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Role of coagulation in membrane filtration of wastewater for reuse
DESALINATION Desalination 173 (2005) 301-307
ELSEVIER
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Role of coagulation in membrane filtration of wastewater for reuse Seung-Hyun Kim a*, Seong-Yong Moon a, Cho-Hee Yoon b, Seong-Keun Yim ¢, Jae-Weon Cho d ~Civiland Environmental Engineering Department, bChemicalEngineering Department, Kyungnam University, Republic of Korea Fax: +82 (55) 249-2994; email: [email protected] CKolonEngineering and Construction Co., Ltd., Republic of Korea dEnvironmental Engineering Department, Kwangju Institute of Science and Technology, Republic of Korea Received 22 July 2004; accepted 19 August 2004
Abstract
This study explains the role of coagulation in membrane filtration of wastewater for reuse. For this purpose, a coagulation-ultrafiltmtion (UF) membrane system was used to treat secondary effluent from a nearby wastewater treatment plant using a rotating biological contactor. The study proceeded with the hypothesis that coagulation could affect membrane filtration through two phenomena: change in particle characteristics and contaminant loading reduction. If fouling reduction were observed at a low alum dosage, coagulation would affect membrane performance by changing particle characteristics because contaminant reduction could not be possible at low dosage. If fouling reduction were observed only at a high alum dosage, the role of coagulation would be contaminant loading reduction. Results showed that both phenomena were important. Coagulation improved the membrane performance by changing particle characteristics at a low alum dosage. The improvement was achieved through both a change in particle charaeteristics and contaminant loading reduction at a high alum dosage. Particle size among various characteristics was found the most important for membrane fouling. Coagulation increased particle size, which led to a reduction of fouling. The beneficial effect from coagulation was observed at both fouling steps of pore blocking/adsorption and cake formation. Coagulation pretreatment was also beneficial for the improvement of the permeate quality in terms of organic matter.
Keywords: Coagulation pretreatment; Wastewater reuse; Ultrafiltration; Particle size
*Corresponding author.
0011-9164/05/$- See front matter © 2005 Elsevier B.V. All fights reserved doi: 10.1016/j.desal.2004.08.036
302
S.-H. Kim et al. /Desalination 173 (2005) 301-307
I. Introduction
Membrane filtration is usually employed to treat clean raw water without pretreatment. However, it is essential to include pretreatment when raw water quality is poor. Membrane filtration is, therefore, combined with pretreatment when it is employed for the reuse of wastewater. There are many options for pretreatment such as coagulation, adsorption and ozonation. Coagulation is the most attractive among these options due to several advantages. The most significant advantage is experience. Coagulation has been used in water and wastewater treatment for many years. Operators feel comfortable with this technology. Another advantage is capital investment. Since most water and wastewater treatment include coagulation, additional capital investment is often not necessary. Coagulation appears to yield some beneficial effects when it is used with membrane filtration for wastewater reuse. Patel et al. [1] compared membrane filtration performance with and without coagulation pretreatment for the treatment of secondary effluent from a wastewater treatment plant. They found that coagulation significantly improved permeation rates. The impact from coagulation was less significant on permeate quality. Membrane filtration showed excellent performance of particle removal regardless of coagulation pretreatment. No coliform was found in permeate, and residual turbidity was tess than 0.1 NTU with and without coagulation. Coagulation pretreatment was, however, effective in TOC reduction and reduced TOC by 50%. Other researchers noted similar results. Kohl et al. [2] used coagulation-microfiltration (MF) as a pretreatment to reverse osmosis (RO) to treat chlorinated tertiary effluent from a wastewater treatment plant. An average reduction of TOC was 22%. Wiesner et al. [3] employed ceramic MF (0.05/xm zirconia) as a pretreatment of RO for the treatment of surface water with high turbidity and TOC concentrations. They found that
coagulation improved permeation rates. The improvement depended on the coagulation pH. The best performance was obtained at a pH near neutral value. The beneficial effect from coagulation was also noted in permeate quality. An average permeate turbidity was 1.29 NTU without coagulation, but it was less than 0.5 NTU with coagulation. Alum reduced the permeate's TOC concentration by approximately half. These results showed that coagulation improves permeation rates and permeate quality. It is, however, unclear how coagulation made such an improvement. Previous studies investigated specific applications, but did not attempt to identify the role of coagulation pretreatment in membrane filtration. This study was designed to discover the cause for the role of coagulation in membrane filtration of wastewater for reuse.
2. Approach
The effects of coagulation can be summarized as follows: • change in particle characteristics • reduced contaminant loading Coagulation changes particle characteristics such as size, charge, and shape [4]. Such change can be related to the improvement of permeation rates and/or in permeate quality. Coagulation, especially followed by sedimentation, evidently removes considerable amounts of contaminants that cause fouling. The reduction of such foulingcausing contaminants can lead to the improvement in permeation rates and/or in permeate quality. The purpose of this study is to identify which is the main cause of coagulation pretreatment in membrane filtration. An experiment was designed with different coagulant dosages (1, 10, and 100 mg'L -1) to achieve this purpose. If fouling reduction were observed at 1 mg'L -1, the role of coagulation would be a change in particle charac-
303
S.-H. Kim et aL / Desalination 173 (2005) 301-307
teristics because contaminant reduction could not be possible at such low dosage. If fouling reduction were observed only at a high alum dosage (10 or 100 mg.L-l), the role of coagulation would be contaminant loading reduction.
3. Materials and methods
Secondary effluent from a nearby wastewater treatment plant employing a rotating biological contactor was used as the raw water in this study. Table 1 shows the characteristics of this source water. This is a very lowly buffered wastewater. The average alkalinity value was 30 mg'L -1. Both turbidity and organic levels were low. Relatively high UV-254 value compared to TOC concentration suggests that organic matter of this source water is mostly of a hydrophobic nature. Coagulation was simulated in jar tests. Rapid mixing was provided for 1 min at a G value of 230 s-I after coagulant addition and slow mixing followed for 5 min at a G value of 50 s -1. Flocculated water was then allowed to settle for 30 min before being introduced into the UF membrane system. The membrane is made of polysulfone with a pore size of 0.01 ~m. The suction-type membrane system was operated at a constant flux of 50 L'(m2"h) -I at 25°C, while pressure was
allowed to increase. Operation was terminated when the suction pressure reached 200 mmHg. The permeation rate and permeate quality were used to evaluate the performance of membrane filtration. Pressure rate, which indicates the pressure change over the filtration time until the target pressure of 200 mmHg was reached, was used to evaluate fouling progress. Specific cake resistance was measured in order to check the cake resistance. Turbidity and organic matter were used to evaluate permeate quality. Turbidity was measured in order to evaluate the particle removal performance of the membrane system. TOC and UV-254 concentrations were measured in order to evaluate the organic removal performance of the membrane system. Particle size and zeta potential were measured using Malvern's Zetasizer in order to examine change in particle characteristics after coagulation.
4. Results and discussion 4.1. Fouling reduction at a low alum dosage
Fig. 1 shows the variation of permeation rates with filtration time at different alum dosages. Table 2 compares pressure rates at different alum dosages. Fig. 1 and Table 2 clearly show that coagulation pretreatment reduced fouling. Pressure rapidly increased without coagulation
Table 1 Raw water characteristics used in this study Item
Value
pH Turbidity, NTU SS, mg.L-l TOC, mg.L-~ UV-254, em-~ COD, mg.L-I BOD, mg.L-~ Alkalinity as CaCO3,mg.L-' Hardness as CaCO3,mg.L-~
7.04-7.83 2.0-4.5 4.0-4.5 1.51-8.61 0.072-0.084 7 4 30 101
Table 2 Comparison results of pressure rates (mg.L-') with and without coagulation pretreatment Alum dosage
Pressure rate (AP), mmHg.h-I
0 1 10 100
36 21 13 4.8
(42%) (64%) (83%)
Values in parenthesesindicatepercent improvementover 0 mg/L.
304
S.-H. rtim et al. / Desalination 173 (2005) 301-307
254 concentrations. This result suggests that the role of coagulation in membrane filtration is a change in particle characteristics, which then leads to fouling reduction. Particle size and zeta potential were then measured in order to pinpoint the particle characteristic responsible for the improvement. Table 3 shows these values at different alum dosages. Even a slight addition of alum was able to change particle characteristics. Alum coagulation increased particle size, but decreased negative zeta potential. Both particle size and surface charge influence permeation rate. An increase in particle size should result in an increase in permeate flux because of a decrease in the specific resistance of
and pressure rate was calculated 36 rnmHg'h -1. Even a slight addition of alum could reduce fouling and improve the permeation rate. Alum coagulation at 1 mg'L- ~reduced the pressure rate to 21 mmHg.h- L In terms o f pressure rate, coagulation pretreatment improved membrane performance by 42% compared to no pretreatment. The settled water qualities o f TOC and UV-254 concentrations were then examined in order to check the hypothesis that coagulation at such low dosage caused any reduction in contaminant loading. Table 3 summarizes the settled water qualities. These results confirmed the hypothesis. Alum coagulation at 1 mg'L-~ could not bring in any improvement of TOC and
] I
Z 7~ -50
~9.- I 0 0
rt
X
0 mg/L
~
1 mg/L
,b, lOmg/L
X
lOOmg/L
rt
ii rt~
X
X
5
i -!5o 4 ~
tr rt ~rt rti,4 '
0
trrt ® ,,
i t L
,L"
2
4
6 FiJtration time, h
8
10
12
Fig. 1. Pressure change with time as a function of alum dosage.
Table 3 Settled water qualities with and without coagulation pretreatment Alum dosage
0 mg'L -j
1 mg'L -I
TOC, mg.L-t UV-254, cm ~ Average size, #m Zeta potential, mV
3.02 0.085 50 -27.2
3.32 0.085 280 - 13.5
(- 10%) (0%)
Values in parentheses indicate the percent improvement over 0 mg/L.
10 mg.L i
100 mg'L -1
2.89 0.077 415 - 12.2
1.33 0.055 734 1.7
(4%) (9%)
(56%) (35%)
305
S.-H. Kim et al. / Desalination 173 (2005) 301-307
extended with increasing alum dosage. Particle size was increased further and zeta potential was decreased further as alum dosage was increased. This change in particle characteristics could also help improve permeation rate. This result suggests that both a change in particle characteristics and reduced contaminant loading would be responsible for the improvement of the permeation rate when coagulation pretreatment is practiced at a high alum dosage. When coagulation is practiced at a high alum dosage so that a contaminant reduction was possible, coagulation improved the membrane performance by changing particle characteristics and reducing contaminant loading.
the cake [5]. Cakes composed of negatively charged particles may be more permeable than those composed of uncharged particles [5]. The increased particle size is, therefore, beneficial for permeation rate, while the decreased zeta potential is not. The improvement of permeation rate by coagulation pretreatment suggests that particle size would be more important for membrane fouling than zeta potential. 4.2. Fouling reduction at a high alum dosage
Table 2 shows that pressure rate decreased with increasing alum dosage. Alum coagulation at 10 and 100 mg.L- ~ improved the permeation rate by 64% and by 83%, respectively. The effect of coagulation at high dosage was also noted in water quality improvement. According to Table 3, organic removal improved with increasing alum dosage. Removal efficiencies of TOC and UV-254 increased to 56% and 35% at an alum coagulation of 100 mg'L -~. This result means that a reduction of fouling-causing contaminants is possible at high alum dosages. A comparison of Tables 2 and 3 shows that the effect of coagulation pretreatment was more evident in permeation rate than organic reduction. Alum coagulation at 10 mg.L -~ improved the organic removal efficiency by less than 10%, but the permeation rate by 64%. This suggests that reduced contaminant loading might not be the only reason for the improvement of the permeation rate. Particle characteristics were therefore examined with increasing alum dosage. The result shown in Table 3 indicates that the change
4.3. Pore blocking/adsorption and cake formation
Fig. 1 shows that membrane fouling progressed in two steps: rapid decline at first and slow decline later. Pore blocking and adsorption onto the membrane matrix are believed to cause the initial rapid decline, while cake formation causes the slow decline. Organics can penetrate and adsorb onto the membrane matrix, which causes fouling by blocking membrane pores. Contaminants sit on the membrane surface and form a cake layer, which causes fouling. Pore blocking and adsorption are important for membrane fouling at the initial stage, while cake formation is important at a later stage. The pressure rate was therefore divided into two stages: the initial stage (APp) and the final stage (APe). According to Table 4, coagulation was important at both stages. Coagulation effect-
Table 4 Comparisonof pressure rates due to pore blockingand cake formationat different alum dosages (mg.L-1) Alum dosage
0
1
10
100
Pressure rate due to pore blocking (APp),rnmHg.h-~ Pressure rate due to cake formation(APe),mmHg.h-~ Specific cake resistance, 10~3kg.m-1
77 23 --
31 16 25.4
33 4.3 6.25
6.7 1.8 1.84
306
S.-H. Kim et al. / Desalination 173 (2005) 301-307
ively reduced the pressure rate due to pore blocking (APp) as well as that due to cake formation (APe). Specific cake resistance confirmed these results. The decreasing pattern of the specific cake resistance was in line with that of the pressure rate due to cake formation (APe). This result suggests that coagulation improved the permeation rate of the cake layer. The increased particle size is believed to cause the improvement. Coagulation increased particle size, which interfered with particles entering the membrane matrix. The cake layer, composed of large particles resulting from coagulation, yielded less resistance to flow.
0.010 --0-- Permeate ~
0.008
Feed
c o 0,006
0.0o4
~
re"
;~{
0.002 0,000 1,0O0
0
2,000
3,0O0
4,00O
Molecular weight
Fig. 2. Comparison of organic portions in feed and permeate.
4.4. Permeate quality It was predicted that membrane filtration would have excellent performance in particle removal, but a poor performance in organic removal. Indeed, membrane filtration yielded excellent particle removal. Permeate turbidity was always less than 0.05 NTU (data are not shown here). Membrane filtration was able to remove a certain portion of organics, as shown in Fig. 2. This figure compares organic portions of feed and permeate by molecular weight. The membrane system preferentially removed large organics. According to Fig. 2, this membrane removed organics larger than a molecular weight of approximately 3,000. In terms of TOC and UV-254 concentrations, the removal efficiencies were 37% and 18% without coagulation. Coagulation pretreatment was also beneficial for water quality improvement of the membrane system. Fig. 3 shows the organic removal efficiencies by coagulation and membrane filtration. This figure clearly shows that organic removal efficiency was improved with coagulation pretreatment. As the alum dosage increased, the overall organic removal efficiency increased, but the removable portion by membrane filtration decreased. This result suggests that coagulation and membrane filtration could target the same
o
E
~S o
t--
m~um C/OSage
lOO mglL
Fig. 3. TOC removalby coagulationand membranefiltration at differentalum dosages. organic constituents. Since both coagulation and membrane filtration preferentially removed large organics, very little was left at a high alum dosage. On the other hand, there still remained large organics after coagulation at low alum dosages.
5. Conclusions
According to this study, coagulation pretreatment effectively improved membrane performance. Coagulation reduced membrane fouling by changing particle characteristics and removing fouling-causing contaminants. Fouling reduction
S.-H. Kim et al. / Desalination 173 (2005) 301-307
was observed both at low and high alum dosages. When alum was added at a low dosage so that contaminant reduction was not possible, coagulation pretreatment reduced membrane fouling through a change in particle characteristics. Coagulation changed particle characteristics, which led to fouling reduction. When alum was added at a high dosage so that contaminant reduction was possible, both change in particle characteristics and reduced contaminant loading were important for fouling reduction. Of various characteristics, particle size was found the most significant for fouling reduction. An increase in particle size by coagulation was the main cause for fouling reduction. Coagulation was involved in both stages of fouling steps. Coagulation increased particle size, which interfered with particle entry into the membrane matrix. The cake layer, composed of large particles resulting from coagulation, yielded less resistance to flow. Coagulation pretreatment also improved the organic removal performance of the membrane system because coagulation effectively removed organics in raw water. It seemed that coagulation and membrane filtration preferentially removed large organics.
Acknowledgements This work was supported by a grant (4-1-1) from the Sustainable Water Resources Research
307
Center (SWRRC) of the 21 st Century Frontier R&D program through the Water Reuse Technology Center (WRTC) at Kwangju Institute of Science & Technology (K-JIST).
References [ 1] R. Patel, A.C. Penisson, R.D. Hill and M.R. Wiesner, Membrane microfiltration of secondary wastewater effluent, in: Chemical Water and Wastewater Treatment III, R. Klute and H.H. Hahn, eds., SpringerVerlag, Berlin, 1994, pp. 29-36. [2] H.R. Kohl, Evaluating reverse osmosis membrane performance on secondary effluent treated by membrane microfiltrtion, Proc. Annual Conference of the American Water Works Association, San Antonio, Texas, 1993. [3] M.R. Wiesner, S. Veerapaneni and D. Brejchova, Improvements in membrane microfiltration using coagulation pretreatment, in: R. Klute and H. Hahn, eds., Chemical Water and Wastewater Treatment II, Springer-Verlag, Berlin, 1992, pp. 281-296. [4] S-H Kim, H-I Lee and C-H Yoon, Evaluation of flocculation performance using floe characteristics. J. Korean Soc. Water Wastewater, 17(1) (2003) 29-33. [5] AWWA Membrane Technology Research Committee, Committee Report: Membrane processes in potable water treatment, J. AWWA, 84(1) (1992) 59--67.
ELSEVIER
www.elsevieEcom/locate/desal
Role of coagulation in membrane filtration of wastewater for reuse Seung-Hyun Kim a*, Seong-Yong Moon a, Cho-Hee Yoon b, Seong-Keun Yim ¢, Jae-Weon Cho d ~Civiland Environmental Engineering Department, bChemicalEngineering Department, Kyungnam University, Republic of Korea Fax: +82 (55) 249-2994; email: [email protected] CKolonEngineering and Construction Co., Ltd., Republic of Korea dEnvironmental Engineering Department, Kwangju Institute of Science and Technology, Republic of Korea Received 22 July 2004; accepted 19 August 2004
Abstract
This study explains the role of coagulation in membrane filtration of wastewater for reuse. For this purpose, a coagulation-ultrafiltmtion (UF) membrane system was used to treat secondary effluent from a nearby wastewater treatment plant using a rotating biological contactor. The study proceeded with the hypothesis that coagulation could affect membrane filtration through two phenomena: change in particle characteristics and contaminant loading reduction. If fouling reduction were observed at a low alum dosage, coagulation would affect membrane performance by changing particle characteristics because contaminant reduction could not be possible at low dosage. If fouling reduction were observed only at a high alum dosage, the role of coagulation would be contaminant loading reduction. Results showed that both phenomena were important. Coagulation improved the membrane performance by changing particle characteristics at a low alum dosage. The improvement was achieved through both a change in particle charaeteristics and contaminant loading reduction at a high alum dosage. Particle size among various characteristics was found the most important for membrane fouling. Coagulation increased particle size, which led to a reduction of fouling. The beneficial effect from coagulation was observed at both fouling steps of pore blocking/adsorption and cake formation. Coagulation pretreatment was also beneficial for the improvement of the permeate quality in terms of organic matter.
Keywords: Coagulation pretreatment; Wastewater reuse; Ultrafiltration; Particle size
*Corresponding author.
0011-9164/05/$- See front matter © 2005 Elsevier B.V. All fights reserved doi: 10.1016/j.desal.2004.08.036
302
S.-H. Kim et al. /Desalination 173 (2005) 301-307
I. Introduction
Membrane filtration is usually employed to treat clean raw water without pretreatment. However, it is essential to include pretreatment when raw water quality is poor. Membrane filtration is, therefore, combined with pretreatment when it is employed for the reuse of wastewater. There are many options for pretreatment such as coagulation, adsorption and ozonation. Coagulation is the most attractive among these options due to several advantages. The most significant advantage is experience. Coagulation has been used in water and wastewater treatment for many years. Operators feel comfortable with this technology. Another advantage is capital investment. Since most water and wastewater treatment include coagulation, additional capital investment is often not necessary. Coagulation appears to yield some beneficial effects when it is used with membrane filtration for wastewater reuse. Patel et al. [1] compared membrane filtration performance with and without coagulation pretreatment for the treatment of secondary effluent from a wastewater treatment plant. They found that coagulation significantly improved permeation rates. The impact from coagulation was less significant on permeate quality. Membrane filtration showed excellent performance of particle removal regardless of coagulation pretreatment. No coliform was found in permeate, and residual turbidity was tess than 0.1 NTU with and without coagulation. Coagulation pretreatment was, however, effective in TOC reduction and reduced TOC by 50%. Other researchers noted similar results. Kohl et al. [2] used coagulation-microfiltration (MF) as a pretreatment to reverse osmosis (RO) to treat chlorinated tertiary effluent from a wastewater treatment plant. An average reduction of TOC was 22%. Wiesner et al. [3] employed ceramic MF (0.05/xm zirconia) as a pretreatment of RO for the treatment of surface water with high turbidity and TOC concentrations. They found that
coagulation improved permeation rates. The improvement depended on the coagulation pH. The best performance was obtained at a pH near neutral value. The beneficial effect from coagulation was also noted in permeate quality. An average permeate turbidity was 1.29 NTU without coagulation, but it was less than 0.5 NTU with coagulation. Alum reduced the permeate's TOC concentration by approximately half. These results showed that coagulation improves permeation rates and permeate quality. It is, however, unclear how coagulation made such an improvement. Previous studies investigated specific applications, but did not attempt to identify the role of coagulation pretreatment in membrane filtration. This study was designed to discover the cause for the role of coagulation in membrane filtration of wastewater for reuse.
2. Approach
The effects of coagulation can be summarized as follows: • change in particle characteristics • reduced contaminant loading Coagulation changes particle characteristics such as size, charge, and shape [4]. Such change can be related to the improvement of permeation rates and/or in permeate quality. Coagulation, especially followed by sedimentation, evidently removes considerable amounts of contaminants that cause fouling. The reduction of such foulingcausing contaminants can lead to the improvement in permeation rates and/or in permeate quality. The purpose of this study is to identify which is the main cause of coagulation pretreatment in membrane filtration. An experiment was designed with different coagulant dosages (1, 10, and 100 mg'L -1) to achieve this purpose. If fouling reduction were observed at 1 mg'L -1, the role of coagulation would be a change in particle charac-
303
S.-H. Kim et aL / Desalination 173 (2005) 301-307
teristics because contaminant reduction could not be possible at such low dosage. If fouling reduction were observed only at a high alum dosage (10 or 100 mg.L-l), the role of coagulation would be contaminant loading reduction.
3. Materials and methods
Secondary effluent from a nearby wastewater treatment plant employing a rotating biological contactor was used as the raw water in this study. Table 1 shows the characteristics of this source water. This is a very lowly buffered wastewater. The average alkalinity value was 30 mg'L -1. Both turbidity and organic levels were low. Relatively high UV-254 value compared to TOC concentration suggests that organic matter of this source water is mostly of a hydrophobic nature. Coagulation was simulated in jar tests. Rapid mixing was provided for 1 min at a G value of 230 s-I after coagulant addition and slow mixing followed for 5 min at a G value of 50 s -1. Flocculated water was then allowed to settle for 30 min before being introduced into the UF membrane system. The membrane is made of polysulfone with a pore size of 0.01 ~m. The suction-type membrane system was operated at a constant flux of 50 L'(m2"h) -I at 25°C, while pressure was
allowed to increase. Operation was terminated when the suction pressure reached 200 mmHg. The permeation rate and permeate quality were used to evaluate the performance of membrane filtration. Pressure rate, which indicates the pressure change over the filtration time until the target pressure of 200 mmHg was reached, was used to evaluate fouling progress. Specific cake resistance was measured in order to check the cake resistance. Turbidity and organic matter were used to evaluate permeate quality. Turbidity was measured in order to evaluate the particle removal performance of the membrane system. TOC and UV-254 concentrations were measured in order to evaluate the organic removal performance of the membrane system. Particle size and zeta potential were measured using Malvern's Zetasizer in order to examine change in particle characteristics after coagulation.
4. Results and discussion 4.1. Fouling reduction at a low alum dosage
Fig. 1 shows the variation of permeation rates with filtration time at different alum dosages. Table 2 compares pressure rates at different alum dosages. Fig. 1 and Table 2 clearly show that coagulation pretreatment reduced fouling. Pressure rapidly increased without coagulation
Table 1 Raw water characteristics used in this study Item
Value
pH Turbidity, NTU SS, mg.L-l TOC, mg.L-~ UV-254, em-~ COD, mg.L-I BOD, mg.L-~ Alkalinity as CaCO3,mg.L-' Hardness as CaCO3,mg.L-~
7.04-7.83 2.0-4.5 4.0-4.5 1.51-8.61 0.072-0.084 7 4 30 101
Table 2 Comparison results of pressure rates (mg.L-') with and without coagulation pretreatment Alum dosage
Pressure rate (AP), mmHg.h-I
0 1 10 100
36 21 13 4.8
(42%) (64%) (83%)
Values in parenthesesindicatepercent improvementover 0 mg/L.
304
S.-H. rtim et al. / Desalination 173 (2005) 301-307
254 concentrations. This result suggests that the role of coagulation in membrane filtration is a change in particle characteristics, which then leads to fouling reduction. Particle size and zeta potential were then measured in order to pinpoint the particle characteristic responsible for the improvement. Table 3 shows these values at different alum dosages. Even a slight addition of alum was able to change particle characteristics. Alum coagulation increased particle size, but decreased negative zeta potential. Both particle size and surface charge influence permeation rate. An increase in particle size should result in an increase in permeate flux because of a decrease in the specific resistance of
and pressure rate was calculated 36 rnmHg'h -1. Even a slight addition of alum could reduce fouling and improve the permeation rate. Alum coagulation at 1 mg'L- ~reduced the pressure rate to 21 mmHg.h- L In terms o f pressure rate, coagulation pretreatment improved membrane performance by 42% compared to no pretreatment. The settled water qualities o f TOC and UV-254 concentrations were then examined in order to check the hypothesis that coagulation at such low dosage caused any reduction in contaminant loading. Table 3 summarizes the settled water qualities. These results confirmed the hypothesis. Alum coagulation at 1 mg'L-~ could not bring in any improvement of TOC and
] I
Z 7~ -50
~9.- I 0 0
rt
X
0 mg/L
~
1 mg/L
,b, lOmg/L
X
lOOmg/L
rt
ii rt~
X
X
5
i -!5o 4 ~
tr rt ~rt rti,4 '
0
trrt ® ,,
i t L
,L"
2
4
6 FiJtration time, h
8
10
12
Fig. 1. Pressure change with time as a function of alum dosage.
Table 3 Settled water qualities with and without coagulation pretreatment Alum dosage
0 mg'L -j
1 mg'L -I
TOC, mg.L-t UV-254, cm ~ Average size, #m Zeta potential, mV
3.02 0.085 50 -27.2
3.32 0.085 280 - 13.5
(- 10%) (0%)
Values in parentheses indicate the percent improvement over 0 mg/L.
10 mg.L i
100 mg'L -1
2.89 0.077 415 - 12.2
1.33 0.055 734 1.7
(4%) (9%)
(56%) (35%)
305
S.-H. Kim et al. / Desalination 173 (2005) 301-307
extended with increasing alum dosage. Particle size was increased further and zeta potential was decreased further as alum dosage was increased. This change in particle characteristics could also help improve permeation rate. This result suggests that both a change in particle characteristics and reduced contaminant loading would be responsible for the improvement of the permeation rate when coagulation pretreatment is practiced at a high alum dosage. When coagulation is practiced at a high alum dosage so that a contaminant reduction was possible, coagulation improved the membrane performance by changing particle characteristics and reducing contaminant loading.
the cake [5]. Cakes composed of negatively charged particles may be more permeable than those composed of uncharged particles [5]. The increased particle size is, therefore, beneficial for permeation rate, while the decreased zeta potential is not. The improvement of permeation rate by coagulation pretreatment suggests that particle size would be more important for membrane fouling than zeta potential. 4.2. Fouling reduction at a high alum dosage
Table 2 shows that pressure rate decreased with increasing alum dosage. Alum coagulation at 10 and 100 mg.L- ~ improved the permeation rate by 64% and by 83%, respectively. The effect of coagulation at high dosage was also noted in water quality improvement. According to Table 3, organic removal improved with increasing alum dosage. Removal efficiencies of TOC and UV-254 increased to 56% and 35% at an alum coagulation of 100 mg'L -~. This result means that a reduction of fouling-causing contaminants is possible at high alum dosages. A comparison of Tables 2 and 3 shows that the effect of coagulation pretreatment was more evident in permeation rate than organic reduction. Alum coagulation at 10 mg.L -~ improved the organic removal efficiency by less than 10%, but the permeation rate by 64%. This suggests that reduced contaminant loading might not be the only reason for the improvement of the permeation rate. Particle characteristics were therefore examined with increasing alum dosage. The result shown in Table 3 indicates that the change
4.3. Pore blocking/adsorption and cake formation
Fig. 1 shows that membrane fouling progressed in two steps: rapid decline at first and slow decline later. Pore blocking and adsorption onto the membrane matrix are believed to cause the initial rapid decline, while cake formation causes the slow decline. Organics can penetrate and adsorb onto the membrane matrix, which causes fouling by blocking membrane pores. Contaminants sit on the membrane surface and form a cake layer, which causes fouling. Pore blocking and adsorption are important for membrane fouling at the initial stage, while cake formation is important at a later stage. The pressure rate was therefore divided into two stages: the initial stage (APp) and the final stage (APe). According to Table 4, coagulation was important at both stages. Coagulation effect-
Table 4 Comparisonof pressure rates due to pore blockingand cake formationat different alum dosages (mg.L-1) Alum dosage
0
1
10
100
Pressure rate due to pore blocking (APp),rnmHg.h-~ Pressure rate due to cake formation(APe),mmHg.h-~ Specific cake resistance, 10~3kg.m-1
77 23 --
31 16 25.4
33 4.3 6.25
6.7 1.8 1.84
306
S.-H. Kim et al. / Desalination 173 (2005) 301-307
ively reduced the pressure rate due to pore blocking (APp) as well as that due to cake formation (APe). Specific cake resistance confirmed these results. The decreasing pattern of the specific cake resistance was in line with that of the pressure rate due to cake formation (APe). This result suggests that coagulation improved the permeation rate of the cake layer. The increased particle size is believed to cause the improvement. Coagulation increased particle size, which interfered with particles entering the membrane matrix. The cake layer, composed of large particles resulting from coagulation, yielded less resistance to flow.
0.010 --0-- Permeate ~
0.008
Feed
c o 0,006
0.0o4
~
re"
;~{
0.002 0,000 1,0O0
0
2,000
3,0O0
4,00O
Molecular weight
Fig. 2. Comparison of organic portions in feed and permeate.
4.4. Permeate quality It was predicted that membrane filtration would have excellent performance in particle removal, but a poor performance in organic removal. Indeed, membrane filtration yielded excellent particle removal. Permeate turbidity was always less than 0.05 NTU (data are not shown here). Membrane filtration was able to remove a certain portion of organics, as shown in Fig. 2. This figure compares organic portions of feed and permeate by molecular weight. The membrane system preferentially removed large organics. According to Fig. 2, this membrane removed organics larger than a molecular weight of approximately 3,000. In terms of TOC and UV-254 concentrations, the removal efficiencies were 37% and 18% without coagulation. Coagulation pretreatment was also beneficial for water quality improvement of the membrane system. Fig. 3 shows the organic removal efficiencies by coagulation and membrane filtration. This figure clearly shows that organic removal efficiency was improved with coagulation pretreatment. As the alum dosage increased, the overall organic removal efficiency increased, but the removable portion by membrane filtration decreased. This result suggests that coagulation and membrane filtration could target the same
o
E
~S o
t--
m~um C/OSage
lOO mglL
Fig. 3. TOC removalby coagulationand membranefiltration at differentalum dosages. organic constituents. Since both coagulation and membrane filtration preferentially removed large organics, very little was left at a high alum dosage. On the other hand, there still remained large organics after coagulation at low alum dosages.
5. Conclusions
According to this study, coagulation pretreatment effectively improved membrane performance. Coagulation reduced membrane fouling by changing particle characteristics and removing fouling-causing contaminants. Fouling reduction
S.-H. Kim et al. / Desalination 173 (2005) 301-307
was observed both at low and high alum dosages. When alum was added at a low dosage so that contaminant reduction was not possible, coagulation pretreatment reduced membrane fouling through a change in particle characteristics. Coagulation changed particle characteristics, which led to fouling reduction. When alum was added at a high dosage so that contaminant reduction was possible, both change in particle characteristics and reduced contaminant loading were important for fouling reduction. Of various characteristics, particle size was found the most significant for fouling reduction. An increase in particle size by coagulation was the main cause for fouling reduction. Coagulation was involved in both stages of fouling steps. Coagulation increased particle size, which interfered with particle entry into the membrane matrix. The cake layer, composed of large particles resulting from coagulation, yielded less resistance to flow. Coagulation pretreatment also improved the organic removal performance of the membrane system because coagulation effectively removed organics in raw water. It seemed that coagulation and membrane filtration preferentially removed large organics.
Acknowledgements This work was supported by a grant (4-1-1) from the Sustainable Water Resources Research
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Center (SWRRC) of the 21 st Century Frontier R&D program through the Water Reuse Technology Center (WRTC) at Kwangju Institute of Science & Technology (K-JIST).
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