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The characterization and water purification behavior of gradient ceramic membranes
Separation and Purification Technology 32 (2003) 73 /79 www.elsevier.com/locate/seppur
The characterization and water purification behavior of gradient ceramic membranes Xiandong Sui a, Xiaorong Huang b,* a
b
Mechanical Engineering College, South China University of Technology, Guangzhou, China Chemical Engineering College, South China University of Technology, Guangzhou 510640, China
Abstract In this paper, the characterization of gradient diatomaceous earth membranes prepared by centrifugal casting was investigated. The size of the ceramic particles change gradually from large to small along the radical cross-section of the tubular membrane, so the pore size of the tubular membrane change gradually from large to small along the radical direction. There are hundreds of layers of different size ceramic particles along the radical cross-section of the membrane tube. The gradient membrane offers high flux, high porosity with narrow pore size distribution and defectfree. The average pore size is 0.15 mm and the maximum do not exceed 0.3 mm. The water purification tests showed that this kind of ceramic membranes perform good behaviors: 100% of the pathogenetic bacterium such as E. coli , Salmonelle, microzyme, staphylotoxin, Pseudomonas aeruginosa and mold, etc. in water can be removed, and 100% of rust, worm and suspension particles in water also can be removed. The gradient ceramic membranes have a good flux efficiency recovery just by simple mechanical brushing. The tests showed that a gradient ceramic membrane filter in 225 mm length, 55 mm outer diameter and 7 mm thickness can filtrate more than 50 M3 of drinking water. # 2003 Published by Elsevier Science B.V. Keywords: Gradient ceramic membrane; Membrane characterization; Diatomaceous earth; Water purification; Ceramic membrane
1. Introduction The asymmetric multi-layer structure has often been used for preparation of ceramic membranes. The support layer provides the mechanical strength with larger pore size and several millimeter in thickness, control layer is the last top layer with very tiny pore size, between them are transition layers. Ceramic membranes with asymmetric multi-layer structure have following shortages, first, their cost is high due to the * Corresponding author. Tel.: /86-20-8711-2390. E-mail address: [email protected] (X. Huang).
multi-layer forming and sintering process, and secondly, defects occur easily between layers because of the ceramic particle size changing suddenly and easy peel off from last support layer. Those shortages especially the cost, limit their application. Industry and research departments are searching new ceramic membrane which can be manufactured in one-step fabrication process, gradient ceramic membrane is the best choice for this purpose because ceramic particle size is changing gradually. Perfect ceramic membranes should satisfy both cost and structure requirements, that is the cost
1383-5866/03/$ - see front matter # 2003 Published by Elsevier Science B.V. doi:10.1016/S1383-5866(03)00057-1
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X. Sui, X. Huang / Separation and Purification Technology 32 (2003) 73 /79
should be as low as possible, and the structure of the membranes should be no visible layers border, thus the peel off between layers can be effectively eliminated. That is to say the best structure of the ceramic membranes is no layers border. Some researchers are working on gradient ceramic membranes field in recent years. Darcovich [1] developed a method to cast functionally gradient porous ceramics by sedimentation. Ceramic disks /45 mm in diameter and 3.3 mm thick were shown to have a functionally gradient crosssection with a continuously increasing mean particle size from top to bottom. Nijmeijer [2] used centrifugal casting to produce tubular membrane supports with 13 mm in length and 16 mm in diameter from a suspension of submicron aalumina powder. Biesheuvel [4] prepared graded membrane supports by using centrifugal casting method. The mean particle size in the final structure increases gradually with the increase of radical coordinate. Steenkamp [3] prepared the membrane tube with the pore size of 50 nm and 30% porosities by using different powder particle sizes and powder mixtures. Centrifugal casting is an effective way to manufacture hollow cylindrical products with a solid and defect-free structure such as iron pipes, cylinders. The ceramic membrane or membrane supports prepared by centrifugal casting have following advantages. (1) Without visible layers border. Because the size of the ceramic particles changes gradually from large to small along the radical cross-section of the tube, the layers border cannot be seen. (2) Without defects inside the membrane tube. The ceramic particles were forced to accumulate and form the membrane tube one particle after another under the centrifugal pressure, no hole or other structure shortages can be seen across the section of the membrane tube. (3) Low cost. The ceramic membrane made by centrifugal casting is cheaper than extrusion. Two reasons can explain these: first, the membrane tube forms from a one-step process which do not need multi time forming and sintering process; secondly, requirement for the ceramic particles used to prepare membrane is not so strict comparing with the common extrusion method. The size of
the ceramic particles can be variable at a very large wide range. This kind of ceramic powder is easy to get and the cost is rather low. Huang [5] prepared a-alumina membrane with the ceramic particles size from 0.1 to 40 mm. (4) The inner surface of the membrane tube is very smooth and has a very uniform and regular pore structure.
2. Experimental
2.1. The gradient ceramic membrane element The gradient ceramic membranes which were used for characterization and water filtration performance were commercialized ceramic filter cartridges manufactured by Springhead Industry and Commerce Co., Ltd (Shunde China) shown in Fig. 1. The filter element was 225 mm in length, outer diameter 55 mm and thickness 7 mm. Mean pore size was 0.15 mm, maximum is 0.28, porosity is 68%. The control layer is the inner surface of the ceramic membrane filter.
Fig. 1. Industry scale production of the gradient ceramic membrane tubes.
X. Sui, X. Huang / Separation and Purification Technology 32 (2003) 73 /79
2.2. Characterization of the gradient ceramic membrane The structure of the diatomaceous earth (DE) has been investigated using JEM 100CXII conventional transmission emission microscope (TEM). Microstructure and fractured surface morphology of the ceramic membrane tube were characterized using XL30N, Philips and Hitachi S550 SEM. 2.3. Water permeability The municipal supply of drinking water was filtrated with dead end filtration mode. The operating pressure is between 0.1 and 0.3 MPa. The fouling of the membrane was resulted from deposition of the agglomerated contaminants at the surface of the outer layer of the membrane. The water flux efficiency of this kind of gradient ceramic membrane can be recovered almost entirely by the simple mechanical cleaning. The brush time could exceed 100 times. When the clog occurred, the filter element was pulled out, washing and brushing the cartridge surface in the small current of water with a hard wash cloth, the deposited pollutants on the cartridge surface can be removed. A fresh and clean surface appears after washing and brushing.
3. Results and discussion 3.1. Microstructure of the diatomaceous earth DE is a naturally occurring substance comprised of the fossilized remains of diatoms. Diatoms are microscopic sized hard shelled creatures found in both marine and fresh waters. They weave microscopic shells which they use for the protection and locomotion. These shells are covered with a pattern of tiny holes so regular that even the slightest change in their design usually signifies a different species. As the diatoms died, their shells survived, slowly piling up in deposits at the bottom of geological lakes and lagoons. When these lakes dried up, what remained were huge deposits of
75
‘diatomaceous earth’. The material is used extensively as an abrasive, a filtering material in food and beer industry, an inert ingredient of explosive, and an insulating material for boilers and steam pipes. DE also can be used as coatings for centrifugal casting [6]. Fig. 2 is the TEM pictures of the DE, (a) is morphology of DE (b) is a complete piece of DE, (c) is a special one in which every small hole contains several tiny secondary hole, the amplified image shown in Fig. 2(d). Fig. 3 is SEM of the DE. From Fig. 3(b), we can see that there are many nanometer size particles on the DE plate and part of them enter the tiny hole of the DE. 3.2. Microstructure of gradient ceramic membrane The tubular gradient ceramic membranes manufactured by centrifugal casting technique were made from DE. The movement of the particulate phase towards the mould wall results in the segregation of the ceramic particles, the size of the ceramic particles change gradually from large to small along the radical cross-section of the tubular membrane, so the pore size of the tubular membrane change gradually from large to small along the radical direction. Fig. 4(a) is the tubular membrane cross-section. The inner surface of the membrane tube is very smooth. There are hundreds layers of different size ceramic particles along the radical cross-section of the membrane tube and there is no visible layers border. Fig. 4(b) is gradient layers distribution of the membrane tube, the inner surface were formed by the finest particles and the outer surface were composed by the largest ceramic particles. Under the pressure of the centrifugal force the largest ceramic particles were forced to accumulate in the outer surface of the tube and formed the outer layer as shown in Fig. 5(a), and the middle size of the particles accumulated in the middle layer and formed the middle layer of the membrane as shown in Fig. 5(b), the finest particles accumulated in the inner layer and formed the control layer shown in Fig. 5(c). From Fig. 5, it is obvious that the more near to the outer surface, the bigger ceramic particle is. The control layer is the inner layer of the tube. The
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X. Sui, X. Huang / Separation and Purification Technology 32 (2003) 73 /79
Fig. 2. TEM morphology of the DE.
Fig. 3. SEM of the DE.
X. Sui, X. Huang / Separation and Purification Technology 32 (2003) 73 /79
77
Fig. 6. Uniform and regular pore size distribution of the inner surface of the membrane tube.
Fig. 4. Fractured cross-section and gradient layer distribution of the membrane tube.
gradient distribution of the ceramic particles resulted that the pore size of the membrane tube changes gradually from large to small. Route of the filtration is not only the gap among the particles but also the tiny hole inside the DE particle. The tiny hole of the DE particle inside join the gap among the particles, so the flux of the
DE membrane filter is very high and the porosity of the filter cartridge is very high more than 65%. The pore size distribution of the gradient ceramic membrane is rather narrow because the control layer (inner surface) of the membrane tube has a very uniform and regular pore structure shown in Fig. 6. Fig. 7 is the pore size distribution of the gradient ceramic membrane. Average pore size is 0.15 mm and maximum less than 0.3 mm.
3.3. Water filtration behavior of the ceramic membrane Membrane filtration, widely used in the industrial and food processing industries, is now considered a promising technology for water
Fig. 5. Gradient distribution of the size ceramic particles along the radical cross-section of the membrane tube.
X. Sui, X. Huang / Separation and Purification Technology 32 (2003) 73 /79
78
Fig. 7. Pore size distribution of the gradient ceramic membrane.
treatment. Depending upon membrane pore sizes and pressures, a variety of contaminants can be removed. The municipal supply of drinking water was purified with dead end filtration mode with an mean pore size 0.15 mm gradient ceramic membrane filter. Comparison of the water quality between raw water and filtered water after 50 M3 water filtration carried out have been shown in Table 1. The experiment results showed that 100% pathogenetic bacterium such as E. coli , Salmonelle, microzyme, staphylotoxin, Pseudomonas aeruginosa and mold, etc. can be removed by the gradient ceramic membrane. Even 200 000 cuf/ml pathogenic bacteria like E. coli , Salmonelle, staphylotoxin and Pseudomonas aeruginosa can be removed entirely. The experiment results also showed that the rust, parasites and suspension particles exceed 50 nm can be removed entirely. Safe and clean
Fig. 8. Filter element before and after filtration of 50 M3 drinking water.
drinking water, retaining its natural mineral content can be obtained without boiling. Fig. 8 shows the filter element before and after 50 M3 water filtration. It is clear that outer diameter of the filter element is reduced after 100 times mechanical brush. The diameter of original filter element was 54 mm and after 50 M3 water filtration, it became 48 mm, that is the thickness of the filter element reduced about 3 mm. Fig. 9 is the result of flux recovery of the ceramic membrane during 100 mechanical brush. Brushing of the gradient ceramic membrane filter was made once a time every 8 h. Flux changes of the ceramic membrane were remarkable in the initial several brush. The flux of the membrane tended to stable from 10 to 100 times brushing. The amount of the flux kept at about 1 m3/(m2 h bar), flux was just about 50% comparing to the beginning use of the membrane filter. Shown in Fig. 9 is the flux recovery of the ceramic membrane after mechanical brushing. It can be seen from the figure that the ceramic membrane
Table 1 Comparison of the water quality before and after filtration Water
Before filtration
After filtration
/50 nm particles (mg/l) Total of colony (cfu/ml)
56 600
0 0
Mold
With
Without
200 000/ml
Without
Pathogenic bacteriaa
a
Added.
E. coli Salmonelle Staphylotoxin Pseudomonas aeruginosa
X. Sui, X. Huang / Separation and Purification Technology 32 (2003) 73 /79
79
Fig. 9. Flux recovery of the ceramic membrane after 100 times mechanical brush (total filtrated water amount was 50 M3).
filter has a good flux recovery behavior after simple mechanical brush.
4. Conclusions Gradient ceramic membrane could be prepared by centrifugal casting. The inner surface was formed by the finest particles and the outer surface was composed by the largest ceramic particles, there is no layers border between them. Pore size of the tubular membrane changed gradually from large to small along the radical direction due to the segregation of the ceramic particles. The inner surface of the membrane tube is very smooth and the pore size distribution of the inner surface is very uniform and regular. The pore size distribution of the gradient ceramic membrane is rather narrow, average pore size is 0.15 mm and maximum less than 0.3 mm. Gradient DE membrane filter performs a good filtration behavior for drinking water. Hundred percent of pathogenetic bacterium such as E. coli , Salmonelle, microzyme, staphylotoxin, Pseudomo-
nas aeruginosa and mold, etc. in water can be removed by the filtration of the membrane filter. The rust, parasites and suspension particles more than 50 nm can be removed entirely. Safe and clean drinking water, retaining its natural mineral content can be obtained without boiling. Mechanical brush is an effective method to recover flux of gradient DE membrane filter. Thickness of the filter element reduced about 3 mm after 100 times mechanical brush, and the water filtration amount could exceed 50 M3.
References [1] K. Darcovich, C.R. Cloutier, J. Am. Ceram. Soc. 82 (1999) 8. [2] A. Nijmeijer, C. Huiskes, G.M. Natascha, et al., Am. Ceram. Soc. Bull. 77 (4) (1998) 95. [3] G.C. Steenkamp, H.W.J.P. Neomagus, H.M. Krieg, K. Keizer, Sep. Purif. Technol. 25 (2001) 407 /413. [4] P.M. Bieseuvel, V. Breedveld, A.P. Higler, H. Verweij, Chem. Eng. Sci. 56 (2001) 3517 /3525. [5] X. Huang, Z. Huang, Membr. Sci. Technol. 16 (2) (1996) 31 /36 (in Chinese). [6] X. Sui, Foundry 6 (1992) 1 /5 (in Chinese).
The characterization and water purification behavior of gradient ceramic membranes Xiandong Sui a, Xiaorong Huang b,* a
b
Mechanical Engineering College, South China University of Technology, Guangzhou, China Chemical Engineering College, South China University of Technology, Guangzhou 510640, China
Abstract In this paper, the characterization of gradient diatomaceous earth membranes prepared by centrifugal casting was investigated. The size of the ceramic particles change gradually from large to small along the radical cross-section of the tubular membrane, so the pore size of the tubular membrane change gradually from large to small along the radical direction. There are hundreds of layers of different size ceramic particles along the radical cross-section of the membrane tube. The gradient membrane offers high flux, high porosity with narrow pore size distribution and defectfree. The average pore size is 0.15 mm and the maximum do not exceed 0.3 mm. The water purification tests showed that this kind of ceramic membranes perform good behaviors: 100% of the pathogenetic bacterium such as E. coli , Salmonelle, microzyme, staphylotoxin, Pseudomonas aeruginosa and mold, etc. in water can be removed, and 100% of rust, worm and suspension particles in water also can be removed. The gradient ceramic membranes have a good flux efficiency recovery just by simple mechanical brushing. The tests showed that a gradient ceramic membrane filter in 225 mm length, 55 mm outer diameter and 7 mm thickness can filtrate more than 50 M3 of drinking water. # 2003 Published by Elsevier Science B.V. Keywords: Gradient ceramic membrane; Membrane characterization; Diatomaceous earth; Water purification; Ceramic membrane
1. Introduction The asymmetric multi-layer structure has often been used for preparation of ceramic membranes. The support layer provides the mechanical strength with larger pore size and several millimeter in thickness, control layer is the last top layer with very tiny pore size, between them are transition layers. Ceramic membranes with asymmetric multi-layer structure have following shortages, first, their cost is high due to the * Corresponding author. Tel.: /86-20-8711-2390. E-mail address: [email protected] (X. Huang).
multi-layer forming and sintering process, and secondly, defects occur easily between layers because of the ceramic particle size changing suddenly and easy peel off from last support layer. Those shortages especially the cost, limit their application. Industry and research departments are searching new ceramic membrane which can be manufactured in one-step fabrication process, gradient ceramic membrane is the best choice for this purpose because ceramic particle size is changing gradually. Perfect ceramic membranes should satisfy both cost and structure requirements, that is the cost
1383-5866/03/$ - see front matter # 2003 Published by Elsevier Science B.V. doi:10.1016/S1383-5866(03)00057-1
74
X. Sui, X. Huang / Separation and Purification Technology 32 (2003) 73 /79
should be as low as possible, and the structure of the membranes should be no visible layers border, thus the peel off between layers can be effectively eliminated. That is to say the best structure of the ceramic membranes is no layers border. Some researchers are working on gradient ceramic membranes field in recent years. Darcovich [1] developed a method to cast functionally gradient porous ceramics by sedimentation. Ceramic disks /45 mm in diameter and 3.3 mm thick were shown to have a functionally gradient crosssection with a continuously increasing mean particle size from top to bottom. Nijmeijer [2] used centrifugal casting to produce tubular membrane supports with 13 mm in length and 16 mm in diameter from a suspension of submicron aalumina powder. Biesheuvel [4] prepared graded membrane supports by using centrifugal casting method. The mean particle size in the final structure increases gradually with the increase of radical coordinate. Steenkamp [3] prepared the membrane tube with the pore size of 50 nm and 30% porosities by using different powder particle sizes and powder mixtures. Centrifugal casting is an effective way to manufacture hollow cylindrical products with a solid and defect-free structure such as iron pipes, cylinders. The ceramic membrane or membrane supports prepared by centrifugal casting have following advantages. (1) Without visible layers border. Because the size of the ceramic particles changes gradually from large to small along the radical cross-section of the tube, the layers border cannot be seen. (2) Without defects inside the membrane tube. The ceramic particles were forced to accumulate and form the membrane tube one particle after another under the centrifugal pressure, no hole or other structure shortages can be seen across the section of the membrane tube. (3) Low cost. The ceramic membrane made by centrifugal casting is cheaper than extrusion. Two reasons can explain these: first, the membrane tube forms from a one-step process which do not need multi time forming and sintering process; secondly, requirement for the ceramic particles used to prepare membrane is not so strict comparing with the common extrusion method. The size of
the ceramic particles can be variable at a very large wide range. This kind of ceramic powder is easy to get and the cost is rather low. Huang [5] prepared a-alumina membrane with the ceramic particles size from 0.1 to 40 mm. (4) The inner surface of the membrane tube is very smooth and has a very uniform and regular pore structure.
2. Experimental
2.1. The gradient ceramic membrane element The gradient ceramic membranes which were used for characterization and water filtration performance were commercialized ceramic filter cartridges manufactured by Springhead Industry and Commerce Co., Ltd (Shunde China) shown in Fig. 1. The filter element was 225 mm in length, outer diameter 55 mm and thickness 7 mm. Mean pore size was 0.15 mm, maximum is 0.28, porosity is 68%. The control layer is the inner surface of the ceramic membrane filter.
Fig. 1. Industry scale production of the gradient ceramic membrane tubes.
X. Sui, X. Huang / Separation and Purification Technology 32 (2003) 73 /79
2.2. Characterization of the gradient ceramic membrane The structure of the diatomaceous earth (DE) has been investigated using JEM 100CXII conventional transmission emission microscope (TEM). Microstructure and fractured surface morphology of the ceramic membrane tube were characterized using XL30N, Philips and Hitachi S550 SEM. 2.3. Water permeability The municipal supply of drinking water was filtrated with dead end filtration mode. The operating pressure is between 0.1 and 0.3 MPa. The fouling of the membrane was resulted from deposition of the agglomerated contaminants at the surface of the outer layer of the membrane. The water flux efficiency of this kind of gradient ceramic membrane can be recovered almost entirely by the simple mechanical cleaning. The brush time could exceed 100 times. When the clog occurred, the filter element was pulled out, washing and brushing the cartridge surface in the small current of water with a hard wash cloth, the deposited pollutants on the cartridge surface can be removed. A fresh and clean surface appears after washing and brushing.
3. Results and discussion 3.1. Microstructure of the diatomaceous earth DE is a naturally occurring substance comprised of the fossilized remains of diatoms. Diatoms are microscopic sized hard shelled creatures found in both marine and fresh waters. They weave microscopic shells which they use for the protection and locomotion. These shells are covered with a pattern of tiny holes so regular that even the slightest change in their design usually signifies a different species. As the diatoms died, their shells survived, slowly piling up in deposits at the bottom of geological lakes and lagoons. When these lakes dried up, what remained were huge deposits of
75
‘diatomaceous earth’. The material is used extensively as an abrasive, a filtering material in food and beer industry, an inert ingredient of explosive, and an insulating material for boilers and steam pipes. DE also can be used as coatings for centrifugal casting [6]. Fig. 2 is the TEM pictures of the DE, (a) is morphology of DE (b) is a complete piece of DE, (c) is a special one in which every small hole contains several tiny secondary hole, the amplified image shown in Fig. 2(d). Fig. 3 is SEM of the DE. From Fig. 3(b), we can see that there are many nanometer size particles on the DE plate and part of them enter the tiny hole of the DE. 3.2. Microstructure of gradient ceramic membrane The tubular gradient ceramic membranes manufactured by centrifugal casting technique were made from DE. The movement of the particulate phase towards the mould wall results in the segregation of the ceramic particles, the size of the ceramic particles change gradually from large to small along the radical cross-section of the tubular membrane, so the pore size of the tubular membrane change gradually from large to small along the radical direction. Fig. 4(a) is the tubular membrane cross-section. The inner surface of the membrane tube is very smooth. There are hundreds layers of different size ceramic particles along the radical cross-section of the membrane tube and there is no visible layers border. Fig. 4(b) is gradient layers distribution of the membrane tube, the inner surface were formed by the finest particles and the outer surface were composed by the largest ceramic particles. Under the pressure of the centrifugal force the largest ceramic particles were forced to accumulate in the outer surface of the tube and formed the outer layer as shown in Fig. 5(a), and the middle size of the particles accumulated in the middle layer and formed the middle layer of the membrane as shown in Fig. 5(b), the finest particles accumulated in the inner layer and formed the control layer shown in Fig. 5(c). From Fig. 5, it is obvious that the more near to the outer surface, the bigger ceramic particle is. The control layer is the inner layer of the tube. The
76
X. Sui, X. Huang / Separation and Purification Technology 32 (2003) 73 /79
Fig. 2. TEM morphology of the DE.
Fig. 3. SEM of the DE.
X. Sui, X. Huang / Separation and Purification Technology 32 (2003) 73 /79
77
Fig. 6. Uniform and regular pore size distribution of the inner surface of the membrane tube.
Fig. 4. Fractured cross-section and gradient layer distribution of the membrane tube.
gradient distribution of the ceramic particles resulted that the pore size of the membrane tube changes gradually from large to small. Route of the filtration is not only the gap among the particles but also the tiny hole inside the DE particle. The tiny hole of the DE particle inside join the gap among the particles, so the flux of the
DE membrane filter is very high and the porosity of the filter cartridge is very high more than 65%. The pore size distribution of the gradient ceramic membrane is rather narrow because the control layer (inner surface) of the membrane tube has a very uniform and regular pore structure shown in Fig. 6. Fig. 7 is the pore size distribution of the gradient ceramic membrane. Average pore size is 0.15 mm and maximum less than 0.3 mm.
3.3. Water filtration behavior of the ceramic membrane Membrane filtration, widely used in the industrial and food processing industries, is now considered a promising technology for water
Fig. 5. Gradient distribution of the size ceramic particles along the radical cross-section of the membrane tube.
X. Sui, X. Huang / Separation and Purification Technology 32 (2003) 73 /79
78
Fig. 7. Pore size distribution of the gradient ceramic membrane.
treatment. Depending upon membrane pore sizes and pressures, a variety of contaminants can be removed. The municipal supply of drinking water was purified with dead end filtration mode with an mean pore size 0.15 mm gradient ceramic membrane filter. Comparison of the water quality between raw water and filtered water after 50 M3 water filtration carried out have been shown in Table 1. The experiment results showed that 100% pathogenetic bacterium such as E. coli , Salmonelle, microzyme, staphylotoxin, Pseudomonas aeruginosa and mold, etc. can be removed by the gradient ceramic membrane. Even 200 000 cuf/ml pathogenic bacteria like E. coli , Salmonelle, staphylotoxin and Pseudomonas aeruginosa can be removed entirely. The experiment results also showed that the rust, parasites and suspension particles exceed 50 nm can be removed entirely. Safe and clean
Fig. 8. Filter element before and after filtration of 50 M3 drinking water.
drinking water, retaining its natural mineral content can be obtained without boiling. Fig. 8 shows the filter element before and after 50 M3 water filtration. It is clear that outer diameter of the filter element is reduced after 100 times mechanical brush. The diameter of original filter element was 54 mm and after 50 M3 water filtration, it became 48 mm, that is the thickness of the filter element reduced about 3 mm. Fig. 9 is the result of flux recovery of the ceramic membrane during 100 mechanical brush. Brushing of the gradient ceramic membrane filter was made once a time every 8 h. Flux changes of the ceramic membrane were remarkable in the initial several brush. The flux of the membrane tended to stable from 10 to 100 times brushing. The amount of the flux kept at about 1 m3/(m2 h bar), flux was just about 50% comparing to the beginning use of the membrane filter. Shown in Fig. 9 is the flux recovery of the ceramic membrane after mechanical brushing. It can be seen from the figure that the ceramic membrane
Table 1 Comparison of the water quality before and after filtration Water
Before filtration
After filtration
/50 nm particles (mg/l) Total of colony (cfu/ml)
56 600
0 0
Mold
With
Without
200 000/ml
Without
Pathogenic bacteriaa
a
Added.
E. coli Salmonelle Staphylotoxin Pseudomonas aeruginosa
X. Sui, X. Huang / Separation and Purification Technology 32 (2003) 73 /79
79
Fig. 9. Flux recovery of the ceramic membrane after 100 times mechanical brush (total filtrated water amount was 50 M3).
filter has a good flux recovery behavior after simple mechanical brush.
4. Conclusions Gradient ceramic membrane could be prepared by centrifugal casting. The inner surface was formed by the finest particles and the outer surface was composed by the largest ceramic particles, there is no layers border between them. Pore size of the tubular membrane changed gradually from large to small along the radical direction due to the segregation of the ceramic particles. The inner surface of the membrane tube is very smooth and the pore size distribution of the inner surface is very uniform and regular. The pore size distribution of the gradient ceramic membrane is rather narrow, average pore size is 0.15 mm and maximum less than 0.3 mm. Gradient DE membrane filter performs a good filtration behavior for drinking water. Hundred percent of pathogenetic bacterium such as E. coli , Salmonelle, microzyme, staphylotoxin, Pseudomo-
nas aeruginosa and mold, etc. in water can be removed by the filtration of the membrane filter. The rust, parasites and suspension particles more than 50 nm can be removed entirely. Safe and clean drinking water, retaining its natural mineral content can be obtained without boiling. Mechanical brush is an effective method to recover flux of gradient DE membrane filter. Thickness of the filter element reduced about 3 mm after 100 times mechanical brush, and the water filtration amount could exceed 50 M3.
References [1] K. Darcovich, C.R. Cloutier, J. Am. Ceram. Soc. 82 (1999) 8. [2] A. Nijmeijer, C. Huiskes, G.M. Natascha, et al., Am. Ceram. Soc. Bull. 77 (4) (1998) 95. [3] G.C. Steenkamp, H.W.J.P. Neomagus, H.M. Krieg, K. Keizer, Sep. Purif. Technol. 25 (2001) 407 /413. [4] P.M. Bieseuvel, V. Breedveld, A.P. Higler, H. Verweij, Chem. Eng. Sci. 56 (2001) 3517 /3525. [5] X. Huang, Z. Huang, Membr. Sci. Technol. 16 (2) (1996) 31 /36 (in Chinese). [6] X. Sui, Foundry 6 (1992) 1 /5 (in Chinese).