Effect of shear rate on fouling in a Vibratory Shear Enhanced Processing (VSEP) RO system

Journal of Membrane Science 366 (2011) 148–157

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Effect of shear rate on fouling in a Vibratory Shear Enhanced Processing (VSEP) RO system Wei Shi ∗ , Mark M. Benjamin Dept. of Civil and Environmental Engineering, University of Washington, Seattle, WA 98195-2700, United States

a r t i c l e

i n f o

Article history: Received 23 July 2010 Received in revised form 24 September 2010 Accepted 27 September 2010 Available online 23 October 2010 Key words: VSEP Reverse osmosis Shear rate

a b s t r a c t Membrane fouling due to formation of inorganic scales limits the application of reverse osmosis (RO) technology for desalination. This study continued our investigation of the performance of RO membranes in a Vibratory Shear Enhanced Processing (VSEP) system during the treatment of a simulated brackish water source and a brine. Increasing vibration amplitude decreased membrane fouling, increased rejection of most solutes and changed the morphology of the scales from a tightly packed layer to a more scattered distribution of particles that is easier for water to permeate. Decreasing pH reduced fouling slightly, and the presence of natural organic matter displayed no substantial effect. When all but a thin ring of the membrane surface was sealed by impermeable epoxy resin, fouling decreased as the shear rate increased. In these tests, the fouling rate was almost identical in paired tests in which the same average shear rate was achieved by different combinations of ring radius and vibration amplitude. The shear rate appears to control fouling by determining the structure of the scale deposit that forms early in a filtration run. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Like all membrane-based separations, desalination by reverse osmosis (RO) is limited by the tendency of the membranes to become fouled by biological, organic and inorganic contaminants. The study described here comprised research into the use of a Vibratory Shear Enhanced Processing (VSEP) membrane system (New Logic Research, Emeryville, CA) to treat either saline raw water or the brine generated by more conventional RO treatment of such water. VSEP technology has been used successfully in more than 200 commercial-scale industrial applications treating extremely challenging wastewater [1–5], but has been investigated much less extensively for potential applications in the potable water industry. A brief review of these applications can be found in a recent paper published by our group [6]. In that study, membrane vibration reduced fouling of RO membranes and improved solute rejection in the treatment of a simulated brackish water source and a brine. SEM images indicated that vibration changed the morphology of the scale deposited on the membrane surface from a uniform layer of needle-like solids to a smoother layer that was apparently more hydraulically conductive. Similarly, Ahmed et al. [7] reported recently that increasing vibration amplitude in a VSEP NF system improved rejection of arsenic species and reduced fouling.

∗ Corresponding author. Tel.: +1 713 348 2149. E-mail address: [email protected] (W. Shi). 0376-7388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2010.09.051

The mechanism of fouling reduction in VSEP systems is undoubtedly related to the shear imparted to the solution at the membrane surface. Akoum et al. [8] presented an approach for estimating that shear rate, suggesting that the time-average shear at a specific radius could be calculated as R =

 0.5

2AR 2 (F)1.5 Rmax 

(1)

where R is the radius of interest, F the vibration frequency,  the fluid kinematic viscosity, and A the total displacement (twice the vibration amplitude) at the membrane periphery (where R = Rmax ). In the same study, the authors investigated the radial distribution of permeate flux during microfiltration of yeast using a series of membranes that were partially sealed with epoxy, so that only rings at certain radii were open to water permeation. When these systems were fed a suspension containing 20 g/L yeast and were operated at different TMPs, the stable permeate flux increased with increasing ring radius. The local permeate flux (JR ) was related to  R by JR = K(R )0.426

(2)

Jaffrin et al. [9] compared the performance of a VSEP system with that of two rotating disk systems for microfiltration of baker’s yeast and ultrafiltration of skim milk. In one of the disk systems, the disks were smooth on both sides, and in the other, they had radial vanes on the side facing the membrane to generate greater shear. Data from the three systems converged to a single correlation between permeate flux and average shear rate, suggesting that

W. Shi, M.M. Benjamin / Journal of Membrane Science 366 (2011) 148–157 Table 1 Specifications of membranes used in tests. Membrane

BW-30

FE

MWCO (da) Material pH Tmax (◦ C) Cl2 Tolerance (mg/L) Fluxa (L/m2 h) Vendor

50 Polyamide 1–12 70 <0.1 71 Filmtec

30 Polyamide 2–11 60 <0.1 90 Saehan

a At a TMP of 2068 kPa. (Note: to convert from L/m2 h to gal/ft2 d, multiply by 0.59.)

the effect of shear on permeate flux was independent of the flow pattern generating the shear. This inference was supported by similar tests involving nanofiltration of dairy process waters [2] and ultrafiltration of oil-in-water emulsions [10]. The study reported here investigated salt removal and membrane fouling in a VSEP RO system treating a simulated brackish solution and a simulated RO brine. The effects of vibration amplitude, feed solution composition, and shear rate were examined. 2. Materials and methods 2.1. Membranes The membranes used in all tests were circular flat sheets with an outer diameter of 28.3 cm and a 7.8-cm-diameter hole cut out of the center for the vibrating shaft, yielding an effective surface area of 580 cm2 . Two types of RO membranes were used in the tests, with the characteristics summarized in Table 1. The FE membrane was initially selected for all tests, but was unavailable toward the end of the project. Therefore, BW-30 membranes, which had fouling behavior and salt rejection characteristics similar to the FE membranes, were used in the painted-membrane tests. 2.2. Reagents and feed solutions All chemicals used in the preparation of feed were reagent grade. Two synthetic feed solutions, one a simulated brackish drinking water source and the other a simulated brine from a conventional RO treatment process, were prepared. The baseline composition of the brackish solution is shown in Table 2. The conductivity of the solution was 1065 ␮S/cm at 25 ◦ C. The brine contained the same constituents in the same relative concentrations as the brackish solution, but it was 10 times as concentrated; i.e., the brine simulated the concentrate solution that would be generated by treating the brackish solution under conditions of 90% recovery and 100% salt rejection. In a few experiments, the initial composition of the brackish water was altered. In one set of experiments, the pH of this solution Table 2 Synthetic brackish water feed composition. Chemical pH

BaCl2 ·2H2 O CaCl2 CaSO4 FeSO4 ·7H2 O MgSO4 ·7H2 O Na2 SO4 NaHCO3 SiO2

149

was adjusted to 6.0, 7.0, or 8.0 with HCl or NaOH. In another, a solution was prepared containing all the constituents of the baseline water at twice the baseline concentrations and was mixed 1:1 with water from Lake Washington (Seattle, WA). The mixture therefore had approximately the same inorganic composition as the simulated brackish water, but also contained natural organic matter (NOM) at a concentration of approximately 1.0 mg/L as dissolved organic carbon (DOC). Another solution with a higher NOM concentration was prepared by adding all chemicals for the baseline brackish solution into water from Lake Pleasant (Bothell, WA), so that it had the same inorganic composition as the baseline brackish water, but an NOM concentration of 14.8 mg/L as DOC. These tests were conducted only at an initial pH of 8.0. 2.3. VSEP unit structure and operation An L-mode VSEP unit (laboratory mode, with only one membrane in the VSEP stack) was employed for all tests. A detailed description of this unit can be found in our previous papers [6,11]. In this unit, the membrane rotation can be adjusted to sweep an angle between 0◦ and 13◦ (6.5◦ in each direction from the rest point), corresponding to displacements of 0–3.18 cm (1.59 cm in each direction) at the outer membrane perimeter. The rotation frequency is close to 55 Hz regardless of the displacement. At the maximum rotation amplitude, the membrane velocity ranges from 0 to 210 cm/s, with an average of 134 cm/s. The tests investigating treatment of the synthetic brackish solution used 80 L of feed, and those investigating the synthetic brine used 60 L. In the baseline tests, the system was operated at a TMP of 965 kPa and the maximum rotation amplitude. In tests exploring the effects of vibration amplitude on system performance, amplitudes of 0, 0.32, 0.95, and 1.59 cm at the membrane perimeter were investigated. The temperature of the concentrate was controlled at ∼30 ◦ C by a refrigerated circulator. In all the tests, the concentrate was returned to the feed reservoir, so the feed became progressively more concentrated as the test proceeded. The average flux across the whole membrane (J) was measured by collecting and weighing the permeate flow intermittently during the run. Samples of the feed and permeate were collected intermittently throughout the run and were stored at 4 ◦ C for subsequent chemical analysis. At the end of most experiments, membrane specimens for SEM imaging were obtained from four locations on the membrane, approximately equally spaced radially between the inner and outer edges of the membrane disk. The painted membrane tests were designed to explore the effect of shear rate on RO fouling. In these tests, the majority of membrane surface was sealed with epoxy resin so that only a thin ring (about 1 cm thick) at a certain radius, accounting for about 10% of the total membrane surface area, remained open for water permeation. The tests comprised four sets of paired experiments, in each of which the same shear rate on the permeation area was achieved using two different combinations of ring radius and vibration amplitude (Table 3). The time-average shear rate on the permeation ring was Table 3 Vibration amplitudes and permeation ring radii used in painted membrane tests.

8.0 Concentration (g/L)

(mol/L)

0.0111 0.269 0.0442 2.5 × 10−5 0.246 0.328 0.252 0.0107

4.45 × 10−5 2.42 × 10−3 3.26 × 10−4 8.95 × 10−6 9.99 × 10−4 2.31 × 10−3 3.00 × 10−3 1.79 × 10−4

Group

Experiment

Vibration amplitude (cm)

Ring radius (cm)

Shear rate (s−1 )

1

1 2 1 2 1 2 1 2

0.64 0.32 1.27 0.64 1.27 0.95 1.59 1.27

5 10 5 10 10 13 10 13

1.0 × 104

2 3 4

2.1 × 104 4.1 × 104 5.3 × 104

150

W. Shi, M.M. Benjamin / Journal of Membrane Science 366 (2011) 148–157

estimated according to Eq. (1). These tests were conducted only with the brine solution as feed.

3. Results and discussion 3.1. Baseline tests

2.4. Analytical: SEM, XRD, EDAX, individual ions SEM images were acquired using a JEOL 7000 electron microscope. Portions of the specimens were analyzed using energy dispersive X-ray analysis (EDAX) to identify the elemental composition of any scales that were present on the membrane surface. Crystal structures of solids that had accumulated in the feed solution when filtration was terminated were analyzed by X-ray diffraction (XRD), using a Bruker D8 Focus X-ray Diffractometer. Total dissolved concentrations of specific elements were measured using ICP-MS (Perkin Elmer ELAN DRC-e) or ion chromatography (IC, Dionex 500). Solids were removed from the concentrate samples by 12 h of quiescent settling or 0.45-␮m filtration before the supernatant was taken for analysis. Conductivity of the feed and permeate samples was measured with an OAKTON® Waterproof ECTestr® conductivity meter. 2.5. Data processing The progress of the filtration process was quantified in terms of either recovery (r, Eq. (3)) or specific volume filtered (Vsp , Eq. (4)). Cumulative Permeate Volume Initial Feed Volume Cumulative Permeate Volume Membrane Area

(4)

System performance was then assessed by plotting the permeate flux and effective specific flux (Eq. (5)) as a function of recovery. Average Flux J = Effective TMP TMP − 

(7)

(5)

(J{Vsp,2 }/J0 ) − (J{Vsp,1 }/J0 ) J{Vsp,2 } − J{Vsp,1 } = Vsp,2 − Vsp,1 J0 (Vsp,2 − Vsp,1 ) (m2 /L);

where rfoul = volume-based fouling rate (L/m2 h); J{Vsp } = flux at a certain Vsp (L/m2 h).

0.06

50

0.05

40

0.04 30 0.03 20

A

10

0.02

Flux

0.01

Effective Specific Flux 0

0 0.0

0.2

0.4

0.6

0.8

1.0

Recovery 25

0.05

20

0.04

15

0.03

10

0.02

B 5

0.01

Flux Effective Specific Flux

0

(6)

J0 = initial flux

Effective Specific Flux (L/m2-h-kPa)

where J = average permeate flux across the whole membrane (L/m2 h); Jsp,eff = effective specific permeate flux (L/m2 h kPa);  = osmotic pressure difference across the membrane (kPa). The speciation of the solutions was modeled using the measured total concentrations of the solutes as inputs to the chemical equilibrium software package Phreeqc® . The output of modeling included concentrations of individual species. The sum of the molarities of all individual species was then inserted into the van’t Hoff equation to compute the osmotic pressure () of the solution. Because the effect of the changing osmotic pressure on the flux is eliminated in Eq. (5) (by subtracting the transmembrane osmotic pressure differential from the total TMP in the denominator), the effective specific flux reflects the overall hydraulic conductivity between the bulk concentrate and permeate, and any decline can be attributed to the combined effect of concentration polarization (CP) and membrane fouling. In the painted membrane tests, recovery was small (usually less than 5%) throughout the test because of the small permeation area, so the specific volume filtered (Vsp ) was used to indicate filtration progress. Also, in these tests, the composition of the feed solution and therefore its osmotic pressure were almost constant because of the low water recovery. The relative flux, defined as the permeate flux normalized to the initial flux, was plotted against Vsp to indicate the rate of fouling development. The volume-based fouling rate over a given range of Vsp was then defined as the reduction in relative flux over that Vsp range divided by the change in Vsp (Eq. (6)). rfoul =

Cb Cm,R

Effective Specific Flux (L/m2-h-kPa)

Jsp,eff =

JR = −kmt,R ln

Flux (L/m2-h)

Vsp =

(3)

Flux (L/m2-h)

r=

3.1.1. Water flux The flux and effective specific flux profiles for treatment of the brackish solution and brine in the baseline tests are plotted in Fig. 1. The reduction in gross flux reflects the combined effects of fouling and the buildup of osmotic pressure in the feed solution, whereas the change in the effective specific flux isolates the effect of fouling alone. The test with the brackish water was terminated at a recovery of 90%, at which time the gross flux had declined by 55%, but the effective specific flux had declined by only 20%. The osmotic pressure difference across the membrane was about 48 kPa at the beginning of the test (zero recovery) and 448 kPa at 90% recovery. Thus, the majority of flux reduction was caused not by membrane fouling or CP, but by the increase in the osmotic pressure of the concentrate and the corresponding decrease in the driving force for permeation. A similar trend was observed when the brine was treated, except that the flux decreased more rapidly than when the brackish water was treated. This test was terminated at 75% recovery, at which point the osmotic pressure difference had increased from 379 to 827 kPa, and gross and effective specific fluxes had declined by 90% and 20%, respectively. The flux data were tested for their fit to the film model with the assumption that solute concentration of the permeate was negligible compared that of the concentrate. The steady state permeate flux at radius R would be thus given by

0 0.0

0.2

0.4

0.6

0.8

1.0

Recovery Fig. 1. Profiles of flux and effective specific flux for RO treatment of (A) the brackish water and (B) the brine, using the FE membrane.

W. Shi, M.M. Benjamin / Journal of Membrane Science 366 (2011) 148–157

60

Brackish Water Brine

50

Flux (L/m2-h)

40

30

20

10

0 0

0.4

0.8

1.2

1.6

2

ln (Cb /Cb,0) Fig. 2. Variation of permeate flux as a function of ln(Cb /Cb,0 ) for filtration of the brackish solution and the brine.

where kmt,R = the mass transfer coefficient in the concentration polarization (CP) layer at R; Cb = solute concentration in the bulk concentrate; Cm,R = solute concentration at the membrane surface at R. Eq. (7) predicts that, in a system at steady state, the flux at any radius depends on the ratio of the solute concentration in the bulk concentrate to that at the membrane surface (Cb /Cm ). In this study, Cb was continuously increasing during each test due to the circulation of rejected materials back to the feed solution, so the system was not at steady state. Nevertheless, if the CP layer responded rapidly to changes in the feed composition, Eq. (7) would apply at any instant. Values of Cb were measured, but those of Cm are not known. In addition, as suggested by the equation, the mass transfer coefficient, the concentration at the membrane surface, and the flux could all vary as a function of radius in a VSEP system. Here, unless stated otherwise, the approximation was made that Eq. (7) can be written using area-average values of those parameters (kmt , Cm , and J, respectively) to describe the prediction of the film model for the performance of the whole membrane, i.e. J = −kmt ln

Cb Cm

(8)

Two limiting cases for the variation of Cm during the test can be considered: (1) Cm remained constant even though Cb increased, and (2) Cm changed in proportion to Cb throughout the test. For a constant kmt , the first limiting case would lead to a linear relationship between flux and the logarithm of Cb , and the second would lead to a constant flux. To test these possibilities, it is convenient to rewrite Eq. (7) with Cb and Cm in reduced form, by dividing each by the initial solute concentration in the bulk concentrate (Cb,0 ), yielding J = −kmt ln

Cb Cm + kmt ln Cb,0 Cb,0

(9)

The fluxes for both the brackish water and the brine are plotted against the logarithm of Cb /Cb,0 in Fig. 2, using conductivity as an indicator of the total solute concentration. The flux profile in the test with the brine is characterized by two stages—an initial period with almost constant flux, and a second period in which the flux decreased approximately linearly with ln(Cb /Cb,0 ). A similar, twostage pattern is suggested by the data for the brackish water, but is less certain. The nearly constant flux in the initial stage is consistent with the limiting condition of Cm increasing in proportion to Cb ,

151

whereas the linear flux decline in the latter stage fits the limiting condition of constant Cm . If Cm was constant during the second stage, the test results with both feed solutions would be consistent with a kmt of 4.2 × 10−4 cm/s during that stage. This value is lower than those reported for typical RO processes with crossflow (generally on the order of 10−3 cm/s), [12,13] even though the shear rate at the membrane surface was orders of magnitude higher in the current study than in conventional RO systems. Other factors being equal, an increase in the shear rate would be expected to increase kmt [14]. A possible explanation for the lower value of kmt in the current work is that more solids accumulated on the membrane than in prior studies. The accumulation of solids on the membranes has rarely been reported in such studies; scale deposition in the VSEP tests is discussed shortly. If kmt was constant during the second stage of the tests, the solute concentration (conductivity) at the membrane surface (Cm ) during that stage can be estimated from the y-intercept of the linear portion of the curves in Fig. 2. Based on Eq. (9), this concentration is Cm = Cb,0 exp

 J∗  kmt

(10)

where J* is the y-intercept obtained by extrapolation of the linear relationship between J and ln Cb /Cb,0 = 0 during the second stage. The calculated values of Cm are 3.7 × 104 ␮S/cm for the brackish water and 7.6 × 104 ␮S/cm for the brine, corresponding to polarization factors of 4.5 and 1.2 for the respective solutions at the end the tests. The assumption of constant kmt in the CP layer is more reasonable during the first stage of the tests, when scales were first forming and accumulating on the membrane, than later, when kmt would be expected to decline due to cake-enhanced concentration polarization as the scale layer grew [15,16]. If kmt was decreasing during the second stage, the observed behavior (i.e., the results in Fig. 2) would still be consistent with a steady increase in Cm during that stage, but at a rate that was less than proportional to the increase in Cb . The fact that the flux decline began earlier with the brackish water than the brine is difficult to explain, since solids presumably accumulated earlier in the run and more extensively when the system was fed the brine. 3.1.2. Solute rejection and solid precipitation Excellent overall salt rejection was achieved during these tests using either feed solution, with >90% rejection of most major solutes when averaged across the whole run [6]. Other aspects of solute rejection and solid precipitation also followed similar trends in experiments with the two feed solutions. The discussion here focuses on the tests with brine; results for the brackish water are available in reference [17]. The fluxes of Na, Mg, Cl and SO4 across the membrane were approximately constant early in the test, but they increased dramatically at recoveries greater than approximately 0.5 (Fig. 3A). The increases were 160% for Na, 75% for Cl, 350% for Mg and 90% for SO4 over the course of the test. This result suggests that the concentrations of these ions at the membrane surface increased as filtration progressed, consistent with the hypothesis that scale growth caused the mass transfer coefficients of these ions (as well as water) in the CP layer to decrease during the latter portions of the test. The fluxes of Ca and Si, however, followed different trends (Fig. 3B). The flux of Ca decreased from an initial value of 400 mg/m2 h to about 300 mg/m2 h at 40% recovery, and then remained stable until the end of the test, and that of Si remained stable in the range of 40–50 mg/m2 h throughout the test. These results suggest that the concentrations of Ca and Si at the membrane surface might have been nearly constant during the test, presumably

W. Shi, M.M. Benjamin / Journal of Membrane Science 366 (2011) 148–157

Cl SO4

10000

500

A

400

Na Mg

8000

300 6000 200 4000 100

2000

250

200

SO4 Mass (g)

12000

Mg Flux (mg/m2-h)

Cl, SO4, Na Flux (mg/m2-h)

152

Precipitate

150

100

Dissolved (Feed) 50

A

0

0 0.0

0.2

0.4

0.6

0.8

Permeate

0

Recovery

0.0

0.2

500 Ca

0.6

0.8

80

Si

400

60

300

Ca Mass (g)

Flux (mg/m2-h)

0.4

Recovery

200

B

100

Precipitate(CaCO3) Precipitate(CaSO4)

40

Dissolved (Feed)

20

0 0.0

0.2

0.4

0.6

0.8

B

Recovery

Permeate

0 Fig. 3. Fluxes of major solutes through the membrane during treatment of the brine. The flux of Mg is plotted on the secondary axis of Fig. 3A.

0.2

0.4

0.6

0.8

Recovery 3200

Si Mass (mg)

governed by the presence of solids containing those species (most likely CaCO3 or CaSO4 controlling the Ca concentration, and SiO2 controlling the Si concentration). The mass partitioning of some of the solutes among the bulk feed solution, the permeate, and precipitated solids is summarized in Fig. 4. The solid horizontal line at the top represents the total mass of each species in the system initially, and the difference between adjacent curves is the mass in the particular phase as labeled. SO4 and CO3 were assumed to precipitate only as the corresponding Ca solids. The amounts shown for the precipitates include both the solids in suspension and those that deposited on the membrane surface. The vast majority of this mass remained in suspension, with only approximately 0.5% of the total solids deposited; the distribution of individual types of solids between the suspension and the membrane surface was not investigated. Sulfate-containing precipitates were present in the initial feed solution, and the total mass of such solids remained relatively stable thereafter (Fig. 4A). By contrast, the mass of Ca in precipitates increased continuously, from an initial value representing about 50% of the total Ca in the system, to over 80% at 75% recovery (Fig. 4B). Almost all of this increase was attributable to precipitation of CaCO3 , which accounted for ∼50% of the Ca in the solids at the beginning of the run and ∼70% at the end. The partitioning of Si among different phases is shown in Fig. 4C. The initial brine contained about 25 mg/L of dissolved Si, representing about 50% of the total Si. Like Ca, Si was converted from soluble to insoluble forms as the feed became more concentrated. At 75% recovery, the soluble and insoluble (precipitated or adsorbed) Si in the concentrate accounted for 20% and 75%, respectively, of the Si mass initially present, with the remaining 5% having passed into the permeate. Even assuming the insoluble Si was all present as solid SiO2 , it accounted for less than 2% of the total mass of solids in the system. The inferences regarding the identities of the precipitated solids were supported by XRD spectra of the solids collected from the

0.0

Precipitate

2400

1600

Dissolved(Feed)

800

C

Permeate

0 0.0

0.2

0.4

0.6

0.8

Recovery Fig. 4. Partitioning of solutes in different phases during treatment of the brine.

concentrate suspension at the end of the test, which indicated the presence of aragonite (CaCO3 ) and gypsum (CaSO4 ·2H2 O) (Fig. 5). EDAX analysis of the solids deposited on the membrane (Fig. 6) detected Ca, Si, C, and O, but not S, on the surface of those solids. This result suggests that CaCO3 and some form of Si were present in the scales on the membrane surface. The failure to detect Si-containing suspended solids by XRD indicates that the Si was not present as an identifiable crystalline phase, but rather in other structures such as non-crystalline (i.e., amorphous) solids or as adsorbed species. The absence of S in the EDAX spectra was attributed to masking of the S peak by a peak associated with Pt. 3.2. Effects of operational parameters 3.2.1. Vibration amplitude The effects of the vibration amplitude on the flux and effective specific flux profiles for treatment of the brine are shown in Fig. 7.

W. Shi, M.M. Benjamin / Journal of Membrane Science 366 (2011) 148–157

153

25 0 0.32 cm 0.95 cm 1.59 cm

Flux (L/m2-h)

20

15

10

5

0 0.0

A 0.2

0.4

0.6

0.8

Recovery 0.05

0 0.32 cm 0.95 cm 1.59 cm

Effective Specific Flux (L/m2-h-kPa)

Fig. 5. XRD spectrum of solids collected at the end of the baseline tests.

0.04

0.03

0.02

0.01

0.00 0.0

B 0.2

0.4

0.6

0.8

Recovery Fig. 7. (A) Flux and (B) effective specific flux profiles for treatment of the brine at different vibration amplitudes. Fig. 6. A typical EDAX spectrum of the scales that formed on the membrane surface in the baseline tests. The Pt peaks are present because the solids were coated with Pt for SEM observation.

Increasing vibration amplitude decreased membrane fouling and led to reduced permeate concentrations as well as increased rejection of most solutes (Table 4). The scale morphology changed from a tightly packed layer in the non-vibrating system to a more open, scattered layer in the system with the maximum vibration (Fig. 8). The incremental effect was slight as the vibration amplitude was increased from 0 to 0.95 cm, but more dramatic when it was then increased to 1.59 cm. 3.2.2. Effects of pH and the presence of NOM In tests of the brackish water at initial pH’s of 6, 7 and 8, all three feed solutions converged toward a pH near 7.5 as the runs proceeded (Fig. 9). The pH change can be accounted for by a combination of CO2 loss to the atmosphere (which leads to an increase in pH) and precipitation of CaCO3 (which causes pH to decline). For a given concentration of total dissolved carbonate (the sum of the concentrations of H2 CO3 , HCO3 − , and CO3 2− ), the concentration of

H2 CO3 steadily decreases and that of CO3 2− steadily increases with increasing pH. Therefore, CO2 volatilization becomes less favorable and CaCO3 precipitation becomes more favorable as pH increases. The changing balance between these two processes can account for the drift in pH toward a constant, intermediate value in the different solutions. The saturation indices of the solutions with respect to CaCO3 and CO2 are shown in Fig. 10. The solution at pH 6 was undersaturated with CaCO3 initially, and it remained so until near the end of the experiment. By contrast, this solution was initially saturated with respect to CO2 and became progressively more supersaturated as filtration proceeded. This supersaturation developed primarily as a result of the removal of nearly pure water from the solution and the concomitant increase in the CO2 concentration. Once the CO2 concentration increased above the value for equilibrium with the atmosphere, CO2 evolved out of solution, causing pH to increase. The solution at pH 7 followed a similar trend, except that the variation in pH was less significant because CaCO3 became supersaturated and began precipitating in the middle of the experiment, thereby partially counteracting the effect of CO2 release. At pH 8,

Table 4 Permeate concentrations and average rejections of major solutes at 50% recovery during treatment of the brine. Permeate concentration (mg/L); rejection A (cm)a

Mg

Ca

Na

Si

Cl

SO4

0 0.32 0.95 1.59

6.4; 0.98 7.0; 0.98 5.3; 0.98 4.4; 0.99

38.9; 0.94 33.1; 0.95 25.2; 0.96 20.2; 0.97

296; 0.86 256; 0.88 218; 0.89 213; 0.90

0.50; 0.95 0.38; 0.98 0.25; 0.99 0.34; 0.97

361; 0.81 241; 0.89 189; 0.91 186; 0.92

122; 0.96 345; 0.91 234; 0.94 318; 0.92

a

Vibration amplitude.

154

W. Shi, M.M. Benjamin / Journal of Membrane Science 366 (2011) 148–157

Fig. 8. SEM images of membrane fouled at vibration amplitude of 0 (NV), 0.32 cm (V1), 0.95 cm (V2) and 1.59 cm (V3) during treatment of the brine. All specimens are taken from a location 11.6 cm from the center of the membrane.

was not vibrated, and the test achieved only 36% water recovery as a result. In the vibrating system, however, fouling was significantly reduced, allowing the recovery to reach about 90%. In fact, when the membrane was vibrated, fouling developed at almost the same rate

Saturation Index (CaCO3)

the solution was saturated with CaCO3 and slightly supersaturated with CO2 throughout the test, and the decrease in pH indicates that CaCO3 precipitation had a greater impact than CO2 release in this system. The profiles of relative flux for treatment of these solutions are plotted in Fig. 11. Fouling developed at almost the same rate in all three tests, with perhaps slightly more fouling in the test with an initial pH of 8 than in the other two tests. The minimal effect of pH on fouling, despite the fact that CaCO3 becomes less soluble as pH increases, probably reflects the convergence of the pH values in the three systems, as noted above. Profiles of relative flux in tests in which the feed was a mixture of synthetic brackish water and lake water containing NOM are plotted in Fig. 12. The corresponding profile at pH 8 for the brackish solution without NOM is included for comparison. The RO membrane was severely fouled in the test with NOM from Lake Pleasant at an initial DOC concentration of 14.8 mg/L when the membrane

1 0 -1 -2 -3

pH 6

A

-4

pH 7 pH 8

-5 0.0

0.2

0.4

0.6

0.8

1.0

Recovery 1.0

Saturation Index (CO2)

9

pH

8

7 Initial pH 6 Initial pH 7 Initial pH 8

6

0.5

0.0

pH 6

-0.5

pH 7

B

5

pH 8

-1.0

0.0

0.2

0.4

0.6

0.8

1.0

Recovery Fig. 9. Change in pH of the brackish solution during the treatment tests at different initial pH’s.

0.0

0.2

0.4

0.6

0.8

1.0

Recovery Fig. 10. Saturation indices of (A) CaCO3 and (B) CO2 during treatment of the brackish solution during the tests at different initial pH’s.

1.0

0.0

0.8

-0.2

ln(J/J0)

Relative Flux

W. Shi, M.M. Benjamin / Journal of Membrane Science 366 (2011) 148–157

0.6

155

-0.4 R² = 0.97 -0.6

0.4

-0.8

pH 6 0.2

pH 7

-1.0

pH 8

9.0

9.4

9.8

0.0 0.0

0.2

0.4

0.6

0.8

1.0

Recovery

10.2

10.6

11.0

ln(shear rate) Fig. 14. Relative flux at Vsp = 300 L/m2 for painted membrane tests at different shear rates.

Fig. 11. Relative flux profiles for treatment of the brackish solution at different initial pH’s.

3.3. Painted membrane test 1.0

The relative flux profiles for the painted membrane tests with the brine are plotted against the specific permeate volume in Fig. 13. Four sets of paired experiments were conducted, in each of which the same shear rate on the permeation area was achieved using two different combinations of ring radius and vibration amplitude. Increasing shear rate reduced membrane fouling, but in all cases, the profiles of relative flux were similar throughout the filtration process for the two experiments at a given shear rate. This result supports the hypothesis that the shear rate is the governing parameter that determines membrane fouling in the VSEP-RO systems and is consistent with the prior studies that emphasized the role

Relative Flux

0.8

0.6

0.4 No NOM, w/ vibration L. Washington, w/ vibration

0.2

L. Pleasant, w/ vibration

0.0

0.2

0.4

0.6

0.8

1.0

Recovery Fig. 12. Relative flux profiles in tests with the brackish water at different NOM concentrations. All tests were conducted at an initial pH of 8.

1.0

0.20 Low R, R high A 0.16

High R, R low A

0.12 0.08 0.04

A

0.00 0 0.E+00

2.E+04

4.E+04

6.E+04

− ) Shear Rate (s−1

0.6 R, A 5, 0.64 10, 0.32 5, 1.27 10, 0.64 13, 0.95 10, 1.27 10, 1.59 13, 1.27

0.4

0.2

0.0 0

100

200

300

400

500

Specific Volume (L/m2) Fig. 13. Relative flux profiles during the painted membrane tests. R represents radius in cm, and A vibration amplitude at the periphery in cm.

Fouling Rate of Stage 2 (m2/L)

Relative Flux (J/J0)

0.8

Fouling g Rate of Stage g 1 ((m2/L))

L. Pleasant, w/o vibration 0.0

0.0012 Low R, high A High R, low A

0.0009

R² = 0.94

0.0006

0.0003

B

0 0.E+00

2.E+04

4.E+04

6.E+04

S Shear Rate (s−1) with all three feed waters regardless the presence or concentration of NOM, indicating that fouling of the membrane was insensitive to NOM, at least for the experimental conditions explored in this study.

Fig. 15. Fouling rate at different shear rates for the first (A) and second (B) fouling stages. “Low R, high A” designates the test in each group that has a lower ring radius and higher vibration amplitude, and “High R, low A” designates the other test at a similar shear rate.

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W. Shi, M.M. Benjamin / Journal of Membrane Science 366 (2011) 148–157

Fig. 16. SEM image of the permeation area at (A1) R 5 cm and A 0.64 cm; (A2) R 10 cm and A 0.32 cm; (B1) R 5 cm and A 1.27 cm; (B2) R 10 cm and A 0.64 cm; (C1) R 10 cm and A 1.27 cm; (C2) R 13 cm and A 0.95 cm; (D1) R 10 cm and A 1.59 cm; and (D2) R 13 cm and A 1.27 cm. R represents the radius, and A the vibration amplitude at the periphery.

W. Shi, M.M. Benjamin / Journal of Membrane Science 366 (2011) 148–157

of shear rate in determining fouling of UF and MF membranes in different dynamic filtration systems. The relative fluxes at a specific permeate volume of 300 L/m2 (toward the end of the tests) for all the tests are shown in Fig. 14. The relative flux was related to the time-average shear rate by the following power-law relationship, similar to the result described earlier by Akoum et at. [8] (Eq. (2)): JR /JR,0 = 0.0137R0.38

(11)

In almost all of these tests, the decline in flux (and thus the development of membrane fouling) followed a two-stage pattern, with a steep decline during approximately the first 50–100 L/m2 filtered and a less rapid but stable rate of decrease through the rest of the test. For instance, in one test at the lowest shear rate investigated (vibration amplitude of 0.32 cm and ring radius of 10 cm), the relative flux decreased by 33% during filtration of the first 60 L/m2 of feed, but only by another 25% during filtration of the next 250 L/m2 . For the test at a vibration amplitude of 1.59 cm and ring radius of 10 cm (the highest shear rate investigated), the corresponding numbers are a 10% drop in flux within the first 50 L/m2 filtered, but only 10% additional drop when another 400 L/m2 was filtered. The fouling rates during these two stages are plotted against the shear rate in Fig. 15. The fouling rate during the initial stage was highest in the system with the lowest shear rate, and was lower but approximately the same in all the other systems. However, the fouling rate later in the runs was orders of magnitude lower than at the beginning and was approximately linearly related to the shear rate. SEM images of the permeation areas for the painted membrane tests are shown in Fig. 16. The scales have a similar morphology for the two tests in each paired set, but change significantly as a function of the shear rate. The scales were mostly continuous and compact at the lowest shear rate (1.0 × 104 s−1 ), became scattered as it increased to 2.1 × 104 s−1 , and became more scattered and sparser as it increased further. This dependence of the morphology and amount of scales on shear is consistent with the tests in which the entire membrane was open for permeation [6]. The transition in scale morphology from a continuous layer to discrete patches and the dramatic decrease in the initial fouling rate as the shear rate increased from 1.0 × 104 to 2.1 × 104 s−1 suggest that the initial fouling stage with a rapid flux drop might be a critical period in which the deposit “skeleton” is established, determining the pattern of subsequent deposition. The development of scales in the second stage follows this pattern by growing on the existing skeleton, so that it increases the amount of deposit but affects the morphology only minimally. The shear rate is undoubtedly the key parameter that determines the shape of the skeleton, and the critical value that controls whether the layer is continuous or scattered appears to be in the range between 1.0 × 104 and 2.1 × 104 s−1 , at least for the systems investigated here. 4. Conclusions Single-stage treatment of synthetic feed solutions in a VSEP RO unit achieved water recoveries of 90% for a synthetic brackish solution, and up to 75% for a synthetic brine. Average rejections of major ions were >90%. Before and during treatment, a substantial mass of solids precipitated, most of which was aragonite (CaCO3 ) and gypsum (CaSO4 ·2H2 O). The vast majority of these solids resided in the concentrate suspension rather than depositing on the membrane surface. Increasing vibration amplitude decreased membrane fouling, increased solute rejection and changed the morphology of the

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scales on the membrane surface from a tightly packed layer to a more porous structure that is easier for water to permeate. Neither pH over the range from 6 to 8, nor the presence of NOM at concentrations up to 15 mg/L as DOC had a significant effect on fouling of vibrating membranes, but NOM did foul the membrane severely when it was not vibrated. The shear rate at the membrane surface was the major factor governing fouling of the RO membranes. Fouling developed more slowly as shear rate increased, but was very similar in several paired tests in which the same average shear rate was achieved by different combinations of vibration amplitude and membrane radius. The shear rate appeared to reduce fouling in these tests by controlling the deposit morphology when the deposit was first laid down. Acknowledgments This work was supported by the Water Research Foundation (WRF). The authors thank New Logic Research, Inc., for providing the test unit. Yujung Chang and Pierre Kwan of HDR, Inc., and Sommer Carter of NLR provided valuable technical assistance during the project. The views expressed are those of the authors and do not necessarily reflect those of WRF. References [1] O. Akoum, M.Y. Jaffrin, L. Ding, M. Frappart, Treatment of dairy process waters using a vibrating filtration system and NF and RO membranes, J. Membr. Sci. 235 (2004) 111–122. [2] M. Frappart, O. Akoum, L. Ding, M.Y. Jaffrin, Treatment of dairy process waters modelled by diluted milk using dynamic nanofiltration with a rotating disk module, J. Membr. Sci. 282 (2006) 465–472. [3] J. Lee, S. Kim, J. Choi, H. Hwang, M. Cha, J. Kim, Tertiary treatment of biologically treated piggery wastewater using vibratory shear-enhanced RO membrane, Water Sci. Technol. 49 (5–6) (2004) 435–442. [4] Y. Yoon, Y.S. Ok, D.Y. Kim, J.G. Kim, Agricultural recycling of the by-product concentrate of livestock wastewater treatment plant processed with VSEP RO and bio-ceramic SBR, Water Sci. Technol. 49 (5–6) (2004) 405–412. [5] T. Huuhilo, P. Vaisanen, J. Nuortila-Jokinen, M. Nystrom, Influence of shear on flux in membrane filtration of integrated pulp and paper mill circulation water, Desalination 141 (3) (2001) 245–258. [6] W. Shi, M. Benjamin, Fouling of RO membranes in a vibratory shear enhanced filtration process (VSEP) system, J. Membr. Sci. 331 (1–2) (2009) 11–20. [7] S. Ahmed, M.G. Rasul, M.A. Hasib, Y. Watanabe, Performance of nanofiltration membrane in a vibrating module (VSEP-NF) for arsenic removal, Desalination 252 (1–3) (2010) 127–134. [8] O. Akoum, M.Y. Jaffrin, L.H. Ding, P. Paullier, C. Vanhoutte, An hydrodynamic investigation of microfiltration and ultrafiltration in a vibrating membrane module, J. Membr. Sci. 197 (1) (2002) 37–52. [9] M.Y. Jaffrin, L.H. Ding, O. Akoum, A. Brou, A hydrodynamic comparison between rotating disk and vibratory dynamic filtration systems, J. Membr. Sci. 242 (1–2) (2004) 155–167. [10] N. Moulai-Mostefa, O. Akoum, M. Nedjihoui, L. Ding, M.Y. Jaffrin, Comparison between rotating disk and vibratory membranes in the ultrafiltration of oil-inwater emulsions, Desalination 206 (1–3) (2007) 494–498. [11] W. Shi, M. Benjamin, Membrane interactions with NOM and an adsorbent in a vibratory shear enhanced filtration process (VSEP) system, J. Membr. Sci. 312 (1–2) (2008) 23–33. [12] I. Sutzkover, D. Hazzon, R. Semia, Simple technique for measuring the concentration polarization level in a reverse osmosis system, Desalination 131 (1–2) (2000) 117–127. [13] Z.V.P. Murthy, S.K. Gupta, Estimation of transformation coefficient using a combined non-linear membrane transport and film theory model, Desalination 109 (1) (1997) 39–49. [14] R. Bian, K. Yamamoto, Y. Watanabe, The effect of shear rate on controlling the concentration polarization and membrane fouling, Desalination 131 (1–3) (2000) 225–236. [15] T.H. Chong, F.S. Wong, A.G. Fane, Enhanced concentration polarization by unstirred fouling layers in reverse osmosis: Detection by sodium chloride tracer response technique, J. Membr. Sci. 287 (2) (2007) 198–210. [16] E.M.V. Hoek, M. Elimelech, Cake-enhanced concentration polarization: a new fouling mechanism for salt-rejecting membranes, Environ. Sci. Technol. 37 (24) (2003) 5581–5588. [17] M.M. Benjamin, W. Shi, P. Kwan, Y. Chang, Evaluation of VSEP to Enhance Water Recovery During Treatment of Brackish Water and RO Concentrate, WaterRF, Denver, Colo, 2010.