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Alternative technique for removal of phosphorus in wastewater using chemically surface-modified silica filter
Journal of Industrial and Engineering Chemistry 18 (2012) 1560–1563
Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Short communication
Alternative technique for removal of phosphorus in wastewater using chemically surface-modified silica filter DaeGun Kim a, InSang Yoo a,b, ByungSeok Park a,b, YongHyun Lee c, SaeHoon Kim b, Duk Chang d, Young Sunwoo e, HyungSoo Shin d, YangDam Eo e, KiHo Hong e,* a
Green Energy and Environment Laboratory (GE2), Gyo-dong 1835-3, Gangneung-si, Gangwon-do 120-100, Republic of Korea Department of Ceramic Engineering, Gangneung-Wonju National University, Daehangro 120, Gangneung-si, Gangwon-do 210-702, Republic of Korea Yeongwol Eco-Materials Industry Foundation, Palgoeri 1241, Yeongwol-eup, Yeongwol-gun, Ganwon-do 230-884, Republic of Korea d Department of Environmental Engineering, Konkuk University, 1 Hwayang-dong, Gwangjin-gu, Seoul 143-701, Republic of Korea e Department of Advanced Technology Fusion, Konkuk University, 1 Hwayang-dong, Gwangjin-gu, Seoul 143-701, Republic of Korea b c
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 November 2011 Accepted 2 March 2012 Available online 10 March 2012
An innovative phosphorus removal filter made of silica granules was designed and evaluated for advanced wastewater treatment. The silica granules from natural mineral stones were formed into a porous body by sintering with addition of glass powder. The phosphorus was removed by the ionic exchange mechanism between OH in Si(OH)4 and PO43 in wastewater. The silica filter was very effective in removing phosphorus in wastewater by ion exchange. We found the possibility of a phosphorus removal filter with no sludge production characteristics of flocculation treatment techniques, and it seems to be quite meaningful as a new wastewater treatment. Crown Copyright ß 2012 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry. All rights reserved.
Keywords: Phosphorus removal Silica Silicon hydroxide Surface modification Ionic exchange
1. Introduction Phosphorus is discharged into wastewater mainly as inorganic phosphates such as orthophosphates and polyphosphates by both domestic and industrial activities [1,2], which is essential to the growth of algae and other biological organisms. However, the presence of excess phosphorus in the wastewater is responsible for eutrophication, which leads to short- and long-term environmental and aesthetic problems in lakes, reservoirs, coastal areas, and other confined water bodies [3,4]. Phosphorus can also interfere with coagulation and with lime soda softening [5]. Because of these harmful effects, effluent discharge standards to natural bodies are continuously upgraded. Therefore, the removal of phosphorus is necessary not only to prevent eutrophication but also to maintain the required water quality [6]. Generally, there are two conventional methods for phosphate removal from wastewater. Chemical precipitation and biological nutrient removal (BNR) are the most commonly used methods for removal of phosphate from municipal and industrial wastewater [7,8]. However, both processes have certain disadvantages. In chemical precipitation, higher maintenance cost, problems
* Corresponding author. E-mail address: [email protected] (K. Hong).
associated with the handling of the chemicals, and the disposal of the large amount of produced sludge are the main disadvantages. The biological process requires a highly efficient secondary clarifier and maintenance of a BOD:TP ratio of at least 20:1. The common and important limitation of these two processes is that neither of them can produce an effluent containing less than 0.5 mg/L P [9,10]. Ion exchange can be a good candidate for phosphorus removal, which is defined as the displacement of one ion by another [11]. The displaced ion is originally a part of an insoluble material and the displacing ion is originally in a soluble material. For example, the OH ion in Si(OH)4 can be the first and the phosphorus ion, the latter. At the completion of the process, the two ions have switched places as the OH ion is dissolved in water and Si3(PO4)4 appears on the solid surface. There will be no requirement for a large area to set-up a wastewater treatment system and there are other major advantages: low energy consumption, easy upgrading of existing facilities, continuous separation, better effluent quality, and avoidance of any chemical addition. As discussed in a previous study [12], a phosphorus removal filter was designed using porous aluminum surface-modified by alkali treatment and stabilization process, in which Al(OH)3 was formed by surface modification. Phosphorus in artificial wastewater was removed through the following ion exchange mechanism; Al(OH)3 + PO4 ! AlPO4 + 3OH. In this case, higher phosphorus removal efficiency was successfully accomplished. However, the
1226-086X/$ – see front matter. Crown Copyright ß 2012 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry. All rights reserved. doi:10.1016/j.jiec.2012.03.003
D.G. Kim et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 1560–1563
1561
experiments had limitations such as low permeability due to low porosity and application of very small amount of artificial wastewater. In this study, a phosphorus removal filter was fabricated by using silica granules from natural mineral. The porous silica body was surface-modified to form silicon hydroxide on the surface of the silica, which can be a phosphate removal filter for advanced wastewater treatment because phosphate ions may be removed by the ion exchange with hydroxyl ions of silicon hydroxide. The evaluation of phosphate removal by filtration was performed under conditions of an immersion state of the filters in wastewater and a flowing state through filters. Also, filter recycling was evaluated with retreatment of the surface modification process. 2. Experimental Silica stones, which were excavated at Yeongwol in Korea, were crushed and sieved to obtain a regular size distribution below 1 mm. They were washed by distilled water, dried in an oven, and then mixed with glass powder (10% of the weight of silica) as a bonding agent. A mixture of 25 g was formed into a cylindrical shape with a diameter of 30 mm and height of 27.5 mm in a stainless steel die without pressing. The form was sintered at 800 8C for 2 h to bond silica granules with each other while introducing macropores within. Sodium hydroxide (NaOH, 93%, Duksan Pure Chemicals Co. Ltd.) was used as a reagent for alkali surface modification. The sintered porous silica was surfacemodified in boiled NaOH solution of 0.1 M for 30 min and then it was immersed in boiling water for 30 min to form silicon hydroxide on the surface of the porous silica. For the evaluation of phosphorus removal performance, wastewater was collected from the end of the grit chamber in a small sized wastewater treatment plant, which could not effectively remove the phosphorus so that the concentration of phosphorus was above 3.56 mg/L. The evaluation of phosphorus removal performance was conducted under conditions of immersion state and flowing state. For immersion state evaluation, the porous silica filters were stacked up to 16ea, and wastewater with total phosphorus (TP) of 4.93 mg/L was used, 47 mL for 1ea, 80 mL for 2ea, 145 mL for 4ea, 265 mL for 8ea, and 700 mL for 16ea. The times of immersion state were 1 and 3 h. For flowing state evaluation, 32ea porous silica filters were stacked in an acryl tube and 500 mL of wastewater with TP of 3.56 mg/L was consecutively sent through the filter stack in the tube. TP was measured at each wastewater filter point, after accumulation of each 500 mL. After filtering the wastewater, the filters were re-treated by alkali solution by the same method and then filtering was performed once again. The measurement of phosphate was conducted using the ascorbic acid method from the American Public Health Association (APHA) Standard Methods [13].
Fig. 1. Photograph of phosphorus removal silica filter.
The sintered porous silica body was chemically surfacemodified by boiled alkali solution and water to form silicon hydroxide on the surface of silica. Fig. 2 shows the surface microstructure of the original silica and surface-modified silica. The surface of the original silica was very smooth as presented in Fig. 2(a), while the surface-modified silica had whiskers grown on the smooth surface like grass as revealed in Fig. 2(b).
3. Results and discussion The macroscopic photograph of prepared porous silica filter as presented in Fig. 1. When the silica granules were mixed with glass powder, the glass powder adhered to the surface of silica granules like powder-coating. For filling the mixture into a stainless steel die, silica granules got in touch with their neighbors and there were glass powder on the contact points. The glass powder was melted at the sintering temperature so that the silica granules would bond with their neighbors as necking in a liquid phase sintering process. Thus, macropores are formed among the silica granules while making a pore-network throughout the sample. The porosity of the silica filter was around 58% and there would not be any closed pore inside.
Fig. 2. Surface morphologies of (a) as-received original silica and (b) silica surfacemodified by scanning electron microscopy.
D.G. Kim et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 1560–1563
1562
a
5
1h 3h
TP (mg/l)
Intensity (A.U.)
4
surface-modified as-received 110
108
106
3
2
1 104
102
100
98
0
Binding energy (eV)
0
b
(1)
In the subsequent treatment in boiling water, Si(OH)4 could be formed on the surface by the following chemical reaction: (2)
SiO2 in the natural silica may be transformed into silicates by dissolution and hydration process in such chemical treatment, so that the smooth surface morphology changes into a grass-like surface as presented in Fig. 2. Actually, such a ‘‘grass’’ cover would increase the specific surface area with a rough surface. Water flow in the filter, especially on the surface of porous silica in the filter, may also be modified with enhanced turbulence. To estimate the phosphorus filtering capability of each filter, phosphorus adsorption into the filter was evaluated for the immersed state in wastewater. The amount of phosphorus adsorption increased with accumulating filter numbers from 1 to 16. At this time, TP decreased from 4.93 mg/L to 2.18 mg/L for 1 h and to about 1 mg/L for 3 h as indicated in Fig. 4(a). The amount of removed phosphorus could be calculated considering the accumulated filter numbers for each immersion time frame. Fig. 4(b) presents the phosphorus amount removed from the wastewater of 4.93 mg/L with respect to accumulating numbers of filters. The removed phosphorus concentration linearly increased with increasing numbers of filters. Also, immersion time affected the amount of removed phosphorus. When 16 filters were accumulated, the amount of removed phosphorus for 1 h was higher than that of 8 filters for 3 h. As a result, it meant that increasing the filter numbers could effectively reduce the filtering time. This is very important criterion when designing a filtering system; if it takes a long time for the ion exchange of OH in Si(OH)4 to PO43, especially for the orthophosphate ions in the wastewater, the filtering would be very difficult to employ as a phosphorus removal filter because the filtering time should be very short, such as several tens of seconds in practical applications. Thirty-two filters were set into an acryl tube. 500 mL of wastewater with TP of 3.56 mg/L was sent through them and this was repeated for up to 4 L. The time for 500 mL of wastewater to
8
10
12
14
16
18
14
16
18
3
1h
2
1
0 0
2Na2 SiO3 þ 6H2 O ! 2SiðOHÞ4 þ 4NaOH
6
3h Removed P (mg)
SiO2 þ 2NaOH ! Na2 SiO3 þ H2 O
4
Filter Number (ea.)
Fig. 3. X-ray photoelectron spectra of as-received original silica and the surfacemodified one.
Fig. 3 presents the spectra of X-ray photoelectron spectroscopy analysis for the original silica and the surface-modified silica. The surface of original silica was composed of mainly pure SiO2. After the surface modification, the surface of silica had some changes; evidence of silicates existence was observed as indicated by the arrow in Fig. 3. First, when SiO2 was in the boiled NaOH solution, there could be a chemical reaction such as below:
2
2
4
6
8
10
12
Filter Number (ea.) Fig. 4. (a) Change in total phosphorus and (b) amount of removed phosphorus with accumulated filter numbers at immersion state in wastewater for 1 and 3 h.
pass through all filters was less than 20 s. Also, the used filters were retreated by the same method for the surface modification process, and wastewater filtering was performed once again in the same way. TP was measured for each 500 mL of wastewater filtering as presented in Fig. 5. At the first filtering step, TP was reduced from 3.56 mg/L to 0.56 mg/L. The filtering was very effective up to 1.5 L as TP decreased to 0.83 mg/L, while the reduction of TP decreased to 1.91 mg/L at the 2 L filtering step and 2.33 mg/L at 2.5 L. The effect of phosphorus filtering gradually decreased up to the 4 L filtering step reaching 2.61 mg/L as depicted in Fig. 5(a). After recycling the filters, TP increased from 2.10 mg/L at the first filtering step to 2.99 mg/L at the 2 L filtering step and it was maintained at about 3 mg/L up to the 4 L filtering step. The accumulated value of removed phosphorus content is illustrated in Fig. 5(b). The amount of removed phosphorus increased at the first filtering evaluation and total content of removed phosphorus was 7.37 mg in 4 L of wastewater. In case of recycling, it reached 2.81 mg. The removal efficiency was about 52% at the first filtering evaluation and around 20% at the second because the total amount of phosphorus in 4 L wastewater was 14.24 mg. The surface modified silica filter was very effective into removing phosphorus from wastewater while the degradation of filtering characteristics was evident when increasing the filtration and even with recycling. However, the phosphorus was not removed at all under the condition of pristine silica filters with no surface-modification by alkali treatment. Obviously, improved performance is possible when filtering the phosphorus in
D.G. Kim et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 1560–1563
a
4
TP (mg/l)
3
2
1
1st 2nd
0 0
1
2
3
4
Influent (l)
b
8 1st 2nd
Accumulated removal P (mg)
7 6 5 4 3
1563
area in the filter could be formed among the silica granules while making a pore-network through the sample. The silica filter was very effective in removing phosphorus from wastewater by ion exchange. Under the condition of immersion state when estimating the phosphorus removal capability of each filter, experimental results showed that the removed phosphorus concentration linearly increased with increasing numbers of filters and that immersion time affected the amount of removed phosphorus. This means that increasing the filter numbers could effectively reduce the filtering time. However, the secondary treatability of the silica granule filter retreated by the same method as for the surface modification process was decreased around 20% after retreatment. In future study, therefore, improvement of the saturation rate and removal after retreatment of filter should be studied. However, the results showed the possibility of this phosphorus removal filter without the sludge production characteristics of flocculation treatment techniques, and we feel that it very meaningful as a new wastewater treatment. Especially in small unit facilities for wastewater treatment, in which there is a relatively small amount of influent with a high concentration of phosphorus, such a filtering system to remove phosphorus can be effectively applied in a relatively narrow space. Thus, this surface-modified silica filter can be an alternative option for phosphorus removal in wastewater treatment plants.
2 Acknowledgement
1 0
1
2
3
4
Influent (l)
This work is financially supported by Korea Minister of Ministry of Land, Transport and Maritime Affairs (MLTM) as ‘U-City Master and Doctor Course Grant Program’.
Fig. 5. (a) Change in total phosphorus and (b) accumulated phosphorus removal amount with increasing influent of wastewater.
References wastewater for a longer lifetime and with effective recycling. However, the results show the possibility of this phosphorus removal filter and we feel that it is very meaningful as a new wastewater treatment. Especially in small unit facilities for wastewater treatment, in which there is a relatively small amount of influent with a high concentration of phosphorus, such a filtering system to remove phosphorus can be effectively applied in a relatively narrow space. Because the phosphorus is removed through an ion exchange mechanism in this system, this eliminates the need for a sludge treatment process in the wastewater treatment plant. It does not require a rebuilding of the facilities because the filtering system can be easily added on to the established one. 4. Conclusions In this study, the feasibility of a new phosphorus removal filter made of silica granules for advanced wastewater treatment was demonstrated. The macropores with an increased specific surface
[1] C.N. Sawyer, P.L. McCarty, G.F. Parkin, Chemistry for Environmental Engineering, 5th ed., McGraw-Hill, Inc., New York, 2003. [2] Metcalf & Eddy Inc., Wastewater Engineering: Treatment and Reuse, 4th ed., McGraw-Hill, Inc., New York, 2003. [3] D.M. Anderson, P.M. Glibert, J.M. Burkholder, Estuaries 25 (2002) 704. [4] V.H. Smith, Environ. Sci. Pollut. Res. 10 (2003) 126. [5] U.S. EPA, Design Manual: Phosphorus Removal, Center for Environmental Research Information, Cincinnati, OH, 1987. [6] Water Environment Federation, Biological and Chemical Systems for Nutrient Removal, Water Environment Federation, Alexandria, VA, 1998. [7] R.I. Sedlak, Phosphorus and Nitrogen Removal from Municipal Wastewater: Principles and Practice, 2nd ed., Lewis Publishers, New York, 1991. [8] A.I. Omoike, G.W. Vanloon, Water Res. 33 (1999) 3617. [9] C.W. Randall, J.L. Barnard, H.D. Stensel, Design and Retrofit of Wastewater Treatment Plants for Biological Nutrient Removal, Technic Publishing Co., Inc., Lancaster, PA, 1992. [10] Water Environment Foundation, Wastewater Treatment Plant Design, Water Environment Federation, Alexandria, 2003. [11] A.P. Sincero, G.A. Sincero, Physical–Chemical Treatment of Water and Wastewater, CRC Press, 2003. [12] Y.I. Seo, Y.J. Lee, K.H. Hong, D. Chang, D.-G. Kim, K.H. Lee, Y.D. Kim, J. Hazard. Mater. 173 (2010) 789. [13] American Public Health Association, American Water Works Association, Water Environmental Federation, Standard Methods for the Examination of Water and Wastewater, 21st ed., APHA, Washington, DC, 2005.
Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Short communication
Alternative technique for removal of phosphorus in wastewater using chemically surface-modified silica filter DaeGun Kim a, InSang Yoo a,b, ByungSeok Park a,b, YongHyun Lee c, SaeHoon Kim b, Duk Chang d, Young Sunwoo e, HyungSoo Shin d, YangDam Eo e, KiHo Hong e,* a
Green Energy and Environment Laboratory (GE2), Gyo-dong 1835-3, Gangneung-si, Gangwon-do 120-100, Republic of Korea Department of Ceramic Engineering, Gangneung-Wonju National University, Daehangro 120, Gangneung-si, Gangwon-do 210-702, Republic of Korea Yeongwol Eco-Materials Industry Foundation, Palgoeri 1241, Yeongwol-eup, Yeongwol-gun, Ganwon-do 230-884, Republic of Korea d Department of Environmental Engineering, Konkuk University, 1 Hwayang-dong, Gwangjin-gu, Seoul 143-701, Republic of Korea e Department of Advanced Technology Fusion, Konkuk University, 1 Hwayang-dong, Gwangjin-gu, Seoul 143-701, Republic of Korea b c
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 November 2011 Accepted 2 March 2012 Available online 10 March 2012
An innovative phosphorus removal filter made of silica granules was designed and evaluated for advanced wastewater treatment. The silica granules from natural mineral stones were formed into a porous body by sintering with addition of glass powder. The phosphorus was removed by the ionic exchange mechanism between OH in Si(OH)4 and PO43 in wastewater. The silica filter was very effective in removing phosphorus in wastewater by ion exchange. We found the possibility of a phosphorus removal filter with no sludge production characteristics of flocculation treatment techniques, and it seems to be quite meaningful as a new wastewater treatment. Crown Copyright ß 2012 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry. All rights reserved.
Keywords: Phosphorus removal Silica Silicon hydroxide Surface modification Ionic exchange
1. Introduction Phosphorus is discharged into wastewater mainly as inorganic phosphates such as orthophosphates and polyphosphates by both domestic and industrial activities [1,2], which is essential to the growth of algae and other biological organisms. However, the presence of excess phosphorus in the wastewater is responsible for eutrophication, which leads to short- and long-term environmental and aesthetic problems in lakes, reservoirs, coastal areas, and other confined water bodies [3,4]. Phosphorus can also interfere with coagulation and with lime soda softening [5]. Because of these harmful effects, effluent discharge standards to natural bodies are continuously upgraded. Therefore, the removal of phosphorus is necessary not only to prevent eutrophication but also to maintain the required water quality [6]. Generally, there are two conventional methods for phosphate removal from wastewater. Chemical precipitation and biological nutrient removal (BNR) are the most commonly used methods for removal of phosphate from municipal and industrial wastewater [7,8]. However, both processes have certain disadvantages. In chemical precipitation, higher maintenance cost, problems
* Corresponding author. E-mail address: [email protected] (K. Hong).
associated with the handling of the chemicals, and the disposal of the large amount of produced sludge are the main disadvantages. The biological process requires a highly efficient secondary clarifier and maintenance of a BOD:TP ratio of at least 20:1. The common and important limitation of these two processes is that neither of them can produce an effluent containing less than 0.5 mg/L P [9,10]. Ion exchange can be a good candidate for phosphorus removal, which is defined as the displacement of one ion by another [11]. The displaced ion is originally a part of an insoluble material and the displacing ion is originally in a soluble material. For example, the OH ion in Si(OH)4 can be the first and the phosphorus ion, the latter. At the completion of the process, the two ions have switched places as the OH ion is dissolved in water and Si3(PO4)4 appears on the solid surface. There will be no requirement for a large area to set-up a wastewater treatment system and there are other major advantages: low energy consumption, easy upgrading of existing facilities, continuous separation, better effluent quality, and avoidance of any chemical addition. As discussed in a previous study [12], a phosphorus removal filter was designed using porous aluminum surface-modified by alkali treatment and stabilization process, in which Al(OH)3 was formed by surface modification. Phosphorus in artificial wastewater was removed through the following ion exchange mechanism; Al(OH)3 + PO4 ! AlPO4 + 3OH. In this case, higher phosphorus removal efficiency was successfully accomplished. However, the
1226-086X/$ – see front matter. Crown Copyright ß 2012 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry. All rights reserved. doi:10.1016/j.jiec.2012.03.003
D.G. Kim et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 1560–1563
1561
experiments had limitations such as low permeability due to low porosity and application of very small amount of artificial wastewater. In this study, a phosphorus removal filter was fabricated by using silica granules from natural mineral. The porous silica body was surface-modified to form silicon hydroxide on the surface of the silica, which can be a phosphate removal filter for advanced wastewater treatment because phosphate ions may be removed by the ion exchange with hydroxyl ions of silicon hydroxide. The evaluation of phosphate removal by filtration was performed under conditions of an immersion state of the filters in wastewater and a flowing state through filters. Also, filter recycling was evaluated with retreatment of the surface modification process. 2. Experimental Silica stones, which were excavated at Yeongwol in Korea, were crushed and sieved to obtain a regular size distribution below 1 mm. They were washed by distilled water, dried in an oven, and then mixed with glass powder (10% of the weight of silica) as a bonding agent. A mixture of 25 g was formed into a cylindrical shape with a diameter of 30 mm and height of 27.5 mm in a stainless steel die without pressing. The form was sintered at 800 8C for 2 h to bond silica granules with each other while introducing macropores within. Sodium hydroxide (NaOH, 93%, Duksan Pure Chemicals Co. Ltd.) was used as a reagent for alkali surface modification. The sintered porous silica was surfacemodified in boiled NaOH solution of 0.1 M for 30 min and then it was immersed in boiling water for 30 min to form silicon hydroxide on the surface of the porous silica. For the evaluation of phosphorus removal performance, wastewater was collected from the end of the grit chamber in a small sized wastewater treatment plant, which could not effectively remove the phosphorus so that the concentration of phosphorus was above 3.56 mg/L. The evaluation of phosphorus removal performance was conducted under conditions of immersion state and flowing state. For immersion state evaluation, the porous silica filters were stacked up to 16ea, and wastewater with total phosphorus (TP) of 4.93 mg/L was used, 47 mL for 1ea, 80 mL for 2ea, 145 mL for 4ea, 265 mL for 8ea, and 700 mL for 16ea. The times of immersion state were 1 and 3 h. For flowing state evaluation, 32ea porous silica filters were stacked in an acryl tube and 500 mL of wastewater with TP of 3.56 mg/L was consecutively sent through the filter stack in the tube. TP was measured at each wastewater filter point, after accumulation of each 500 mL. After filtering the wastewater, the filters were re-treated by alkali solution by the same method and then filtering was performed once again. The measurement of phosphate was conducted using the ascorbic acid method from the American Public Health Association (APHA) Standard Methods [13].
Fig. 1. Photograph of phosphorus removal silica filter.
The sintered porous silica body was chemically surfacemodified by boiled alkali solution and water to form silicon hydroxide on the surface of silica. Fig. 2 shows the surface microstructure of the original silica and surface-modified silica. The surface of the original silica was very smooth as presented in Fig. 2(a), while the surface-modified silica had whiskers grown on the smooth surface like grass as revealed in Fig. 2(b).
3. Results and discussion The macroscopic photograph of prepared porous silica filter as presented in Fig. 1. When the silica granules were mixed with glass powder, the glass powder adhered to the surface of silica granules like powder-coating. For filling the mixture into a stainless steel die, silica granules got in touch with their neighbors and there were glass powder on the contact points. The glass powder was melted at the sintering temperature so that the silica granules would bond with their neighbors as necking in a liquid phase sintering process. Thus, macropores are formed among the silica granules while making a pore-network throughout the sample. The porosity of the silica filter was around 58% and there would not be any closed pore inside.
Fig. 2. Surface morphologies of (a) as-received original silica and (b) silica surfacemodified by scanning electron microscopy.
D.G. Kim et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 1560–1563
1562
a
5
1h 3h
TP (mg/l)
Intensity (A.U.)
4
surface-modified as-received 110
108
106
3
2
1 104
102
100
98
0
Binding energy (eV)
0
b
(1)
In the subsequent treatment in boiling water, Si(OH)4 could be formed on the surface by the following chemical reaction: (2)
SiO2 in the natural silica may be transformed into silicates by dissolution and hydration process in such chemical treatment, so that the smooth surface morphology changes into a grass-like surface as presented in Fig. 2. Actually, such a ‘‘grass’’ cover would increase the specific surface area with a rough surface. Water flow in the filter, especially on the surface of porous silica in the filter, may also be modified with enhanced turbulence. To estimate the phosphorus filtering capability of each filter, phosphorus adsorption into the filter was evaluated for the immersed state in wastewater. The amount of phosphorus adsorption increased with accumulating filter numbers from 1 to 16. At this time, TP decreased from 4.93 mg/L to 2.18 mg/L for 1 h and to about 1 mg/L for 3 h as indicated in Fig. 4(a). The amount of removed phosphorus could be calculated considering the accumulated filter numbers for each immersion time frame. Fig. 4(b) presents the phosphorus amount removed from the wastewater of 4.93 mg/L with respect to accumulating numbers of filters. The removed phosphorus concentration linearly increased with increasing numbers of filters. Also, immersion time affected the amount of removed phosphorus. When 16 filters were accumulated, the amount of removed phosphorus for 1 h was higher than that of 8 filters for 3 h. As a result, it meant that increasing the filter numbers could effectively reduce the filtering time. This is very important criterion when designing a filtering system; if it takes a long time for the ion exchange of OH in Si(OH)4 to PO43, especially for the orthophosphate ions in the wastewater, the filtering would be very difficult to employ as a phosphorus removal filter because the filtering time should be very short, such as several tens of seconds in practical applications. Thirty-two filters were set into an acryl tube. 500 mL of wastewater with TP of 3.56 mg/L was sent through them and this was repeated for up to 4 L. The time for 500 mL of wastewater to
8
10
12
14
16
18
14
16
18
3
1h
2
1
0 0
2Na2 SiO3 þ 6H2 O ! 2SiðOHÞ4 þ 4NaOH
6
3h Removed P (mg)
SiO2 þ 2NaOH ! Na2 SiO3 þ H2 O
4
Filter Number (ea.)
Fig. 3. X-ray photoelectron spectra of as-received original silica and the surfacemodified one.
Fig. 3 presents the spectra of X-ray photoelectron spectroscopy analysis for the original silica and the surface-modified silica. The surface of original silica was composed of mainly pure SiO2. After the surface modification, the surface of silica had some changes; evidence of silicates existence was observed as indicated by the arrow in Fig. 3. First, when SiO2 was in the boiled NaOH solution, there could be a chemical reaction such as below:
2
2
4
6
8
10
12
Filter Number (ea.) Fig. 4. (a) Change in total phosphorus and (b) amount of removed phosphorus with accumulated filter numbers at immersion state in wastewater for 1 and 3 h.
pass through all filters was less than 20 s. Also, the used filters were retreated by the same method for the surface modification process, and wastewater filtering was performed once again in the same way. TP was measured for each 500 mL of wastewater filtering as presented in Fig. 5. At the first filtering step, TP was reduced from 3.56 mg/L to 0.56 mg/L. The filtering was very effective up to 1.5 L as TP decreased to 0.83 mg/L, while the reduction of TP decreased to 1.91 mg/L at the 2 L filtering step and 2.33 mg/L at 2.5 L. The effect of phosphorus filtering gradually decreased up to the 4 L filtering step reaching 2.61 mg/L as depicted in Fig. 5(a). After recycling the filters, TP increased from 2.10 mg/L at the first filtering step to 2.99 mg/L at the 2 L filtering step and it was maintained at about 3 mg/L up to the 4 L filtering step. The accumulated value of removed phosphorus content is illustrated in Fig. 5(b). The amount of removed phosphorus increased at the first filtering evaluation and total content of removed phosphorus was 7.37 mg in 4 L of wastewater. In case of recycling, it reached 2.81 mg. The removal efficiency was about 52% at the first filtering evaluation and around 20% at the second because the total amount of phosphorus in 4 L wastewater was 14.24 mg. The surface modified silica filter was very effective into removing phosphorus from wastewater while the degradation of filtering characteristics was evident when increasing the filtration and even with recycling. However, the phosphorus was not removed at all under the condition of pristine silica filters with no surface-modification by alkali treatment. Obviously, improved performance is possible when filtering the phosphorus in
D.G. Kim et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 1560–1563
a
4
TP (mg/l)
3
2
1
1st 2nd
0 0
1
2
3
4
Influent (l)
b
8 1st 2nd
Accumulated removal P (mg)
7 6 5 4 3
1563
area in the filter could be formed among the silica granules while making a pore-network through the sample. The silica filter was very effective in removing phosphorus from wastewater by ion exchange. Under the condition of immersion state when estimating the phosphorus removal capability of each filter, experimental results showed that the removed phosphorus concentration linearly increased with increasing numbers of filters and that immersion time affected the amount of removed phosphorus. This means that increasing the filter numbers could effectively reduce the filtering time. However, the secondary treatability of the silica granule filter retreated by the same method as for the surface modification process was decreased around 20% after retreatment. In future study, therefore, improvement of the saturation rate and removal after retreatment of filter should be studied. However, the results showed the possibility of this phosphorus removal filter without the sludge production characteristics of flocculation treatment techniques, and we feel that it very meaningful as a new wastewater treatment. Especially in small unit facilities for wastewater treatment, in which there is a relatively small amount of influent with a high concentration of phosphorus, such a filtering system to remove phosphorus can be effectively applied in a relatively narrow space. Thus, this surface-modified silica filter can be an alternative option for phosphorus removal in wastewater treatment plants.
2 Acknowledgement
1 0
1
2
3
4
Influent (l)
This work is financially supported by Korea Minister of Ministry of Land, Transport and Maritime Affairs (MLTM) as ‘U-City Master and Doctor Course Grant Program’.
Fig. 5. (a) Change in total phosphorus and (b) accumulated phosphorus removal amount with increasing influent of wastewater.
References wastewater for a longer lifetime and with effective recycling. However, the results show the possibility of this phosphorus removal filter and we feel that it is very meaningful as a new wastewater treatment. Especially in small unit facilities for wastewater treatment, in which there is a relatively small amount of influent with a high concentration of phosphorus, such a filtering system to remove phosphorus can be effectively applied in a relatively narrow space. Because the phosphorus is removed through an ion exchange mechanism in this system, this eliminates the need for a sludge treatment process in the wastewater treatment plant. It does not require a rebuilding of the facilities because the filtering system can be easily added on to the established one. 4. Conclusions In this study, the feasibility of a new phosphorus removal filter made of silica granules for advanced wastewater treatment was demonstrated. The macropores with an increased specific surface
[1] C.N. Sawyer, P.L. McCarty, G.F. Parkin, Chemistry for Environmental Engineering, 5th ed., McGraw-Hill, Inc., New York, 2003. [2] Metcalf & Eddy Inc., Wastewater Engineering: Treatment and Reuse, 4th ed., McGraw-Hill, Inc., New York, 2003. [3] D.M. Anderson, P.M. Glibert, J.M. Burkholder, Estuaries 25 (2002) 704. [4] V.H. Smith, Environ. Sci. Pollut. Res. 10 (2003) 126. [5] U.S. EPA, Design Manual: Phosphorus Removal, Center for Environmental Research Information, Cincinnati, OH, 1987. [6] Water Environment Federation, Biological and Chemical Systems for Nutrient Removal, Water Environment Federation, Alexandria, VA, 1998. [7] R.I. Sedlak, Phosphorus and Nitrogen Removal from Municipal Wastewater: Principles and Practice, 2nd ed., Lewis Publishers, New York, 1991. [8] A.I. Omoike, G.W. Vanloon, Water Res. 33 (1999) 3617. [9] C.W. Randall, J.L. Barnard, H.D. Stensel, Design and Retrofit of Wastewater Treatment Plants for Biological Nutrient Removal, Technic Publishing Co., Inc., Lancaster, PA, 1992. [10] Water Environment Foundation, Wastewater Treatment Plant Design, Water Environment Federation, Alexandria, 2003. [11] A.P. Sincero, G.A. Sincero, Physical–Chemical Treatment of Water and Wastewater, CRC Press, 2003. [12] Y.I. Seo, Y.J. Lee, K.H. Hong, D. Chang, D.-G. Kim, K.H. Lee, Y.D. Kim, J. Hazard. Mater. 173 (2010) 789. [13] American Public Health Association, American Water Works Association, Water Environmental Federation, Standard Methods for the Examination of Water and Wastewater, 21st ed., APHA, Washington, DC, 2005.