- Email: [email protected]
Effect of the removal of DOMs on the performance of a coagulation-UF membrane system for drinking water production
DESALINATION Desalination
ELSEVIER
145 (2002) 237-245 www.elsevier.com/locate/desal
Effect of the removal of DOMs on the performance of a coagulation-UF membrane system for drinking water production Pyung-kyu Park”, Chung-hak Lee”*, Sang-June Choib, Kwang-Ho Choo”, Seung-Hyun Kimd, Cho-Hee Yoone “School of Chemical Engineering, Seoul National University, Seoul 151-744, Korea Tel. +82 (2) 880-7075; Fax +82 (2) 888-1604; email: [email protected] “Department of Environmental Engineering, Kyungpook National University, Daegu 702-701, Korea =Department of Architectural, Civil &Environmental Engineering, Daegu University, Gyeongsan 712-714, Korea “Civil and Environmental Engineering Department, eDepartment of Chemical Engineering, Kyungnam University, Masan 631-701, Korea Received 4 February 2002; accepted 27 March 2002
Abstract Coagulation with only rapid mixing in a separate tank (ordinary coagulation) and coagulation with no mixing tank (in-line coagulation) were applied prior to ultrafiltration with an inside-out type hollow fiber membrane. In result the filterability at the former conditions was superior in both the crossflow and dead-end modes. Thus the relative importance of the removal rate of DOMs was investigated. Precoating the surface of the membranes with metal hydroxide particles of coagulants was also examined. This method of utilizing coagulants resulted in a smaller consumption of coagulant in the coagulation-UF system. Keywords: Ultrafiltration;
Coagulation;
In-line coagulation;
1. Introduction
Dissolved organic matters
ficantly changed the surface water treatment
The development of microfiltration (MF) and ultrafiltration (UF) membrane processes has signi*Corresponding
Precoating;
author.
Presented at the International July 7-12, 2002.
Congress on Membranes
in a relatively short time [l]. However, they are not in widespread use because of some problems associated with their use, which include a low removal rate of and membrane fouling caused by and Membrane
Processes
00 I l-9 164/02/$- See front matter 0 2002 Elsevier Science B.V. All rights reserved PII: SO01 l-9164(02)00418-6
industry
(ICOM),
Toulouse, France,
238
P-k. Park et al. /Desalination
DOMs (Dissolved Organic Matters) in the raw water. As a result, the addition of a coagulant as a pretreatment prior to membrane filtration has been proposed for the purpose of not only improving the removal of DOMs but reducing membrane fouling [2,3]. In this study, prior to UP with an inside-out type hollow fiber membrane, two modes of coagulation were examined, (i) coagulation of raw water in a separate tank with rapid mixing only and (ii) in-line coagulation, and then the effects of the two procedures were compared with respect to the removal rate of DOMs and the membrane performance. Precoating the surface of the membranes with metal hydroxide particles of coagulants was also examined to enhance the coagulation-UF system. 2. Materials and methods 2.1. Raw watel; membrane, and coagulants The raw water samples used in this study were collected from an intake located at the Han River, the water quality of which is presented in Table 1. The membrane was a hollow-fiber ultrafiltration membrane made from polysulfone, the nominal pore size and inner diameter of which was 0.01 pm and 1 mm, respectively. The effective surface of a module with 8 fibers was 0.078 m2. PACl (polyaluminum chloride) was used as a coagulant at a concentration of 1 .O-10 ppm as Al,O,, and ferric chloride was also used as a comparison with PACI. The range of velocity gradient (G-value) in the mixing tank was
Table 1 Characteristics of the raw water used in this study Parameters
Range
Turbidity, NTU DOC, ppm UV254absorbance, cm-’ Concentration of SS, ppm Alkalinity, ppm as CaC03 PH
2-5 2.3-2.9 0.029-0.036 6.8-14.8 41-70 7.2-7.8
145 (2002) 237-245
Fig. 1. Schematics of bench-scale coagulation-UF system.
58-461 s’, and the hydraulic residence time (HRT) was varied over the range of 12-60 min. 2.2. Bench-scale
coagulation-UF
system
A schematic diagram of the bench-scale coagulation-UF system used in this study is shown in Fig. 1. All pumps were peristaltic. In the crossflow mode, a circulation pump was equipped so as to circulate water within the circulation loop, including the lumen-side of hollow fibers, while in the deadend mode it was removed. Because the loop was closed, the flux remained constant without any control even in the crossflow mode. 3. Operating conditions 3.1. Filtration conditions The water flux was fixed to 100 L/m*h (25°C). The trans-membrane pressure (TMP) would be expected to increase in the course of time due to membrane fouling, and so the TMP profile was monitored up to 70 kPa. In the crossflow mode, the crossflow velocity was 1 m/s. Backwashing was performed at 160 kPa with the permeate. Initially this was carried out for 1 min at lhr intervals in both the crossflow and dead-end modes. However, in the dead-end mode, floes produced as the result of coagulation blocked the lumens of some fibers. Therefore backwashing was executed for 20 s at 20 min intervals in the case of the dead-end mode. After backwashing, the lumens were flushed with raw or coagulated water. The recovery of raw water was about 92%.
P-k. Park et al. /Desalination
3.2. Coagulation
239
3.2.3. Precoating with metal hydroxide particles
conditions
3.2.1. Selection of coagulant hydraulic condition of coagulation
145 (2002) 237-245
dosage
and
Batch experiments were first carried out in order to select the optimum dosage of coagulant. The PACl dosage was changed under sufficient mixing conditions (HRT of 60 min and G-value of 3.50 s-l) and the DOC and UVZS4absorbance of coagulated water were then measured to select the most effective PACl dosage, which was determined to be 4.1 ppm as AJO,, Continuous experiments without membrane filtration according to HRT and velocity gradient were carried out at the selected dosage of PACl 4.1 ppm. The HRT and velocity gradient were varied by changing the rotating speed of the agitator and the flow rate through the mixing tank, respectively. UV,,, absorbance and the particle size distribution in the coagulated water were then measured.
3.2.2. Ordinary and in-line coagulation The bench-scale coagulation-UF system was operated at the two selected coagulation conditions determined in the previous experiments. One condition (HRT of 60 min and G-value of 350 s-‘) was applied such that PACl was dosed into the raw water line toward the mixing tank (line 1 in Fig. 1) and coagulation was executed in the mixing tank by only rapid mixing with an HRT of 60 min, a G-value of 350 s-l. The other was carried out similarly but with an HRT of 12 min with no agitation. Hereafter, the former and latter conditions will be designated as Ordinary and Znline coagulation, respectively. At these conditions the filtration performance was observed in both the crossflow and dead-end modes. In the case of the crossflow mode an additional experiment at ordinary coagulation conditions with a PACl dose of 10 ppm was also executed to increase the removal of DOMs and compared with the filterability at a PACl dose of 4.1 ppm.
Unlike these conditions, coagulants were also applied to form a cake or coating layer made up of metal hydroxide particles. The coating layer which was formed on the membrane surface would be predicted to be permeable and easily removable, and to remove DOMs by adsorption before being detached by backwashing and flushing [4]. Therefore, aluminum hydroxide particles were prepared by diluting a PACl stock solution with ultrapure water and adjusting the pH to 7.0(+0.2), and this slurry were dosed to the feeding line toward membrane module (line 2 in Fig. 1). After the coated layer had been formed in this way, only raw water was filtered through the precoated membrane. Because the layer was removed by backwashing and flushing at the last step of a cycle, the slurry was dosed for the first 1 min of every 20-min cycle. The concentration of PACl was set to a somewhat large value so that the total amount dosed within a cycle could be the same for both ordinary and in-line coagulation as well as at precoating conditions. Therefore a concentra-tion of 4.1 ppm as Al,O, at precoating conditions does not indicate the actual concentration but the concentration corresponding to 4.1 ppm as Al,O, at ordinary and in-line coagulation conditions. With a similar filtration test at 1.0 ppm and 2.0 ppm as Al,O, the removal of DOMs and the filterability as a function of the amount of hydroxide particles were investigated. Lastly, in order to compare aluminum hydroxide particles with iron hydroxide particles, the same filtration experiment was carried out with ferric chloride at 13.0 ppm as FeCl,, which value was selected so as to equalize the moles of AP+ and Fe3+. 4. Results and discussion 4.1. Coagulation according to HRT and velocity gradient In the continuous coagulation of raw water without membrane filtration according to HRT and velocity gradient, the removal of UV,,, ab-
240
P.-k. Park et al. /Desalination
145 (2002) 237-245
sorbance did not change significantly (33-38%), and even with no agitating (0 rpm) this was 33-34% (Fig. 2). Particle size distribution patterns were also similar regardless of the HRT and the velocity gradients except 0 s-’ (Fig. 3a). Therefore filtration with the UF membrane was not carried out at all the conditions but only two: one being ordinary coagulation conditions and the other, inline coagulation conditions. 4.2. Ordinary and in-line coagulation in crossjlow mode Under the crossflow mode, membrane filterability was obviously enhanced by the coagulation pretreatment and, of the two conditions with a PACl dosage of 4.1 ppm that for ordinary coagulation was superior to that for in-line coagulation (Fig. 4). And ordinary coagulation conditions at 10 ppm gave the best filterability. The reduction in the filterability is due to membrane fouling, and the major foulants in raw or coagulated water are particulate matters such as coagulated floes and dissolved or colloidal matters such as DOMs. The characteristics and variation of particulate matters including floes can be evaluated indirectly by measuring the particle size distribution. A clear distinction in the size distribution of samples taken from the mixing tank (Fig. 3a) was found between ordinary (58-350 s’) and in-line coagulation (0 s-‘) conditions. However, as shown in Fig. 3b, the particle size of each sample collected from the circulation loop had a similar distribution pattern with an average of about 10 pm. The difference between the particle size distributions of samples from the mixing tank appeared to diminish owing to the breakage and reflocculation of chemical floes by the shear of the pump under the same flow regime in the circulation loop. This should be due to floes in the circulation loop, not in the mixing tank, which had a direct influence on the membrane surface, and might have very similar properties. Therefore, it appears that the physical properties, e.g., size of coagulated floes, or particulates was not a key
Removal (%) 0
38
0
36 34
n
33
G value(5 ‘)
Fig. 2. Removal rate of samples from the mixing tank according to HRTs and velocity gradients. I
I _____ I
0 s-1
129
o
: 1 0 0 0 0 0 10
/ c 1.1
-
j
0
000.0
Particle diameter (pm)
------
PACI 4,lppm(Ordinary
_.-.-
PACI lO.Oo~m10rdinarv
Particle diameter
coag.) coa ‘9. -
(pm)
Fig. 3. Size distribution of a coagulated suspension according to HRTs and velocity gradients: samples were taken from (a) the mixing tank and (b) the circulation loop.
I?-k. Park et al. /Desalination
A PACI 4,lppm(Ordinary . PACI lO.OtxMOrdinatv
Table 2 The removal rate of UVzs, absorbance in the mixing tank as a function of coagulation conditions
coag.) cq&$J
Y
a
70
’
60
241
145 (2002) 237-245
Removal rate, % Crossflow mode
Dead-end mode
In-line coagulation (4.1 mm*)
33.3
34.3
Ordinary coagulation (4.1 mm)
35.7
36.3
Ordinary coagulation (10 mm)
38.3
-
50 40
’ 0
10
20
30
40
Filtration Time (hr)
Fig. 4. Variation in TMP as a function of coagulation conditions in the crossflow mode. factor for the discrepancy in the filterability of coagulated suspensions formed under the different coagulation conditions. In addition, the backtransport under crossflow mode would be expected to take particulate matters away from the membrane surface [5]. Fig. 5 shows the backtransport velocity calculated using the experimental conditions in this study. The inside diameter of the hollow fibers is 1 mm, and, thus, the small diameter result in a laminar flow. Therefore equations developed for laminar flow can be used. Based on the data shown in Fig. 5, particles with a size of over 2 pm could not theoretically approach the membrane surface at constant flux of 100 L/m2h and a crossflow velocity of 1 m/s.
*Concentration of PACl, as Al,O,
In conclusion, the size of particulate matters became equalized in the circulation loop in spite of the different conditions used for coagulation in the mixing tank. Furthermore particles with a size of over 2 pm could not affect membrane permeability. This suggests that the removal of DOMs was responsible for the differences in filterability. As seen in Table 2, the removal rate of UV,,, absorbance in the crossflow mode was slightly dependent on coagulation conditions and was in the range of 33.3-38.3%. Although the difference was not so great, its order matched well with that of filterability. The accumulation of DOMs by continuous filtration might lead to the difference in TMP patterns shown in Fig. 4. 4.3. Ordinary and in-line coagulation end mode
Particle diameter (Wn)
Fig. 5. Backtransport velocity at the crossflow velocity of 1 m/s.
in dead-
In the dead-end mode, like the crossflow mode, membrane filterability was improved by the use of coagulation, and that for ordinary coagulation was superior to that for in-line coagulation (Fig. 6). However, contrary to the crossflow mode, the floes may remain intact, reaching and accumulating on the membrane surface in the dead-end mode, so their characteristics which varied according to coagulation conditions would be expected to exert a somewhat significant effect on the filtration.
242
P-k. Park et al. /Desalination
145 (2002) 237-245
d l
Raw water
o PACI 4.lppm(ln-line Q & ;
l.E+15
ii z
coag.)
0”
E
70 82
60
2 .-
50
l.E+14 G l.Et13
1 Eel2
ti
40
kt
30
l.E+ll 10
0
10
20
30
40
50
30
40
50 60 70
1CiJ
60 TMP (kPa)
FiltrationTime (hr)
Fig. 6. Variation in TMP as a function of coagulation conditions in the dead-end mode.
Fig. 7. Specific cake resistances of raw water, coagulated water, and hydroxide particles (measured at 25°C).
The specific cake resistance (a) could be a representative property of cake layer comprised of chemical floes. Thus, the specific cake resistances of these floes formed under the two conditions at 4.1 ppm were measured and the results are shown in Fig. 7. The resistance for ordinary coagulation conditions was about 10 times smaller than that for in-line coagulation conditions. This difference might be caused by differences in hydraulic conditions [6]. At ordinary coagulation conditions, the GT value was very large, leading to dense and spherical floes that could form permeable cake layers. However, at in-line coagulation conditions, the GT value was very small, leading to the formation of loose and elongated floes (or flocculi) that probably formed less permeable cake layers under pressurization. These specific cake resistances can be converted to cake resistances (Rc) in the resistance-in-series model of filtration using the equation below:
to be 1.8x1012m-’ at 25°C. Therefore for ordinary coagulation Rc was less than 0.3% of value of R,,,, while for in-line coagulation Rc was around 1.2-2% of the value of R”,, which was applicable to about 1 kPa of TMP and this increased with increase in TMP. Therefore, during ultrafiltration, the filterability gap at the two coagulation conditions would become greater. Similarly to the crossflow mode, the removal of DOMs was different with different coagulation conditions. Table 2 shows that the removal rate of UV,,, at ordinary coagulation conditions was greater than that for in-line coagulation conditions, and its order was the same as the filterability. In conclusion the difference in filterability appears to be due to the variation in both the characteristics of floes and the removal rate of DOMs as a function of coagulation conditions in the dead-end mode.
CZCV R, =4,
In this experiment, an amount of PACl corresponding to 4.1 ppm as Al,O, at ordinary and/or in-line coagulation conditions was used to precoat the membrane in the form of aluminum hydroxide particles. As a result, the filtration time when the TMP increase to 70 kPa was about 7 h (Fig. 8), indicating that the filterability was worse than that of ordinary (about 50 h) or in-line coagulation conditions (about 40 h). Furthermore it did not improved significantly compared to that
(1)
where C, V, and A,,, are the concentration of suspended solids, the accumulated volume of the filtrate and surface area of the membrane, respectively. The values for Rc were 1.8x10”-5.4~10” m-l for ordinary coagulation, and 2.1 x 1O’O-3.7x 1O’Om-’ for in-line coagulation. Meanwhile the inherent resistance of the membrane (R,,,) was determined
4.4. Precouting with metal hydroxide particles
E-k. Park et al. /Desalination
243
145 (2002) 237-245
1Backwashing l
PM 1.0 ppn(prf.mating) ~
Fig. 8. Variation in TMP as a function of precoating conditions with PACI. in a filtration test using ultrapure water instead of slurry of aluminum hydroxide particles as a controlled experiment (about 6 h). However as shown in Table 3 the removal of UVzs, absorbance in the permeate was higher than that at ordinary or in-line coagulation conditions. The reason for this is that resistance of aluminum hydroxide particles themselves was worse in spite of the higher removal rate of DOMs. Fig. 7 shows that the specific cake resistance of aluminum hydroxide particles was about 5~10’~ m/kg, which is even greater than that for coagulated floes. In addition, the Rc value calculated in the same manner as mentioned above was about 1.1 x lOi’ m-l, which was around 6% of the R , corresponding to a TMP of 3 kPa or so. The addiconal increase of TMP was actually observed as seen in Fig. 9. After backwashing and flushing, the TMP was immediately increased by around 2-3 kPa with respect to that
Table 3 The removal rate of UV,,, in the permeate (dead-end mode) Removal rate, % Precoating
PACI 1.Oppm* PACl2.0 ppm PAC14.1 ppm FeCl, 13.0 ppm**
27.3 38.8 49.6 30.3
In-line coag. Ordinarv coasz
PACI 4. I ppm PACl4. I oum
44.1 44.1
*as AJO,
** as FeCl,
and flushing
-5
0
5
IO
15
a,
Rbablihebwl1qde(rhl)
Fig. 9. Variation in TMP within 1 cycle of filtration as a function of precoating conditions.
in the last part of the previous cycle using PACl 4.1 ppm. Even though fouling within a cycle after precoating did not progress due to the high removal rate of DOMs, the sudden increase in TMPremained nearly constant and was maintained up to the last time of the cycle. This is the reason why the cake layer formed by accumulating aluminum hydroxide particles continued to function as resistance to filtration. Therefore another test was carried out at 1.Oand 2.0 ppm in order to reduce the amount of aluminum hydroxide particles, and the filterability was then better than that at 4.1 ppm as seen in Fig. 8. The lower the concentration of PACl, the worse the removal of DOMs as shown in Table 3, but the superior filterability was due to a lowered resistance of aluminum hydroxide particles. Fig. 9 shows that the TMP for 1.O ppm increased less than that for 4.1 ppm and diminished to about the value in the last part of the previous cycle. In the sense that filterability could be improved reasonably by a smaller (one forth) amount of coagulant, it appears that the precoating method could decrease the consumption of coagulant. Finally, 13.0 ppm ferric chloride as FeCl,, a concentration corresponding to PACl4.1 ppm as AJO,, was used to form iron hydroxide particles for precoating the membrane surface. Fig. 10 shows that the filterability of ferric chloride 13.0 ppm
t?-k. Park et al. /Desalination
244
70 3 % 60 ;
50
(’ 0
5
FeCl3 13.0ppm(precoating)l 10
15
Filtration Time (hr)
Fig. 10. Comparison of TMP between precoating conditions with PACl and ferric chloride.
was better than that for PACl at 4.1 ppm, the reason for which is that the iron hydroxide particles resulted in a very low value for specific cake resistance as seen in Fig. 7, or formed a very permeable cake layer. The corresponding RCwas about 2~10~ m-l, which was less than 0.01% of the value of R,,,. Although equivalent molar amounts of PACl and ferric chloride was dosed, and even more iron hydroxide was formed than aluminum hydroxide because of solubility differences, the resistance of cake layer made up of iron hydroxide was even less than that of aluminum hydroxide: the reason for this is that the water content of the particles are different. A class of metal cations like A13+has a great preference for binding to water, rather than to other ligands, such as ammonia or cyanide, while transition metal cations like Fe3+ favors water to a lesser degree than does A13+[7,8]. Therefore aluminum hydroxide particles may form gelled and less porous cake layers, while iron hydroxide particles result in more porous and permeable cake layers. The TMP increase during precoating was relatively small for this reason and after precoating decreased to very low level even though the low removal rate of DOMs (Table 3) caused a slight increase in TMP (Fig. 9).
145 (2002) 237-245
ment, and of the two conditions using aPAC1 dosage of 4.1 ppm as Al,O, that at ordinary coagulation conditions was superior to that for in-line coagulation conditions. The removal of DOMs appears to be responsible for the difference in filterability. In the dead-end mode, like the crossflow mode, membrane filterability was improved by the use of coagulation pretreatment, and that at ordinary coagulation was better than that for in-line coagulation. In this case the characteristics of the floes as well as the removal of DOMs as a function of coagulation conditions had an effect on filtration. In the precoating experiments filterability using PACl at 4.1 ppm was worse than that of ordinary or in-line coagulation conditions because of the resistance of aluminum hydroxide particles themselves. However, the reduction of the dosage to 2.0 and 1.O ppm caused a reasonable improvement in filterability. Thus, it appears that the precoating method could lead to decrease in the consumption of coagulant. Meanwhile filterability for 13.0 ppm ferric chloride as FeCl, was better than that for 4.1 ppm PACl, because the iron hydroxide particles formed a very permeable cake layer. Thus, in the application of the precoating method as a membrane pretreatment, iron hydroxide particles were found to be more suitable than aluminum hydroxide particles.
Acknowledgements The authors would like to thank the Korean Ministry of Environmnet for the financial support under grant 11104-002. Aquasource S.A., Toulouse, France, is acknowledged for providing hollow fiber membranes.
References [l]
5. Conclusions In the crossflow mode the filterability of a membrane is enhanced by coagulation pretreat-
[2]
S. Freeman, From zero to sixty in only seven years: the rapid increase in MF/UF membrane surface water treatment, Proc. 2001 AWWAMembraneTechnology Conference, March 4-7, 200 1. M.R. Wiesner, M.M. Clark and J. Mallevialle,
P-k. Park et al. /Desalination
[3]
[4]
[5]
Membrane filtration of coagulation suspensions, J. Environ. Eng., 15 (1989) 20. J.G Jacangelo, J. DeMacro, D.M. Owen and S.J. Randtke, Selected processes for removing NOM: an overview, J. AWWA, 87 (1995) 64. G. Galjaard, J. van Paassen, P. Bujis and F. Schoonenberg, Enhanced pre-coat engineering (EPCE) for micro-and ultrafiltration: the solution for fouling? Water Sci. & Tech.: Water Supply, 1 (2001) 151. J.D. Lee, S.H. Lee, M.H. Jo, PK. Park, C.H. Lee and J.W. Kwak, Effect of coagulation conditions on
145 (2002) 237-245
[6] [7]
[8]
245
membrane filtration characteristics in coagulation-MF process for water treatment, Environ. Sci. & Tech., 34 (2000) 3780. AWWAResearch Foundation, Mixing in Coagulation and Flocculation, AWWA, Denver, 199 1. P.L.Thompson and W.L. Paulson, Dewaterability of alum and ferric coagulation sludges, J. AWWA, 90 (1998) 164. W. Stumm and J.J. Morgan, Aquatic chemistry: an introduction emphasizing chemical equilibria in natural waters, John Wiley & Sons, Inc., New York, 1981.
ELSEVIER
145 (2002) 237-245 www.elsevier.com/locate/desal
Effect of the removal of DOMs on the performance of a coagulation-UF membrane system for drinking water production Pyung-kyu Park”, Chung-hak Lee”*, Sang-June Choib, Kwang-Ho Choo”, Seung-Hyun Kimd, Cho-Hee Yoone “School of Chemical Engineering, Seoul National University, Seoul 151-744, Korea Tel. +82 (2) 880-7075; Fax +82 (2) 888-1604; email: [email protected] “Department of Environmental Engineering, Kyungpook National University, Daegu 702-701, Korea =Department of Architectural, Civil &Environmental Engineering, Daegu University, Gyeongsan 712-714, Korea “Civil and Environmental Engineering Department, eDepartment of Chemical Engineering, Kyungnam University, Masan 631-701, Korea Received 4 February 2002; accepted 27 March 2002
Abstract Coagulation with only rapid mixing in a separate tank (ordinary coagulation) and coagulation with no mixing tank (in-line coagulation) were applied prior to ultrafiltration with an inside-out type hollow fiber membrane. In result the filterability at the former conditions was superior in both the crossflow and dead-end modes. Thus the relative importance of the removal rate of DOMs was investigated. Precoating the surface of the membranes with metal hydroxide particles of coagulants was also examined. This method of utilizing coagulants resulted in a smaller consumption of coagulant in the coagulation-UF system. Keywords: Ultrafiltration;
Coagulation;
In-line coagulation;
1. Introduction
Dissolved organic matters
ficantly changed the surface water treatment
The development of microfiltration (MF) and ultrafiltration (UF) membrane processes has signi*Corresponding
Precoating;
author.
Presented at the International July 7-12, 2002.
Congress on Membranes
in a relatively short time [l]. However, they are not in widespread use because of some problems associated with their use, which include a low removal rate of and membrane fouling caused by and Membrane
Processes
00 I l-9 164/02/$- See front matter 0 2002 Elsevier Science B.V. All rights reserved PII: SO01 l-9164(02)00418-6
industry
(ICOM),
Toulouse, France,
238
P-k. Park et al. /Desalination
DOMs (Dissolved Organic Matters) in the raw water. As a result, the addition of a coagulant as a pretreatment prior to membrane filtration has been proposed for the purpose of not only improving the removal of DOMs but reducing membrane fouling [2,3]. In this study, prior to UP with an inside-out type hollow fiber membrane, two modes of coagulation were examined, (i) coagulation of raw water in a separate tank with rapid mixing only and (ii) in-line coagulation, and then the effects of the two procedures were compared with respect to the removal rate of DOMs and the membrane performance. Precoating the surface of the membranes with metal hydroxide particles of coagulants was also examined to enhance the coagulation-UF system. 2. Materials and methods 2.1. Raw watel; membrane, and coagulants The raw water samples used in this study were collected from an intake located at the Han River, the water quality of which is presented in Table 1. The membrane was a hollow-fiber ultrafiltration membrane made from polysulfone, the nominal pore size and inner diameter of which was 0.01 pm and 1 mm, respectively. The effective surface of a module with 8 fibers was 0.078 m2. PACl (polyaluminum chloride) was used as a coagulant at a concentration of 1 .O-10 ppm as Al,O,, and ferric chloride was also used as a comparison with PACI. The range of velocity gradient (G-value) in the mixing tank was
Table 1 Characteristics of the raw water used in this study Parameters
Range
Turbidity, NTU DOC, ppm UV254absorbance, cm-’ Concentration of SS, ppm Alkalinity, ppm as CaC03 PH
2-5 2.3-2.9 0.029-0.036 6.8-14.8 41-70 7.2-7.8
145 (2002) 237-245
Fig. 1. Schematics of bench-scale coagulation-UF system.
58-461 s’, and the hydraulic residence time (HRT) was varied over the range of 12-60 min. 2.2. Bench-scale
coagulation-UF
system
A schematic diagram of the bench-scale coagulation-UF system used in this study is shown in Fig. 1. All pumps were peristaltic. In the crossflow mode, a circulation pump was equipped so as to circulate water within the circulation loop, including the lumen-side of hollow fibers, while in the deadend mode it was removed. Because the loop was closed, the flux remained constant without any control even in the crossflow mode. 3. Operating conditions 3.1. Filtration conditions The water flux was fixed to 100 L/m*h (25°C). The trans-membrane pressure (TMP) would be expected to increase in the course of time due to membrane fouling, and so the TMP profile was monitored up to 70 kPa. In the crossflow mode, the crossflow velocity was 1 m/s. Backwashing was performed at 160 kPa with the permeate. Initially this was carried out for 1 min at lhr intervals in both the crossflow and dead-end modes. However, in the dead-end mode, floes produced as the result of coagulation blocked the lumens of some fibers. Therefore backwashing was executed for 20 s at 20 min intervals in the case of the dead-end mode. After backwashing, the lumens were flushed with raw or coagulated water. The recovery of raw water was about 92%.
P-k. Park et al. /Desalination
3.2. Coagulation
239
3.2.3. Precoating with metal hydroxide particles
conditions
3.2.1. Selection of coagulant hydraulic condition of coagulation
145 (2002) 237-245
dosage
and
Batch experiments were first carried out in order to select the optimum dosage of coagulant. The PACl dosage was changed under sufficient mixing conditions (HRT of 60 min and G-value of 3.50 s-l) and the DOC and UVZS4absorbance of coagulated water were then measured to select the most effective PACl dosage, which was determined to be 4.1 ppm as AJO,, Continuous experiments without membrane filtration according to HRT and velocity gradient were carried out at the selected dosage of PACl 4.1 ppm. The HRT and velocity gradient were varied by changing the rotating speed of the agitator and the flow rate through the mixing tank, respectively. UV,,, absorbance and the particle size distribution in the coagulated water were then measured.
3.2.2. Ordinary and in-line coagulation The bench-scale coagulation-UF system was operated at the two selected coagulation conditions determined in the previous experiments. One condition (HRT of 60 min and G-value of 350 s-‘) was applied such that PACl was dosed into the raw water line toward the mixing tank (line 1 in Fig. 1) and coagulation was executed in the mixing tank by only rapid mixing with an HRT of 60 min, a G-value of 350 s-l. The other was carried out similarly but with an HRT of 12 min with no agitation. Hereafter, the former and latter conditions will be designated as Ordinary and Znline coagulation, respectively. At these conditions the filtration performance was observed in both the crossflow and dead-end modes. In the case of the crossflow mode an additional experiment at ordinary coagulation conditions with a PACl dose of 10 ppm was also executed to increase the removal of DOMs and compared with the filterability at a PACl dose of 4.1 ppm.
Unlike these conditions, coagulants were also applied to form a cake or coating layer made up of metal hydroxide particles. The coating layer which was formed on the membrane surface would be predicted to be permeable and easily removable, and to remove DOMs by adsorption before being detached by backwashing and flushing [4]. Therefore, aluminum hydroxide particles were prepared by diluting a PACl stock solution with ultrapure water and adjusting the pH to 7.0(+0.2), and this slurry were dosed to the feeding line toward membrane module (line 2 in Fig. 1). After the coated layer had been formed in this way, only raw water was filtered through the precoated membrane. Because the layer was removed by backwashing and flushing at the last step of a cycle, the slurry was dosed for the first 1 min of every 20-min cycle. The concentration of PACl was set to a somewhat large value so that the total amount dosed within a cycle could be the same for both ordinary and in-line coagulation as well as at precoating conditions. Therefore a concentra-tion of 4.1 ppm as Al,O, at precoating conditions does not indicate the actual concentration but the concentration corresponding to 4.1 ppm as Al,O, at ordinary and in-line coagulation conditions. With a similar filtration test at 1.0 ppm and 2.0 ppm as Al,O, the removal of DOMs and the filterability as a function of the amount of hydroxide particles were investigated. Lastly, in order to compare aluminum hydroxide particles with iron hydroxide particles, the same filtration experiment was carried out with ferric chloride at 13.0 ppm as FeCl,, which value was selected so as to equalize the moles of AP+ and Fe3+. 4. Results and discussion 4.1. Coagulation according to HRT and velocity gradient In the continuous coagulation of raw water without membrane filtration according to HRT and velocity gradient, the removal of UV,,, ab-
240
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145 (2002) 237-245
sorbance did not change significantly (33-38%), and even with no agitating (0 rpm) this was 33-34% (Fig. 2). Particle size distribution patterns were also similar regardless of the HRT and the velocity gradients except 0 s-’ (Fig. 3a). Therefore filtration with the UF membrane was not carried out at all the conditions but only two: one being ordinary coagulation conditions and the other, inline coagulation conditions. 4.2. Ordinary and in-line coagulation in crossjlow mode Under the crossflow mode, membrane filterability was obviously enhanced by the coagulation pretreatment and, of the two conditions with a PACl dosage of 4.1 ppm that for ordinary coagulation was superior to that for in-line coagulation (Fig. 4). And ordinary coagulation conditions at 10 ppm gave the best filterability. The reduction in the filterability is due to membrane fouling, and the major foulants in raw or coagulated water are particulate matters such as coagulated floes and dissolved or colloidal matters such as DOMs. The characteristics and variation of particulate matters including floes can be evaluated indirectly by measuring the particle size distribution. A clear distinction in the size distribution of samples taken from the mixing tank (Fig. 3a) was found between ordinary (58-350 s’) and in-line coagulation (0 s-‘) conditions. However, as shown in Fig. 3b, the particle size of each sample collected from the circulation loop had a similar distribution pattern with an average of about 10 pm. The difference between the particle size distributions of samples from the mixing tank appeared to diminish owing to the breakage and reflocculation of chemical floes by the shear of the pump under the same flow regime in the circulation loop. This should be due to floes in the circulation loop, not in the mixing tank, which had a direct influence on the membrane surface, and might have very similar properties. Therefore, it appears that the physical properties, e.g., size of coagulated floes, or particulates was not a key
Removal (%) 0
38
0
36 34
n
33
G value(5 ‘)
Fig. 2. Removal rate of samples from the mixing tank according to HRTs and velocity gradients. I
I _____ I
0 s-1
129
o
: 1 0 0 0 0 0 10
/ c 1.1
-
j
0
000.0
Particle diameter (pm)
------
PACI 4,lppm(Ordinary
_.-.-
PACI lO.Oo~m10rdinarv
Particle diameter
coag.) coa ‘9. -
(pm)
Fig. 3. Size distribution of a coagulated suspension according to HRTs and velocity gradients: samples were taken from (a) the mixing tank and (b) the circulation loop.
I?-k. Park et al. /Desalination
A PACI 4,lppm(Ordinary . PACI lO.OtxMOrdinatv
Table 2 The removal rate of UVzs, absorbance in the mixing tank as a function of coagulation conditions
coag.) cq&$J
Y
a
70
’
60
241
145 (2002) 237-245
Removal rate, % Crossflow mode
Dead-end mode
In-line coagulation (4.1 mm*)
33.3
34.3
Ordinary coagulation (4.1 mm)
35.7
36.3
Ordinary coagulation (10 mm)
38.3
-
50 40
’ 0
10
20
30
40
Filtration Time (hr)
Fig. 4. Variation in TMP as a function of coagulation conditions in the crossflow mode. factor for the discrepancy in the filterability of coagulated suspensions formed under the different coagulation conditions. In addition, the backtransport under crossflow mode would be expected to take particulate matters away from the membrane surface [5]. Fig. 5 shows the backtransport velocity calculated using the experimental conditions in this study. The inside diameter of the hollow fibers is 1 mm, and, thus, the small diameter result in a laminar flow. Therefore equations developed for laminar flow can be used. Based on the data shown in Fig. 5, particles with a size of over 2 pm could not theoretically approach the membrane surface at constant flux of 100 L/m2h and a crossflow velocity of 1 m/s.
*Concentration of PACl, as Al,O,
In conclusion, the size of particulate matters became equalized in the circulation loop in spite of the different conditions used for coagulation in the mixing tank. Furthermore particles with a size of over 2 pm could not affect membrane permeability. This suggests that the removal of DOMs was responsible for the differences in filterability. As seen in Table 2, the removal rate of UV,,, absorbance in the crossflow mode was slightly dependent on coagulation conditions and was in the range of 33.3-38.3%. Although the difference was not so great, its order matched well with that of filterability. The accumulation of DOMs by continuous filtration might lead to the difference in TMP patterns shown in Fig. 4. 4.3. Ordinary and in-line coagulation end mode
Particle diameter (Wn)
Fig. 5. Backtransport velocity at the crossflow velocity of 1 m/s.
in dead-
In the dead-end mode, like the crossflow mode, membrane filterability was improved by the use of coagulation, and that for ordinary coagulation was superior to that for in-line coagulation (Fig. 6). However, contrary to the crossflow mode, the floes may remain intact, reaching and accumulating on the membrane surface in the dead-end mode, so their characteristics which varied according to coagulation conditions would be expected to exert a somewhat significant effect on the filtration.
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145 (2002) 237-245
d l
Raw water
o PACI 4.lppm(ln-line Q & ;
l.E+15
ii z
coag.)
0”
E
70 82
60
2 .-
50
l.E+14 G l.Et13
1 Eel2
ti
40
kt
30
l.E+ll 10
0
10
20
30
40
50
30
40
50 60 70
1CiJ
60 TMP (kPa)
FiltrationTime (hr)
Fig. 6. Variation in TMP as a function of coagulation conditions in the dead-end mode.
Fig. 7. Specific cake resistances of raw water, coagulated water, and hydroxide particles (measured at 25°C).
The specific cake resistance (a) could be a representative property of cake layer comprised of chemical floes. Thus, the specific cake resistances of these floes formed under the two conditions at 4.1 ppm were measured and the results are shown in Fig. 7. The resistance for ordinary coagulation conditions was about 10 times smaller than that for in-line coagulation conditions. This difference might be caused by differences in hydraulic conditions [6]. At ordinary coagulation conditions, the GT value was very large, leading to dense and spherical floes that could form permeable cake layers. However, at in-line coagulation conditions, the GT value was very small, leading to the formation of loose and elongated floes (or flocculi) that probably formed less permeable cake layers under pressurization. These specific cake resistances can be converted to cake resistances (Rc) in the resistance-in-series model of filtration using the equation below:
to be 1.8x1012m-’ at 25°C. Therefore for ordinary coagulation Rc was less than 0.3% of value of R,,,, while for in-line coagulation Rc was around 1.2-2% of the value of R”,, which was applicable to about 1 kPa of TMP and this increased with increase in TMP. Therefore, during ultrafiltration, the filterability gap at the two coagulation conditions would become greater. Similarly to the crossflow mode, the removal of DOMs was different with different coagulation conditions. Table 2 shows that the removal rate of UV,,, at ordinary coagulation conditions was greater than that for in-line coagulation conditions, and its order was the same as the filterability. In conclusion the difference in filterability appears to be due to the variation in both the characteristics of floes and the removal rate of DOMs as a function of coagulation conditions in the dead-end mode.
CZCV R, =4,
In this experiment, an amount of PACl corresponding to 4.1 ppm as Al,O, at ordinary and/or in-line coagulation conditions was used to precoat the membrane in the form of aluminum hydroxide particles. As a result, the filtration time when the TMP increase to 70 kPa was about 7 h (Fig. 8), indicating that the filterability was worse than that of ordinary (about 50 h) or in-line coagulation conditions (about 40 h). Furthermore it did not improved significantly compared to that
(1)
where C, V, and A,,, are the concentration of suspended solids, the accumulated volume of the filtrate and surface area of the membrane, respectively. The values for Rc were 1.8x10”-5.4~10” m-l for ordinary coagulation, and 2.1 x 1O’O-3.7x 1O’Om-’ for in-line coagulation. Meanwhile the inherent resistance of the membrane (R,,,) was determined
4.4. Precouting with metal hydroxide particles
E-k. Park et al. /Desalination
243
145 (2002) 237-245
1Backwashing l
PM 1.0 ppn(prf.mating) ~
Fig. 8. Variation in TMP as a function of precoating conditions with PACI. in a filtration test using ultrapure water instead of slurry of aluminum hydroxide particles as a controlled experiment (about 6 h). However as shown in Table 3 the removal of UVzs, absorbance in the permeate was higher than that at ordinary or in-line coagulation conditions. The reason for this is that resistance of aluminum hydroxide particles themselves was worse in spite of the higher removal rate of DOMs. Fig. 7 shows that the specific cake resistance of aluminum hydroxide particles was about 5~10’~ m/kg, which is even greater than that for coagulated floes. In addition, the Rc value calculated in the same manner as mentioned above was about 1.1 x lOi’ m-l, which was around 6% of the R , corresponding to a TMP of 3 kPa or so. The addiconal increase of TMP was actually observed as seen in Fig. 9. After backwashing and flushing, the TMP was immediately increased by around 2-3 kPa with respect to that
Table 3 The removal rate of UV,,, in the permeate (dead-end mode) Removal rate, % Precoating
PACI 1.Oppm* PACl2.0 ppm PAC14.1 ppm FeCl, 13.0 ppm**
27.3 38.8 49.6 30.3
In-line coag. Ordinarv coasz
PACI 4. I ppm PACl4. I oum
44.1 44.1
*as AJO,
** as FeCl,
and flushing
-5
0
5
IO
15
a,
Rbablihebwl1qde(rhl)
Fig. 9. Variation in TMP within 1 cycle of filtration as a function of precoating conditions.
in the last part of the previous cycle using PACl 4.1 ppm. Even though fouling within a cycle after precoating did not progress due to the high removal rate of DOMs, the sudden increase in TMPremained nearly constant and was maintained up to the last time of the cycle. This is the reason why the cake layer formed by accumulating aluminum hydroxide particles continued to function as resistance to filtration. Therefore another test was carried out at 1.Oand 2.0 ppm in order to reduce the amount of aluminum hydroxide particles, and the filterability was then better than that at 4.1 ppm as seen in Fig. 8. The lower the concentration of PACl, the worse the removal of DOMs as shown in Table 3, but the superior filterability was due to a lowered resistance of aluminum hydroxide particles. Fig. 9 shows that the TMP for 1.O ppm increased less than that for 4.1 ppm and diminished to about the value in the last part of the previous cycle. In the sense that filterability could be improved reasonably by a smaller (one forth) amount of coagulant, it appears that the precoating method could decrease the consumption of coagulant. Finally, 13.0 ppm ferric chloride as FeCl,, a concentration corresponding to PACl4.1 ppm as AJO,, was used to form iron hydroxide particles for precoating the membrane surface. Fig. 10 shows that the filterability of ferric chloride 13.0 ppm
t?-k. Park et al. /Desalination
244
70 3 % 60 ;
50
(’ 0
5
FeCl3 13.0ppm(precoating)l 10
15
Filtration Time (hr)
Fig. 10. Comparison of TMP between precoating conditions with PACl and ferric chloride.
was better than that for PACl at 4.1 ppm, the reason for which is that the iron hydroxide particles resulted in a very low value for specific cake resistance as seen in Fig. 7, or formed a very permeable cake layer. The corresponding RCwas about 2~10~ m-l, which was less than 0.01% of the value of R,,,. Although equivalent molar amounts of PACl and ferric chloride was dosed, and even more iron hydroxide was formed than aluminum hydroxide because of solubility differences, the resistance of cake layer made up of iron hydroxide was even less than that of aluminum hydroxide: the reason for this is that the water content of the particles are different. A class of metal cations like A13+has a great preference for binding to water, rather than to other ligands, such as ammonia or cyanide, while transition metal cations like Fe3+ favors water to a lesser degree than does A13+[7,8]. Therefore aluminum hydroxide particles may form gelled and less porous cake layers, while iron hydroxide particles result in more porous and permeable cake layers. The TMP increase during precoating was relatively small for this reason and after precoating decreased to very low level even though the low removal rate of DOMs (Table 3) caused a slight increase in TMP (Fig. 9).
145 (2002) 237-245
ment, and of the two conditions using aPAC1 dosage of 4.1 ppm as Al,O, that at ordinary coagulation conditions was superior to that for in-line coagulation conditions. The removal of DOMs appears to be responsible for the difference in filterability. In the dead-end mode, like the crossflow mode, membrane filterability was improved by the use of coagulation pretreatment, and that at ordinary coagulation was better than that for in-line coagulation. In this case the characteristics of the floes as well as the removal of DOMs as a function of coagulation conditions had an effect on filtration. In the precoating experiments filterability using PACl at 4.1 ppm was worse than that of ordinary or in-line coagulation conditions because of the resistance of aluminum hydroxide particles themselves. However, the reduction of the dosage to 2.0 and 1.O ppm caused a reasonable improvement in filterability. Thus, it appears that the precoating method could lead to decrease in the consumption of coagulant. Meanwhile filterability for 13.0 ppm ferric chloride as FeCl, was better than that for 4.1 ppm PACl, because the iron hydroxide particles formed a very permeable cake layer. Thus, in the application of the precoating method as a membrane pretreatment, iron hydroxide particles were found to be more suitable than aluminum hydroxide particles.
Acknowledgements The authors would like to thank the Korean Ministry of Environmnet for the financial support under grant 11104-002. Aquasource S.A., Toulouse, France, is acknowledged for providing hollow fiber membranes.
References [l]
5. Conclusions In the crossflow mode the filterability of a membrane is enhanced by coagulation pretreat-
[2]
S. Freeman, From zero to sixty in only seven years: the rapid increase in MF/UF membrane surface water treatment, Proc. 2001 AWWAMembraneTechnology Conference, March 4-7, 200 1. M.R. Wiesner, M.M. Clark and J. Mallevialle,
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Membrane filtration of coagulation suspensions, J. Environ. Eng., 15 (1989) 20. J.G Jacangelo, J. DeMacro, D.M. Owen and S.J. Randtke, Selected processes for removing NOM: an overview, J. AWWA, 87 (1995) 64. G. Galjaard, J. van Paassen, P. Bujis and F. Schoonenberg, Enhanced pre-coat engineering (EPCE) for micro-and ultrafiltration: the solution for fouling? Water Sci. & Tech.: Water Supply, 1 (2001) 151. J.D. Lee, S.H. Lee, M.H. Jo, PK. Park, C.H. Lee and J.W. Kwak, Effect of coagulation conditions on
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membrane filtration characteristics in coagulation-MF process for water treatment, Environ. Sci. & Tech., 34 (2000) 3780. AWWAResearch Foundation, Mixing in Coagulation and Flocculation, AWWA, Denver, 199 1. P.L.Thompson and W.L. Paulson, Dewaterability of alum and ferric coagulation sludges, J. AWWA, 90 (1998) 164. W. Stumm and J.J. Morgan, Aquatic chemistry: an introduction emphasizing chemical equilibria in natural waters, John Wiley & Sons, Inc., New York, 1981.