- Email: [email protected]
ultrafiltration process
Desalination 185 (2005) 327–333
Effect of Al coagulant type on natural organic matter removal efficiency in coagulation/ultrafiltration process Malgorzata Kabsch-Korbutowicz Wroclaw University of Technology, Institute of Environment Protection Engineering, Wybrzeze Wyspianskiego 27, 50–370 Wroclaw, Poland Tel. +48 (71) 3203639; Fax +48 (71) 3282980; email: [email protected] Received 7 February 2005; accepted 21 February 2005
Abstract Coagulation followed by ultrafiltration has been studied in order to improve water quality for surface water treatment. Three aluminium-based coagulants: Al2(SO4)3, NaAlO2, and polyaluminium chloride (PAC) have been used. The influence of coagulant type and the water pH on investigated process performance was analyzed. Two ultrafiltration membranes (cut-off 30 kDa) made of regenerated cellulose and polyethersulphone were used in the tests. The nature of aluminium-based coagulant and solution pH strongly affected the performance of coagulation/ultrafiltration process. Under the given dose of aluminium (3.59 g Al/m3), the best results were observed when alum or prehydrolized coagulant were used. For those coagulants the best results of NOM separation in the integrated process were observed in the pH range 6–8. Application of sodium aluminate resulted in much worse elimination of organic matter and a rather high residual aluminium concentration in coagulation as well as in the coagulation/ultrafiltration process. Keywords: Natural organic matter; Hybrid process; Coagulation; Ultrafiltration
1. Introduction In the recent years, considerable effort has been made in water treatment technologies to develop methods with greater natural organic matter (NOM) removal efficiencies. Reduction in the level of NOM before disinfection minimizes the formation of the disinfection *Corresponding author.
by-products and reduces the disinfectant residual that is required to control the bacterial regrowth in the distribution systems. This contributes to the improvement of water quality delivered to the consumers. Conventional water treatment process, comprising of coagulation/flocculation, sedimentation and filtration, has been the most common one for NOM removal from the natural waters.
0011-9164/05/$– See front matter Ó 2005 Elsevier B.V. All rights reserved doi:10.1016/j.desal.2005.02.083
328
M. Kabsch-Korbutowicz / Desalination 185 (2005) 327–333
The effectiveness of NOM removal through coagulation is strongly affected by many factors, such as: nature and properties of NOM particles, type and dose of coagulant, pH, ionic strength and temperature. Moreover, a portion of the coagulant (metal salts or polyelectrolytes) that has been added to water is not removed during the treatment and remains as a residual in the treated water. Membrane filtration technology is one of the most noticeable water treatment advances of the recent years. Microfiltration and ultrafiltration, the low-pressure membrane processes, have been presented as very effective for the removal of particles, turbidity, bacteria and cysts from the natural waters [1]. However, those processes are not effective enough in the colour removal and they suffer from fouling [2–6]. This has led to great interest in the hybrid membrane processes involving chemical and/or physical techniques to remove effectively NOM and reduce the incidence of any organic fouling [7–10]. The aim of this work was to evaluate the suitability of different types of aluminiumbased coagulants in NOM removal by the coagulation/ultrafiltration process. The influence of pH on the effectiveness of organic substances removal and amount of residual aluminium has been determined.
Table 1 The feed water properties Parameter
Mean value 3
Colour, g/m Abs 254 nm, cm1 TOC, g C/m3
68 0.425 9.43
2.2. Coagulants All coagulants, the comprised aluminium sulphate Al2(SO4)3 nH2O (14.3 n 15), the polyaluminium chloride PAC10WA (the basicity of 60 5%) and the sodium aluminate NaAlO2 were provided by Zlotniki Chemical Co. The stock solutions of the coagulants (1%) were prepared by dissolving an appropriate concentrate in the deionised water. 2.3. Membranes In this study two ultrafiltration membranes made by Millipore were used. Their characterisation is included in the Table 2. A cut-off of the investigated membranes amounted to 30 kDa. 2.4. Coagulation Coagulation was performed by the jar test method with 3-min rapid mixing and 20-min slow mixing. After that the samples were allowed to sediment for an hour. In all experiments the coagulant dose amounted to 3.59 g Al/m3.
2. Materials and methods 2.1. Feed water
2.5. Ultrafiltration
Experiments were carried out with a model solution prepared from the natural water flowing out from The Great Batorow Peatbag (southwest Poland) and the dechlorinated tap water. Properties of the feed water are presented in the Table 1. The pH of the tested solution ranged from 5 to 10, and was adjusted via addition of 0.1 M NaOH or HCl.
Experiments were carried out in a laboratory system [11] at a transmembrane pressure Table 2 The characterisation of membranes Membrane type Material PES (PBTK) C (PLTK)
J* (m3/m2d)
polyethersulphone 4.330 regenerated cellulose 6.361
*Pure water flux at 0.1 MPa.
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M. Kabsch-Korbutowicz / Desalination 185 (2005) 327–333
of 0.1 MPa. The main part of the system was an Amicon 8400 ultrafiltration cell pressuring with gas coming out from a cylinder. The effective surface of the membrane amounted to 4.52103 m2. In an integrated coagulation/ultrafiltration process a decanted sample, after being coagulated and left for an hour sedimentation, was ultrafiltered at a pressure of 0.1 MPa. 2.6. Analytical methods The efficiency of the examined processes was determined by measuring the amount of the organic matter in the samples before and after the process. The NOM concentration was monitored by the measurement of TOC (TOC 5050 Analyser, Shimadzu), the absorbance of UV at 254 nm, and the colour intensity (Shimadzu QP2000 spectrophotometer). The amount of residual aluminium was determined by a reaction with eriochrome cyanine R followed by spectrophotometric detection of the coloured product at 535 nm.
to be strongly influenced by the coagulant type and the membrane characteristics. Among the tested coagulants, the best results in separation of the organic substances were noticed for alum and polyaluminium chloride. After coagulation the TOC removal efficiency amounted to 42.1 and 44.1%, for alum and PAC, respectively. Integration of the coagulation with the ultrafiltration for the PES membrane resulted in an increase of the TOC removal efficiency up to 66.2 and 58.9%. In experiments performed with membrane made from regenerated cellulose (C), the TOC removal was lower (64.2 and 52.2%). When sodium alumininate was used as the coagulant, in all cases (i.e. coagulation with or without ultrafiltration) the efficiency of NOM separation was lower. After addition to water, Al coagulant dissociates and the Al3þ ions undergo the hydrolysis reactions as shown below [12]: Al3þ
!AlðOHÞ2þ
þH2 O;Hþ
!AlðOHÞ3
þH2 O;Hþ
!AlðOHÞþ 2
þH2 O;Hþ
!AlðOHÞ 4
þH2 O;Hþ
3. Results 3.1. The effect of coagulant type As presented in the Fig. 1 and the Table 3, the efficiency of the NOM separation in the integrated coagulation/ultrafiltration process was found coagulation
80
coagulation+PES coagulation+C
RTOC, %
60 40 20 0 UF
Al2(SO4)3
PAC10WA
NaAlO2
Fig. 1. The effect of the coagulant and the membrane type on the TOC removal efficiency (the coagulant dose 3.59 g Al/m3, pH = 7.0).
The degree to which the reactions are proceeded and the nature of the produced species may depend on the Al ions concentration, pH, temperature and the presence of any other ions. Among the hydrolysis reaction products monomeric (Al3þ, Al(OH)2þ, Al(OH)2þ, Al(OH)3, Al(OH)4) and polymeric (Al2(OH)24þ, Al3(OH)45þ, Al13O4(OH)247þ) forms may be detected [13]. In the solutions of the prehydrolyzed coagulants (PAC), the polymeric species with high positive charges occur more frequently among the hydrolysis products than in the solutions of the non-prehydrolyzed coagulants, which are added to water at a natural pH [14]. Although prehydrolyzed and nonprehydrolyzed coagulants follow the same mechanism of the natural organic matter removal, it is noticeable that the larger size
330
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Table 3 The effect of the aluminium coagulant type on the colour and the abs 254 nm removal efficiency in the coagulation/UF process for the PES and the C membrane coagulation
coagulation/PES membrane
coagulation/C membrane
–
RB (%)
Rabs (%)
RB (%)
Rabs (%)
RB (%)
Rabs (%)
UF Al2(SO4)3 PAC 10WA NaAlO2
– 49.2 57.1 39.6
– 50.0 48.2 28.4
52.7 89.0 92.3 59.8
46.8 82.1 84.2 48.5
32.3 85.2 82.2 55.9
27.1 78.9 75.4 43.7
3
coagulation coagulant dose=3.59 gAl/m
2.5 Al residual, g Al/m 3
of PAC hydrolysis products is [15], the greater is the probability of their collision with the organic colloids as well as the possibility of retention by the ultrafiltration membrane. Addition of the sodium aluminate caused the increase of the pH value (by 0.62) which resulted in the increase of the dissociation degree of the NOM particles. At the same time a decrease of the quantity of positively charged products of the coagulant hydrolysis was noticed. As a result, low elimination of the organic matter particles was observed. Based on the information reported by membrane manufacturer PES membranes are more hydrophobic and have more dense structure as compared to C membranes, causing higher effectiveness of NOMs separation. In order to determine the influence of coagulant type on the amount of residual aluminium in treated water, the amount of aluminium in water after coagulation and coagulation followed by ultrafiltration was measured (Fig. 2). In the assessment of the aluminium-based coagulants performance not only the removal efficiencies of the organic pollutants should be taken into account, but also the amount of the residual coagulant in the treated water. Due to the fact that aluminium is a suspected causative agent of the neurological disorders as Alzheimer’s disease [16], the amount of the residual coagulant must be strictly controlled. The
3
coagulation+PES coagulation+C
pH=7.0 2 1.5 1 0.5 0 Al2(SO4)3
PAC 10WA
NaAlO2
coagulant
Fig. 2. The effect of coagulant type on the concentration of the residual aluminium.
results of our experiment show that the lowest coagulant pollution was stated when the prehydrolyzed coagulant was used. Application of alum as a coagulant resulted in an increase of the residual alum concentration. The worst result, that is a very high concentration of aluminium, was noticed when the coagulation was performed with the sodium aluminate. For all of the coagulants that have been tested and for both of the membranes, integration of coagulation with ultrafiltration caused a decrease of the residual aluminium concentration. In these cases the concentration of aluminium for alum and PAC was at a level accepted by the authorities (0.2 g Al/m3). Slightly better results were observed when the PAC 10WA was used as a coagulant, suggesting that the UF membranes retained more effectively polymeric products of the coagulant hydrolysis.
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3.2. The effect of pH
a)
Al2(SO4)3
80
PAC10WA NaAlO2
RTOC, %
60
40
20
0 5
6
7
pH 8
9
10
Fig. 3. The influence of solution pH on the TOC removal efficiency in coagulation process.
UF Al2(SO4)3+UF
80
PAC10WA+UF R TOC, %
60
NaAlO2+UF
40 20
PES membrane 0 5
6
7
pH
8
9
10
b) UF
80
Al2(SO4)3+UF PAC10WA+UF
60
R TOC, %
The effect of the solution pH on the removal of the natural organic matter during coagulation and coagulation followed by ultrafiltration was studied at the pH values equal to 5.0, 6.0, 7.0, 8.0, 9.0 and 10.0. The results of these tests are presented in the Figs. 3, 4a and b. As it can be inferred from the Fig. 3, the removal of the organic substances from water during coagulation for all of the coagulants that have been tested were strongly affected by the pH of the solution. It may be stated that a lower removal rate was observed at a higher solution pH. The results of the experiment indicate that the optimal pH values for the NOM removal ranged from pH 5–6. For the solution of pH 5, the TOC removal efficiencies amounted to 59.9, 62.3 and 44.9, for alum, PAC and sodium aluminate, respectively, while the colour removal efficiency amounted to 69.1, 73.9 and 66.2. At acidic conditions the positively charged products of the coagulant hydrolysis tend to neutralize the negatively charged NOM particles. The neutralization mechanism of the coagulation is predominant. At higher pH values, when Al(OH)3 fraction dominates in water, the NOM removal efficiency is dependant on
NaAlO2+UF
40 20
C membrane 0 5
6
7
8
9
10
pH
Fig. 4. The influence of solution pH on the TOC removal efficiency in the coagulation/ultrafiltration process. (a, PES membrane and b, C membrane).
the adsorption of the humic substances on the Al(OH)3 crystals, and sweep flocculation plays the dominant role. In such conditions a much higher dose of coagulant must be used in order to produce a considerable quantity of the amorphous aluminium hydroxide. At the alkaline pH values some soluble NOMmetal complexes may be formed. Solution pH has also strongly affected performance of the coagulation/ultrafiltration process. Basing on the presented results (Fig. 4) it may be stated that at pH 6–8 the efficiency of the NOM removal is almost constant and very similar for alum and PAC. With the increase of solution pH, the TOC removal efficiency decreases, and at pH 10 no effect of coagulation on the integrated process performance was observed.
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M. Kabsch-Korbutowicz / Desalination 185 (2005) 327–333 Al2(SO4)3 PAC10WA NaAlO2
Al residual, g Al/m
3
3,5
coagulant dose=3.59 gAl/m3
3 2,5 2 1,5 1 0,5 0 5
6
7
8
9
10
pH Fig. 5. The effect of the coagulant type and the pH on the residual aluminium concentration after coagulation.
a) Al2(SO4)3+UF
3,5 Al residual, g Al/m
3
coagulant dose=3.59 gAl/m
PAC10WA+UF
3
3
NaAlO2+UF
2,5
PES
2 1,5 1
4. Conclusions
0,5 0 5
6
7
pH
8
9
10
b) Al2(SO4)3+UF 3,5 Al residual, g Al/m3
It may be supposed that the pH has a strong influence on the amount of residual aluminium as well in coagulation as in coagulation/ultrafiltration process (Figs. 5 and 6). Whenever alum was used as a coagulant, the lowest concentration of the residual aluminium (0.09 g Al/m3) was observed at pH 6. The increase of solution reaction resulted in increase of the residual coagulant. For PAC the lowest concentration of aluminium was observed at pH 7 and increase of pH brought about an increase of its concentration. According to Duan and Gregor [13] the minimum solubility of the Al hydrolysis products is observed at pH values from 6 to 7. The application of the sodium aluminate produced, at pH values ranging above 6, a considerable secondary coagulant pollution. Integration of coagulation with ultrafiltration resulted in a decrease of the aluminium concentration in the treated water. For PES and the C membrane, whenever alum or PAC was applied, the acceptable Al concentration (below 0.2 g/m3) was observed in much wider pH range (from 6 to 8).
PAC10WA+UF
3
coagulant dose=3.59 gAl/m
3
NaAlO2+UF
2,5
C
2 1,5 1 0,5 0 5
6
7
8
9
10
pH
Fig. 6. The influence of solution pH on the residual aluminium concentration in the coagulation/ultrafiltration process. (a, PES membrane and b, C membrane).
The following observations and conclusions arose from the present study: application of coagulation before ultrafiltration resulted in improvement of the natural organic matter removal (except for the most alkaline conditions), the nature of aluminium-based coagulant and the solution pH strongly affected the performance of the coagulation/ultrafiltration process; Under a given dose of aluminium, the best results were observed whenever the alum or prehydrolized coagulant were used. Application of the sodium aluminate resulted in much poorer elimination of the organic matter in coagulation as well as in the coagulation/
M. Kabsch-Korbutowicz / Desalination 185 (2005) 327–333
ultrafiltration process. For all of the coagulants the best results of the NOM separation in the integrated process were observed at pH range 6–8. the type of coagulant and the water pH had a great influence on the amount of the residual aluminium in purified water. For alum and PAC the lowest aluminium concentration was observed at pH 6, when Al(OH)3 played a dominant role.
References [1] J.G. Jacangelo, E.M. Aieta, K.E. Carns, E.W. Cummings and J. Mallevialle, Assessing hollowfiber ultrafiltration for particle removal. JAWWA, 81(11) (1989) 68–75. [2] E. Aoustin, A.I. Scha¨fer, A.G. Fane and T.D. Waite, Ultrafiltration of natural organic matter. Separ Purif. Technol., 22–23 (2001) 63–78. [3] M. Zhang, Li Ch., M.M. Benjamin and Y. Chang, Fouling and natural organic matter removal in adsorbent/membrane systems for drinking water treatment. EST, 37 (2003) 1663–1669. [4] Laıˆ ne´. J.-M., M.M. Clark and J. Mallevialle, Ultrafiltration of lake water: effect of pretreatment on the partitioning of organics, THMFP and flux. J.AWWA, 82(12) (1990) 82–87. [5] J.G. Jacangelo, J. DeMarco, D.M. Owen and S.J. Randke, Selected processes for removing NOM. an overview. J.AWWA, 89(1) (1995) 64–76. [6] N.H. Lee, G. Amy, J.-P. Croute´ and H. Buisson, Identification and understanding of fouling in low-pressure membrane (MF/UF) filtration by
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natural organic matter (NOM). Water Res., 38 (2004) 4511–4523. [7] J.C. Vickers, M.A. Thompson and U.G. Kelkar, The use of membrane filtration in conjunction with coagulation process for improved NOM removal. Desalination, 102 (1995) 57–61. [8] M. Tomaszewska and S. Mozia, Removal of organic matter from water by PAC/UF system. Water Res., 36(16) (2002) 4137–4143. [9] K. Glucina and A. Alvarez, Laıˆ ne´ J.-M. Assessment of an integrated membrane system for surface water treatment. Desalination, 132 (2000) 73–82. [10] G. Klomfas and K. Konieczny, Fouling phenomena in unit and hybrid processes for potable water treatment. Desalination, 163 (2004) 311–322. [11] M. Kabsch-Korbutowicz, Application of ultrafiltration integrated with coagulation for improved NOM removal. Desalination 175(1) (2005) 13–22. [12] K.N. Exall and G.W. vanLoon, Effects of raw water conditions on solute-state aluminium speciation during coagulant dilution. Water Res., 37 (2003) 3341–3350. [13] J. Duan and J. Gregory, Coagulation by hydrolysing metal salts. Adv. Colloid Interface Sci., 100–102 (2003) 475–502. [14] M. Rak and M. Swiderska-Broz, Efficiency of alum and prehydrolyzed polyaluminium chlorides as coagulating agents: a comparative study. Environmental Protection Eng., 27 (2001) 5–15. [15] K.N. Exall and G.W. vanLoon, Using coagulants to remove organic matter. J.AWWA, 92(11) (2000) 93–102. [16] P.T. Srinivasan, T. Viraraghavan and K.S. Subramanian, Aluminium in drinking water: an overview. Water SA, 25(1) (1999) 47–55.
Effect of Al coagulant type on natural organic matter removal efficiency in coagulation/ultrafiltration process Malgorzata Kabsch-Korbutowicz Wroclaw University of Technology, Institute of Environment Protection Engineering, Wybrzeze Wyspianskiego 27, 50–370 Wroclaw, Poland Tel. +48 (71) 3203639; Fax +48 (71) 3282980; email: [email protected] Received 7 February 2005; accepted 21 February 2005
Abstract Coagulation followed by ultrafiltration has been studied in order to improve water quality for surface water treatment. Three aluminium-based coagulants: Al2(SO4)3, NaAlO2, and polyaluminium chloride (PAC) have been used. The influence of coagulant type and the water pH on investigated process performance was analyzed. Two ultrafiltration membranes (cut-off 30 kDa) made of regenerated cellulose and polyethersulphone were used in the tests. The nature of aluminium-based coagulant and solution pH strongly affected the performance of coagulation/ultrafiltration process. Under the given dose of aluminium (3.59 g Al/m3), the best results were observed when alum or prehydrolized coagulant were used. For those coagulants the best results of NOM separation in the integrated process were observed in the pH range 6–8. Application of sodium aluminate resulted in much worse elimination of organic matter and a rather high residual aluminium concentration in coagulation as well as in the coagulation/ultrafiltration process. Keywords: Natural organic matter; Hybrid process; Coagulation; Ultrafiltration
1. Introduction In the recent years, considerable effort has been made in water treatment technologies to develop methods with greater natural organic matter (NOM) removal efficiencies. Reduction in the level of NOM before disinfection minimizes the formation of the disinfection *Corresponding author.
by-products and reduces the disinfectant residual that is required to control the bacterial regrowth in the distribution systems. This contributes to the improvement of water quality delivered to the consumers. Conventional water treatment process, comprising of coagulation/flocculation, sedimentation and filtration, has been the most common one for NOM removal from the natural waters.
0011-9164/05/$– See front matter Ó 2005 Elsevier B.V. All rights reserved doi:10.1016/j.desal.2005.02.083
328
M. Kabsch-Korbutowicz / Desalination 185 (2005) 327–333
The effectiveness of NOM removal through coagulation is strongly affected by many factors, such as: nature and properties of NOM particles, type and dose of coagulant, pH, ionic strength and temperature. Moreover, a portion of the coagulant (metal salts or polyelectrolytes) that has been added to water is not removed during the treatment and remains as a residual in the treated water. Membrane filtration technology is one of the most noticeable water treatment advances of the recent years. Microfiltration and ultrafiltration, the low-pressure membrane processes, have been presented as very effective for the removal of particles, turbidity, bacteria and cysts from the natural waters [1]. However, those processes are not effective enough in the colour removal and they suffer from fouling [2–6]. This has led to great interest in the hybrid membrane processes involving chemical and/or physical techniques to remove effectively NOM and reduce the incidence of any organic fouling [7–10]. The aim of this work was to evaluate the suitability of different types of aluminiumbased coagulants in NOM removal by the coagulation/ultrafiltration process. The influence of pH on the effectiveness of organic substances removal and amount of residual aluminium has been determined.
Table 1 The feed water properties Parameter
Mean value 3
Colour, g/m Abs 254 nm, cm1 TOC, g C/m3
68 0.425 9.43
2.2. Coagulants All coagulants, the comprised aluminium sulphate Al2(SO4)3 nH2O (14.3 n 15), the polyaluminium chloride PAC10WA (the basicity of 60 5%) and the sodium aluminate NaAlO2 were provided by Zlotniki Chemical Co. The stock solutions of the coagulants (1%) were prepared by dissolving an appropriate concentrate in the deionised water. 2.3. Membranes In this study two ultrafiltration membranes made by Millipore were used. Their characterisation is included in the Table 2. A cut-off of the investigated membranes amounted to 30 kDa. 2.4. Coagulation Coagulation was performed by the jar test method with 3-min rapid mixing and 20-min slow mixing. After that the samples were allowed to sediment for an hour. In all experiments the coagulant dose amounted to 3.59 g Al/m3.
2. Materials and methods 2.1. Feed water
2.5. Ultrafiltration
Experiments were carried out with a model solution prepared from the natural water flowing out from The Great Batorow Peatbag (southwest Poland) and the dechlorinated tap water. Properties of the feed water are presented in the Table 1. The pH of the tested solution ranged from 5 to 10, and was adjusted via addition of 0.1 M NaOH or HCl.
Experiments were carried out in a laboratory system [11] at a transmembrane pressure Table 2 The characterisation of membranes Membrane type Material PES (PBTK) C (PLTK)
J* (m3/m2d)
polyethersulphone 4.330 regenerated cellulose 6.361
*Pure water flux at 0.1 MPa.
329
M. Kabsch-Korbutowicz / Desalination 185 (2005) 327–333
of 0.1 MPa. The main part of the system was an Amicon 8400 ultrafiltration cell pressuring with gas coming out from a cylinder. The effective surface of the membrane amounted to 4.52103 m2. In an integrated coagulation/ultrafiltration process a decanted sample, after being coagulated and left for an hour sedimentation, was ultrafiltered at a pressure of 0.1 MPa. 2.6. Analytical methods The efficiency of the examined processes was determined by measuring the amount of the organic matter in the samples before and after the process. The NOM concentration was monitored by the measurement of TOC (TOC 5050 Analyser, Shimadzu), the absorbance of UV at 254 nm, and the colour intensity (Shimadzu QP2000 spectrophotometer). The amount of residual aluminium was determined by a reaction with eriochrome cyanine R followed by spectrophotometric detection of the coloured product at 535 nm.
to be strongly influenced by the coagulant type and the membrane characteristics. Among the tested coagulants, the best results in separation of the organic substances were noticed for alum and polyaluminium chloride. After coagulation the TOC removal efficiency amounted to 42.1 and 44.1%, for alum and PAC, respectively. Integration of the coagulation with the ultrafiltration for the PES membrane resulted in an increase of the TOC removal efficiency up to 66.2 and 58.9%. In experiments performed with membrane made from regenerated cellulose (C), the TOC removal was lower (64.2 and 52.2%). When sodium alumininate was used as the coagulant, in all cases (i.e. coagulation with or without ultrafiltration) the efficiency of NOM separation was lower. After addition to water, Al coagulant dissociates and the Al3þ ions undergo the hydrolysis reactions as shown below [12]: Al3þ
!AlðOHÞ2þ
þH2 O;Hþ
!AlðOHÞ3
þH2 O;Hþ
!AlðOHÞþ 2
þH2 O;Hþ
!AlðOHÞ 4
þH2 O;Hþ
3. Results 3.1. The effect of coagulant type As presented in the Fig. 1 and the Table 3, the efficiency of the NOM separation in the integrated coagulation/ultrafiltration process was found coagulation
80
coagulation+PES coagulation+C
RTOC, %
60 40 20 0 UF
Al2(SO4)3
PAC10WA
NaAlO2
Fig. 1. The effect of the coagulant and the membrane type on the TOC removal efficiency (the coagulant dose 3.59 g Al/m3, pH = 7.0).
The degree to which the reactions are proceeded and the nature of the produced species may depend on the Al ions concentration, pH, temperature and the presence of any other ions. Among the hydrolysis reaction products monomeric (Al3þ, Al(OH)2þ, Al(OH)2þ, Al(OH)3, Al(OH)4) and polymeric (Al2(OH)24þ, Al3(OH)45þ, Al13O4(OH)247þ) forms may be detected [13]. In the solutions of the prehydrolyzed coagulants (PAC), the polymeric species with high positive charges occur more frequently among the hydrolysis products than in the solutions of the non-prehydrolyzed coagulants, which are added to water at a natural pH [14]. Although prehydrolyzed and nonprehydrolyzed coagulants follow the same mechanism of the natural organic matter removal, it is noticeable that the larger size
330
M. Kabsch-Korbutowicz / Desalination 185 (2005) 327–333
Table 3 The effect of the aluminium coagulant type on the colour and the abs 254 nm removal efficiency in the coagulation/UF process for the PES and the C membrane coagulation
coagulation/PES membrane
coagulation/C membrane
–
RB (%)
Rabs (%)
RB (%)
Rabs (%)
RB (%)
Rabs (%)
UF Al2(SO4)3 PAC 10WA NaAlO2
– 49.2 57.1 39.6
– 50.0 48.2 28.4
52.7 89.0 92.3 59.8
46.8 82.1 84.2 48.5
32.3 85.2 82.2 55.9
27.1 78.9 75.4 43.7
3
coagulation coagulant dose=3.59 gAl/m
2.5 Al residual, g Al/m 3
of PAC hydrolysis products is [15], the greater is the probability of their collision with the organic colloids as well as the possibility of retention by the ultrafiltration membrane. Addition of the sodium aluminate caused the increase of the pH value (by 0.62) which resulted in the increase of the dissociation degree of the NOM particles. At the same time a decrease of the quantity of positively charged products of the coagulant hydrolysis was noticed. As a result, low elimination of the organic matter particles was observed. Based on the information reported by membrane manufacturer PES membranes are more hydrophobic and have more dense structure as compared to C membranes, causing higher effectiveness of NOMs separation. In order to determine the influence of coagulant type on the amount of residual aluminium in treated water, the amount of aluminium in water after coagulation and coagulation followed by ultrafiltration was measured (Fig. 2). In the assessment of the aluminium-based coagulants performance not only the removal efficiencies of the organic pollutants should be taken into account, but also the amount of the residual coagulant in the treated water. Due to the fact that aluminium is a suspected causative agent of the neurological disorders as Alzheimer’s disease [16], the amount of the residual coagulant must be strictly controlled. The
3
coagulation+PES coagulation+C
pH=7.0 2 1.5 1 0.5 0 Al2(SO4)3
PAC 10WA
NaAlO2
coagulant
Fig. 2. The effect of coagulant type on the concentration of the residual aluminium.
results of our experiment show that the lowest coagulant pollution was stated when the prehydrolyzed coagulant was used. Application of alum as a coagulant resulted in an increase of the residual alum concentration. The worst result, that is a very high concentration of aluminium, was noticed when the coagulation was performed with the sodium aluminate. For all of the coagulants that have been tested and for both of the membranes, integration of coagulation with ultrafiltration caused a decrease of the residual aluminium concentration. In these cases the concentration of aluminium for alum and PAC was at a level accepted by the authorities (0.2 g Al/m3). Slightly better results were observed when the PAC 10WA was used as a coagulant, suggesting that the UF membranes retained more effectively polymeric products of the coagulant hydrolysis.
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3.2. The effect of pH
a)
Al2(SO4)3
80
PAC10WA NaAlO2
RTOC, %
60
40
20
0 5
6
7
pH 8
9
10
Fig. 3. The influence of solution pH on the TOC removal efficiency in coagulation process.
UF Al2(SO4)3+UF
80
PAC10WA+UF R TOC, %
60
NaAlO2+UF
40 20
PES membrane 0 5
6
7
pH
8
9
10
b) UF
80
Al2(SO4)3+UF PAC10WA+UF
60
R TOC, %
The effect of the solution pH on the removal of the natural organic matter during coagulation and coagulation followed by ultrafiltration was studied at the pH values equal to 5.0, 6.0, 7.0, 8.0, 9.0 and 10.0. The results of these tests are presented in the Figs. 3, 4a and b. As it can be inferred from the Fig. 3, the removal of the organic substances from water during coagulation for all of the coagulants that have been tested were strongly affected by the pH of the solution. It may be stated that a lower removal rate was observed at a higher solution pH. The results of the experiment indicate that the optimal pH values for the NOM removal ranged from pH 5–6. For the solution of pH 5, the TOC removal efficiencies amounted to 59.9, 62.3 and 44.9, for alum, PAC and sodium aluminate, respectively, while the colour removal efficiency amounted to 69.1, 73.9 and 66.2. At acidic conditions the positively charged products of the coagulant hydrolysis tend to neutralize the negatively charged NOM particles. The neutralization mechanism of the coagulation is predominant. At higher pH values, when Al(OH)3 fraction dominates in water, the NOM removal efficiency is dependant on
NaAlO2+UF
40 20
C membrane 0 5
6
7
8
9
10
pH
Fig. 4. The influence of solution pH on the TOC removal efficiency in the coagulation/ultrafiltration process. (a, PES membrane and b, C membrane).
the adsorption of the humic substances on the Al(OH)3 crystals, and sweep flocculation plays the dominant role. In such conditions a much higher dose of coagulant must be used in order to produce a considerable quantity of the amorphous aluminium hydroxide. At the alkaline pH values some soluble NOMmetal complexes may be formed. Solution pH has also strongly affected performance of the coagulation/ultrafiltration process. Basing on the presented results (Fig. 4) it may be stated that at pH 6–8 the efficiency of the NOM removal is almost constant and very similar for alum and PAC. With the increase of solution pH, the TOC removal efficiency decreases, and at pH 10 no effect of coagulation on the integrated process performance was observed.
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M. Kabsch-Korbutowicz / Desalination 185 (2005) 327–333 Al2(SO4)3 PAC10WA NaAlO2
Al residual, g Al/m
3
3,5
coagulant dose=3.59 gAl/m3
3 2,5 2 1,5 1 0,5 0 5
6
7
8
9
10
pH Fig. 5. The effect of the coagulant type and the pH on the residual aluminium concentration after coagulation.
a) Al2(SO4)3+UF
3,5 Al residual, g Al/m
3
coagulant dose=3.59 gAl/m
PAC10WA+UF
3
3
NaAlO2+UF
2,5
PES
2 1,5 1
4. Conclusions
0,5 0 5
6
7
pH
8
9
10
b) Al2(SO4)3+UF 3,5 Al residual, g Al/m3
It may be supposed that the pH has a strong influence on the amount of residual aluminium as well in coagulation as in coagulation/ultrafiltration process (Figs. 5 and 6). Whenever alum was used as a coagulant, the lowest concentration of the residual aluminium (0.09 g Al/m3) was observed at pH 6. The increase of solution reaction resulted in increase of the residual coagulant. For PAC the lowest concentration of aluminium was observed at pH 7 and increase of pH brought about an increase of its concentration. According to Duan and Gregor [13] the minimum solubility of the Al hydrolysis products is observed at pH values from 6 to 7. The application of the sodium aluminate produced, at pH values ranging above 6, a considerable secondary coagulant pollution. Integration of coagulation with ultrafiltration resulted in a decrease of the aluminium concentration in the treated water. For PES and the C membrane, whenever alum or PAC was applied, the acceptable Al concentration (below 0.2 g/m3) was observed in much wider pH range (from 6 to 8).
PAC10WA+UF
3
coagulant dose=3.59 gAl/m
3
NaAlO2+UF
2,5
C
2 1,5 1 0,5 0 5
6
7
8
9
10
pH
Fig. 6. The influence of solution pH on the residual aluminium concentration in the coagulation/ultrafiltration process. (a, PES membrane and b, C membrane).
The following observations and conclusions arose from the present study: application of coagulation before ultrafiltration resulted in improvement of the natural organic matter removal (except for the most alkaline conditions), the nature of aluminium-based coagulant and the solution pH strongly affected the performance of the coagulation/ultrafiltration process; Under a given dose of aluminium, the best results were observed whenever the alum or prehydrolized coagulant were used. Application of the sodium aluminate resulted in much poorer elimination of the organic matter in coagulation as well as in the coagulation/
M. Kabsch-Korbutowicz / Desalination 185 (2005) 327–333
ultrafiltration process. For all of the coagulants the best results of the NOM separation in the integrated process were observed at pH range 6–8. the type of coagulant and the water pH had a great influence on the amount of the residual aluminium in purified water. For alum and PAC the lowest aluminium concentration was observed at pH 6, when Al(OH)3 played a dominant role.
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