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Ground kenaf core as a filtration aid
Industrial Crops and Products 13 (2001) 155– 161 www.elsevier.com/locate/indcrop
Ground kenaf core as a filtration aid Sarah A. Lee, Mark A. Eiteman * Department of Biological and Agricultural Engineering, Driftmier Engineering Center, Uni6ersity of Georgia, Athens, GA 30602, USA Received 7 June 1999; accepted 2 June 2000
Abstract The objective of this study was to examine the filtration characteristics of ground kenaf core, a waste material generated during the production of kenaf bast fibers for paper. The constant-pressure precoat filtration characteristics of ground kenaf core were compared to commercial diatomaceous earth (DE). Three challenge solutions were studied: a yeast solution, a bacterial solution, and a standard silica-particle solution. The kenaf and DE both satisfactorily permitted removal of all silica particles from solution without noticable flux degradation over the course of the filtration. The kenaf and DE also removed yeast particles. In this case, the flux loss with time was lower with the DE precoated filter than in the kenaf precoated filter. The DE precoat excluded less than 10% of the bacterial particles from solution, whereas the kenaf removed about 40% of these small particles. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Diatomaceous earth; Filtration; Kenaf; Hibiscus cannabinus; Filter aid; Flux
1. Introduction Kenaf (Hibiscus cannabinus) is an industrial crop of increasing importance. In 1995, approx. 1000 ha were grown in the United States for commercial purposes (Kugler, 1996). The kenaf plant is composed of an internal core comprising 60 – 75% of the plant and outer bast fibers totaling 25 – 40% (Sellers et al., 1993). Recent research has demonstrated numerous potential uses for each of these two materials, which often must be sepa-
* Corresponding author. Tel.: +1-706-5427801; fax: +1706-5428806. E-mail address: [email protected] (M.A. Eiteman).
rated from each other. Products from the kenaf core include oil/chemical adsorbents in place of polypropylene (Goforth, 1994), horticultural mixes (Wang, 1994), bedding material for animals (Kugler, 1996), and insulation paneling (Ramaswamy and Easter, 1997). The absorption properties of kenaf core have been shown to enhance bioremediation (Borazjani and Diehl, 1994). Low-density core panels have potential for sound absorption or thermal insulation (Sellers et al., 1994). Mats for grass seeding and erosion control, and printing and writing papers have become commercial products from bast fibers (Kaldor et al., 1990 Ramaswamy and Easter, 1997). Bast fibers can be mixed with polypropylene for making nonwoven textile products includ-
0926-6690/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 6 6 9 0 ( 0 0 ) 0 0 0 6 2 - 5
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ing wallpaper backing and furniture underlays (Ramaswamy and Boyd, 1994), or blended with cotton for apparel (Ramaswamy and Easter, 1997). The whole plant has high protein, good digestibility and may be pelletized (Webber and Bledsoe, 1991). During the production of kenaf bast fibers for paper, a significant volume of waste kenaf core dustings are generated. Loose, inert particles of uniform size and structure such as ground kenaf have potential in filtration. Examples of the broad application of filters include the filtration of drinking water, swimming pool water or in industrial settings such as sugar refineries and breweries (Dickey, 1961). Because of the material’s light density, inertness, insolubility, low expense and homogeneity, ground kenaf core might be a suitable filtration aid for certain applications. Filter aids are used to improve filtration rates and the retention of particles by forming a porous, permeable and rigid lattice structure on the surface of a filtration medium (Jackson, 1961; Leppla, 1962). A filtration aid may be mixed with the challenge solution to be filtered prior to filtration, or alternatively it can be used as a precoat, applied before filtration. The principal purpose of precoating is to protect the filter medium from clogging by compressible particles in the challenge solution. In continuous applications, particles can be removed from the filter medium and replaced with fresh precoat. The objectives of this study were to examine the filtration characteristics of ground kenaf core and compare the performance with those of a commercial filtration aid.
2. Materials and methods Kenaf core chips containing B5% bast were obtained from Kenaf Product Marketing Incorporated (Atlanta, GA). An air stream was used to separate the large core chips from the bast fibers and smaller chips. The large chips were then ground in a heavy duty blender (Waring Model CBIO, New Hartford, CT). The resulting powder was separated into fractions with a vibratory siever (Retsch Model AS200, Haan, Germany) at
an amplitude of 1 mm, with material larger than 106 mm being reground. Filtration experiments were conducted in a constant pressure system in accordance with ASTM F796-88 (ASTM, 1988). Two filter precoats were examined: Diatomaceous Earth (DE, with a manufacturer claim of 28 mm average particle size, Alar Engineering Corporation, Mokena, IL), and kenaf fines (ground core which passed through a 38 mm sieve). Actual particle size distribution was determined by laser diffraction (Sympatec, Inc., Princeton, NJ). Each precoat to be tested was deposited onto an industrial filter (Ertel Engineering Company, Kingston, NY) with a surface area of 0.02206 m2. The filter was supported by a sheet of perforated metal, sealed with a rubber gasket. The entire assembly was clamped together and leveled. Two precoat densities were examined: 0.0227 and 0.0340 kg/m2. For precoat deposition, a sample of the precoat was suspended in 1 l of water and deposited onto the filter media at 699 3.5 kPa. Solutions to be filtered were pumped at a constant pressure of 699 3.5 kPa. Filtrate mass was recorded every 1.0 s with a balance (Mettler Model PM4000, Hightstown, NJ) and stored on a computer. The pour densities of the DE and kenaf fines were measured by filling a 100 ml glass graduated cylinder with each material without packing and measuring the tared mass. The compressed density was measured by placing a known mass of each material into a graduated 50 ml tube, centrifuging (300g for 10 min), and measuring the final volume. Based on the accuracy of volume measurement, the errors of the calculated compressed density were estimated to be 910%. The quantity of material leached from kenaf and DE particles was estimated by placing 4 g of each material into 400 ml of distilled deionized water at 25°C for 18 h, then centrifuging the solutions (1000g for 10 min). A sample of each supernatant was used for a chemical oxygen demand test (COD, Greenberg et al., 1992). Three challenge solutions were selected for study: yeast, bacteria and silica. The yeast solution contained Saccharomyces cere6isiae grown for 15–18 h at 28°C in a modified Wort media (mass/l): 7.5 g malt extract, 0.78 g peptone, 6.35 g
S.A. Lee, M.A. Eiteman / Industrial Crops and Products 13 (2001) 155–161
dextrose, 2.35 g glycerol, 1.0 g K2HPO4 and 1.0 g NH4C1, and then adjusted to a pH of 4.8 with phosphoric acid. Just prior to filtration, the solution was diluted with water to obtain a challenge solution having an optical absorbance of 0.50 at 620 nm which corresponds to a yeast dry mass concentration of 0.23 g/l or a number concentration of 1.2× 106 cells/l. The bacterial solution contained Lactobacillus acidophilus grown for 15 – 18 h at 30°C in media of (mass/l): 2 g dextrose, 2 g tryptone, 2 g yeast extract, 1 g K2HP04, adjusted to a pH of 6.0 with HCl. Analogous to the yeast challenge solution, this solution was diluted to an optical absorbance of 0.50 at 620 nm, which corresponds to a bacterial mass concentration of 0.16 g/l or a number concentration of 2.0×108 cells/ml. The silica challenge solution was prepared in accordance with NSF 50-1996 (NSF, 1996). This solution contained SIL-CO-SIL 106 silica (US Silica Co. Berkeley Springs, WV) at a turbidity of 40 NTU.
3. Results and discussion Photographs of each of the two filter precoat media examined in this study are shown in Fig. 1(a)(b). At the 500× magnification of these electron micrographs, the porous structure of each of the materials is clearly indicated. The diatomaceous earth (DE) particles were generally cylindrical, whereas the kenaf particles were irregular tubular fragments. The pour density of kenaf fines was measured to be 12392 kg/m3, and the pour density of DE was measured to be 2189 4 kg/m3. The compressed density of kenaf fines was 188 kg/m3, whereas the compressed density of DE was 286 kg/m3. Thus, the density of the kenaf samples increased by about 50% and the DE increased by about 30% on compression. The solution mixed with kenaf particles for 18 h leached oxidizable material at 55 mg COD/g kenaf. The solution mixed with DE particles did not leach a measureable amount of oxidizable material. The particle size distribution for samples of DE and kenaf fines is shown in Fig. 2. The kenaf fines volume-mean diameter was 26.190.5 mm and the DE volume-mean diameter was 25.090.8 mm.
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Also, the kenaf fines had a volume-median diameter of 22.39 0.2 mm, whereas DE had a volumemedian diameter of 20.490.5 mm. The two samples were similar in size distribution. Three challenge solutions having different particle sizes were selected for examining the filtration characteristics of DE and kenaf fines. The filtrate volumes collected with time for the two materials and two precoat densities using the yeast-containing solution are compared in Fig. 3. Although the four precoat treatments performed similarly for either the DE or kenaf, the higher precoat density yielded the greater filtrate volume at any given time. Also, the DE yielded the greater filtrate volume at both precoat densities. For the Lactobacillus challenge solutions, the two kenaf precoat densities and the two DE precoat densities performed similarly (Fig. 4). However, far less filtrate volume was collected for the kenaf media at any given time. For the silica particles, the filtration media performed equally, regardless of precoat density (Fig. 5). Each set of data in Figs. 3–5 was transformed to yield the volumetric filter flux as a function of time. The average fluxes for each of the four media as functions of filtrate volume for the yeast and bacterial challenge solutions are shown in Figs. 6 and 7, respectively. For the yeast solution initially, the flux through the kenaf media was about 20% less than the flux through the DE media for the lower precoat density. For the higher precoat density, the flux initially through the kenaf precoat was about 40% less. The fluxes through all four treatments decreased as additional filtrate passed through the media. For the bacterial solution, the initial fluxes through all precoat media were greater than for the yeast solution. The flux through the kenaf precoat decreased with filtrate volume similarly to the observations for the yeast solution. However, for the DE media the flux maintained its initial value for the duration of the collection. For all precoat media using the silica challenge solutions (Fig. 5), the fluxes maintained their initial rate of about 0.95× 10 − 3 m3/m2s. The hydrodynamic characteristics of the filtration media are shown in Figs. 3–7, but not how well the media actually exclude the particles. To
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Fig. 1. Electron micrographs of diatomaceous earth (a), kenaf fines (b).
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Fig. 2. Volume basis particle size distribution for diatomaceous earth ( ) and kenaf fines ( ). Data are based on three replicate analyses. Fig. 4. Filtrate volume collected during a constant-pressure (69 kPa) filtration of L. acidophilus challenge solutions using kenaf fines or diatomaceous earth as filter precoat. Two precoat densities were examined, 0.0227 and 0.0340 k g/m3.
determine the effectiveness of each of the three media in this regard, the optical absorbance for the filtrate as it just passed through the filter was measured as a function of filtration time. For the yeast challenge solution, the DE and kenaf media each removed virtually all of these particles throughout the duration of a 15 min filtration. Yeast particles are spherical and have 2 – 3 mm diameter. Similar results were obtained for the larger (100 mm) silica particles. Thus, the two types of precoat media, DE and kenaf, were similar in their abilities to exclude these relatively
‘large’ particles. For the bacterial challenge solution, the DE precoat of 0.0340 kg/m2 excluded only 6% of the particles after 3 min of filtration, whereas the kenaf precoat of the same density was able to exclude 45% of the particles at 3 min and still 36% of the particles after 15 min. The Lactobacillus challenge solution which contains long cylindrical particles of 0.5 mm or less on the narrowest diameter, indicates the limits of filtra-
Fig. 3. Filtrate volume collected during a constant-pressure (69 kPa) filtration of yeast challenge solutions with kenaf fines or diatomaceous earth as precoat. Two precoat densities were examined, 0.0227 and 0.0340 kg/m3.
Fig. 5. Filtrate volume collected during a constant-pressure (69 kPa) filtration of silica challenge solutions using kenaf fines or diatomaceous earth as filter precoat. Two precoat densities were examined, 0.0227 and 0.0340 k g/m3.
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the smaller yeast particles, kenaf and DE precoat both retained these particles completely. In this case, however, the kenaf precoat provided a lower flux than the DE precoat. For the smallest bacterial particles, the kenaf precoat retained the particles only partially, and the DE precoat did not retain these particles. Because only a very small fraction of the particles were retained by the DE precoat, the flux remained at the maximum initial value. The results indicate that kenaf and DE have similar filtration characteristics. Ground kenaf core, as a by-product of kenaf bast fiber production operations, may be used in filtration applications which have some tolerance for leached dissolved solids. Fig. 6. Volumetric filter flux as a function of filtrate volume during a constant-pressure (69 kPa) filtration of yeast challenge solutions using kenaf fines or diatomaceous earth as filter precoat. Two precoat densities were examined, 0.0227 and 0.0340 kg/m3.
tion for the media in this study without the addition of flocculants. In summary, for the filtration of 100 mm silica particles, both kenaf and DE precoats met the specifications for swimming pools and spas. For
Fig. 7. Volumetric filter flux as a function of filtrate volume during a constant-pressure (69 kPa) filtration of L. acidophilus challenge solutions using kenaf fines or diatomaceous earth as filter precoat. Two precoat densities were examined, 0.0227 and 0.0340 kg/m3.
Acknowledgements The authors are grateful to the Georgia Experiment Station, Georgia Pulp and Paper Industry Initiative, and A. Kaldor of Kenaf Product Marketing, Inc. for assistance in this research.
References ASTM F796-88, 1988. Standard Practice for Determing the Performance of a Filter Medium Employing a Single-Pass, Constant-Pressure, Liquid Test. Am. Soc. Testing Materials, West Consholiocken. Borazjam, A., Diehl, S., 1994. Kenaf core as an enhancer of bioremediation. In: Goforth, C.E., Fuller, M.J. (Eds.), A Summary of Kenaf Production and Product Development Research, 1989– 1993. Mississippi Agriculture and Forestry Experiment Station Bulletin 1011, Mississippi State University, pp. 26 – 27. Dickey, G.D., 1961. Filtration. Reinhold, New York, pp. 183– 187. Goforth, C.E., 1994. The evaluation of kenaf as an oil sorbent. In: Goforth, C.E., Fuller, M.J. (Eds.). A Summary of Kenaf Production and Product Development Research, 1989– 1993. Mississippi Agriculture and Forestry Experiment Station Bulletin 1011, Mississippi State University, p. 25. Greenberg, A.E., Clesceri, L.S., Eaton, A.D., 1992. Closed reflux, colorimetric method for chemical oxygen demand, 5220D. In: Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, DC, pp. 9 – 10. Jackson Jr, T.M., 1961. Filter aids speed up difficult filtrations. Chem. Eng. 68 (3), 141– 146.
S.A. Lee, M.A. Eiteman / Industrial Crops and Products 13 (2001) 155–161 Kaldor, A.F., Karlgren, C., Verwest, H., 1990. Kenaf — a fast growing fiber source for papermaking. Tappi J 73 (11), 205– 209. Kugler, D.E., 1996. Kenaf commercialization: 1986–1995. In: Janick, J. (Ed.), Progress in New Crops. ASHS Press, Alexandria, VA, pp. 129–132. Leppla, P.W., 1962. Filter aids to impove filtration. Ind. Eng. Chem. 54 (5), 40 – 43. NSF 50-1996, 1996. Circulation system components and related materials for swimming pools, spas/hot tubs, National Sanitation Foundation, Ann Arbor, MI. Ramaswamy, G.N., Boyd, C.R., 1994. Kenaf as a textile fiber: Processing, fiber quality, and product development. In: Goforth, C.E., Fuller, M.J. (Eds.), A Summary of Kenaf Production and Product Development Research, 1989– 1993. Mississippi Agriculture and Forestry Experiment Station Bulletin 1011, Mississippi State University, pp. 31– 33.
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Ramaswamy, G.N., Easter, E.P., 1997. Durability and aesthetic properties of kenaf/cotton blend fabrics. Textile Res. J. 67 (11), 803– 808. Sellers Jr, T., Miller, G.D., Fuller, M.J., 1993. Kenaf core as a board raw material. Forest Products J. 43 (7/8), 69 – 71. Sellers, Jr, T., Miller, G.D., Fuller, M.J., 1994. Kenaf core as a board raw material. In: Goforth, C.E., Fuller, M.J. (Eds.). A Summary of Kenaf Production and Product Development Research, 1989– 1993. Mississippi Agriculture and Forestry Experiment Station Bulletin 1011, Mississippi State University, pp. 28 – 29. Wang, Y.-T., 1994. Using ground kenaf stem core as a major component of container media. J. Am. Soc. Hort. Sci. 119 (5), 931– 935. Webber, C.L., Bledsoe, R.E., 1991. Kenaf. Production, harvesting, processing, and products. In: Janick, J., Simon, J.E. (Eds.), New Crops. Wiley, New York, pp. 416– 421.
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Ground kenaf core as a filtration aid Sarah A. Lee, Mark A. Eiteman * Department of Biological and Agricultural Engineering, Driftmier Engineering Center, Uni6ersity of Georgia, Athens, GA 30602, USA Received 7 June 1999; accepted 2 June 2000
Abstract The objective of this study was to examine the filtration characteristics of ground kenaf core, a waste material generated during the production of kenaf bast fibers for paper. The constant-pressure precoat filtration characteristics of ground kenaf core were compared to commercial diatomaceous earth (DE). Three challenge solutions were studied: a yeast solution, a bacterial solution, and a standard silica-particle solution. The kenaf and DE both satisfactorily permitted removal of all silica particles from solution without noticable flux degradation over the course of the filtration. The kenaf and DE also removed yeast particles. In this case, the flux loss with time was lower with the DE precoated filter than in the kenaf precoated filter. The DE precoat excluded less than 10% of the bacterial particles from solution, whereas the kenaf removed about 40% of these small particles. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Diatomaceous earth; Filtration; Kenaf; Hibiscus cannabinus; Filter aid; Flux
1. Introduction Kenaf (Hibiscus cannabinus) is an industrial crop of increasing importance. In 1995, approx. 1000 ha were grown in the United States for commercial purposes (Kugler, 1996). The kenaf plant is composed of an internal core comprising 60 – 75% of the plant and outer bast fibers totaling 25 – 40% (Sellers et al., 1993). Recent research has demonstrated numerous potential uses for each of these two materials, which often must be sepa-
* Corresponding author. Tel.: +1-706-5427801; fax: +1706-5428806. E-mail address: [email protected] (M.A. Eiteman).
rated from each other. Products from the kenaf core include oil/chemical adsorbents in place of polypropylene (Goforth, 1994), horticultural mixes (Wang, 1994), bedding material for animals (Kugler, 1996), and insulation paneling (Ramaswamy and Easter, 1997). The absorption properties of kenaf core have been shown to enhance bioremediation (Borazjani and Diehl, 1994). Low-density core panels have potential for sound absorption or thermal insulation (Sellers et al., 1994). Mats for grass seeding and erosion control, and printing and writing papers have become commercial products from bast fibers (Kaldor et al., 1990 Ramaswamy and Easter, 1997). Bast fibers can be mixed with polypropylene for making nonwoven textile products includ-
0926-6690/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 6 6 9 0 ( 0 0 ) 0 0 0 6 2 - 5
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ing wallpaper backing and furniture underlays (Ramaswamy and Boyd, 1994), or blended with cotton for apparel (Ramaswamy and Easter, 1997). The whole plant has high protein, good digestibility and may be pelletized (Webber and Bledsoe, 1991). During the production of kenaf bast fibers for paper, a significant volume of waste kenaf core dustings are generated. Loose, inert particles of uniform size and structure such as ground kenaf have potential in filtration. Examples of the broad application of filters include the filtration of drinking water, swimming pool water or in industrial settings such as sugar refineries and breweries (Dickey, 1961). Because of the material’s light density, inertness, insolubility, low expense and homogeneity, ground kenaf core might be a suitable filtration aid for certain applications. Filter aids are used to improve filtration rates and the retention of particles by forming a porous, permeable and rigid lattice structure on the surface of a filtration medium (Jackson, 1961; Leppla, 1962). A filtration aid may be mixed with the challenge solution to be filtered prior to filtration, or alternatively it can be used as a precoat, applied before filtration. The principal purpose of precoating is to protect the filter medium from clogging by compressible particles in the challenge solution. In continuous applications, particles can be removed from the filter medium and replaced with fresh precoat. The objectives of this study were to examine the filtration characteristics of ground kenaf core and compare the performance with those of a commercial filtration aid.
2. Materials and methods Kenaf core chips containing B5% bast were obtained from Kenaf Product Marketing Incorporated (Atlanta, GA). An air stream was used to separate the large core chips from the bast fibers and smaller chips. The large chips were then ground in a heavy duty blender (Waring Model CBIO, New Hartford, CT). The resulting powder was separated into fractions with a vibratory siever (Retsch Model AS200, Haan, Germany) at
an amplitude of 1 mm, with material larger than 106 mm being reground. Filtration experiments were conducted in a constant pressure system in accordance with ASTM F796-88 (ASTM, 1988). Two filter precoats were examined: Diatomaceous Earth (DE, with a manufacturer claim of 28 mm average particle size, Alar Engineering Corporation, Mokena, IL), and kenaf fines (ground core which passed through a 38 mm sieve). Actual particle size distribution was determined by laser diffraction (Sympatec, Inc., Princeton, NJ). Each precoat to be tested was deposited onto an industrial filter (Ertel Engineering Company, Kingston, NY) with a surface area of 0.02206 m2. The filter was supported by a sheet of perforated metal, sealed with a rubber gasket. The entire assembly was clamped together and leveled. Two precoat densities were examined: 0.0227 and 0.0340 kg/m2. For precoat deposition, a sample of the precoat was suspended in 1 l of water and deposited onto the filter media at 699 3.5 kPa. Solutions to be filtered were pumped at a constant pressure of 699 3.5 kPa. Filtrate mass was recorded every 1.0 s with a balance (Mettler Model PM4000, Hightstown, NJ) and stored on a computer. The pour densities of the DE and kenaf fines were measured by filling a 100 ml glass graduated cylinder with each material without packing and measuring the tared mass. The compressed density was measured by placing a known mass of each material into a graduated 50 ml tube, centrifuging (300g for 10 min), and measuring the final volume. Based on the accuracy of volume measurement, the errors of the calculated compressed density were estimated to be 910%. The quantity of material leached from kenaf and DE particles was estimated by placing 4 g of each material into 400 ml of distilled deionized water at 25°C for 18 h, then centrifuging the solutions (1000g for 10 min). A sample of each supernatant was used for a chemical oxygen demand test (COD, Greenberg et al., 1992). Three challenge solutions were selected for study: yeast, bacteria and silica. The yeast solution contained Saccharomyces cere6isiae grown for 15–18 h at 28°C in a modified Wort media (mass/l): 7.5 g malt extract, 0.78 g peptone, 6.35 g
S.A. Lee, M.A. Eiteman / Industrial Crops and Products 13 (2001) 155–161
dextrose, 2.35 g glycerol, 1.0 g K2HPO4 and 1.0 g NH4C1, and then adjusted to a pH of 4.8 with phosphoric acid. Just prior to filtration, the solution was diluted with water to obtain a challenge solution having an optical absorbance of 0.50 at 620 nm which corresponds to a yeast dry mass concentration of 0.23 g/l or a number concentration of 1.2× 106 cells/l. The bacterial solution contained Lactobacillus acidophilus grown for 15 – 18 h at 30°C in media of (mass/l): 2 g dextrose, 2 g tryptone, 2 g yeast extract, 1 g K2HP04, adjusted to a pH of 6.0 with HCl. Analogous to the yeast challenge solution, this solution was diluted to an optical absorbance of 0.50 at 620 nm, which corresponds to a bacterial mass concentration of 0.16 g/l or a number concentration of 2.0×108 cells/ml. The silica challenge solution was prepared in accordance with NSF 50-1996 (NSF, 1996). This solution contained SIL-CO-SIL 106 silica (US Silica Co. Berkeley Springs, WV) at a turbidity of 40 NTU.
3. Results and discussion Photographs of each of the two filter precoat media examined in this study are shown in Fig. 1(a)(b). At the 500× magnification of these electron micrographs, the porous structure of each of the materials is clearly indicated. The diatomaceous earth (DE) particles were generally cylindrical, whereas the kenaf particles were irregular tubular fragments. The pour density of kenaf fines was measured to be 12392 kg/m3, and the pour density of DE was measured to be 2189 4 kg/m3. The compressed density of kenaf fines was 188 kg/m3, whereas the compressed density of DE was 286 kg/m3. Thus, the density of the kenaf samples increased by about 50% and the DE increased by about 30% on compression. The solution mixed with kenaf particles for 18 h leached oxidizable material at 55 mg COD/g kenaf. The solution mixed with DE particles did not leach a measureable amount of oxidizable material. The particle size distribution for samples of DE and kenaf fines is shown in Fig. 2. The kenaf fines volume-mean diameter was 26.190.5 mm and the DE volume-mean diameter was 25.090.8 mm.
157
Also, the kenaf fines had a volume-median diameter of 22.39 0.2 mm, whereas DE had a volumemedian diameter of 20.490.5 mm. The two samples were similar in size distribution. Three challenge solutions having different particle sizes were selected for examining the filtration characteristics of DE and kenaf fines. The filtrate volumes collected with time for the two materials and two precoat densities using the yeast-containing solution are compared in Fig. 3. Although the four precoat treatments performed similarly for either the DE or kenaf, the higher precoat density yielded the greater filtrate volume at any given time. Also, the DE yielded the greater filtrate volume at both precoat densities. For the Lactobacillus challenge solutions, the two kenaf precoat densities and the two DE precoat densities performed similarly (Fig. 4). However, far less filtrate volume was collected for the kenaf media at any given time. For the silica particles, the filtration media performed equally, regardless of precoat density (Fig. 5). Each set of data in Figs. 3–5 was transformed to yield the volumetric filter flux as a function of time. The average fluxes for each of the four media as functions of filtrate volume for the yeast and bacterial challenge solutions are shown in Figs. 6 and 7, respectively. For the yeast solution initially, the flux through the kenaf media was about 20% less than the flux through the DE media for the lower precoat density. For the higher precoat density, the flux initially through the kenaf precoat was about 40% less. The fluxes through all four treatments decreased as additional filtrate passed through the media. For the bacterial solution, the initial fluxes through all precoat media were greater than for the yeast solution. The flux through the kenaf precoat decreased with filtrate volume similarly to the observations for the yeast solution. However, for the DE media the flux maintained its initial value for the duration of the collection. For all precoat media using the silica challenge solutions (Fig. 5), the fluxes maintained their initial rate of about 0.95× 10 − 3 m3/m2s. The hydrodynamic characteristics of the filtration media are shown in Figs. 3–7, but not how well the media actually exclude the particles. To
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Fig. 1. Electron micrographs of diatomaceous earth (a), kenaf fines (b).
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159
Fig. 2. Volume basis particle size distribution for diatomaceous earth ( ) and kenaf fines ( ). Data are based on three replicate analyses. Fig. 4. Filtrate volume collected during a constant-pressure (69 kPa) filtration of L. acidophilus challenge solutions using kenaf fines or diatomaceous earth as filter precoat. Two precoat densities were examined, 0.0227 and 0.0340 k g/m3.
determine the effectiveness of each of the three media in this regard, the optical absorbance for the filtrate as it just passed through the filter was measured as a function of filtration time. For the yeast challenge solution, the DE and kenaf media each removed virtually all of these particles throughout the duration of a 15 min filtration. Yeast particles are spherical and have 2 – 3 mm diameter. Similar results were obtained for the larger (100 mm) silica particles. Thus, the two types of precoat media, DE and kenaf, were similar in their abilities to exclude these relatively
‘large’ particles. For the bacterial challenge solution, the DE precoat of 0.0340 kg/m2 excluded only 6% of the particles after 3 min of filtration, whereas the kenaf precoat of the same density was able to exclude 45% of the particles at 3 min and still 36% of the particles after 15 min. The Lactobacillus challenge solution which contains long cylindrical particles of 0.5 mm or less on the narrowest diameter, indicates the limits of filtra-
Fig. 3. Filtrate volume collected during a constant-pressure (69 kPa) filtration of yeast challenge solutions with kenaf fines or diatomaceous earth as precoat. Two precoat densities were examined, 0.0227 and 0.0340 kg/m3.
Fig. 5. Filtrate volume collected during a constant-pressure (69 kPa) filtration of silica challenge solutions using kenaf fines or diatomaceous earth as filter precoat. Two precoat densities were examined, 0.0227 and 0.0340 k g/m3.
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the smaller yeast particles, kenaf and DE precoat both retained these particles completely. In this case, however, the kenaf precoat provided a lower flux than the DE precoat. For the smallest bacterial particles, the kenaf precoat retained the particles only partially, and the DE precoat did not retain these particles. Because only a very small fraction of the particles were retained by the DE precoat, the flux remained at the maximum initial value. The results indicate that kenaf and DE have similar filtration characteristics. Ground kenaf core, as a by-product of kenaf bast fiber production operations, may be used in filtration applications which have some tolerance for leached dissolved solids. Fig. 6. Volumetric filter flux as a function of filtrate volume during a constant-pressure (69 kPa) filtration of yeast challenge solutions using kenaf fines or diatomaceous earth as filter precoat. Two precoat densities were examined, 0.0227 and 0.0340 kg/m3.
tion for the media in this study without the addition of flocculants. In summary, for the filtration of 100 mm silica particles, both kenaf and DE precoats met the specifications for swimming pools and spas. For
Fig. 7. Volumetric filter flux as a function of filtrate volume during a constant-pressure (69 kPa) filtration of L. acidophilus challenge solutions using kenaf fines or diatomaceous earth as filter precoat. Two precoat densities were examined, 0.0227 and 0.0340 kg/m3.
Acknowledgements The authors are grateful to the Georgia Experiment Station, Georgia Pulp and Paper Industry Initiative, and A. Kaldor of Kenaf Product Marketing, Inc. for assistance in this research.
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