Journal of Membrane Science, 52 (1990) 173-190 Elsevier Science Publishers B.V., Amsterdam

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FOULING IN TANGENTIAL-FLOW ULTRAFILTRATION: THE EFFECT OF COLLOID SIZE AND COAGULATION PRETREATMENT

VBRONIQUE LAHOUSSINE-TURCAUD

Centre de Recherche, Lyonnaise des Eaur, Le Pecq (France)

MARK R. WIESNER*

Department of Environmental Science and Engineering, Rice University, P.O. Box 1892, Houston, TX 77251 (U.S.A.)

JEAN-YVES BOTTERO

Centre de Recherche SUF la Valorisation des Minerais, Vandoeuvre-les-Nancy (France)

(Received September 21,1989; accepted in revised form March 22,199O)

Summary

The fouling characteristics of dispersions of dissolved organic materials and colloids are inves- tigated in a series of ultrafiltration experiments. Dispersions of tannic acid, humic acid and kaolin are filtered using a cross-flow hollow fiber module. In some cases these materials are modified through the addition of a polymeric aluminum coagulant. The flux characteristics of these dis- persions, ranging in particle size from several nanometer to several hundred micrometer, are in qualitative agreement with theory; particles near 0.2 pm in diameter produce rapid fouling, while particles greater than 3 pm in size have little effect on flux. Coagulation of the model dispersions is effective in maintaining membrane flux. However, coagulation pretreatment of Seine River water produced smaller improvements in membrane flux.

Introduction

Membrane processes such as ultrafiltration (UF) and microfiltration (MF) offer attractive alternatives to conventional water and wastewater treatment. If membrane processes are to be successfully applied to water and wastewater treatment, the characteristics of the raw water that control membrane per- formance must be identified. This paper presents the results of cross-flow ultrafiltration experiments aimed at evaluating the effect on membrane flux of the size of colloidal or macromolecular materials. The effectiveness of co- agulation as a pretreatment for membrane filtration of surface water is also investigated.

*To whom correspondence should be addressed.

0376-7388/90/$03.50 0 1990 - Elsevier Science Publishers B.V.

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In the following sections, some of the previous work on particle transport in porous ducts is summarized. Order-of-magnitude estimates of several proposed particle transport mechanisms are presented and the conditions that theoret- ically favor or inhibit the deposition of particles on the surface of a membrane are evaluated experimentally.

Particle transport in porous ducts

Materials deposited within the matrix of membrane pores may reduce the permeability of the membrane. Additionally, they are operationally difficult to remove. Particles with an effective size that is larger than the pores of a mem- brane will be rejected at the surface of the membrane and therefore should not reduce the permeability of the membrane per se. However, material deposited on or adsorbed to the surface of the membrane may create an additional layer of resistance to permeate flux and particle size will affect the hydraulic resis- tance of this layer. For a unit depth of deposit, larger particles should produce a layer of lower resistance than smaller particles. If all other factors were equal, larger particles would be expected to produce less fouling than smaller parti- cles. However, particles size also plays a role in determining the net deposition of particles on membranes and several mechanisms have been proposed by which particles may be transported away from the surface of the membranes.

As particles concentrate near the surface of the membrane, they may diffuse back into the bulk flow. By the Stokes-Einstein equation for the particle dif- fusion coefficient, the diffusive flux of particles away from the membrane is predicted to increase with decreasing particle diameter. Thus, the diffusive flux of small particles from the membrane surface should be greater than that of larger particles.

Green and Belfort [ 1 ] extended the earlier work of Cox and Brenner [ 21 to describe the effect of lift forces on rotating particles as a mechanism of back- transport in membrane filtration. In contrast with transport by Brownian dif- fusion, particle flux away from the membrane by lateral migration is predicted to increase with increasing particle diameter since the lift force is proportional to the particle diameter cubed. Green and Belfort assumed that the lateral force acting on a single particle could be obtained by vector addition of the inertial lift force derived for flow in a non-porous duct and the permeation drag force derived from the solution of the Navier-Stokes equations in a porous duct. Altena and Belfort [ 31 developed a more rigorous approach to the prob- lem using a particle trajectory analysis and in later work [4] found good cor- relation between theoretical and experimentally observed trajectories.

Several models for back-transport have been presented that consider shear- induced transport or scouring of particles deposited on the membrane [ 5-101. Although the details of these models may differ, they are all fundamentally based on the concept that the shear force of fluid moving across the membrane

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initiates particle migration and at a critical point overcomes the advective force of fluid moving through the membrane (as well as any gravity force or inter- particle attraction). Zydney and Colton [ 51 propose that particles in a shear flow are displaced as the result of complex particle-particle interactions aris- ing from the rotation of each particle, the resultant drag forces exerted on neighboring particles, and the interception of particles moving along different stream lines. Each displacement is the product of a random configuration of particles and so the motion of an individual particle should behave as a random walk and is characterized by an effective diffusion coefficient. The frequency of particle-particle interactions will be greater for higher concentrations of particles. As a result, there will be a tendency for particles to be “nudged” out of areas of higher concentration to areas of lower concentration. Eckstein et al. [ 111 found that the effective diffusivity of particles in shear flow increased with the square of particle diameter and linearly with shear rate. Based on these observations, Davis and Leighton [6] proposed an expression for the effective, or “shear-induced” diffusivity of particles in porous ducts.

Fane [9] observed that if Brownian diffusion and shear mechanisms were considered simultaneously, a minimum in the back-transport of particles and perhaps therefore the flux would be expected at some particle diameter; Brownian diffusion favours the transport of smaller particles while shear transport increases with increasing diameter. Fane supported this qualitative observation with data from experiments in the stirred batch filtration of sus- pensions of particles varying in size from 0.025 to 20 pm. A minimum in per- meate flux was reported for particle diameters near 0.1 ,um.

Cohen and Probstein [8] compared the relative importance of several back- transport mechanisms to the permeation rate through a membrane. Following a similar approach, Wiesner et al. [ 121 calculated order-of-magnitude esti- mates in capillary fibers for the velocity of particle back-transport by Brown- ian motion, un, lateral migration, uL, and shear-induced diffusion, us, as a func- tion of particle size:

us =k77(3&?&) (1)

uL =u;d;/(32vR2) (2)

us =O.O5u&(4R2) (3)

where d,, is the particle diameter, p is the absolute viscosity, k is Boltzman’s coefficient, T is the absolute temperature, R is the radius of a hollow fiber or capillary membrane (or equivalently the distance to the centerline between parallel porous walls), u,, is the centerline maximum velocity in the hollow fiber, and v is the kinematic viscosity. Calculations for the hollow fiber mem- branes used in this work are shown in Fig. 1.

In Fig. 1 a minimum in the back-transport rate of particles from the mem- brane occurs at a particle diameter of approximately 0.2 pm. For particles

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log particle diameter (pm)

Fig. 1. Back-transport as a function of particle diameter in a hollow fiber membrane. Particles smaller than 3 pm and 18 pm are likely to deposit on the membrane for fluxes typical of ultrafil- tration and microfiltration respectively.

smaller than 0.2 pm the theoretical back-transport rate is dominated by Brownian diffusion. Larger particles are transported away from the membrane by lateral migration and shear- induced diffusion. The back-transport veloci- ties shown in Fig. 1 can be compared with the permeate fluxes in hollow fiber membranes to estimate the conditions in which back-transport will offset ad- vective deposition on the membrane. Typical ultrafiltration fluxes are on the order of 1O-2.5 cm/set. Particles larger than 3 pm have a back-transport veloc- ity due to lateral migration that is greater than the permeation velocity. Thus, these larger particles are not expected to contribute to the fouling of ultrafil- tration membranes. An increase in flux increases the range of particle sizes over which deposition is anticipated. For example, the smallest particle that should not contribute to the fouling of a microfiltration hollow fiber membrane is approximately 18 pm for the conditions chosen.

These theoretical predictions of the “fouling potential” of a suspension of particles were evaluated in a series of cross-flow membrane filtration experi- ments. Experiments were designed to test the fouling characteristics of mate- rials varying in size over a range typically encountered in water and wastewater treatment and to test the feasibility of improving membrane performance in water treatment by increasing the particle size prior to membrane filtration using inorganic coagulants.

Experimental procedures

Ultrafiltration unit Cross-flow ultrafiltration experiments were performed using polysulphone

hollow fiber modules (Lyonnaise des Eaux, Toulouse, France ) . The modules

have and effective length of 20 cm and contain 10 fibers. The fibers in modules used in one series of experiments have an internal diameter of 0.06 cm ( 2 0.001 cm). Modules used in a second series of experiments contain fibers with an internal diameter of approximately 0.099 cm ( 2 0.002 ). The molecular weight cutoff of these membranes is approximately 1,000 corresponding to a particle diameter of approximately 1 nm. An adjustable pulseless gear pump provides a flow of 14 L/hr to the module at a differential pressure of 0.5 bar (7.25 psi). The volumetric flow rate corresponds to linear velocities in the hollow fibers of 133 cm/set and 50 cm/set with Reynolds numbers (at 20°C) of approxi- mately 800 and 500 for the 0.06 cm and 0.099 cm diameter fibers respectively. The unit is configured to operate in either a single pass or recycle (feed and bleed) mode. A second gear pump on the recycle line is used to control the recycle velocity. Pressure gauges and variable area flow meters monitor the feed and recycle lines. The permeate line passes to a graduated cylinder used to measure permeate flow rate.

Dispersions for the single pass experiments were prepared in batch using a 100-L baffled tank used to feed the membrane unit. Experiments done in a recycle mode were fed by alternating two 2-L baffled reactors also used to pre- pare a given dispersion in batch. The 100-L tank is scaled up from the 2-L reactors based on similitude for mixing.

Dispersions The ultrafiltration flux characteristics of nine different dispersions of col-

loidal and/or macromolecular materials were evaluated. The composition of these dispersions is summarized in Table 1. Unless otherwise noted, all solu- tions were made from ultrapure water (deionized, low-organic Millipore water)

TABLE 1

Dispersions treated by ultrafiltration

Material Approximate Concentration size

Zeta potential

Humic acid (HA) 6nm zone I > 100 pm zone II 0.2 ,um zone III > 100 firn

Tannic acid (TA) 12nm Kaolin (Ka ) 3Pm TA+Ka 2nm,3pm TA + Ka + tap water lnm-3pm

TA + Ka+ tap water + Al > 100 pm

10 mg/L, pH 7.1

low4 M Al, 10 mg HA/L, pH 7.1 1O-3.5 M Al, 10 mg HA/L, pH 6.5 10-3.5 M Al, 10 HA/L, mg pH 7.5 10 mg/L, pH 5.1 30 mg/L, pH 6.1 10 mg TA/L, 30 mg Ka/L, pH 5.1 10 TA/L, 30 mg Ka/L, mg 1O-2.7 M Ca, pH 7.7

-

igO mV [= +25 mV <=O mV

F< - 10 mV &-10mV c<-10mV

10 mg TA/L, 30 mg Ka/L, 10e2.’ M Ca, [r 0 mV lo-* M Al, pH 7.7

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filtered on a 0.22 pm membrane. Adjustments in pH were made by either NaOH or HCl addition. Fluka humic acid was dissolved in water (500 mg/L) at pH 11 and left to mix for 24 hr. This mixture was then filtered through a 0.22 pm membrane. The filtrate was used to make up 10 mg/L solutions of humic acid (HA) which served as one of the experimental feeds. Most of the organic car- bon present in this commercial humic acid has a molecular weight in the range of 50,000-100,000. Three additional dispersions were prepared from the humic acid by adding a polyaluminum coagulant at two different doses and by varying pH. Detailed information on the preparation, particle size and electrophoretic mobility of these dispersions of coagulated humic acid have been presented elsewhere [ 12,13 J . These preparations will be referred to as zone I, zone II and zone III dispersions. Zone I particles are formed at a pH of approximately 7.1 and at the lowest dose of Al of the three zones ( lop4 M). These particles are approximately 1 ,um in size, with a zeta potential close to zero, and, under conditions of slow stirring, easily aggregate to form large ( > 100 pm), settle- able floe. Zone II particles are approximately 0.2 pm in diameter and form a stable colloidal dispersion. These positively-charged particles are formed at pH 6.5 and are stabilized by a higher dose of Al ( 10-3.5 M) than that used to form zone I particles. Zone III particles are formed at approximately the same Al dose as zone II particles, but at a value of pH (7.6) that is higher than that used to form either the zone I or zone II particles. These particles are over a micrometer in size, but are present as aggregates of large ( > 100pm) settleable floe under conditions of gentle stirring.

A 10 mg/L solution of tannic acid (pH 5.2) was used as a model of smaller molecular weight material and 30 mg/L of kaolin was used as a model for larger inorganic particles. Two additional dispersions composed of both tannic acid and kaolin were used, one was prepared in ultrapure water, and the second in tap water. The tap water is essentially Seine River water treated by conven- tional flocculation, settling, filtration and ozonation. Therefore, the tap water preparation contains most of the inorganic materials present in the Seine river including approximately 100 mg Ca/L ( 1O-2.7 M) as well as an organic fraction that is not removed during treatment. The ninth dispersion was prepared by coagulating the tap water, tannic acid and kaolin mixture at an aluminum dose of 10m4 M Al and a pH of 7.7. Coagulation of this material produced large settleable floe ( > 100 pm) that incorporated approximately 90% of the tannic acid.

Characterization of particle size Electron microscopy, photon correlation spectroscopy, electrical sensing

zone, light scattering, and ultrafiltration sieving techniques were used to de- termine the effective size of materials in these dispersions in batch prepara- tions. In addition, the size of aggregates in the membrane module inlet, con- centrate and recycle were monitored continuously using an in-lien sensor and

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particle-sizing device (Malvern Master-Sizer, Malvern Instruments Ltd., Worcestershire, England). This device uses full Mie scattering theory to de- scribe particle size distributions in the range 0.5 pm-600 pm.

Filtration procedure A new membrane module was used for each experiment. In all cases, exper-

iments began with a one-hour filtration of ultrapure water to establish the initial permeability of the membrane. The flux during this initial period was typically stable. However, in some cases flux increased by almost 20% during this period before stabilizing. A given dispersion was then introduced to the system and the permeate flux was monitored over a period of approximately 6 hr. Ultrapure water was then re-introduced to the system for 1 hr (surface wash), followed by a caustic (0.01 N NaOH) wash for 2 hr, and a final deter- mination of permeability using ultrapure water. All experiments were con- ducted at room temperature (15-30”(J) and the data reported with viscosity corrections to 20°C. The experiments with humic acid and coagulated humic acid were run in a single pass mode. All other experiments, including a replicate set of humic experiments, were run with the system in a recycle configuration. As the system approached a quasi-steady state operation under recycle condi- tions, approximately 4 L/hr of new dispersion was introduced, 1 L/hr of per- meate was produced, 10 L/hr of concentrate was recycled to the membrane inlet, and 3 L/hr of concentrate was wasted from the system.

Results and discussion

Qualitative predictions of flux behavior The approximate sizes of materials used in the experimental portion of this

study and the theoretical back-transport velocities as a function of particle diameter are shown in Fig. 2. Tannic acid is estimated to have a Stokes-di- ameter of approximately 2 nm. This is close to the cutoff of the membrane and therefore some leakage of tannic acid across the membrane is anticipated. As this material moves through the membrane there is the possibility for adsorp- tion within the membrane matrix. The back-transport of tannic acid by dif- fusion is relatively high, and may partially overcome the convective transport of tannic acid up to the membrane; deposition of tannic acid on the membrane surface is only moderately favorable.

While a mean size of 6 nm is estimated for the Fluka humic acid, it is in fact extremely heterogeneous so that some of the humic material in solution would be expected to be much smaller. Cornel et al. [ 141 report that 36% of the car- bon in Aldrich humic acid, which is reported to be similar to the Fluka product [ 151, has a molecular weight less than 5,000. Thus it is possible that some of the Fluka humic acid may also bleed through the ultrafiltration membrane and adsorb within the membrane matrix. The larger fractions of humic material

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log particle diameter (pm)

Fig. 2. The approximate sizes of materials present in the dispersions used in experiments com-

pared with the back-transport estimates for a 0.06 cm hollow fiber. The back-transport of large floe (zone I, III and flocculated tannic acid) is greater than the advective flux of these particles

toward the membrane. Zone II particles should be transported least-effectively from the mem- brane surface.

will retro-diffuse at a slower rate than the tannic acid. As a result, there is a greater potential for deposition of humic material on the membrane surface.

The zone II particles lie within the minimum of back-transport potential. These fairly uniform-sized particles are two orders of magnitude larger than the membrane cutoff and are unlikely to foul the interior of the membrane. They are predicted to form a cake on the membrane surface that will in turn produce resistance to flow across the membrane. Most of the kaolin particles should be rejected at the membrane surface. The kaolin is large enough so that the magnitude of back-transport approaches that of the permeation rate. Therefore, cake formation by the kaolin, if it occurs, should be small. Large floes are predicted to have a back-transport velocity that is as much as 3 orders of magnitude greater than the permeation rate. These large aggregates should not block pores; nor are they predicted to deposit on the surface of the membrane.

Single pass filtration of humic acid dispersions Ultrafiltration of the 10 mg/L solution of humic acid produces a 55% loss in

flux after 300 min of treatment (Fig. 3). The flux declines in three stages. An immediate 25% drop in permeability occurs within the first 5 min after intro- ducing the humic acid to the membrane. This is followed by a rapid decrease in flux over the next 2 hr of filtration. During the last 4 hr, flux decreases at a slower rate. When a surface wash is applied to the membrane by re-introducing ultrapure water in the feed stream, a portion of the initial flux is restored. The flux observed after surface wash corresponds approximately to the flux ob-

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Fig. 3. Flux decline across an ultrafiltration membrane treating a 10 mg/L dispersion of humic acid. The initial membrane flux is established during a 60-min startup period in which ultrapure water is filtered. After introducing the humic acid, flux declines in three stages that terminate at approximately the levels of flux observed after surface wash, and chemical wash.

served at the end of the second stage of flux decline. Approximately 70% of the initial flux is recuperated by washing the membrane with a solution of base. These results suggest that the third stage of flux decline is due to a reversible deposition of materials on the surface of the membrane. Diffusive back-trans- port is not great enough to overcome convective transport and a layer of humic material develops near the membrane surface. During the surface-wash, humic acid is no longer present in the influent and materials accumulated near the membrane are able to diffuse back into the bulk flow. The early decreases in flux may be due to a operationally irreversible deposition of material in mem- brane pores, and sorption of the humic acid on the surface of the hollow fiber.

In comparison with the untreated humic acid, the permeate flux was much lower after 300 min of filtering the zone II preparation (Fig. 4 ) . The flux curve for the zone II particles does not show a large immediate decrease in flux as was observed for the humic acid. The aluminium-humate particles in the zone II preparation are large enough, and the extent of complexation of the humic acid is sufficient to avoid penetration of most of the humic material into mem- brane pores. Nonetheless, the initial stage of flux decline is rapid. This may be due in part to favorable conditions for the deposition of positively charge par- ticles on the negatively charged membrane. However, after an initial layer of particles are deposited on the membrane surface, diffuse-layer interactions between deposited and approaching particles will work against further depo- sition in a similar fashion that they impart stability to these particles in the colloidal suspension. It is therefore significant that after 100 min of operation, the reduction in flux for the zone II particles occurs at a higher rate than that

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Fig. 4. A comparison of the decline in membrane flux produced by untreated humic acid, and humic acid coagulated under three different conditions of polyaluminum dose and pH (zones I, II, and III). Zones I and III produced similarly small decreases in flux.

observed for humic acid. The later reductions in flux are reversed by surface wash indicating that zone II particles are deposited on the surface of the mem- brane. Particles of this size (0.2 pm) are too large to be removed from the membrane surface by Brownian diffusion, and are too small to be removed by lateral migration or fluid shear. Despite unfavorable diffuse layer interactions working against particle deposition, convective transport of the zone II parti- cles results in an accumulation of these particles on the membrane. The com- bination of surface wash and chemical wash with base restored the membrane flux to 87% of its initial value.

By coagulating humic acid with polyaluminum to produce floe-aggregates of zone I and zone III particles it is possible to maintain flux at a relatively high rate (Fig. 4). The two coagulated preparations produce very similar flux curves. After 300 min of filtering these suspensions, membrane flux declines only about 17%. In contrast with the zone II and humic acid dispersions, flux is not re- covered by surface or chemical wash. During filtration, flow in the hollow fiber is effective in transporting large floe away from the membrane surface and so there is little material to be removed from the surface. Approximately 90% of the humic acid is associated with the floe thereby reducing the amount of humic material that can adsorb on the membrane. The 17% reduction in flux that does occur may be due to the depositon of stray material in membrane pores and/or the sorption of unflocculated humic material on membrane surfaces.

Ultrafiltration with recycle In practice, the concentrate from the membrane module is often recycled to

the inlet where it is mixed with the raw water. The velocity gradients (G) and

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detention times (z) within the recycle loop are of a magnitude that may cause particles to break up or aggregate, thereby changing the particle size distribu- tion entering the membrane module. The dimensionless product, G7, is calcu- lated to have a value of 6 for the membrane module used in this study. This corresponds to a value of G for the module of 34 see-l which is of the same magnitude as that present in the feed reactors due to slow stirring (G= 19 set-‘). The detention time through the module is approximately 0.2 sec. In comparison, G7 calculated for the recycle loop, excluding mixing in the pump, is 1,700. This high value is due to the relatively long detention time in the recycle line (70 set). The average velocity gradient in the recycle line is 24 set-l with a maximum of only 32 set-‘. Since these velocity gradients are also of the same order as that provided for the stirred feed reactors, significant changes in the particle size distribution and therefore, membrane perform- ance, are not likely to occur due to mixing in the recycle line. Zone I and zone II particles were filtered with the unit configured for recycle. The flux curves generated with recycle were virtually identical to those generated without re- cycle (Fig. 5). The ultrafiltration unit was operated at very low recoveries ( - 25% ), and the effect of recycling more concentrated materials, as would occur in practice, remains to be tested. All of the remaining experiments re- ported were performed with the unit configured for recycle.

The fouling characteristics of the tannic acid solution are similar to those of the humic acid (Figs. 6, 7). Immediate and operationally irreversible reduc- tions in flux occur when the tannic acid is introduced to the membrane. These reductions are attributed to deposition of tannic acid in membrane pores. The initial blockage of pores with tannic acid is slightly greater than that observed for humic acid and is consistent with the smaller size of this material (Fig. 7 ). Also consistent with its small size and, therefore high diffusivity, is the small

-

q D

8 0

. 0

-

q

.

8

Time (min)

Fig. 5. A comparison of the fluxes produced in single-pass arations of coagulated humic acid.

and recycle ultrafiltration of two prep-

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10 mg,l ,annic am mtroduced

Time (min)

Fig. 6. Flux decline across an ultrafiltration membrane treating a 10 mg/L dispersion of tannic acid.

Fig. 7. A comparison of the decline in membrane flux produced by humic acid, kaolin, tannic acid, tannic acid and kaolin, and tannic acid, kaolin and tap water dispersions. The mixture of kaolin and tannic acid produced a flux curve nearly identical to that produced by tannic acid alone.

(5% ) increase in flux produced by the surface wash of the membrane. During filtration, tannic acid diffuses away from the membrane at a relatively high rate and, to some extent, compensates for the advective flux of material up to the surface of the membrane. Thus, very little flux is recuperated when tannic acid is removed from the influent. Flux is recovered by washing the membrane with NaOH. This is consistent with the increased solubility of tannic acid at higher values of pH. Most of the reduction in flux appears to be due to the sorption of tannic acid on the membrane surface or in membrane pores.

The 30 mg/L suspension of kaolin produces very little fouling (Fig. 7 ) . After

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300 min of filtration the permeate flux decreases by only 14%; most of the decrease occurring within the first 5 min. Based on a mean diameter of 3 ,um the back-transport of kaolin from the membrane by shear-induced diffusion and lateral migration is calculated to be slightly greater than that of tannic acid by Brownian diffusion. In both cases, back-transport is relatively high and surface wash has very little effect on membrane flux. Unlike the tannic acid, the kaolin particles are too large to enter membrane pores and thus a large decrease in flux due to pore deposition is not observed.

Tannic acid and kaolin in ultrapure water combine to produce a flux curve that is nearly identical to that of the tannic acid alone (Fig. 7). Flux reductions are attributed primarily to the deposition of tannic acid in membrane pores. However, when the same mixture is made using tap water, much more fouling is observed, after 300 min of filtration, flux is reduced to 17% of the initial value (Fig. 7). The flux curve for the tannic acid, kaolin and tap water prepa- ration is characterized by an initial 35% drop in flux that is not reversible by either the surface- or chemical-washing procedures. Very little of the flux de- cline can be attributed to surface deposition. Most of the flux reduction that follows the initial loss in permeability is reversible only by the chemical wash. As well as increasing the fouling potential, preparation of the tannic acid and kaolin suspension using tap water rather than ultrapure water increases the mobility of tannic acid through the membrane. Absorbance of UV light at 254 nm is proportional to the concentration of tannic acid and is approximately 2 times greater in the permeate of the tap water preparation than in the permeate produced from ultrapure water preparation of tannic acid and kaolin. Rela- tively high concentrations of Ca in the tap water (approx. 10-2.7 M) may con- tribute to membrane fouling by tannic acid by decreasing the hydrodynamic radius of the molecule thereby increasing the accessibility of the tannic acid to the interior of membrane pores. Also, as acidic functional groups that impart charge to tannic (and humic) acids become saturated with metals such as cal- cium, the stability of the tannic acid in a polar solvent such as water is de- creased. Calcium has been found to enhance the adsorption of humic materials on activated carbon [16], MnO,(solid), Fe(OH),(amorph), CaC03(solid) [ 171 and other solids. It is probable that high concentrations of calcium in the raw water enhance the adsorption of tannic acid or humic materials on mem- brane surfaces as well.

The tap water preparation of 10 mg/L tannic acid and 30 mg/L kaolin was flocculated at pH 7.7 with lop4 M Al of polyaluminum coagulant. Approxi- mately 90% of the tannic acid was incorporated into the floe. The flocculated suspension produced relatively little fouling; after 340 min of filtration, flux had declined to 80% of the initial rate. Surface wash produced a very small increase in flux while chemical wash restored all but 11% of the initial flux. As in the case of the flocculated humic acid, the maintenance of membrane flux can be attributed to the removal of small colloids and macromolecules through

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flocculation. The flocculated material is too large to enter membrane pores, is easily transported away from the membrane and, in the event that it does deposit on the membrane, produces little resistance to flow.

Ultrafiltration of Seine River water Seine River water, pre-filtered on a 200 ,um screen was treated with and

without coagulation prior to ultrafiltration. Ultrafiltration experiments were performed using the polyaluminum coagulant and, in one case, ferric chloride. The removal of organic materials by ultrafiltration was estimated by measur- ing the absorbance of UV light at 254 nm in the feed and permeate waters. Without the addition of coagulant to the raw water approx. 40% of the organic materials were rejected by the membrane. Within the first hour of filtration, permeate flux dropped to approximately 43% of its initial value and then con- tinued to decline steadily. After 300 min of filtering uncoagulated Seine River water, the flux had dropped to 26% of its initial value (Fig. 8). When the Seine river water was pretreated with a polyaluminum dose of 10-3.7 M as Al at a pH of 7.6, the flux declined at a slower rate and the UV absorbance of the permeate water was lower than for the uncoagulated water. However, after 300 min of operation, flux had dropped to the same level as that observed for the uncoa- gulated water (25% of the initial flux). While surface wash was ineffective in recuperating flux for the uncoagulated water, coagulation pretreatment re- sulted in a higher recuperation of the initial flux after surface wash (36% ) .

These results might be explained by the complexation of aluminium hy- drolysis products by humic materials when the river water is coagulated with polyaluminum. Assuming that all functional groups on the humic acid are com- plexed, humic material would need to shed the complexed aluminum species for specific adsorption on the membrane to occur. The free energy of formation

Fig. 8. Summary of ultrafiltration experiments on Seine River water.

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for the aluminium-humic complexes is likely to be less than the free energy of adsorption of the uncomplexed humic material. Thus, attachment of the com- plexed humic material to the membrane surface will most likely be due to weaker dispersion forces and hydrogen bonding rather than electron sharing between functional groups on the humic material and sites on the membrane. The re- duction of UV absorbance across the membrane is greater with coagulation pretreatment than without. This suggests that some of these complexes are composed of humic materials that, in their uncomplexed state, are smaller than the cutoff of the membrane.

As in the case of the zone II aluminium-humate particles, the complexed humic materials from the Seine River are large enough to be rejected by the membrane yet they are apparently too small to be transported to a significant degree by lateral migration or shear-induced diffusion. It is coincidental that the reductions in flux after 300 min for the coagulated and uncoagulated river water are similar. Some of the material that deposits on the membrane when coagulant is not added, is likely to be incorporated into larger floe when coag- ulant is added. This material will be transported away from the membrane. Conversely, some of the organic material that, without coagulation, is small enough to diffuse away from the membrane will, as an aluminum-complexed species, have a smaller diffusion coefficient and therefore deposit on the mem- brane more readily. In general, these two processes will not cancel out each other. In contrast with the model humic acid and tannic acid systems, coagu- lation of the Seine River water was less effective in complexing organic mate- rials. The use of coagulation to maintain membrane flux is limited by the ex- tent to which small colloids and humic materials are complexed and aggregated. At a higher dose of polyaluminum coagulant ( lop3 M Al) and at a lower pH (5.5) more UV-absorbing materials were removed by the membrane; the re- duction in UV-absorbance increased by 10% (Fig. 8). A greater reduction in UV absorbance corresponds to an improved coagulation of organic materials and an improvement in the permeate flux after 300 min of operation. Addi- tional membrane flux was not recuperated after surface-washing the mem- brane. Compared with the lower polyaluminum dose, most of the improvement in flux appears to be due to formation of larger, better transported particles rather than greater complexation of humic materials; the flux at the higher polyaluminum dose roughly corresponds to that observed at the lower dose after surface wash.

Pre-treatment of Seine River water with FeCl, produced a flux curve and reduction in UV-254 absorbance (70% ) similar to that achieved using the higher polyaluminum dose (Fig. 8). The permeate flux of the Seine water after 300 min of filtration was approximately 12% greater with ferric chloride pretreat- ment ( 10-3.2 M Fe and pH 6.5) than without pretreatment. However, 62% of the initial flux was still unrecoverable. Surface wash produced no improvement in permeate flux.

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In all of the ultrafiltration experiments with Seine River water, the greatest losses in flux occur during the first 30 min of contact with the raw or coagulated water. These rapid drops in membrane flux are consistent with fouling by pore blockage and adsorption on the membrane surface. Most of the materials re- sponsible for these initial losses in flux are apparently not incorporated into the large floe formed by adding polyaluminum or iron coagulants.

Conclusions

The membrane fouling behavior of dispersions made up of particles ranging in size from several nanometers to several hundred micrometers is qualita- tively consistent with predictions of fouling potential based on particle trans- port considerations. The theoretical interpretations of flux reduction in this work rely primarily on physical mechanisms. However, the degree of irrever- sible fouling observed in experiments using humic and tannic acids under- scores the importance of chemical interactions in membrane fouling. The sur- face chemistry of larger colloidal material is also likely to play an important role in particle back-transport where double layer repulsion is significant.

Fouling by physical mechanisms is predicted when the transport of particles from the surface of the membrane through Brownian diffusion, shear-induced diffusion and lateral migration is calculated to be less than the advective trans- port of these particles up to the surface of the membrane. For the experimental conditions in this study a minimum in back-transport, and therefore a maxi- mum potential for fouling, is calculated to occur at particle diameters near 0.2 ,um. These particles are too large to be transported by Brownian diffusion, and are too small to be pulled from the surface by shear forces or rotational lift. They are also too large to penetrate into the matrix of an ultrafiltration mem- brane and thus, most of the reduction in flux can be reversed hydrodynami- cally. Smaller, more mobile materials are also predicted to deposit on the mem- brane surface, but to a lesser extent. However, as particle size approaches the effective size of membrane pores, fouling due to adsorption and pore blockage occurs.

At a given permeation rate there should be a limiting particle size, above which particles will not deposit on the membrane. For the conditions appli- cable to the hollow fiber ultrafiltration membranes used in this study, order- of-magnitude estimates of particle transport indicate that particles larger than approximately 3 pm should not foul membranes. These particles are effectively removed from the membrane by shear-induced diffusion and lateral migration and, if present at the surface of the membrane, should contribute negligible resistance to flow. Experimentally, kaolin (approximately 3 bum in diameter) and coagulated humic and tannic acids ( > 80 ,um) were found to produce little if any fouling. We conclude that the removal of submicrometer materials to form larger particles by coagulation may improve the performance of mem-

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brane filtration units used to treat waters that contain colloidal organic and inorganic materials.

In water treatment, naturally occurring organic macromolecules, such as humic and fulvic acids, are likely to foul membranes by adsorbing in pores or on the membrane surface. A measure of the Total Organic Carbon (TOC) of the raw water may therefore serve as an indicator of the rate at which such irreversible losses in membrane capacity will occur. Coagulation pre-treatment of raw waters will be effective in reducing these losses if the TOC is extensively complexed. If, in addition, the resultant particles are aggregated to form large floes, flux reductions due to surface deposition may also be reduced.

As a measure of the scattering of visible light (1 N 500 nm), turbidity will be particularly sensitive to those particles in the raw water that are most likely to deposit on the surface of ultrafiltration membranes; particles from approxi- mately 0.1 to 1 micrometer in size are transported least effectively from mem- brane surfaces. Coagulation pretreatment should be effective in aggregating these particles to a size that is continuously removed from the surface of the membrane.

Coagulation of Seine River water with polyaluminum or ferric chloride be- fore ultrafiltration marginally improved permeate flux. Nonetheless, after 300 min of operation, permeate flux dropped to approximately 37% of the initial flux. We conclude that the relatively small effect of aluminium and iron coag- ulants on membrane performance in these experiments is linked to the relative inefficiency of coagulating the TOC in this surface water; no more than 70% the total organic carbon in the raw water as measured by the absorbance of UV light at 254 nm was associated with the aluminium or iron precipitates. Sig- nificant benefits from coagulation prior to membrane filtration can only be realized for waters containing colloidal and macromolecular materials that are more effectively incorporated into large floes.

Acknowledgements

Support for this work was provided by the Research Center of the Lyonnaise des Eaux, Le Pecq, France where the first author performed her doctoral re- search, and by the National Science Foundation, (Grant No. BCS-8909722). Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the National Science Foundation.

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