Enhancing microfiltration through an inorganic tubular membrane by gas sparging

Journal of Membrane Science 165 (2000) 47–57

Enhancing microfiltration through an inorganic tubular membrane by gas sparging L. Vera a , R. Villarroel a , S. Delgado a , S. Elmaleh b,∗ a

b

Departamento de Ingenier´ıa Qu´ımica, Universidad de La Laguna, 38200 La Laguna, Spain UMR 5569, UM II/CNRS/IRD, Hydrosciences Montpellier, Equipe Génie des Procédés, CC 24, Université Montpellier II, 34095 Montpellier Cedex 5, France Received 26 October 1998; received in revised form 24 June 1999; accepted 24 June 1999

Abstract A novel technique is tested for reducing tubular mineral membrane fouling by injecting gas into a cross-flow stream. The injected gas is thought to form complex hydrodynamic conditions inside the microfiltration module which increase the wall shear stress, preventing the membrane fouling and enhancing the microfiltration mass transfer. The experimental study was carried out with a ferric hydroxide suspension and a biologically treated wastewater, both of them filtered through a tubular inorganic membrane (Carbosep M14). The sparging led to an increase of the permeate flux with a slug flow structure for the two kinds of suspension. New dimensionless quantities of shear stress number and resistance number were developed by generalized dimensional analysis of steady state flux in sparged and unsparged cross-flow filtration. An unique formalism allowed interpretation of the experimental results both in classical diphasic filtration and with gas sparging. The variation in the dimensionless numbers demonstrated the benefit of gas sparging. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Microfiltration; Water treatment; Inorganic membranes; Hydrodynamics; Fouling

1. Introduction Water quality standards are becoming more and more stringent for all purposes including drinking water or even water reclamation in agriculture. Membrane techniques could easily meet those standards. For example, cross-flow microfiltration is particularly useful for the elimination of suspended solids, turbidity and microorganisms. It is also a promising process for tertiary wastewater treatment [1–3] allowing simultaneous clarification and disinfection [4]. ∗ Corresponding author. Tel.: +33-4-671-43723; fax: +33-4675-44810. E-mail address: [email protected] (S. Elmaleh).

However microfiltration is hampered by mass transfer limitation due to the formation of a highly concentrated layer, i.e. a Stefan layer, which results from solute accumulation near the membrane wall and/or from irreversible fouling. Since the permeate flux is one of the main parameters determining the economic viability of the process, many works were devoted to its enhancement. A large number of techniques were proposed to limit the fouling such as increase of cross-flow velocity [5], use of baffles [6], backflushing [7,8] or transmembrane pressure pulsing [9]. Recent works demonstrated the efficiency of gas sparging through the cross-flowing liquid. Cabassud et al. [10], using a hollow fibre membrane, showed

0376-7388/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 9 9 ) 0 0 2 1 6 - 1

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L. Vera et al. / Journal of Membrane Science 165 (2000) 47–57

that slug flow limited the deposition of bentonite particles, enhancing the permeate flux even at low gas velocity, e.g. 110% increase at 1 m s−1 and 60% at 0.1 m s−1 . This approach was extended to the field of drinking water treatment [11]. Lee et al. [12] doubled and even tripled the permeate flux through a 0.2 ␮m microfiltration membrane or a 300 kDa cut-off ultrafiltration membrane while filtering a bacteria suspension in the presence of gas slug flow. By use of the same technique, Cui et al. [13,14] obtained significant enhancement while filtering proteins, e.g. 60% for undyed 162 kDa dextran, 113% for 162 kDa dyed-dextran and 91% for BSA solutions. Mercier et al. [15], microfiltering bentonite and yeast suspensions, concluded that slug flow is the optimum hydrodynamic regime for increasing permeate flux. It is worthwhile to note that, in all previous works, the hydrodynamic regime inducing the largest enhancement in filtration flux was slug flow regardless of the suspension or the membrane. Moreover, the examples and references given here are only a background since much work was devoted to permeation flux enhancement; an exhaustive bibliographical review, which includes the gas sparging technique, was recently done [16]. The aim of the present work is to apply tangential diphasic gas–liquid flow to suspensions whose types were not already studied. Ferric hydroxide suspension is an example of a suspension of deformable particles. On the other hand, biologically treated effluent from the Santa Cruz, Tenerife, wastewater treatment plant is a real suspension of significant economic importance. The water requirement in Tenerife exceeds the traditional water resource recovered by water galleries or wells that will be more and more limited because they are becoming brackish. An important programme of wastewater reclamation for agricultural purposes was consequently implemented. The project is focused on the reuse of the effluent from the wastewater treatment plant of the city of Santa Cruz to irrigate the southern banana and tomato crops. Cross-flow microfiltration was evaluated as a tertiary treatment, demonstrating its ability to deliver an excellent quality effluent [3]. However, the economic viability of this technique remains hampered by low fluxes and any process allowing intensification of the mass transfer should be evaluated.

2. Materials and methods 2.1. Experimental unit The experimental unit is shown schematically in Fig. 1. Temperature was maintained at 25◦ C with a spiral heat exchanger immersed in a 5 l tank; this temperature is currently observed in the effluent of the Santa Cruz wastewater treatment plant. Nitrogen was added through a nozzle to the liquid stream at the entrance of the membrane tube. The gas and liquid flow rates were monitored with flow-meters. The selected M 14 Carbosep membrane [3], manufactured by Tech-Sep, was an inorganic composite membrane whose zirconia active layer was deposited on a carbon support. This tubular membrane has an internal 6 mm diameter and a length of 40 cm. The rated pore diameter as given by the manufacturer is 0.14 ␮m with an effective filtration area of 0.0075 m2 . The filtration element could be replaced by a 7 mm internal diameter transparent tube which allowed observation of the bubbles shape, size and motion by recording with a fast video camera. 2.2. Suspensions For comparison some runs were carried out with tap water. Then, a 1 g l−1 ferric hydroxide suspension was filtered at pH 9 with recirculation of both the permeate and the retentate. A solution was prepared by dissolving a predetermined quantity of ferric chloride in a given volume of tap water and then the pH was adjusted with sodium hydroxide. Routine cleaning at the end of each run was carried out by the following procedure: 1. 30 min static washing with a 3% (v/v) nitric acid solution; 2. 1 h static washing with tap water. In the second experimental study, the suspension was the effluent of the Santa Cruz wastewater treatment plant which includes a grit removal and a primary sedimentation followed by activated sludge treatment whose volumetric loading rate is about 1.6 kg BOD5 m−3 day. The main characteristic parameters of the effluent are given in Table 1. The high conductivity was essentially due to sodium and hydrogen carbonate.

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Fig. 1. Experimental unit.

In this case, the membrane was regenerated by the following procedure: 1. 30 min static washing with a 3% weight sodium hydroxide solution at 50◦ C; 2. 1 h static washing with tap water; 3. 30 min static washing with a 3% (v/v) nitric acid solution; 4. 1 h static washing with tap water. In both cases, in order to assess the cleanliness of the

membrane, measurement of the pure water permeability was carried out before each filtration run. 2.3. Analyses All the physical and chemical analyses were carried out in accordance with the standard methods [17]. pH was measured with a pH-meter Metrohm and turbidity with a turbidimeter Hach DR 3000.

Table 1 Main characteristics of wastewater and filtrate with and without gas injection Parameters

pH Conductivity, ␮S cm−1 Turbidity, FTU S S content, mg l−1 Total COD, mg l−1 TOC, mg l−1 N–NH3 , mg l−1 N–NO3 − , mg l−1 N–NO2 − , mg l−1 PO4 3− , mg l−1 a

Standard deviation.

Feed

Filtrate without gas injection

Average

σa

8.57 1680 48.00 19.70 81.00 28.75 36.44 1.92 0.79 40.87

0.09 62.18 5.35 2.05 9.34 4.36 1.90 1.19 0.66 15.82

Filtrate with gas injection

Average

σa

Average

σa

8.20 1692 3.25 Nil 29.33 14.80 29.33 1.20 0.71 26.19

0.41 84.21 0.96 Nil 3.79 1.49 2.00 1.39 0.66 3.40

8.68 1645 3.75 Nil 31.33 17.15 33.50 1.52 0.68 27.00

0.08 82.26 0.50 Nil 2.08 2.77 2.53 0.85 0.64 1.77

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Fig. 2. Two-phase flow patterns in vertical tubes.

3. Results and discussion 3.1. Flow patterns The direct and recorded observations through the transparent tubular pipe confirmed published results [10,15]. Each flow pattern corresponded to values of the ratio r = Ug /(Ug + Ul ) where Ug and Ul are the superficial gas velocity and the superficial liquid velocity, respectively, both of them being calculated as if each phase was separately circulating. The main structures which were observed when the gas velocity was increased for a given liquid velocity were: bubble flow, slug flow, churn flow and annular flow (Fig. 2). For the bubble flow, obtained when r < 0.25, the gas phase was uniformly dispersed as small bubbles and this flow showed quasi-steady characteristics. For intermediate values of r (0.25 < r < 0.9), large bubbles were observed with size of the order of the internal diameter of the tube. These bubbles are usually called Taylor bubbles. Due to the reduction in the available cross-section for the liquid phase, a thin liquid film always remained over the surface of the membrane and moved in the opposite direction with respect to the main flow. This phenomenon induces a highly variable large shear rate against the pipe wall. Unsteady characteristics were then obtained at any point in the flow with a frequency depending on the velocity of the Taylor bubbles. Churn flow and annular flow could

not be observed in this study because of a limitation in the gas flow-rate but they are reported at larger values of r [15]. It should be noted that for a given liquid flow-rate, the presence of the gas increases the mean longitudinal velocity of the fluid which, in association with the great variations in the wall shear stress and the turbulence existing in the wake of the Taylor bubbles, can improve the performance. Previous works showed that slug flow is the most efficient regime for significant enhancement of mass transfer [10,15]. 3.2. Membrane rejection Irrespective of the operating parameters, the M14 membrane was a total barrier for the inorganic or organic suspended solids. Moreover, the rejection of other species were not significantly affected by gas sparging producing water of high quality for irrigation (Table 1). 3.3. Permeate flux Gas sparging was not effective while tap water was filtered. On the contrary, the permeation flux was decreased by gas introduction then reached a plateau value (Fig. 3). Gas bubbles could probably penetrate into the relatively large porous structure. Gas sparging should then be solely considered when a deposit or a less porous active layer can prevent gas penetration.

L. Vera et al. / Journal of Membrane Science 165 (2000) 47–57

Fig. 3. Steady state flux against gas velocity for tap water at different transmembrane pressure (Ul = 2 m s−1 ).

Figs. 4 and 5 illustrate the advantage of gas sparging for suspensions. As in conventional single-phase cross-flow microfiltration the initial flux decline could still be observed. The presence of the gas phase does not modify the general behaviour of the flux variation with time. The steady state flux was generally

Fig. 4. Influence of gas velocity on ferric hydroxide microfiltration (Ul = 0.7 m s−1 ; TMP = 2 bar).

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increased when gas was injected and the flux decrease was slower. In the case of ferric hydroxide, a steady flux without initial flux decrease could be observed without gas sparging at 0.7 m s−1 liquid cross-flow velocity and 2 bar transmembrane pressure (Fig. 4). Such a constant flux was also observed while filtering treated wastewater without gas sparging at 1 bar driving pressure and 3 m s−1 liquid cross-flow velocity (Fig. 5). There is a difference in the steady state flux between the two suspensions (Figs. 6 and 7). In the case of wastewater, without gas injection or with gas sparging at gas velocities lower than 2 m s−1 , a limiting flux was reached at 2 bar, the limiting flux being the plateau value reached when the transmembrane pressure is increased. When the gas velocity was higher than 2 m s−1 , no limiting flux was observed. No limiting flux could be observed either while filtering the ferric hydroxide suspension with or without gas injection. The induced resistance, i.e. the hydraulic resistance, which is added to the membrane resistance, could significantly be decreased by gas sparging (Fig. 8). However the effect was more dramatic for wastewater than for ferric hydroxide. Under similar experimental conditions, e.g. 1 bar driving pressure and 1 m s−1 liquid cross-flow velocity, gas sparging in the vicinity of 1 m s−1 reduced the resistance by a factor of 2 for ferric hydroxide and by a factor of 4 for wastewater. This effect was observed even at low gas velocities, e.g. at 0.3 and 0.4 m s−1 while the liquid cross-flow velocity was 1 m s−1 (Fig. 8). The limiting mass transfer processes are quite different. With ferric hydroxide the resistance results mainly from the deposit of deformable particles [18] whereas with biologically treated wastewater the processes can be reduced, in spite of their complexity, to a reversible particle deposit and an irreversible fouling due to adsorption of organic macromolecules [3,19]. With unsparged (classical) filtration of treated wastewater, at 1 bar and 3 m s−1 , the induced resistance was 1.91 × 109 m−1 . Gas sparging was able to reduce the resistance by a factor of 2 (Fig. 8). Gas sparging is therefore able to eliminate part of fouling, which was previously impossible to reduce by solely increasing liquid cross-flow [3]. The effect of liquid cross-flow is indeed limited by the respective sizes of the viscous layer and the Stefan layer; this limitation does not exist in gas–liquid flow where unsteady effects do not allow

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Fig. 5. Influence of gas velocity on treated wastewater microfiltration (Ul = 3 m s−1 ; TMP = 1 bar).

the formation of a Stefan layer. The viscous layer is a layer through which momentum is transported by diffusion (Nusselt model). In unsparged filtration, the liquid flow reduces its size to a minimum generally achieved in turbulent regime. The solute can concentrate in such a layer to constitute what is called a Stefan layer, e.g. a polarisation layer. In sparged filtration,

the gas flow induces unsteady phenomena, which do not allow its formation. The steady-state flux for ferric hydroxide filtration reached a plateau in the slug flow regime (Fig. 9). A similar plateau was also obtained in slug flow by Cabassud et al. [10] but Mercier et al. [15] observed the plateau in the churn turbulent regime. Both authors

Fig. 6. Steady state flux against transmembrane pressure at different superficial gas velocities for ferric hydroxide suspension (Ul = 1 m s−1 ).

Fig. 7. Steady state flux against driving pressure at different superficial gas velocities for treated wastewater (Ul = 1 m s−1 ).

L. Vera et al. / Journal of Membrane Science 165 (2000) 47–57

Fig. 8. Influence of gas velocity on overall induced resistance for ferric hydroxide (Ul = 1 m s−1 ; TMP = 1 bar) and wastewater microfiltration (Ul = 1, 2 and 3 m s−1 ; TMP = 1 bar).

were filtering a bentonite suspension. On the other hand, in this work, the flux was still increasing in slug flow regime for treated wastewater filtration (Fig. 10). Experiments at higher r-values should have been carried out but larger gas flow-rates could not be obtained with the experimental unit. Regardless of the experimental conditions, the permeate flux obtained with gas sparging was always

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larger than under unsparged conditions and the enhancement was maximum for a moderate liquid velocity (0.5–1 m s−1 ) and a high proportion of injected gas (Figs. 9 and 10). Furthermore, the flux intensification was found to decrease with transmembrane pressure for the same gas and liquid flow velocities while filtering ferric hydroxide because of a probable compression of the solid deposit, e.g. the ratio J 0 S /Js between the fluxes obtained at steady state conditions with gas injection and without gas injection is 1.43 at 1 bar and 1.19 at 2 bar (Ul = 0.7 m s−1 and Ug = 0.3 m s−1 ) (Fig. 11). On the other hand, with wastewater, the J 0 S /Js variations with TMP showed a minimum at 2 bar (Fig. 12). Due to the complexity of microfiltration, the results were re-plotted in terms of two non-dimensional parameters [19,20]. For unsparged filtration, the shear stress number Ns = ρ l Ul2 /P, where ρ l is the liquid density and Ul the cross-flow velocity, compares the shear stress against the membrane wall to the transmembrane pressure P. The fouling or resistance number Nf = (µRf Ul )/P = Ul /Jf compares the convective cross-flow transport flux to the permeation flux Jf through a layer whose resistance is the overall resistance Rf induced by all the processes that can limit the mass transfer, e.g. particle deposit, polarisation, adsorption and/or internal clogging. The resistance number can be viewed as the equivalent of the inverse of a Stanton number also called Margoulis number. This

Fig. 9. Variation of the J 0 S /Js ratio against velocity ratio for ferric hydroxide at different liquid velocities (TMP = 1 bar).

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Fig. 10. Variation in the steady flux value against velocity ratio for treated wastewater at different liquid velocities (TMP = 1 bar).

number is used in mass or heat transfer in a tubular exchanger. It is the ratio between the transferred (mass or heat) flux and the cross-flow transported mass (or heat) flux [21]. In the (Ns ,Nf ) plane, a straight line of positive slope means that the mass transport is mostly limited by a compression of the deposit and weakly enhanced by cross-flow which is the case of ferric hydroxide without gas sparging (Fig. 13, r = 0). A straight line of negative slope followed by a steady

Nf implies that fouling cannot be completely eliminated by liquid cross-flow which is the case of treated wastewater (Fig. 14, r = 0). This is probably related to bacteria or metabolite adsorption against the membrane wall [21]. The shear stress number contains ρ l Ul2 , which is the minimum specific energy required to transport the liquid at velocity Ul [22]. This number can be slightly modified in the case of a gas–liquid flow considering

Fig. 11. Effect of the transmembrane pressure on the J 0 S /Js ratio for ferric hydroxide at different gas velocities (Ul = 0.7 m s−1 ). All runs at slug flow regime excepted at 0.2 m s−1 gas velocity (bubble flow regime).

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Fig. 12. Effect of the transmembrane pressure on the J 0 S /Js ratio for treated wastewater at different gas velocities (Ul = 1 m s−1 ). All runs at slug flow regime excepted at 0.3 m s−1 gas velocity (bubble flow regime).

an equivalent fluid whose density is: ρ0 =

Ug ρg + Ul ρl Ug + Ul

(1)

Eq. (1) can be viewed as a thermodynamic mixing rule for the calculation of the mixture property from the properties of the individual components and has been shown to be accurate for many solution properties. For sparged filtration, a shear stress NS 0 number is defined as: N 0S =

ρ 0 (Ug + Ul )2 P

(2)

For sparged filtration, the resistance number is not modified since the cross-flow solute mass transport through the filtration element is ensured by the liquid flow. The experimental data recalculated in terms of the dimensionless groups gave the plots of Figs. 13 and 14. In the case of ferric hydroxide, curves were again straight lines whose gradient was decreasing while r increased (Fig. 13). In the bubble flow

Fig. 13. Resistance number against shear stress number for ferric hydroxide at different r factors. All experimental conditions.

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4. Conclusions

Fig. 14. Resistance number against shear stress number for treated wastewater at different r factors. All experimental conditions.

regime (r < 0.25), the slope was positive indicating that the induced resistance could not be eliminated by cross-flow which is characteristic of the compressible deposits. At the beginning of slug flow regime (r = 0.3), the slope was approaching a zero value. For r factor of 0.4 and 0.5, the straight lines had negative slopes exactly like in diphasic cross-flow filtration of magnesia particle suspensions that can solely limit the permeation by cake deposition [17]. Such negative slopes mean that there could exist N 0 S values for which Nf is 0 and therefore no induced resistance. In the case of treated wastewater, gas sparging did not change the curve shape (Fig. 14). Two regions could be distinguished in the (N 0 S , Nf ) plane (Fig. 14). While NS 0 increased, Nf linearly decreased with a negative slope but remained steady beyond a N 0 S value which implies that the induced resistance could not be completely eliminated by simultaneous gas sparging and liquid cross-flow. However the final Nf value was much lower with gas sparging, i.e. in classical cross-flow filtration, the minimum resistance number value reached 60 000 while gas sparging allowed values of about 10 000 to be obtained. A minimum resistance number value can be compared to the maximum Stanton number value that can be reached by increase in the Reynolds number in a tubular exchanger [21], e.g. at the plateau, the permeate flux is 10 000 smaller than the cross-flow transport flux.

1. The experiments under a large range of flow conditions, with two kinds of suspensions, showed that gas–iquid two-phase flow enhances microfiltration flux. This effect is probably due to the high and transient wall shear stress induced by the sparging. 2. The hydrodynamic regime inducing the largest enhancement in filtration flux is slug flow in the case of ferric hydroxide where a permeate flux plateau is reached at the beginning of the slug flow regime. 3. The flux was an increasing function of r in the case of treated wastewater but no flux plateau was observed even in established slug flow regime. Larger r values should be tested. 4. In the experimental range explored, corresponding to bubble flow and slug flow, the most significant effects occurred at a moderate liquid flow velocity (0.5–1 m s−1 ) and high proportion of injected gas (r > 0.4) and low transmembrane pressure (1 bar). 5. Dimensionless quantities can be used to recalculate the experimental data: a new shear stress number and a resistance number obtained by analogy with previous works on classical cross-flow filtration [20] allowed display of all the experimental results on one graph.

5. List of symbols permeate flux (l m−2 h−1 ) steady state flux without gas sparging (l m−2 h−1 ) 0 J S steady state flux with gas sparging (l m−2 h−1 ) flux through the resistance layer (m s−1 ) Jf resistance number (−) Nf shear stress number without gas sparging (−) Ns N 0 S shear stress number with gas sparging (−) P transmembrane pressure (N m−2 ) r velocity ratio Ug /(Ug + Ul ) overall induced resistance (m−1 ) Rf TMP transmembrane pressure (N m−2 ) superficial velocity of the gas phase (m s−1 ) Ug superficial velocity of the liquid phase (m s−1 ) Ul J Js

L. Vera et al. / Journal of Membrane Science 165 (2000) 47–57

5.1. Greek letters µ ρ ρ0 ρg ρl

dynamic viscosity (kg m−1 s−1 ) density (kg m−3 ) equivalent density (kg m−3 ) gas density (kg m−3 ) liquid density (kg m−3 )

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