magnesium concentration in the feed solution before and after the thermal water softening characterized using ICP-OES and are shown in Table 5. It was suggested that the concentration of calcium in the feed was drastically reduced (from 378 mg/L to 97.6 mg/L) after thermal water softening, by more than 70% was precipitated as CaCO3. Meanwhile the concentration of magnesium in the feed solution barely changed. In this case, at the time the submerged VMDC operation started, the membrane was exposed to a feed solution of significantly lower calcium concentration than the initial feed. Table 5 Calcium and magnesium concentration in the feed solution prior and after thermal water softening

Cations Ca2+ Mg2+

Before thermal softening (mg/L) 378 1152

Feed solution After thermal softening (mg/L) 97.6 1149

The images of CaCO3 deposition on the membrane surface during submerged VMDC without agitation; with and without thermal water softening procedure were given in in Figure 13. After 5 hours of test, the membrane surface of submerged VMDC without thermal water softening was fully covered by CaCO3. The cross section images showed that the CaCO3layer has a non-uniform thickness ranged between 28-53 µm. When the thermal water softening was added prior to the tests, CaCO3deposited at membrane surface was significantly less, with most pores still visible and a thin layer of CaCO3 at the cross section. After 25 hours of test without thermal water softening, a denser layer of CaCO3was formed on the membrane and completely covered the membrane pores. Meanwhile, in the experiment with the added thermal water softening procedure, the CaCO3 on the membrane surface has not formed a continuous layer and some pores still visible. These results suggested that the thermal water softening delayed the heterogeneous nucleation at the membrane surface for a certain period of time, but it did not completely eliminate the heterogeneous nucleation.

Time (h)

No thermal water softening Surface Cross section

Thermal water softening Surface Cross section

24

5

15

25

Figure 13 SEM of membrane surface and cross section during submerged VMDC operation no agitation with and without thermal water softening According to Lyster et al., the rate of nucleation of calcium carbonate at the membrane surface is a function of uncovered membrane surface area and can be calculated as follow: Equation 2 Where θ is the fractional membrane area covered by crystal, N is the crystal number density and t is the observation time [43]. From the observation of SEM images at different times during the submerged VMDC tests, the fractional membrane area covered by CaCO3 and the CaCO3 number density could be determined and the nucleation rate of CaCO3 calculated. For the submerged VMDC test without thermal water softening, the observation was terminated at 3 hours since the membrane fractional covered area has approached unity (Figure 14).

25

(a)

(b)

Figure 14 (a) Crystal number density and (b) fractional covered area of the membrane surface during MD operation As shown in Figure 14, both the crystal number density and the fractional covered area in both tests with and without the thermal water softening increased with time, indicating continuous nucleation with time. However, in the tests without thermal water softening, the crystal number density reached a peak after 2 hours, with a value of 0.0095 crystal/µm2. In addition to that, the fractional covered area reached unity, indicating the absence of visible pores on the membrane surface after 3 hours. In the experiment with thermal water softening, the fractional covered area was only 0.7 after 15 hours. The nucleation rates of CaCO3 during the tests were calculated and the results showed that the nucleation rate of the test without thermal water softening (0.011068 crystals/h/µm2) was 13 times higher than the test with thermal water softening (0.000815 crystals/h/µm2). 4. Conclusions This study demonstrated that sustainable performance was achievable in application of submerged VMDC for treatment of inland brine with transverse vibration of membrane as a form of agitation. The operation was lasted for more than 700 hours and the feed conductivity increased for almost three times its initial value; from 22.9 mS/cm to 61.2 26

mS/cm. Although the submerged VMDC maintained a reasonable performance at high feed conductivity, when the initial feeds of conductivity at 30.7 mS/cm and 44.9 mS/cm were applied, the flux of submerged VMDC operation was declined rapidly due to severe fouling at the membrane surface. This suggested the importance of crystal formation history in the feed solution because the distribution of salts species in the feed solution may have changed during the prolonged operation with low initial feed conductivity. At accelerated tests with feed of 44.9 mS/cm conductivity, transverse vibration and aeration could reduce the concentration and temperature boundary layer thicknesses at membrane surface and increase the initial flux before the occurrence of membrane fouling. However, the reduction of boundary layer thickness resulted on higher temperature at the membrane surface which led to more rapid formation of CaCO3on the membrane with the consequence of rapid blockage of membrane pores, particularly on the experiment with transverse vibration. Apart from the feed agitation to reduce concentration and temperature polarization layer thicknesses, aeration in the feed solution also created competitive heterogeneous nucleation and crystallization sites in the solution, evidenced with almost 100% increase in crystal yield from the feed at the end compared to the test without aeration. The reduced salt concentration in the feed due to high crystal yield led to longer high flux operation time. Delayed introduction of membrane vibration and aeration did not significantly change the onset and the rate of the crystal formation on the membrane surface. The reduction of boundary layer thickness by introduction of feed agitation at the delayed time and the increased temperature at the membrane surface led to faster precipitation of crystal on the membrane as CaCO3 has a negative temperature solubility - correlation. This indicated that once the precipitation of the salt ions had occurred, reduction in polarisation layer thickness did not significant affect the overall crystal formation and growth on the membrane surface. Thermal water softening could reduce the calcium concentration on the feed solution by precipitation prior to submerged VMDC operation and resulted in significantly slower nucleation rate of CaCO3 fouling on the membrane surface. As the fouling process was also dictated with the species in the feed solution and influence by their solubility at the feed temperature, consideration of the properties of the species in the feed solution is also important in achieving overall performance. Acknowledgement This research was supported under Australian Research Council's Discovery Projects funding scheme (DP130104048). Helen Julian gratefully acknowledges the financial support of Indonesian Endowment Fund for Education.

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Highlights • Transverse vibration and feed aeration improved mass transfer. • Feed aeration delayed fouling by increasing crystal formation in the bulk solution. • Thermal water softening reduced the CaCO3 nucleation on the membrane surface. • Crystal formation history played an important role on submerged VMDC performance.

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