Environ. Sci. Technol. 2000, 34, 3780-3788
Effect of Coagulation Conditions on Membrane Filtration Characteristics in Coagulation-Microfiltration Process for Water Treatment JEONG-DAE LEE,† SANG-HO LEE,† MIN-HO JO,† PYUNG-KYU PARK,† C H U N G - H A K L E E , * ,† A N D JONG-WOON KWAK‡ School of Chemical Engineering, Seoul National University, Seoul 151-742, Korea, and Kyunggi Chemicals Co. Ltd, Sosa-Gu, Buchon-Shi, Kyunggi-Do 422-080, Korea
In a coagulation-microfiltration (MF) hybrid process, membrane permeability and permeate water quality were investigated in conjunction with coagulation mechanisms under two kinds of dead-end (e.g., submerged and external pressure type) and cross-flow microfiltration modes. The specific cake resistance of coagulated suspension largely depended on coagulation condition, being lower at chargeneutralization than that at sweep-floc condition. The lower specific cake resistance was attributed to the formation of less compressible but more porous cake with the chargeneutralization condition. Under the dead-end MF modes, the effect of coagulation condition was clearly demonstrated on the rising rate of transmembrane pressure at constant flux, e.g., membrane permeability with charge neutralization turned out to be much better than that with sweep-floc mechanism. The trends of specific cake resistance are in good agreement with the difference in membrane permeabilities between the two dead-end filtration modes. Under the cross-flow microfiltration mode, however, the coagulated suspensions formed via the two different mechanisms showed almost the same steady-state flux. It was because the portion of particles that otherwise deposit or adsorb on the membrane was reduced, and thus the effect of different specific cake resistance was mitigated. It was confirmed by the analysis of back-transport velocities, feed, and permeate water quality and also by the measurement of particle size distributions involved in the coagulation-cross-flow microfiltration. The permeate water quality was also examined in terms of the removal of natural organic matter (UV254, TOC) and residual aluminum concentration.
Introduction Recently, microfiltration (MF) and ultrafiltration (UF) technologies have been in progress as an alternative to conventional drinking water treatment to meet more stringent regulations. MF has been known to be effective for the removal of particles, turbidity, coliform bacteria, Giardia, and Cryptosporidium (1). However, a practical impediment to wide application of MF or UF in water treatment has been * Corresponding author telephone: 82-2-8807075; fax: 82-28881604; e-mail:
[email protected]. † Seoul National University. ‡ Kyunggi Chemicals Co. Ltd. 3780
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the problem of membrane fouling, which is closely associated with the existence of dissolved organic matters and small colloids (less than 1 µm) in the raw water (2). Addition of a coagulant prior to the membrane filtration has been suggested for the purpose of not only reducing membrane fouling but also improving the removal of dissolved organic matters that might otherwise not be removed by MF (2, 3). Coagulation in water treatment is a process of combining small particles into larger aggregates for better settlability. Among several mechanisms of particle destabilization, two were referred frequently for the coagulation of colloid particles or natural organic matter (NOM): charge-neutralization and sweep-floc mechanism (4, 5). The chargeneutralization may result from a specific chemical reaction between positively charged coagulants and the negatively charged colloids and natural organic matters or from a shielding of the negatively charged sites, thus leading to precipitation. Coagulation by charge-neutralization would be accomplished over a narrow pH range (4∼5.5). On the other hand, sweep-floc mechanisms appear in the range of pH 6∼8 where conditions are ripe for rapid formation of amorphous solid-phase Al(OH)3(s). In the sweep-floc condition, removal of turbidity and natural organic matters occurs by adsorption on the precipitate of Al(OH)3(s). The characteristics of floc and treated water largely have been reported to highly depend on the coagulation conditions and mechanisms (6). Many studies have addressed the combined treatment of coagulation and MF. Peuchot and Ben Aim (7) reported that combined coagulation-cross-flow MF process tends to reduce the colloidal membrane fouling at the optimal concentration of coagulant. According to Olivieri et al (8), however, the impact of coagulation addition on flux was difficult to evaluate since sometimes a greater flux decline was shown with coagulation pretreatment. Bian et al. (9) demonstrated that optimum coagulation dosage of alum exists to reduce MF fouling caused by humic acid. Lahoussine-Turcaud et al. (10) suggested that the flux characteristics of coagulated water can be explained in terms of the backtransport velocity of particles; particles near 0.2 µm in diameter produce rapid fouling, while particles greater than 3 µm in size have little effect on flux. These previous reports suggest that the membrane filtrability in MF process would largely depend on the coagulation mechanism, which is governed by the specific coagulation condition. Moreover, it can be thought that the effect of particle back-transport may also be important in cross-flow MF of coagulated water. However, the direct relationship between coagulation conditions and membrane filtration performance was not clearly revealed yet, although it is essential to select an optimal coagulation condition to achieve the highest MF performance in a coagulation-MF process. Therefore, this study was focused on investigating a relationship between membrane permeability and coagulation mechanism under various coagulation conditions such as alum dosage and pH through dead-end and cross-flow MF.
Theory Specific Cake Resistance. The resistance-in-series model was applied to estimate the effect of coagulation pretreatment on MF performance. According to this model, the permeate flux (J) can be expressed as (11)
J)
∆P ∆P ) µRt µ(Rm + Rc + Rf)
10.1021/es9907461 CCC: $19.00
(1)
2000 American Chemical Society Published on Web 08/04/2000
where ∆P is the TMP (transmembrane pressure), µ is the viscosity of permeate, Rt is the total resistance, Rm is the membrane resistance, Rc is the cake resistance, and Rf is the fouling resistance due to irreversible adsorption and pore blocking. The cake resistance, Rc, is related to the specific cake resistance, R, and the mass of cake deposited on the membrane surface, M, as in the following equation:
Rc )
RM RVCb ) Am Am
(3)
m
R)
180(1 - ) 3Fpdp2
(4)
(5)
where R0 is the specific cake resistance at unit pressure and n is the compressibility index. Thus, the slope of linear fits to logarithmic plots of R at various ∆P gives the value of n. There were small fluctuations of Rraw due to the variation of raw water quality every time the samples were collected. Thus, the specific cake resistance was expressed as a normalized form: A relative specific cake resistance (Rrel) was defined as the ratio of specific cake resistance of coagulated water (Rcoagulation) to that of raw water (Rraw):
Rrel )
Rcoagulation Rraw
(6)
Back-Transport Velocity. In cross-flow MF, particle transport depends on two main actions: one action is to move the particles toward the membrane surface (negative direction) and the other one is to shift them away from the membrane surface (positive direction). The negative direction forces include permeation drag (Fd), while the positive direction forces include Brownian diffusion (FB), shearinduced diffusion (Fs), and lateral inertial lift (Fl) (15, 16). Assuming that the density of particles suspended in a flowing stream is the same as that of the liquid, the gravity and buoyancy forces were neglected. The rate of momentum of a particle equals the sum of all the forces imposed on the particles in a fluid stream along a membrane channel. Thus, the net force exerted on a particle along the membrane channel, F, is the sum of all forces noted above (17):
dvp π F ) dp3Fp ) (FB + Fs + Fl) - Fd 6 dt
range
average
SD
3-8 2.0- 2.5 0.040-0.080 5.0-20 7-8 40-55
4 2.2 0.045 7.0 7.5 50
1.4 0.3 0.011 3.4 0.4 3.2
be transformed into the form composed of corresponding velocities: 3 1 dp Fp dvp ) (vB + vs + vl) - J 18 η dt
(8)
where vB, vs, and vl are back-transport velocity by Brownian diffusion, shear-induced migration, and lateral migration, respectively. At a steady state (dvp/dt ) 0), the above equation can be simplified to
Jss ) vB + vs + vl
where is the porosity of cake, Fp is the particle density, and dp is the particle diameter. When the cake layer on membrane surface is compressible, the R of compressible cakes is modeled using a power law as follows (13, 14):
R ) R0∆Pn
parameter turbidity (NTU) TOC (mg/L) UV254 (cm-1) suspended solids (mg/L) pH alkalinity (mg/L as CaCO3)
(2)
where Am is the membrane area, V is the cumulative filtrate volume, Cb is the bulk concentration of particles, and R is the cake resistance normalized to the mass of materials deposited per unit of membrane surface area. R can be determined experimentally by eq 3 if applied TMP(∆P) is constant or estimated theoretically using the Carman-Kozeny equation (eq 4) (12):
µRm RµCb t ) + V V Am∆P 2A 2∆P
TABLE 1. Characteristics of Raw Water
(7)
where vp is the particle transport velocity and t is the time. Dividing eq 7 by 3πηdp, the particle transport equation can
(9)
where Jss is the steady-state flux, which is governed by hydrodynamic back-transport (b). Each back-transport velocity may be calculated as in the following equations:
( ) ( )
vB )
1/3 Cw 0.807D2/3 B τw ln 1/3 Cb L
(10)
vs )
1/3 0.807D2/3 Cw s τw ln 1/3 Cb L
(11)
vl ) 0.577
r3p U2m
(12)
l 2ν
where DB is the Brownian diffusion coefficient () kBT/6πµr2p); Ds is the shear-induced diffusion coefficient () 0.03r2pτw); τw is the wall shear stress; Cw is the concentration of particle at membrane surface; Cb is the concentration of particle in bulk solution; L is the membrane module channel length; l is a channel height; ν is the kinematic viscosity of dispersing medium; Um is the maximum flow velocity at channel entrance; and rp is a radius of particle. The steady-state flux can be estimated from eq 9 since the steady-state flux corresponds to the critical flux at which no more particle deposition takes place with time. It should be noted that concept of particle back-transport and critical flux is only applicable when flux decline occurs by the deposition of particles on membrane. In the case of coagulated water, it is reasonable to assume that the loss in flux is mainly due to the formation of cake layer on membrane.
Materials and Methods Materials. Water samples used in this study were collected every 1-3 weeks from June to October 1998 from Pungnap intake at Han River (a water supply resource for Seoul, Korea). Samples were stored in a refrigerator without any chemical pretreatment before being used in the test system. The maximum storage time was less than 2 weeks because it was confirmed that there was no significant change in particle size distribution, TOC, UV absorbance, pH, and turbidity during that storage time. The raw water quality is presented in Table 1. VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Alum [Al2(SO4)3‚13-14H2O, Shinyo Pure Chemicals, Japan] was used as a coagulant. The concentration of alum stock solution was 25 g/L as alum [Al2(SO4)3‚13.5H2O], and it was diluted to appropriate portion prior to each coagulation experiment. The MF membrane used for cross-flow MF mode was a flat sheet of PVDF membrane (GVWP, Millipore, USA) whose nominal pore size was 0.22 µm, and that for dead-end MF mode was a hollow fiber of polyethylene membrane (Mitsubishi Rayon Co., Japan) whose nominal pore size was 0.1 µm. Analytical Methods. The analytical methods from Standard methods (17) were adopted for the measurements of total organic carbon (TOC), total suspended solids (TSS), pH, alkalinity in raw water, supernatant of coagulated samples and membrane filtrates, respectively. Analysis of TOC was performed by a TOC analyzer (DC-180, Rosemount, USA). Measurement of absorbance at the wavelength of 254 nm (UV254) was carried out using a spectrophotometer (Spectronic 1201, Milton Roy, USA). The turbidity was measured by a nephelometric turbidimeter (DRT-100B, HF Sci. Co., USA). The zeta-potentials of particles in raw and coagulated water were measured using a zeta meter (Delsa 440SX, Coulter Ltd., USA). The volumetric size distributions of particles in raw and coagulated water were evaluated using a particle size analyzer (Malvern Mastersizer/E, U.K.). The samples were measured either directly or after pretreatment. Pretreatment was necessary when measuring the size of the particles ranging from 0.1 to 10 µm, otherwise the small number of large particles could obscure the portion by the large number of small particles. So the samples were pretreated using an 8-µm filter (Satorius, USA) prior to the measurement. Once some cake layer was found to deposit on the prefilter during the pretreatment, the prefilter was replaced with a new one to minimize the possible interference due to the deposited cake layer. Jar Test. Jar tests were carried out to identify the coagulation mechanism as well as to estimate the coagulation efficiency according to alum dose and pH. The alum doses were varied from 0 to 150 mg/L as alum [Al2(SO4)3‚13.5H2O], and the final pH was adjusted from 4.0 to 9.0 using 0.1 N HCl and 0.1 N NaOH solution. Coagulation and subsequent flocculation were performed as follows; alum together with acid (0.1 N HCl) or base (0.1 N NaOH) were added simultaneously to the raw water, followed by a rapid mix of 3 min at 100 rpm and a slow mix of 20 min at 30 rpm and settling for 60 min. The pH was continuously monitored during the jar test, but the pH values reported in this study were final ones measured at the end of settling period. The experimental plan for jar test is shown in Table 2. Specific Cake Resistance. The measurement of specific cake resistance was carried out in an unstirred filtration mode for the coagulated solutions under the various coagulation conditions. The experimental setup is illustrated in Figure 1. After the jar test, each coagulated solution was transferred into the buffer tank and agitated to make a uniform suspension of coagulated flocs prior to the unstirred deadend MF. TMP was regulated at 0.34 bar using nitrogen gas. The permeate flux was determined by weighing permeates on an electronic balance connected to a personal computer equipped with an autoreading program. To investigate the effect of operating pressure on specific cake resistance, the compression-permeability experiments were also carried out in an unstirred MF mode (Figure 1). In this case, TMP was changed from 0.2 to 0.5 bar to evaluate the compressibility factor (n) from eq 5. Dead-End Microfiltration. Schematic diagrams of the experimental setup for the dead-end MF are shown in Figure 2a,b. In the external pressure type (Figure 2a), coagulated 3782
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TABLE 2. Experimental Plan for Jar Test run
pH
alum dose
1
7.1 7.1 7.2 7.2 7.1 7.1 7.1 7.0 5.2 5.2 5.1 5.1 4.9 4.7 7.0 3.9 4.0 4.0 4.0 4.1 4.1
0 5 10 20 50 100 150 0 5 10 20 50 100 150 0 5 10 20 50 100 150
2
3
run
pH
alum dose
4
7.0 5.1 5.1 5.1 5.1 4.9 4.7 7.0 5.4 5.5 5.5 5.5 5.4 5.5 7.0 6.2 6.2 6.1 6.1 6.2 6.1
0 5 10 20 50 100 150 0 5 10 20 50 100 150 0 5 10 20 50 100 150
5
6
run
pH
alum dose
7
7.7 7.0 7.0 7.0 7.0 7.0 7.1 7.7 7.9 7.9 7.9 7.9 7.9 8.0 7.8 8.8 8.8 8.8 8.7 8.7 8.8
0 5 10 20 50 100 150 0 5 10 20 50 100 150 0 5 10 20 50 100 150
8
9
suspension was fed from the flocculation tank to the hollowfiber membrane module by a peristaltic pump. The flow direction in the hollow-fiber module is outside-in, e.g., the feed stream was pumped into the cartridge shell and passes over the fiber walls. The constant flux was maintained at 200 L m-2 h-1, and the rise-up of TMP was monitored. On the other hand, in the submerged type (Figure 2b), a U-shaped hollow-fiber membrane module was immersed in the coagulation tank having 0.063 L of working volume. The permeate was continuously removed by a peristaltic pump under a constant flux (200 L m-2 h-1), constantly monitoring the rise-up of TMP, which indicates the extent of membrane fouling. The effective membrane area was 0.0015 m2 for both types of dead-end MF. Cross-Flow Microfiltration. A schematic diagram of the experimental setup for the cross-flow MF is shown in Figure 2c. The system consisted of flocculation tank with a total working volume of 5 L, a recirculation pump, and a MF module with an effective membrane area of 0.00274 m2. The length of the membrane channel (L) is 12 cm, and the channel height (l) is 0.3 cm. The cross-flow velocity was manipulated by controlling the speed of pump with an inverter (StarvertG, LG, Korea) while the operating pressure was adjusted with a back-pressure valve. Total recycle mode, where both the retentate from the MF loop and the permeate were recycled into the flocculation tank, was adopted to keep the reactor volume constant during the operation time.
Results and Discussion Contour Diagram of Alum Coagulation. Contour diagram of percentage removal of turbidity as a function of alum dosage and pH was developed based on the jar test results of alum coagulation (Figure 3). Two distinct regions with more than 80% removal of turbidity appeared: One region (zone I) occurred in the range of about pH 5 with alum dosage of less than 20 mg/L, and the other one (zone II) occurred in the range of pH 6-8 with alum dosage greater than about 10 mg/L. Zone I represents the region where destablization of particles is achieved predominantly through the chargeneutralization mechanism, which is known to occur at both lower pH and lower alum dosage. Zone II represents the region where the destabilization of suspensions is achieved through the sweep-floc mechanism, e.g., enmeshment in a aluminum hydroxide precipitate, which is known to occur typically at both higher pH and higher alum dosage. The
FIGURE 1. Schematics of experimental setup for a batch unstirred cell (dead-end filtration) to determine the specific cake resistance. analysis of zeta-potentials for flocs formed at different coagulation conditions also supports the different coagulation mechanisms with respect to alum dosage and pH (Figure 4). The flocs formed in the range of pH 5 and alum dosages of 5-20 mg/L showed zero surface potential due to the chargeneutralization of negatively charged colloids by soluble positive hydrolysis aluminum species. Neutralized flocs also appeared at the pH of greater than 7 due to the interaction of positively charged aluminum hydroxide with negatively charged colloids (19). Specific Cake Resistance at Different Coagulation Conditions. MF of coagulated suspensions formed under the various coagulation conditions were carried out with an unstirred cell filtration equipment to evaluate the specific cake resistance of each suspension from eq 3. Figure 5 shows the contour diagram of relative specific cake resistances (Rrel) as a function of pH and alum dosage. If Rrel were smaller than 1, the coagulation pretreatment prior to the MF would have positive effect with respect to the membrane permeability and vice versa. As shown in Figure 5, coagulation conditions greatly influenced on the characteristics of MF. In the range of pH 5 and alum dosage of 5-20 mg/L, Rrel gave the lowest values of less than 0.7, which indicates the least cake resistances and thus the most favorable conditions in terms of membrane permeability in the coagulation-MF process. This region of the lowest Rrel was nearly superimposed on zone I in Figure 3 where the charge-neutralization mechanism was predominant. On the other hand, the values of Rrel were greater than 1 in the region corresponding to zone II in Figure 3 where the sweep-floc mechanism was predominant. These results implies that the coagulation of raw water through the charge-neutralization mechanism may be the critical point to get the highest membrane permeability in this hybrid MF with coagulation as far as the dead-end mode such as submerged or external pressure type is adopted. Factors Affecting Specific Cake Resistance. From the eq 4, the specific cake resistance depends on the floc size (dp) and porosity of cake layer (), which is directly related to compressibility index (n). Thus, the particle size distributions and compressibility index of the coagulated suspensions were measured in order to elucidate the effect of coagulation mechanisms on Rrel. Figure 6 shows the particle size distributions of coagulated suspensions formed under different coagulation conditions. Although the coagulation through any mechanism made the floc size distributions shift to the right direction, little difference in the floc size distribution was observed between two mechanisms. This suggests that the difference in the specific cake resistance should be attributed to any other physicochemical property of flocs such as the porosity of cake layer. Because it was not possible to directly measure the porosity of each cake layer formed on the membrane surface by each coagulation mechanism, the compressibility index of each cake was evaluated, and a comparison was made. The specific cake resistance vs TMP is plotted in Figure 7,
and the compressibility index was obtained from the slope of the straight line. These experiments were performed in the range of 0.2-0.5 bar to encompass the operating TMP of 0.35 bar during microfiltration of coagulated suspension. The compressibility index of cake formed by sweep-floc mechanism (n ) 0.59) was about three times as large as that by charge-neutralization mechanism (n ) 0.19). It was reported that flocs formed by the sweep-floc mechanism have much lower density than those formed by the chargeneutralization mechanism due to their relatively higher water content. Furthermore, they are gelated, more compact, and less porous because they are made up most of the aluminum hydroxide precipitates. On the other hand, flocs formed through the charge-neutralization mechanism consist of complexes of aluminum cation, inorganic colloids, and/or organic substances, and thus they are less compressible (20, 21). Hence, the cake layer formed through the sweep-floc mechanism would certainly be more compressible and thus have smaller porosity than that formed through the chargeneutralization mechanism, which leads to a greater compressibility index (Figure 8). Dead-End Microfiltration. Dead-end MF of coagulated suspensions formed at different coagulation conditions was carried out to investigate the effect of coagulation conditions on membrane permeability. Three different feeds were prepared prior to MF: e.g., (i) raw water, coagulated suspensions by (ii) sweep-floc condition (30 mg/L alum, pH 7.5) and (iii) charge-neutralization (10 mg/L alum, pH 5). MF was carried out at constant flux of 200 L m-2 h-1 under two different modes (external pressure and submerged types), and TMP was monitored with operation time as shown in Figure 9. Under the external pressure mode (Figure 9a), the TMP reached 1.0 bar after about 2100 min for the chargeneutralization condition while it took only 900 min for the sweep-floc condition. In other words, the rising rate of the TMP for sweep-floc condition was more than 2 times higher than that for charge-neutralization condition, e.g., membrane fouling proceeded much faster with sweep-floc condition than with charge-neutralization condition. The impact of the two types of chemical flocs on membrane permeability coincides well with the difference in the specific cake resistance between them described in the previous section. Under the submerged mode (Figure 9b), the operation was stopped when TMP reached about 0.6 bar because the module configuration made it difficult to maintain the flux at constant level over that pressure. Specially in case of charge-neutralization condition, the submerged MF was carried out with the coagulated suspension into which 20 mg/L of Al(OH)3 precipitates was added in order to compensate for the difference of alum dosage between two selected coagulation conditions. It was done intentionally because one may doubt that the different membrane permeability could not come up with the differences of specific cake resistance, but it could come up with the differences of deposited mass in the membrane due to VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Schematics of experimental setup for coagulation-microfiltration hybrid processes. (a) Dead-end MF (external pressure type). (b) Dead-end MF (submerged type). (c) Cross-flow MF. different coagulant doses applied. As shown in Figure 9b, the TMP reached 0.6 bar after about 4800 min for the chargeneutralization condition while it took only 1100 min for the sweep-floc condition. In other words, the rate of membrane permeability loss (e.g., the rising rate of TMP) is about 4 3784
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times greater under the sweep-floc condition than under the charge-neutralization condition. This result wiped out any doubt that the difference in the rising rate of TMP between two coagulated suspensions could also be explained by dosage of coagulant. In summary, for both external pressure
FIGURE 3. Contour diagram of percentage removal of turbidity as a function of alum dosage and pH: turbidity of raw water; 3-4 NTU. FIGURE 6. Particle size distribution of flocs at different coagulation conditions.
FIGURE 4. Region of nearly zero value (-1 to +2 mV) of zetapotential of coagulated flocs.
FIGURE 5. Contour diagram for the relative specific cake resistances after alum coagulation as a function of alum dosage and pH: relative specific cake resistance, rrel ) rcoagulation/rraw. and submerged dead-end MF, the membrane permeability largely depends on the coagulation condition because it gave rise to the different specific cake resistance.
FIGURE 7. Variation of specific cake resistance with TMP (transmembrane pressure) at different coagulation conditions. Meanwhile, in Figure 9a,b, it was also observed that the coagulation of raw water prior to MF obviously enhanced the membrane permeability regardless of the coagulation condition. It could not be explained only by the specific cake resistance. It is because with the charge-neutralization condition the specific cake resistance was decreased through coagulation (e.g., Rrel is around 0.7), but it was rather increased with the sweep-floc condition (e.g., Rrel is around 1.3) as shown in Figure 5. Dissolved organics have been known to be the main foulants causing the internal membrane fouling (Rf) through irreversible adsorption and could be removed through coagulation. The removals of dissolved organics represented by UV254 and TOC were 52-55% and 23-35%, respectively, depending on the coagulation condition as shown in Table 3. However, the internal fouling (Rf) is not reflected by the specific cake resistance. Therefore, partial removal of organics by coagulation may be one of the reasons why the permeability was enhanced under the sweep-floc condition despite greater specific cake resistance than raw water. On the other hand, when the membrane permeability is compared between two different coagulated suspensions, the specific cake resistance could be the key factor governing the membrane permeability because the removals of dissolved organics represented by UV254 and TOC were quite similar irrespective of the coagulation condition. VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 8. Comparison of two types of coagulated flocs formed at different coagulation conditions.
FIGURE 9. Variations of TMP at constant flux during dead-end microfiltration: (a, external pressure type; b, submerged type) of coagulated suspensions made at different coagulation conditions. Constant flux, 200 L m-2 h-1. Cross-Flow Microfiltration. Cross-flow MF of coagulated suspensions made at the same coagulation conditions as for 3786
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dead-end MF was carried out with a plate and frame module under the total recycle mode (Figure 2c). The flux variation during cross-flow MF of the three different feeds was depicted in Figure 10. All the MF experiments were conducted at the constant TMP and cross-flow rate. The flux after 190 min of operation was regarded as steady-state flux since there was no substantial decrease in flux after 190 min for each feed tested. The steady-state flux of coagulated suspensions was higher than that of raw water. It was also observed under dead-end MF and could be explained by the same reason applied to the dead-end MF. However, contrary to expectation, the two kinds of suspensions gave rise to the almost same flux patterns regardless of the coagulation conditions. To elucidate this phenomenon, the back-transport velocities of particles involved in the coagulation-MF system used in this work was calculated based on eqs 9-12 and is depicted in Figure 11. The particle wall concentration (Cw) was chosen to be 0.62 based on the work of Yoon et al. (22). The bulk concentration (Cb) was estimated from the measured value of suspended solids. Particles in the range of 0.1-1 µm have a strong tendency to move toward the membrane; thus, they are major foulants through internal fouling (Rf) or cake deposit (Rc) in the MF of raw water. These particles were also reported to be the major foulants for flux decline (7). On the other hand, considering the initial flux in Figure 11, particles larger than about 45 µm could not approach the membrane surface and thus have nothing to do with flux decline. It could be postulated that the flux enhancement through the alum addition might be achieved since most of these small particles (0.1-1 µm) being greatly responsible for the flux decline might be removed by alum coagulation. To verify this postulation, particle size distributions in raw water and coagulated suspensions during cross-flow MF were analyzed, respectively (Figure 12). Samples were prefiltered with a 8-µm filter prior to the measurement of particle size distribution. As clearly shown in Figure 12, most of the small particles in the range of 1 µm in the raw water disappeared after coagulation. It is worth noting that in the cross-flow MF both the coagulated suspensions made by charge-neutralization and sweep-floc mechanisms gave nearly the same steady-state flux even though their specific cake resistances are quite different from each other as mentioned before. It is because not all particles in the coagulated suspensions would deposit on the membrane surface during continuous cross-flow microfiltration due to the back-transport forces acting on the particles. The size of coagulated flocs was so large (more than 10 µm)
TABLE 3. Water Quality of Feed and Permeate in Alum Coagulation-MF System coagulation condition
MF without coagulation
coagulation-MF (10 mg/L alum, pH 5, charge- neutralization)
turbidity (NTU)/feed turbidity (NTU)/permeate turbidity (NTU)/removal (%) UV254 (cm-1)/feed UV254 (cm-1)/permeate UV254 (cm-1)/removal (%) TOC (mg/L)/feed TOC (mg/L)/permeate TOC (mg/L)/removal (%) aluminum (mg/L)/permeate
3.5 0.13 96.3 0.058 0.036 37.9 2.10 1.90 9.52 0.042
2.0 0.13 93.5 0.044 0.02 54.5 2.01 1.30 35.3 0.045
coagulation-MF (30 mg/L alum, pH 7.5, sweep-floc) 1.6 0.11 93.1 0.040 0.019 52.5 2.04 1.56 23.5 0.145
FIGURE 11. Comparison of different models for back-transport of particles over a range of particle size: temperature, 25 °C; channel height, 3 mm; average flow rate, 0.22 m/s; NRe, 1030.
FIGURE 10. Flux variation at constant TMP during cross-flow microfiltration of coagulated suspensions made at different coagulation conditions. Cross-flow rate, 1.0 L/min; ∆P, 0.34 bar. regardless of coagulation mechanism that they might hardly deposit on the membrane, which was assumed on the basis of the steady-state flux for the raw water. Hence, the effect of the different physicochemical properties of coagulated flocs on the membrane flux seemed not to be demonstrated. NOM Removal and Residual Aluminum Concentration. In Table 3, the percent removal of turbidity and natural organic matter (UV254, TOC) by MF were shown for several coagulation conditions. Irrespective of the alum addition to the raw water, MF could remove turbidity almost completely with the permeate turbidity of approximately 0.1 NTU. The
FIGURE 12. Particle size distribution of coagulated suspensions during microfilration after prefiltration with 8-µm filter. removal of UV254 representing unsaturated organic matter like humic substances was about 38% with only MF and was increased by more than 15% with the alum addition. The TOC removal was only about 10% without coagulation, whereas it was increased by 15-25% with coagulation. The highest removal of TOC occurred at the chargeneutralization condition. It may be because soluble organic substances become less electronegatively charged at lower pH and thus subject to be easily coagulated. VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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The coagulation through the charge-neutralization condition was also advantageous with respect to the residual aluminum concentration. As shown in Table 3, the residual aluminum concentration in the permeate was nearly same as that without alum addition, whereas 0.145 ppm of Al was found with the sweep-floc condition.
Acknowledgments We thank the Korean Ministry of Science and Technology for its financial support under Grant 98-NE-06-05-A-01.
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Received for review July 1, 1999. Revised manuscript received May 11, 2000. Accepted June 15, 2000. ES9907461
