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Treatment of highly turbid water using chitosan and aluminum salts
Separation and Purification Technology 104 (2013) 322–326
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Treatment of highly turbid water using chitosan and aluminum salts Ching-Yao Hu a, Shang-Lien Lo b,⇑, Chia-Ling Chang c, Fu-Ling Chen b, Yu-De Wu a, Jia-lin Ma d a
School of Public Health, Taipei Medical University, 250 Wu-Xin Street, Taipei 110, Taiwan, ROC Graduate Institute of Environmental Engineering, National Taiwan University, Taipei 106, Taiwan, ROC c Department of Water Resources Engineering and Conservation, Feng Chia University, No. 100 Wenhwa Rd., Seatwen, Taichung 40724, Taiwan, ROC d Water Resource Agency, Ministry of Economic Affairs, 9-12F 41-3 Sec.3 Hsin-yi Rd., Taipei, Taiwan 10651, Taiwan, ROC b
a r t i c l e
i n f o
Article history: Received 20 August 2012 Received in revised form 6 November 2012 Accepted 14 November 2012 Available online 4 December 2012 Keywords: Turbidity Sludge Aluminum chloride Chitosan Residual aluminum concentration
a b s t r a c t The high turbidity in surface water may make it difficult for water treatment plants to supply drinking water. Chitosan, a natural linear cationic polymer, and aluminum chloride, a metal salt, and the mixture of the two coagulants were used to treat highly turbid raw water in this study according to the residual turbidity, sludge volume and residual aluminum concentration. The residual turbidity was less than 50 NTU but the sludge/water volume ratio was over 150 mL/L after aluminum salt coagulation (135 mg/L as Al), which could stop the sedimentation process. The amount of sludge produced after chitosan coagulation (5 mg/L) was only about 1/5 of that for aluminum coagulation for the similar turbidity removal. Chitosan coagulation, however, still has two problems that need to be solved. First, the residual turbidity of treated water is still too high for sand filtration. Second, the colloid particles may restabilize if chitosan is overdosed. Adding a comparative low dosage of aluminum salt (13.5 mg/L as Al) with chitosan can successfully solve both of the problems. The sludge volume ratio only increased slightly and the residual turbidity was less than 10 NTU. Moreover, the restabilization of colloids did not occur. The residual aluminum concentration, which could lead to Alzheimer’s disease, can also be reduced significantly after addition of chitosan. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The turbidity of river water in Taiwan and other tropical areas occasionally rises to over 10,000 NTU during typhoons, hurricanes, or rainstorms. This makes it difficult for water purification plants to supply useable water. A lack of water supply may cause sanitary problems and interfere with manufacturing activity which could lead to huge economic damage. How to treat highly turbid water efficiently and economically, therefore, is an important issue for environmental engineers. Coagulation with metal salts, such as aluminum or iron salts, can effectively increase the size of the particles in water, after which they can be removed by sedimentation. This process, however, may produce large amounts of sludge because of the formation of metal hydroxides if large amounts of metal salt are added to treat high turbid water. The water purification process sometimes has to stop in Taiwan due to too much sludge in the sedimentation tank during typhoons. Moreover, the residual aluminum concentration in the water may increase due to the increase of aluminum dose. It may raise the risk of Alzheimer’s disease [1].
⇑ Corresponding author. Tel.: +886 2 23625373; fax: +886 2 23928821. E-mail address: [email protected] (S.L. Lo). 1383-5866/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2012.11.016
Synthetic polymers, such as poly acryl-amide and poly diallyldimethyl ammonium chloride, are alternative coagulants used to remove turbidity in water. They can reduce the turbidity of water but do not increase the volume of sludge because they do not generate metal hydroxides. However, health concerns related to the release of carcinogenic oligomer [2–5] restricts their use. Chitosan, which is a natural linear cationic polymer generated by extensive deacetylation of chitin, is a bio-degradable coagulant and has been used to treat colloidal particles [6–14], COD [15–20], metal ions [16,21–24], humic acids [25], cryptosporidium cysts [26] and other bio-microspheres [27]. It has similar properties to synthetic polymers but has a relatively low toxicity for humans and aquatic species. Moreover, it contains amino and hydroxyl functional groups, which show significant adsorption capacity for various pollutants. Divakaran and Sivasankara Pillai [8] have successfully used chitosan to reduce the turbidity of water. The turbidity removal efficiency was over 90% in their study but the turbidity of the raw water was lower than the target water in this study. Sekine and coworkers used chitosan to treat turbid river water [11]. Their results showed that the turbidity removal efficiency was good when the turbidity was under 1000 NTU, but the coagulation process failed if the turbidity was over 1000 NTU. The use of monocoagulants, such as chitosan, therefore, may not be a good solution for highly turbid water purification. New coagulants or coagulation
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150
200 Turbidity Sludge V 150
100 100
50 50
0
0
2. Materials and methods
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2.3. Coagulation test A jar test apparatus was used for the coagulation and flocculation experiments. For all experiments, the pH of the suspension was adjusted by adding a 0.1 M NaOH or 0.1 M HCl (pH = 7.5 ± 0.1). After the coagulants were added, the water was rapidly mixed (at 100 rpm) for 3 min followed by 20 min of slow mixing (at 30 rpm). After standing for 30 min, the supernatant from the top 5 cm of the suspension was withdrawn for turbidity and residual aluminum measurement. 2.4. Analytical measurements The turbidity of each sample was measured by a turbidity meter (HACH Ratio, USA). The sludge volume was measured using a quiescent column. Residual aluminum ions were analyzed by a TJA IRIS Advantage/1000 Radial ICP spectrometer calibrated against a CLARITAS certified reference solution (SPEX Certi Prep, Inc). A particle size analyzer (Model Zetasizer 3000, Malvern Instrument Ltd., Worcester, UK) was used to determine the particle size distribution before and after coagulation. 3. Results and discussion 3.1. Jar test with different coagulants Figs. 1 and 2 show the variations of residual turbidity and the water-sludge volume ratio after aluminum and chitosan coagulation, respectively. The residual turbidity decreased with increasing coagulant dosage for aluminum coagulation but had an optimum dosage (5 mg/L) for chitosan coagulation. This is due to the different mechanisms of the two coagulants. The mechanisms for aluminum coagulation are compression of the electrical double layer, charge neutralization and flocs sweeping. The main mechanism
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Sludge Volume (ml/L)
Model waters with the desired turbidity were prepared by mixing given amounts of clay or bentonite (Hayashi Co., Japan) with deionized water. NaHCO3 was added to produce the alkalinity at 50 mg/L as CaCO3.
80
Fig. 1. Variation of residual turbidity and sludge volume per liter of water after coagulation with aluminum chloride. (Initial turbidity = 10,000 NTU, settling time = 30 min.)
Residual Turbidity (NTU)
2.2. Preparation of model water
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Al dosage (mg/L)
2.1. Preparation of coagulants Chitosan powder purchased from Wako Pure Chemicals (Tokyo, Japan) was dissolved in 1.0% chloride acid solution by stirring overnight at 150 rpm to make 1.0% (w/w) stock solution. Aluminum salt stock solution was prepared by dissolving 133.5 g of aluminum chloride (AlCl3) in 1 L deionized water ([Al3+] = 1.0 M). Prior to the coagulant mixture experiments, both of the two stock solutions were added to the deionized water and carefully mixed with magnetic stirring to prepare a coagulant mixture solution of desired concentration.
Sludge Volume (ml/L)
Residual Turbidity (NTU)
processes need to be investigated to solve these problems. This study tried to use chitosan with aluminum salt to form a mixture coagulant to treat highly turbid water (over 1000 NTU). The residual turbidity, sludge volume, and residual aluminum concentration were investigated after sedimentation to access the applicability of the coagulant mixture. Finally, a coagulation test using the raw water of Zhi-tan water purification plant during typhoon Megi was performed to examine the effectiveness of the combined procedure.
0 0
5
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25
30
35
40
45
Chitosan dosage (mg/L) Fig. 2. Variation of residual turbidity and sludge volume per liter of water after coagulation with chitosan. (Initial turbidity = 10,000 NTU, settling time = 30 min.)
in this study should be flocs sweeping because the aluminum hydroxide must appear at high aluminum dosage. The addition of large amounts of aluminum salt can produce aluminum hydroxide flocs (as shown in reaction (1)) which settle by gravity, within a reasonable time. þ3
nAlðaqÞ þ 3nOHðaqÞ ! Aln ðOHÞ3nðsÞ
Ksp ¼ 1032 M4
ð1Þ
These coagulant flocs then collide with and drag colloids down with them. The more flocs produced, the better the turbidity removal. The removal of turbidity, therefore, increased with increasing aluminum dosage. The addition of chitosan; however, did not produce hydroxide flocs. The functional groups on the chitosan combine with the active sites of particles and the interaction of a single molecule with many particles produces a bridging effect. This combines them into larger particles which settle under the action of gravity. When a larger amount of chitosan was added; however, the effect of bridging coagulation became relatively weak and the colloids restabilized, possibly due to steric hindrance and repulsion between charged polymers [28,29]. Similar phenomenon also occurred in previous studies which use chitosan to treat river water [6,8,14]. The optimum dose of chitosan (1.0–2.0 mg/L) in their works were less than that (5.0 mg/L) in this study. This phenomenon may be due to the higher turbidity. Chatterjee et al. [30] observe the same optimum dose of chitosan and the initial turbidity in their work was also extremely high (SS = 5.0 g/L). The amount of sludge produced by aluminum coagulation was proportional to the amount of aluminum salt added. Although aluminum hydroxide floc can sweep the colloids in the water, the floc itself is a kind of sludge. The sludge/water volume ratio may rise to 15% if the dosage used for the same turbidity removal as chitosan is
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added. The amount of sludge produced by chitosan coagulation was much less than aluminum coagulation and it did not increase with the dosage of chitosan. Chitosan, therefore, may be more suitable than metal salts for high turbidity raw water treatment. 3.2. Jar test with mixture of coagulants Although chitosan may be a better coagulant for high turbidity raw water treatment, the residual turbidity, after chitosan coagulation, was still too high for the sand filtration process. The backwash frequency increases significantly due to high turbidity. The amount of water supplied may reduce because of the use of too much water for the backwash. The residual turbidity after chitosan coagulation was over 50 NTU, while the appropriate turbidity for rapid sand filtration should be less than 10 NTU [31]. Combining the two coagulants, metal salt and chitosan, may solve this problem. Therefore, aluminum salt and chitosan were mixed together to form a mixture of coagulants. Fig. 3 shows the variations of residual turbidity and sludge volume per liter of water after coagulation with different chitosan dosages and 13.5 mg/L as Al of aluminum salt (the lowest dosage in Fig. 1), which was a relatively low dosage of aluminum in this study and did not produce too much sludge. The residual turbidity was less than 10 NTU after the chitosan dosage was over 5 mg/L and the sludge/water volume ratio of water was only 30–40 mL/L. Moreover, the colloids did not restabilize after coagulation. Comparing to the single coagulant system (as shown in Table 1), coagulation with the chitosan + aluminum salt mixture coagulant got a lower residual turbidity and produced comparatively lower sludge volume. A combination usage of metal salts and polymers may be a promising method for treating extremely turbid raw water. Zemmouri et al. also used the chitosan and aluminum salt to treat surface water [32]. They found the combination usage of the two coagulants have a better removal efficiency than usage of the coagulants alone. They stated that the turbidity removal efficiency of chitosan was highly influenced by the initial turbidity. Chitosan was not suitable to use as a primary coagulant to treat raw water with low turbidity (about 5 NTU) but can effectively remove turbidity when it is used as an aid coagulant to the aluminum salt as the main coagulant. Their findings are similar to our results. The initial turbidities (5–150 NTU) in the raw water of their researches, however, were much lower than that in our research. The optimum dosage (2.0 mg/L) in their studies, therefore, was less than that of our study (5.0 mg/L) and the overdose pheromone of chitosan still exited in their study. They also did not consider the problem of sludge volume while the turbidity of the raw water in their study was not high enough to produce so huge amounts of sludge.
Coagulants
Al (low dose)
Al (high dose)
Chitosan
Al + Chitosan
Al dose Chitosan dose Residual turbidity Sludge volume Residual Al
13.5 mg/L None 166 NTU
135 mg/L None 40.6 NTU
None 5 mg/L 50.5 NTU
13.5 mg/L 5 mg/L 10.0 NTU
44 mL/L 236 lg/L
150 mL/L 446 lg/L
32 mL/L ND
33 mL/L 60.4 lg/L
3.3. Particle size with different coagulants Fig. 4 shows the size distribution of the particles after coagulation with different types of coagulants. The addition of 13.5 mg/L as Al of aluminum salts did not make the particles aggregate, but the addition of 5 mg/L of chitosan did. The particles aggregated to form larger flocs (dm > 100 lm) after coagulation with the coagulant mixture. Some small particles, whose diameter were smaller than 1.0 lm (dm < 1.0 lm), still exited after aluminum or chitosan coagulation. They were the main source of the residual turbidity. Almost no small particles existed after coagulation with coagulant mixture. The combined use of aluminum and chitosan can coagulate almost all of the small particles and make the flocs larger. In addition to generating hydroxide precipitates, adding multivalent metal ions, such as aluminum ions, can compress the electrical double layer or neutralize the negative charge of the colloid surface, and, hence, decrease the exclusive force between the colloids, resulting in a decrease of the potential barrier. With a reduced potential barrier, colloids can come together and aggregate, and, hence, a chitosan molecule can attach to more colloids. The size of the floc, therefore, increases significantly. Similar inference was stated in Altaher’s study [33]. He used chitosan to treat turbid sea water and distilled water and found the turbidity removal efficiency of sea water was much better than that of distilled water. Since the ionic strength of sea water is much higher than distilled water, the compression of the electrical double layer may be the main reason of the improvement of effectiveness in sea water. 3.4. Residual aluminum Another advantage of use the coagulant mixture is the reduction of residual aluminum concentrations. Figs. 5 and 6 show the residual concentrations after aluminum coagulation and aluminum–chitosan coagulation, respectively. As the aluminum dosage increased, the residual aluminum concentration rose to 500 lg/L if the aluminum salt was used alone. It is over the limit
50 Turbidity Sludge V
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0
Sludge Volume (ml/L)
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50
Table 1 Comparison of the effectiveness of coagulation between different coagulants.
0 0
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35
Chitosan dosage (mg/L) Fig. 3. Variation of residual turbidity and sludge volume per liter of water after coagulation with chitosan dosage and aluminum salt. (Initial turbidity = 10,000 NTU, aluminum dosage = 13.5 mg/L, settling time = 30 min.)
Fig. 4. The size distribution of particles with different coagulants (aluminum dosage = 13.5 mg/L, chitosan dosage = 5 mg/L).
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Residual Al con. (µ g/L)
700 600 500 400 300 200 100 0 0
50
100
150
200
Al dosage (mg/L) Fig. 5. Variation of the residual aluminum concentrations after coagulation with different dosages of aluminum salt. (Initial turbidity = 10,000 NTU, settling time = 30 min.)
Fig. 7. Variation of residual turbidity and sludge volume per liter of Zhi-tan water purification plant raw water during typhoon Megi after coagulation with chitosan and aluminum chloride. (Initial turbidity = 4817 NTU, aluminum dosage = 10.0 mg/ L, settling time = 30 min.)
Residual Al con. ( µ g/L)
250 200 150 100 50 0
0
2
4
6
8
10
12
Chitosan dosage (mg/L) Fig. 6. Variation of residual aluminum concentration after coagulation with different dosages of chitosan. (Initial turbidity = 10,000 NTU, aluminum dosage = 13.5 mg/L, settling time = 30 min.)
Table 2 Water quality parameters of the raw water collected from Zhi-tan water purification plant during typhoon Megi. Parameters
Values
Temperature (°C) pH Turbidity (NTU) Alkalinity (mg/L as CaCO3) Conductivity (lS/cm)
26.8 7.12 4817 13.6 65.0
suggested by USEPA (200 lg/L) [34] and may increase the risk of Alzheimer’s disease [1]. Chitosan is an adsorbent for metal ions [21–24]. The amine groups of chitosan can adsorb metal cations in near neutral pH [35]. The residual aluminum concentration after coagulation by the coagulant mixture, therefore, decreased from 200 to 50 lg/L as the chitosan dosage increased. Hence, treating high turbid raw water by coagulation with chitosan can significantly decrease the rise of Alzheimer’s disease. Although chitosan can reduce the toxicity of hazardous metal ions in water, the total organic carbon (TOC) after coagulation with chitosan may increase because the chitosan itself is a kind of organic compound. The increase of TOC may lead to the formation of trihalomethanes or other disinfection by-products after chorine disinfection [14]. Rizzo et al. compare the toxicity for Daphnia magna between chitosan and metal salts after chlorination [36]. They found the water with chitosan coagulation had a higher toxicity than that with metal salt coagulation. The use of chitosan as a coagulant, therefore, needs to consider the risk of disinfection by-products formation.
Fig. 8. Variation of residual aluminum concentration Zhi-tan water purification plant raw water after coagulation with different dosages of chitosan and aluminum chloride. (Initial turbidity = 10,000 NTU, aluminum dosage = 10.0 mg/L, settling time = 30 min.)
3.5. Application on actual high turbid raw water The raw water of Zhi-tan water purification plant during typhoon Megi (international designation: 1013, October 21, 2010) was used to demonstrate the effectiveness of the combined procedures. Table 2 shows the characteristics of the raw water. The turbidity (4817 NTU) and alkalinity (13.6 mg/L as CaCO3) of the raw water were both less than those of model water. The aluminum dosage (10.0 mg/L as Al) and the dissolved aluminum (69.9 lg/L), therefore, were both less than those of model water. The results (Figs. 7 and 8) showed the turbidity and the concentration of dissolved aluminum decreased from 139 to 4 NTU and 70 to 12 lg/L after the addition of chitosan from 0 to 3 mg/L. The sludge volume was around 30 mL per liter of raw water and did not increase with the increase of chitosan dose. This result reveals the combination usage of metal salts and chitosan can also effectively treat the actual high turbid surface water. 4. Conclusion Coagulation with a mixture of coagulants (Chitosan + Aluminum chloride) can effectively treat extremely turbid raw water (turbidity = 10,000 NTU). The residual turbidity was under 10 NTU and the sludge volume ratio was around 30–40 mL per liter of raw water. The residual aluminum concentration after coagulation with the coagulant mixture was much less than coagulation with aluminum salt alone. The above results suggest that greater economic and environmental benefits can be obtained if this
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coagulant mixture is used to replace the traditional coagulant in high turbid water treatment. Further researches may need to done to clarify the risk of the disinfection by products after chlorination of this coagulation process.
Acknowledgment The authors would like to thank the Water resource agency of the Chinese Taiwan for financially supporting this research under Contract No. MOEAWAR 0980036.
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Contents lists available at SciVerse ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Treatment of highly turbid water using chitosan and aluminum salts Ching-Yao Hu a, Shang-Lien Lo b,⇑, Chia-Ling Chang c, Fu-Ling Chen b, Yu-De Wu a, Jia-lin Ma d a
School of Public Health, Taipei Medical University, 250 Wu-Xin Street, Taipei 110, Taiwan, ROC Graduate Institute of Environmental Engineering, National Taiwan University, Taipei 106, Taiwan, ROC c Department of Water Resources Engineering and Conservation, Feng Chia University, No. 100 Wenhwa Rd., Seatwen, Taichung 40724, Taiwan, ROC d Water Resource Agency, Ministry of Economic Affairs, 9-12F 41-3 Sec.3 Hsin-yi Rd., Taipei, Taiwan 10651, Taiwan, ROC b
a r t i c l e
i n f o
Article history: Received 20 August 2012 Received in revised form 6 November 2012 Accepted 14 November 2012 Available online 4 December 2012 Keywords: Turbidity Sludge Aluminum chloride Chitosan Residual aluminum concentration
a b s t r a c t The high turbidity in surface water may make it difficult for water treatment plants to supply drinking water. Chitosan, a natural linear cationic polymer, and aluminum chloride, a metal salt, and the mixture of the two coagulants were used to treat highly turbid raw water in this study according to the residual turbidity, sludge volume and residual aluminum concentration. The residual turbidity was less than 50 NTU but the sludge/water volume ratio was over 150 mL/L after aluminum salt coagulation (135 mg/L as Al), which could stop the sedimentation process. The amount of sludge produced after chitosan coagulation (5 mg/L) was only about 1/5 of that for aluminum coagulation for the similar turbidity removal. Chitosan coagulation, however, still has two problems that need to be solved. First, the residual turbidity of treated water is still too high for sand filtration. Second, the colloid particles may restabilize if chitosan is overdosed. Adding a comparative low dosage of aluminum salt (13.5 mg/L as Al) with chitosan can successfully solve both of the problems. The sludge volume ratio only increased slightly and the residual turbidity was less than 10 NTU. Moreover, the restabilization of colloids did not occur. The residual aluminum concentration, which could lead to Alzheimer’s disease, can also be reduced significantly after addition of chitosan. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The turbidity of river water in Taiwan and other tropical areas occasionally rises to over 10,000 NTU during typhoons, hurricanes, or rainstorms. This makes it difficult for water purification plants to supply useable water. A lack of water supply may cause sanitary problems and interfere with manufacturing activity which could lead to huge economic damage. How to treat highly turbid water efficiently and economically, therefore, is an important issue for environmental engineers. Coagulation with metal salts, such as aluminum or iron salts, can effectively increase the size of the particles in water, after which they can be removed by sedimentation. This process, however, may produce large amounts of sludge because of the formation of metal hydroxides if large amounts of metal salt are added to treat high turbid water. The water purification process sometimes has to stop in Taiwan due to too much sludge in the sedimentation tank during typhoons. Moreover, the residual aluminum concentration in the water may increase due to the increase of aluminum dose. It may raise the risk of Alzheimer’s disease [1].
⇑ Corresponding author. Tel.: +886 2 23625373; fax: +886 2 23928821. E-mail address: [email protected] (S.L. Lo). 1383-5866/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2012.11.016
Synthetic polymers, such as poly acryl-amide and poly diallyldimethyl ammonium chloride, are alternative coagulants used to remove turbidity in water. They can reduce the turbidity of water but do not increase the volume of sludge because they do not generate metal hydroxides. However, health concerns related to the release of carcinogenic oligomer [2–5] restricts their use. Chitosan, which is a natural linear cationic polymer generated by extensive deacetylation of chitin, is a bio-degradable coagulant and has been used to treat colloidal particles [6–14], COD [15–20], metal ions [16,21–24], humic acids [25], cryptosporidium cysts [26] and other bio-microspheres [27]. It has similar properties to synthetic polymers but has a relatively low toxicity for humans and aquatic species. Moreover, it contains amino and hydroxyl functional groups, which show significant adsorption capacity for various pollutants. Divakaran and Sivasankara Pillai [8] have successfully used chitosan to reduce the turbidity of water. The turbidity removal efficiency was over 90% in their study but the turbidity of the raw water was lower than the target water in this study. Sekine and coworkers used chitosan to treat turbid river water [11]. Their results showed that the turbidity removal efficiency was good when the turbidity was under 1000 NTU, but the coagulation process failed if the turbidity was over 1000 NTU. The use of monocoagulants, such as chitosan, therefore, may not be a good solution for highly turbid water purification. New coagulants or coagulation
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2. Materials and methods
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2.3. Coagulation test A jar test apparatus was used for the coagulation and flocculation experiments. For all experiments, the pH of the suspension was adjusted by adding a 0.1 M NaOH or 0.1 M HCl (pH = 7.5 ± 0.1). After the coagulants were added, the water was rapidly mixed (at 100 rpm) for 3 min followed by 20 min of slow mixing (at 30 rpm). After standing for 30 min, the supernatant from the top 5 cm of the suspension was withdrawn for turbidity and residual aluminum measurement. 2.4. Analytical measurements The turbidity of each sample was measured by a turbidity meter (HACH Ratio, USA). The sludge volume was measured using a quiescent column. Residual aluminum ions were analyzed by a TJA IRIS Advantage/1000 Radial ICP spectrometer calibrated against a CLARITAS certified reference solution (SPEX Certi Prep, Inc). A particle size analyzer (Model Zetasizer 3000, Malvern Instrument Ltd., Worcester, UK) was used to determine the particle size distribution before and after coagulation. 3. Results and discussion 3.1. Jar test with different coagulants Figs. 1 and 2 show the variations of residual turbidity and the water-sludge volume ratio after aluminum and chitosan coagulation, respectively. The residual turbidity decreased with increasing coagulant dosage for aluminum coagulation but had an optimum dosage (5 mg/L) for chitosan coagulation. This is due to the different mechanisms of the two coagulants. The mechanisms for aluminum coagulation are compression of the electrical double layer, charge neutralization and flocs sweeping. The main mechanism
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Model waters with the desired turbidity were prepared by mixing given amounts of clay or bentonite (Hayashi Co., Japan) with deionized water. NaHCO3 was added to produce the alkalinity at 50 mg/L as CaCO3.
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Fig. 1. Variation of residual turbidity and sludge volume per liter of water after coagulation with aluminum chloride. (Initial turbidity = 10,000 NTU, settling time = 30 min.)
Residual Turbidity (NTU)
2.2. Preparation of model water
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Al dosage (mg/L)
2.1. Preparation of coagulants Chitosan powder purchased from Wako Pure Chemicals (Tokyo, Japan) was dissolved in 1.0% chloride acid solution by stirring overnight at 150 rpm to make 1.0% (w/w) stock solution. Aluminum salt stock solution was prepared by dissolving 133.5 g of aluminum chloride (AlCl3) in 1 L deionized water ([Al3+] = 1.0 M). Prior to the coagulant mixture experiments, both of the two stock solutions were added to the deionized water and carefully mixed with magnetic stirring to prepare a coagulant mixture solution of desired concentration.
Sludge Volume (ml/L)
Residual Turbidity (NTU)
processes need to be investigated to solve these problems. This study tried to use chitosan with aluminum salt to form a mixture coagulant to treat highly turbid water (over 1000 NTU). The residual turbidity, sludge volume, and residual aluminum concentration were investigated after sedimentation to access the applicability of the coagulant mixture. Finally, a coagulation test using the raw water of Zhi-tan water purification plant during typhoon Megi was performed to examine the effectiveness of the combined procedure.
0 0
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Chitosan dosage (mg/L) Fig. 2. Variation of residual turbidity and sludge volume per liter of water after coagulation with chitosan. (Initial turbidity = 10,000 NTU, settling time = 30 min.)
in this study should be flocs sweeping because the aluminum hydroxide must appear at high aluminum dosage. The addition of large amounts of aluminum salt can produce aluminum hydroxide flocs (as shown in reaction (1)) which settle by gravity, within a reasonable time. þ3
nAlðaqÞ þ 3nOHðaqÞ ! Aln ðOHÞ3nðsÞ
Ksp ¼ 1032 M4
ð1Þ
These coagulant flocs then collide with and drag colloids down with them. The more flocs produced, the better the turbidity removal. The removal of turbidity, therefore, increased with increasing aluminum dosage. The addition of chitosan; however, did not produce hydroxide flocs. The functional groups on the chitosan combine with the active sites of particles and the interaction of a single molecule with many particles produces a bridging effect. This combines them into larger particles which settle under the action of gravity. When a larger amount of chitosan was added; however, the effect of bridging coagulation became relatively weak and the colloids restabilized, possibly due to steric hindrance and repulsion between charged polymers [28,29]. Similar phenomenon also occurred in previous studies which use chitosan to treat river water [6,8,14]. The optimum dose of chitosan (1.0–2.0 mg/L) in their works were less than that (5.0 mg/L) in this study. This phenomenon may be due to the higher turbidity. Chatterjee et al. [30] observe the same optimum dose of chitosan and the initial turbidity in their work was also extremely high (SS = 5.0 g/L). The amount of sludge produced by aluminum coagulation was proportional to the amount of aluminum salt added. Although aluminum hydroxide floc can sweep the colloids in the water, the floc itself is a kind of sludge. The sludge/water volume ratio may rise to 15% if the dosage used for the same turbidity removal as chitosan is
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added. The amount of sludge produced by chitosan coagulation was much less than aluminum coagulation and it did not increase with the dosage of chitosan. Chitosan, therefore, may be more suitable than metal salts for high turbidity raw water treatment. 3.2. Jar test with mixture of coagulants Although chitosan may be a better coagulant for high turbidity raw water treatment, the residual turbidity, after chitosan coagulation, was still too high for the sand filtration process. The backwash frequency increases significantly due to high turbidity. The amount of water supplied may reduce because of the use of too much water for the backwash. The residual turbidity after chitosan coagulation was over 50 NTU, while the appropriate turbidity for rapid sand filtration should be less than 10 NTU [31]. Combining the two coagulants, metal salt and chitosan, may solve this problem. Therefore, aluminum salt and chitosan were mixed together to form a mixture of coagulants. Fig. 3 shows the variations of residual turbidity and sludge volume per liter of water after coagulation with different chitosan dosages and 13.5 mg/L as Al of aluminum salt (the lowest dosage in Fig. 1), which was a relatively low dosage of aluminum in this study and did not produce too much sludge. The residual turbidity was less than 10 NTU after the chitosan dosage was over 5 mg/L and the sludge/water volume ratio of water was only 30–40 mL/L. Moreover, the colloids did not restabilize after coagulation. Comparing to the single coagulant system (as shown in Table 1), coagulation with the chitosan + aluminum salt mixture coagulant got a lower residual turbidity and produced comparatively lower sludge volume. A combination usage of metal salts and polymers may be a promising method for treating extremely turbid raw water. Zemmouri et al. also used the chitosan and aluminum salt to treat surface water [32]. They found the combination usage of the two coagulants have a better removal efficiency than usage of the coagulants alone. They stated that the turbidity removal efficiency of chitosan was highly influenced by the initial turbidity. Chitosan was not suitable to use as a primary coagulant to treat raw water with low turbidity (about 5 NTU) but can effectively remove turbidity when it is used as an aid coagulant to the aluminum salt as the main coagulant. Their findings are similar to our results. The initial turbidities (5–150 NTU) in the raw water of their researches, however, were much lower than that in our research. The optimum dosage (2.0 mg/L) in their studies, therefore, was less than that of our study (5.0 mg/L) and the overdose pheromone of chitosan still exited in their study. They also did not consider the problem of sludge volume while the turbidity of the raw water in their study was not high enough to produce so huge amounts of sludge.
Coagulants
Al (low dose)
Al (high dose)
Chitosan
Al + Chitosan
Al dose Chitosan dose Residual turbidity Sludge volume Residual Al
13.5 mg/L None 166 NTU
135 mg/L None 40.6 NTU
None 5 mg/L 50.5 NTU
13.5 mg/L 5 mg/L 10.0 NTU
44 mL/L 236 lg/L
150 mL/L 446 lg/L
32 mL/L ND
33 mL/L 60.4 lg/L
3.3. Particle size with different coagulants Fig. 4 shows the size distribution of the particles after coagulation with different types of coagulants. The addition of 13.5 mg/L as Al of aluminum salts did not make the particles aggregate, but the addition of 5 mg/L of chitosan did. The particles aggregated to form larger flocs (dm > 100 lm) after coagulation with the coagulant mixture. Some small particles, whose diameter were smaller than 1.0 lm (dm < 1.0 lm), still exited after aluminum or chitosan coagulation. They were the main source of the residual turbidity. Almost no small particles existed after coagulation with coagulant mixture. The combined use of aluminum and chitosan can coagulate almost all of the small particles and make the flocs larger. In addition to generating hydroxide precipitates, adding multivalent metal ions, such as aluminum ions, can compress the electrical double layer or neutralize the negative charge of the colloid surface, and, hence, decrease the exclusive force between the colloids, resulting in a decrease of the potential barrier. With a reduced potential barrier, colloids can come together and aggregate, and, hence, a chitosan molecule can attach to more colloids. The size of the floc, therefore, increases significantly. Similar inference was stated in Altaher’s study [33]. He used chitosan to treat turbid sea water and distilled water and found the turbidity removal efficiency of sea water was much better than that of distilled water. Since the ionic strength of sea water is much higher than distilled water, the compression of the electrical double layer may be the main reason of the improvement of effectiveness in sea water. 3.4. Residual aluminum Another advantage of use the coagulant mixture is the reduction of residual aluminum concentrations. Figs. 5 and 6 show the residual concentrations after aluminum coagulation and aluminum–chitosan coagulation, respectively. As the aluminum dosage increased, the residual aluminum concentration rose to 500 lg/L if the aluminum salt was used alone. It is over the limit
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Table 1 Comparison of the effectiveness of coagulation between different coagulants.
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Chitosan dosage (mg/L) Fig. 3. Variation of residual turbidity and sludge volume per liter of water after coagulation with chitosan dosage and aluminum salt. (Initial turbidity = 10,000 NTU, aluminum dosage = 13.5 mg/L, settling time = 30 min.)
Fig. 4. The size distribution of particles with different coagulants (aluminum dosage = 13.5 mg/L, chitosan dosage = 5 mg/L).
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Residual Al con. (µ g/L)
700 600 500 400 300 200 100 0 0
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Al dosage (mg/L) Fig. 5. Variation of the residual aluminum concentrations after coagulation with different dosages of aluminum salt. (Initial turbidity = 10,000 NTU, settling time = 30 min.)
Fig. 7. Variation of residual turbidity and sludge volume per liter of Zhi-tan water purification plant raw water during typhoon Megi after coagulation with chitosan and aluminum chloride. (Initial turbidity = 4817 NTU, aluminum dosage = 10.0 mg/ L, settling time = 30 min.)
Residual Al con. ( µ g/L)
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12
Chitosan dosage (mg/L) Fig. 6. Variation of residual aluminum concentration after coagulation with different dosages of chitosan. (Initial turbidity = 10,000 NTU, aluminum dosage = 13.5 mg/L, settling time = 30 min.)
Table 2 Water quality parameters of the raw water collected from Zhi-tan water purification plant during typhoon Megi. Parameters
Values
Temperature (°C) pH Turbidity (NTU) Alkalinity (mg/L as CaCO3) Conductivity (lS/cm)
26.8 7.12 4817 13.6 65.0
suggested by USEPA (200 lg/L) [34] and may increase the risk of Alzheimer’s disease [1]. Chitosan is an adsorbent for metal ions [21–24]. The amine groups of chitosan can adsorb metal cations in near neutral pH [35]. The residual aluminum concentration after coagulation by the coagulant mixture, therefore, decreased from 200 to 50 lg/L as the chitosan dosage increased. Hence, treating high turbid raw water by coagulation with chitosan can significantly decrease the rise of Alzheimer’s disease. Although chitosan can reduce the toxicity of hazardous metal ions in water, the total organic carbon (TOC) after coagulation with chitosan may increase because the chitosan itself is a kind of organic compound. The increase of TOC may lead to the formation of trihalomethanes or other disinfection by-products after chorine disinfection [14]. Rizzo et al. compare the toxicity for Daphnia magna between chitosan and metal salts after chlorination [36]. They found the water with chitosan coagulation had a higher toxicity than that with metal salt coagulation. The use of chitosan as a coagulant, therefore, needs to consider the risk of disinfection by-products formation.
Fig. 8. Variation of residual aluminum concentration Zhi-tan water purification plant raw water after coagulation with different dosages of chitosan and aluminum chloride. (Initial turbidity = 10,000 NTU, aluminum dosage = 10.0 mg/L, settling time = 30 min.)
3.5. Application on actual high turbid raw water The raw water of Zhi-tan water purification plant during typhoon Megi (international designation: 1013, October 21, 2010) was used to demonstrate the effectiveness of the combined procedures. Table 2 shows the characteristics of the raw water. The turbidity (4817 NTU) and alkalinity (13.6 mg/L as CaCO3) of the raw water were both less than those of model water. The aluminum dosage (10.0 mg/L as Al) and the dissolved aluminum (69.9 lg/L), therefore, were both less than those of model water. The results (Figs. 7 and 8) showed the turbidity and the concentration of dissolved aluminum decreased from 139 to 4 NTU and 70 to 12 lg/L after the addition of chitosan from 0 to 3 mg/L. The sludge volume was around 30 mL per liter of raw water and did not increase with the increase of chitosan dose. This result reveals the combination usage of metal salts and chitosan can also effectively treat the actual high turbid surface water. 4. Conclusion Coagulation with a mixture of coagulants (Chitosan + Aluminum chloride) can effectively treat extremely turbid raw water (turbidity = 10,000 NTU). The residual turbidity was under 10 NTU and the sludge volume ratio was around 30–40 mL per liter of raw water. The residual aluminum concentration after coagulation with the coagulant mixture was much less than coagulation with aluminum salt alone. The above results suggest that greater economic and environmental benefits can be obtained if this
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coagulant mixture is used to replace the traditional coagulant in high turbid water treatment. Further researches may need to done to clarify the risk of the disinfection by products after chlorination of this coagulation process.
Acknowledgment The authors would like to thank the Water resource agency of the Chinese Taiwan for financially supporting this research under Contract No. MOEAWAR 0980036.
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