Effect of flocculation conditions on membrane permeability in coagulation microfiltration

Transcription

1 Desalination 191 (2006) Effect of flocculation conditions on membrane permeability in coagulation microfiltration Min-Ho Cho a, Chung-Hak Lee a*, Sangho Lee b a School of Chemical and Biological Engineering, Seoul National University, Seoul , Korea Tel. +82 (2) ; Fax +82 (2) ; leech@snu.ac.kr b Construction Environment Research Department, Korea Institute of Construction Technology, Gyeonggi-Do , Korea Received 7 March 2005; accepted 18 August 2005 Abstract A coagulation microfiltration (MF) system has the potential to remove natural organic matter and mitigate membrane fouling. The effect of flocculation conditions on MF performance was investigated using three membrane modules including stirred cell, dead-end MF, and submerged MF. The experimental results were analysed in terms of filterability, particle size, and floc structure as represented by the fractal dimension. In jar tests, specific cake resistance and fractal dimension of flocs decreased as flocculation time increased. In addition, small colloidal particles (<5 µm) were reduced as flocculation progressed. These results indicate that the formation of loose and porous flocs and reduction of small colloidal particles at longer flocculation time led to higher flux in three MF modules. Moreover, flocs generated in two-stage mixing had better filterability than those in single-stage mixing. Keywords: Microfiltration; Coagulation; Flocculation; Floc structure; Fractal dimension 1. Introduction Microfiltration (MF) offers an innovative technology with the potential of providing water that meets current and anticipated water quality standards. However, a major challenge associated *Corresponding author. with MF is removing natural organic matter (NOM) that has been known as a precursor for disinfection by-products (DBPs). MF alone is ineffective to control DBPs because of its large pore size compared to NOM. Another problem is the sensitivity to fouling that arises from the accumulation and deposition of contaminants in feed water. Therefore, the effective control of Presented at the International Congress on Membranes and Membrane Processes (ICOM), Seoul, Korea, August /06/$ See front matter 2006 Published by Elsevier B.V. doi: /j.desal

2 M.-H. Cho et al. / Desalination 191 (2006) fouling is crucial for proper functioning and longterm effectiveness of any membrane system for potable water processing. A combination of membrane filtration with physicochemical processes such as coagulation, adsorption and so on can improve the quality of the produced water and membrane permeability [1,2]. Especially, adding a coagulant prior to MF, which is often called the coagulation MF hybrid process, is an efficient way to resolve these problems. Coagulation of feed water enables the removal of NOM that can be a major fouling material and enhances membrane permeability [3]. Coagulation also improves the filtration characteristics of MF by reducing cake resistance of the deposit on the membrane [4 6]. In coagulation MF hybrid systems, many factors related to the coagulation process significantly affect the filterability of membranes. For instance, coagulation mechanisms, which are varied by concentration of coagulant and ph of the raw water, affect the filterability of flocs [5,7]. In addition, the contribution of NOM to fouling depends on the raw water quality, characteristics of the NOM, and the type and level of any pretreatment. Floc characteristics play an important role in inducing membrane fouling. The physical and chemical properties of flocs are sensitive to flocculation conditions including mixing intensity and flocculation time. Increased shear rate reduces the average steady-state size of flocs [8]. Higher mixing intensity leads to larger fractal dimension, which is related to floc structure [9]. Longer flocculation time results in decreased floc size and reduced fractal dimension of flocs [10]. However, most of the previous works on floc characteristics were done in conventional coagulation/flocculation systems. Thus, little information is available on the floc characteristics in connection with membrane performances, which is a key to understanding coagulation MF systems. Thus, this study focused on investigating floc filterability under various flocculation conditions including flocculation time and mixing conditions in a coagulation MF hybrid system. The fractal concept was applied to define floc structure, and the small-angle laser light scattering (SALLS) method was used to measure the fractal dimension [11 13]. Filtration experiments were performed and compared using three different membrane configurations including stirred cell, external type dead-end MF, and continuous submerged MF. 2. Materials and methods 2.1. Jar tests Water samples used in this study were collected from the surface water pumping line at a water treatment plant at the basin of the Han River, which has been used as a main source of drinking water for the citizens of Seoul, Korea, for a long time. Suspended solids content in raw water was 6.8 mg/l (±0.6), and turbidity was 2 3 NTU. As indicators of NOM, DOC was 2 3 mg/l and UV 254 absorbance was cm!1 during the experimental period. Jar tests were carried out to investigate the characteristics of floc and removal of UV 254 with different flocculation conditions. Polyaluminum chloride (PACl) was used as a coagulant at 3.18 mg/l as Al 2 O 3, which corresponds to 30 mg/l as PACl dose, in all experiments. The coagulant dose was determined by jar tests and had a moderate removal of organic matter. Flocculation was performed in two different modes to examine the influence of shear rate on floc formation: C RM (rapid mixing) + SM (slow mixing): PACl was added to the raw water, followed by a rapid mix at 100 rpm (150 s!1 ) and a slow mix at 30 rpm (G=45 s!1 ). C RM (rapid mixing): Only rapid mixing was undertaken at 100 rpm (150 s!1 ). In the RM+SM mode, the time for rapid mixing was fixed to 3 min and the time for slow

3 388 M.-H. Cho et al. / Desalination 191 (2006) mixing varied from 20 min to 8 h. In RM modes, the time for rapid mixing varied from 20 min to 8 h. Here, the duration for slow mixing and rapid mixing was defined as flocculation time in both RM+SM and RM modes. In this study, some experiments were conducted with a very long residence time to investigate the fundamental characteristics of flocs under various conditions. Floc size distribution, specific cake resistance, and fractal dimension (d F ) of the floc at each flocculation condition were estimated to investigate the changes in floc characteristics. Specific cake resistance was measured using an unstirred cell device. Floc size and fractal dimension were estimated using MasterSizer/E (Malvern, UK). Using this instrument, the fractal dimension was calculated based on the SALLS theory [11 13]. The floc size is the volume median diameter, D (0.5) and polydispersity, i.e., width of the particle size distribution, of flocs was measured by: Span = (particle diameter at 90% cumulative size)! (particle diameter at 10% cumulative size) / (partial diameter at 50% cumulative size) (1) In addition, the number of particles was measured using particle counter (267 WGS, MetOne, USA) Filtration experiments Three membrane systems including a stirred cell, a batch external dead-end filter, and a continuous submerged filter were used to examine the membrane filterability with flocculation time. In the stirred cell filtration, the flocculated suspension prepared at each flocculated condition was transferred to the stirred cell unit when each given flocculation time was ended. The permeate passes through a hydrophilic microfilter with a pore size 0.22 µm (GVWP, Millipore, USA) under P = 40 kpa. To prevent flocs from settling in the buffer tank connected to stirred cell device, weak stirring by a magnetic bar was performed and the filtration was achieved under P = 40 kpa for fast filtration. The volume of flocculated water and the stirring speed were constant for each test. In the batch external dead-end MF, flocculated suspension in each flocculation time of h and 3 5 h was fed from the flocculation reactor to an external-type hollow-fiber membrane module using a peristaltic pump. In the continuous submerged MF experiments, raw water and stock solution of PACl were fed into the reactor continuously through the line static mixer. Filtration and determined mixing with high or low shear rate were simultaneously carried out at a constant reactor volume. The experimental set-up for the continuous submerged MF is illustrated in Fig. 1. To intensify the effect of flocs on membrane fouling, there was no use of methods to prevent fouling such as aeration in two kinds of filtration such as the batch external dead-end MF and the continuous submerged MF. The membrane used in the dead-end and submerged filtration was made of polyethylene with a pore size 0.1 µm (Mitsubishi Rayon, Japan). Flux in the batch and continuous submerged MF experiment was 100 L/m and the transmembrane pressure was measured with time as the degree of membrane fouling. In continuous submerged MF, hydraulic retention time (HRT) was introduced to control the flocculation time. That is long flocculation time was achieved as increasing of HRT. Strictly speaking, HRT is different from the flocculation time. Nevertheless, flocs in the reactor had a long flocculation time as HRT increased relatively because there was no drainage in the continuous system. 3. Results and discussion 3.1. Jar tests results Jar tests were carried out to explore the removal efficiency of NOM and the characteristics of flocs under various flocculation conditions. Here, UV 254 absorbance was used as a

4 M.-H. Cho et al. / Desalination 191 (2006) Fig. 1. Experimental set-up for continuous submerged MF. Aeration for retarding membrane fouling and sludge wastage were not undertaken. Table 1 Changes in UV 254, floc size, and fractal dimension with flocculation time under various mixing conditions (RM+SM, rapid mix 3 min followed by slow mix; RM, rapid mix alone) 20 min 1 h 2 h 4 h 8 h RM + UV 254 (cm!1 ) SM Floc size (µm), (span) 90 (±9) (1.542±0.010) 102 (±11) (1.411±0.012) 127 (±14) (2.304±0.017) 282 (±12) (1.715±0.009) 447 (±23) (1.094±0.006) Fractal dimension (d F ) 2.29 (±0.04) 2.22 (±0.01) 2.16 (±0.01) 1.95 (±0.02) 1.89(±0.01) RM UV 254 (cm!1 ) Floc size (µm) (span) 49 (±5) (1.675±0.007) 51 (±4) (1.592±0.014) 63 (±5) (1.724±0.05) 56 (±6) (1.655±0.011) 55(±2) (1.635±0.003) Fractal dimension (d F ) 2.30 (±0.02) 2.28 (±0.03) 2.05 (±0.02) 1.97 (±0.05) 1.92(±0.01) surrogate of NOM concentration because NOM removal showed a similar tendency to UV 254 removal. UV 254 removal as a function of mixing condition and flocculation time is summarized in Table 1. UV 254 removal seems to be similar regardless of the mixing mode and flocculation time. It appeared that the removal of organics was determined at the early stage of the flocculation process and its dependence on mixing methods was relatively weak. Table 1 also shows the dependence of floc size, span and its fractal dimension on flocculation time and mixing modes. The effect of flocculation time on floc size in RM mode was different from that in the RM+SM mode, as also shown in Fig. 2. With increasing flocculation

5 390 M.-H. Cho et al. / Desalination 191 (2006) (a) (b) Fig. 2. Particle size distribution of flocs at different mixing modes. (a) RM + SM; (b) RM. time, floc size increased in RM+SM mode, but it remained almost constant in the RM mode. Moreover, span did not change with flocculation time in the RM mode, indicating the same particle size distribution. It appears that increasing time for rapid mixing in RM mode was not effective to increase floc size because a high intensity of mixing may limit the growth of flocs with time [8]. On the other hand, the fractal dimension of flocs decreased as flocculation time increased in both RM+SM and RM modes. For instance, the fractal dimension decreased by 6% to 18% with increasing flocculation time from 20 min to 8 h. This clearly indicates that the floc structure represented by fractal dimension continuously changes during flocculation. Based on this result, the floc structure appears to be looser with increasing flocculation time. In addition to UV 254 removal and floc properties, the zeta potential of flocs was measured using a laser light scattering instrument (Delsa 440 SX, Coulter, USA) under various flocculation times (Fig. 3). The results indicate that only flocs at early stages had negative charges. For Fig. 3. Variations in zeta potential according to flocculation time. (RM 3min condition was applied to two flocculation conditions identically.) instance, zeta potential of flocs at 3 minutes of rapid mixing ranged from 0 to -5 mv but decreased to near 0 mv beyond 20 minutes of flocculation time. These electric properties appear to be determined at the early stage of coagulation Specific cake resistance Based on the previous results from jar tests,

6 M.-H. Cho et al. / Desalination 191 (2006) Fig. 4. Decrease in specific cake resistance with an increase of flocculation time. Fig. 5. Dependence of particle counts on flocculation time in the RM+SM mode. Time for slow mixing varied from 20 min to 4 h. the changes in floc properties including floc size and fractal dimension seem to be closely related to the filterability of flocs. To investigate this relationship, specific cake resistances of flocs formed under a variety of flocculation conditions were measured using the unstirred cell equipment. Fig. 4 shows the dependence of specific cake resistance in RM+SM and RM modes, indicating that the specific cake resistances in RM mode are higher than those in RM+SM mode. This is because the floc size in RM+SM mode is larger than that in RM mode as indicated in Table 1. According to the Carman Kozeny equation, the filterability of flocs increases with floc size. Thus, flocs formed in RM+SM mode have higher filterability due to their larger size than those in RM mode. Fig. 4 also demonstrates that the specific cake resistance decreased with flocculation time in both RM+SM and RM modes. For instance, the specific resistance of flocs decreased from kg/m to kg/m as the flocculation increased from 20 min to 8 h in RM+SM mode. Similarly, the specific cake resistance decreased from to with increasing flocculation time in RM mode. Since floc size did not vary with flocculation time as shown in Table 1, the decrease in specific cake resistances cannot be explained by the size effect. Instead, this may be attributed to the changes in floc structure represented by fractal dimension. As shown in Table 1, longer flocculation time results in the formation of flocs having lower fractal dimension. This leads to an increase in floc permeability because flocs with low fractal dimension are generally porous and large [14,15]. Thus, flocs generated at longer flocculation time have lower value of specific cake resistance. It should be noted that the higher permeability of flocs in bulk solution does not always lead to higher filterability or lower specific cake resistance. Since the cake layer may be compressed under pressure, flocs having loose structure may even result in higher specific cake resistance. Nevertheless, compaction of cake layer could be negligible in this study because experiments were carried out under low or moderate pressure conditions (# 40 kpa). Therefore, it is likely that local structures of the cake on membrane remained the same as in the aggregates of the original suspension, at least under the testing conditions in this study. This also agrees with previous work [16].

7 392 M.-H. Cho et al. / Desalination 191 (2006) (a) (b) (c) (d) Fig. 6. Microscopic pictures of flocs generated at different flocculation time in the RM+SM mode. Time for slow mixing varied from 20 min to 4 h. (a) SM 20 min. (b) SM 1 h. (c) SM 2 h. (d) SM 4 h. In addition to the structural differences, the reduced resistance of flocs at longer flocculation time may be attributed to different removal of colloidal particles. As depicted in Fig. 5, the number of small particles ranging from 2 µm to 5 µm decreased with mixing time after flocculation. For example, the removal of the small particles increased by 60 70% with increasing flocculation time from 20 min to 4 h under SM conditions. These results can be visually confirmed by comparing the microscopic pictures of flocs generated at different flocculation time (Fig. 6). The photographs in Fig. 6 were obtained by filtration of 10 ml of sample water using a video microscope system (Sometech Vison, Korea). In accordance with particle counting data, the amount of small particles deposited on the membrane was reduced as flocculation time increased. As mentioned earlier, the specific cake resistance decreases with its composing particle size according to the Carmen-Kozeny equation. Thus, higher removal of colloidal particles as well as decreased fractal dimension at longer flocculation time is likely to allow lower resistance of flocs.

8 M.-H. Cho et al. / Desalination 191 (2006) Filtration results The filtration characteristics of flocs were investigated using three membrane modules including stirred cell MF, dead-end MF, and continuous submerged MF under various flocculation conditions. First, stirred cell experiments were carried out to compare permeate flux in RM+SM and RM modes. In RM+SM mode (Fig. 7), flux was improved by 100% at 15 min of filtration time with increasing flocculation time from 3 min to 8 h. The results were similar in RM mode (Fig. 8). This indicates that longer flocculation time leads to better filterability of flocs in both RM+SM and RM conditions as expected from specific resistance data in Fig. 4. Although stirred cell is simple and convenient to check the filterability of flocs, it may not Fig. 7. Dependence of flux on time at various mixing conditions and flocculation times in stirred cell. Permeate flux in stirred cell filtration at P = 40 kpa, RM+SM; low shear rate (G = 45 s!1 ). Fig. 8. Dependence of flux on time at various mixing conditions and flocculation times in stirred cell. Permeate flux in stirred cell filtration at P = 40 kpa, RM only; high shear rate (G =150 s!1 ).

9 394 M.-H. Cho et al. / Desalination 191 (2006) Fig. 9. Dependence of transmembrane pressure on time at various mixing conditions and flocculation times in deadend MF. Transmembrane pressure in batch dead-end MF at J = 100 L/m 2 -h. Rapid mix (G = 150 s!1 ) was followed by slow mix (G = 45 s!1 ). Fig. 10. Transmembrane pressure as a function of HRT in continuous submerged MF at lower mixing intensity (G = 45 s!1 ). RM+SM mode. properly represent the practical membrane systems. Thus, the filtration results from the stirred cell were compared with those from hollow-fiber membrane modules. Fig. 9 illustrates the dependence of transmembrane pressure on filtration time in a dead-end MF system for various flocculation times. Again, a longer flocculation time results in a slower increase in transmembrane pressure for the dead-end MF. It is evident that filterability of floc increases with flocculation time regardless of mixing conditions. The filtration tests were also performed using submerged MF equipment. Here, coagulation conditions such as mixing intensity remained the same as those in the batch experiment. However, unlike the stirred cell and dead-end systems, in this case coagulant and water were continuously fed to the reactor through the line during the filtration. Thus, the flocculation time is proportional to HRT because there is no drainage of flocculated suspension during filtration. Figs. 10 and 11 demonstrate the dependence of transmembrane pressure on HRT under slow and high mixing conditions. It is evident that filterability of floc increases with flocculation time regardless of Fig. 11. Transmembrane pressure as a function of HRT in continuous submerged MF at higher mixing intensity (G = 150 s!1 ). RM mode. mixing conditions. In other words, the TMP augmented more slowly as the HRT increases under both slow (Fig. 10) and rapid mixing (Fig. 11). Under slow mixing, the particle size distribution shifted to the right (e.g., the higher size) with the increase in HRT (Fig.2a). Thus, the different rate of TMP rise-up could be easily

10 M.-H. Cho et al. / Desalination 191 (2006) explained by the different particle size distributions. However, under rapid mixing, the same patterns of TMP variation as a function of HRT were observed despite the change in the particle size distribution being almost negligible (Fig. 2b). For this reason, it is fair to conclude that the fractal dimension of particles as well as the size play a key role in membrane permeability. The results obtained here indicate the importance of floc structure as well as floc size for membrane performance. Therefore, not only the size but also the fractal dimension of flocs should be considered as a new control parameter in coagulation MF hybrid systems. 4. Conclusions The effects of different process conditions for coagulation on floc properties and membrane performance were examined. The following conclusions can be drawn: 1. Changes in mixing conditions lead to difference in floc filterability. Flocs generated in two-stage mixing (RM+SM) mode had lower specific cake resistance than those in the rapid mixing (RM) mode. This is attributed to the formation of larger flocs in RM+SM mode, which form cake layers with higher permeability. 2. Increasing flocculation time results in lower specific cake resistance in both RM+SM and RM modes. This is because loose flocs were formed at longer flocculation time as indicated by their low fractal dimension. 3. In three membrane modules including stirred cell MF, dead-end MF, and continuous submerged MF, membrane filterability was better for longer flocculation time in both in RM+SM and RM modes as expected from the specific cake resistance data. 4. Understanding floc properties including floc size and fractal dimension seems to be essential to control membrane fouling in coagulation MF hybrid systems. Acknowledgement The authors wish to thank the Mitsubishi Rayon Co. Ltd., Japan, for providing the hollowfiber membranes. References [1] A. Yuasa, Drinking water production by coagulationmicrofiltration and adsorption-ultrafiltration, Water Sci. Technol., 10 (1998) [2] A. Maartens, P. Swart and E. P. Jacobs, Feed-water pretreatment: methods to reduce membrane fouling by natural organic matter, J. Membr. Sci., 163 (1999) [3] T. Carroll, S. King, S.R. Gray, B.A. Bolto and N.A. Booker, The fouling of microfiltration membranes by NOM after coagulation treatment, Water Res., 11 (2000) [4] J.D. Lee, S.H. Lee, M.H. Jo, P.K. Park, C.H. Lee and J.W. Kwak, Effect of coagulation conditions on membrane filtration characteristics in coagulationmicrofiltration process for water treatment, Environ. Sci. Technol., 17 (2000) [5] P. Choksuchart, M. Héran and A. Grasmick, Ultrafiltration enhanced by coagulation in an immersed membrane system, Desalination, 145 (2002) [6] M.R. Wiesner, M.M. Clark and J. Mallevialle, Membrane filtration of coagulated suspension. J. Environ. Eng., 115(1) (1989) [7] S.J. Judd and P. Hillis, Optimisation of combined coagulation and microfiltration for water treatment, Water Res., 12 (2001) [8] P.T Spicer and S.E. Pratsinis, Shear-induced flocculation: the evolution of floc structure and the shape of the size distribution at steady state, Water Res., 5 (1996) [9] R.C. Klimpel and R. Hogg, Effects of flocculation conditions on agglomerate structure, J. Col. Inter. Sci., 1 (1986) [10] R.K. Chakraborti, K.H. Gardner, J.F Atkinson and J.E.V. Benschoten, Changes in fractal dimension during aggregation, Water Res., 4 (2003) [11] R. Amal, J.A. Raper and T.D Waite, Fractal structure of hematite aggregates, J. Col. Inter. Sci., 1 (1990)

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