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Production and recovery of volatile fatty acids from fermentation broth
Energy Conversion & Management 40 (1999) 1543±1553
Production and recovery of volatile fatty acids from fermentation broth N.A. Mostafa Department of Chemical Engineering, Minia University, El-Minia, Egypt Received 19 June 1998; accepted 22 January 1999
Abstract The production of volatile fatty acids (VFAs) by anaerobic fermentation of wheat milling waste residues (akalona) either as a solid or hydrolyzate and whey was studied using fresh rumen ¯uid as a mixed culture for anaerobic digestion of the organic residues. Maximum VFAs production was obtained from akalona followed by whey after 8 days. The lower VFAs production was obtained from akalona hydrolyzate after 7 days. VFAs recovery from fermentation broth by liquid±liquid extraction using a mixed solvent (tri-n-octylphosphine oxide in kerosene) followed by a pure solvent was studied. Experiments were run covering the extraction kinetics of tri-n-octylphosphine in the diluent kerosene under the optimum conditions for extraction of VFAs with mixed solvent and with pure solvent only, and the per cent recovery of VFAs was calculated in both steps. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Volatile fatty acids; Whey; Akalona; Extraction; Anaerobic fermentation
1. Introduction Much of the present world output of acetic acid comes from petroleum, while microbial oxidation of ethanol to acetic acid in the so-called quick vinegar process has long been known. Recently, newer processes have been under development for the manufacture of acetic acid and other volatile aliphatic acids (VFAs) by anaerobic fermentation of cellulosic and other wastes. Bioconversion of cellulose to acetate was accomplished with cocultures of two organisms. One was the cellulolytic species Ruminococcus albus. The other was a hydrogen using acetogen (HA). The major product of the fermentation by R. albus and HA coculture is acetate. High concentrations of acetate (333 mM) were obtained when batch cocultures grown on 5% cellulose were neutralized with Ca(OH)2. Continuous cocultures grown at retention times of 2 0196-8904/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 1 9 6 - 8 9 0 4 ( 9 9 ) 0 0 0 4 3 - 6
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and 3.1 days produced 109 and 102 mM acetate, respectively, when fed 1% cellulose with utilization of 84% of the substrate [1]. Soil obtained from a beech forest formed signi®cant amounts of acetate when incubated in a bicarbonate-buered, mineral salt solution under anaerobic conditions at both 5 and 208C (21 and 38 g of acetate per kg (dry weight) of soil, respectively) [2]. The production of volatile fatty acids (VFAs) by anaerobic fermentation of municipal solid wastes was studied at pilot-plant level. A plug-¯ow reactor (80 l total volume) without solid or liquid recirculation was employed to digest a mixture of two types of organic fraction of the municipal solid waste (OFMSW): OFMSW mechanically selected and OFMSW coming from a market of fruit and vegetables. The acidogenic process was studied at dierent retention times (between 2 and 6 days) in the mesophilic (378C 2 28C) range of temperature. The VFA concentration obtained in the ®rst valve of the tubular reactor ranged from 9.1 to 13.4 g lÿ1 and in the outlet sludge, it oscillated between 11.8 and 23.1 g lÿ1, increasing when increasing the retention time from 2 to 6 days [3]. In acetic acid fermentation by Acetoboeter aceti, the acetic acid produced inhibits the production of acetic acid by this microorganism. To alleviate this inhibitory eect, an electrodialysis fermentation method was developed such that acetic acid is continuously removed from the broth. The fermentation unit has a computerized system for control of the pH and the concentration of ethanol in the fermentation broth. The electrodialysis fermentation system resulted in improved cell growth and higher productivity over an extended period. The maximum productivity of the electrodialysis fermentation was 2.13 g hÿ1, a rate which was 1.35 times higher than that of the non-pH controlled fermentation (1.58 g hÿ1) [4]. A series of microporous membranes and polymer ®lms was examined for mass transfer performance in the membrane based extraction of propionic and acetic acids. A range of organic solvents and acid-complexing carriers was screened for toxic eects on a strain of Propionibacterium acidipropionici. Based on these results, more extensive studies were made of the extraction kinetics of tri-n-octylphosphine oxide in the diluents decane and kerosene in a celgarde X-20 microporous membrane system which was chosen for use in an extractive fermentation with the membrane between the fermentation broth and the bulk extractant in a hollow ®ber module [5]. In the anaerobic digestion of sewage, two types of organisms are present, namely those converting organics to VFAs and those converting the VFAs to CH4 and CO2. The action of heat on cultures from normal sewage sludge (808C for 15 min) usually kills the methane producing organisms, and alternatively, the addition of 2-bromoethane sulphonic acid selectively inhibits the methanogenic organisms in whole sewage sludge [6]. The objective of the present study was to investigate an alternative means for the success of acidogenic fermentation of cellulosic and non-cellulosic organic residues and eliminating methanogensis. As a result, volatile fatty acids, such as acetic, propionic, etc., have been accumulated as the end products. The second part of this study was conducted to investigate the optimum conditions for the extraction of VFAs with mixed solvent and with a pure solvent.
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2. Materials and methods Akalona was obtained from the Middle Egypt Milling Company in El-Minia. The akalona was hydrolyzed by autoclaving (1188C, 20 min) with (10% H2SO4). The hydrolyzate was then neutralized with calcium hydroxide, and the calcium sulfate formed was removed. Whey, which is a waste product from the dairy and cheese industry (containing 5% lactose), was used as a substrate. 2.1. Microorganism All experiments were conducted with a fresh rumen ¯uid of cow which was obtained from the slaughter house. Kerosene, tri-n-octylphosphine oxide, glacial acetic acid, propionic acid, HCl (A.R) and sodium hydroxide were used. 2.2. Fermentation Fermentation runs were conducted in a 1.5 l ¯ask (700 ml working volume) each substrate was inoculated with 200 ml fresh rumen ¯uid which was pre-®ltered and preheated at 808C for 20 min to inhibit the methanogenic bacteria. The fermentation process was maintained at the desired temperature and pH 7 for 7±10 days. Four dierent temperatures (25, 35, 37 and 408C) were tested to show the eect of temperature on the rate and yield of VFAs production. 2.3. Recovery of VFAs The mixed solvent (tri-n-octylphoshine oxide in kerosene) was prepared by dissolving the desired amount of tri-n-octylphosphine oxide (TOPO) in kerosene using a magnetic stirrer for complete mixing. The dierent concentrations of mixed solvent used in this study were 5, 10, 15 and 20% TOPO in kerosene. Partition coecients for each of propionic acid and acetic acid were determined for extractant containing 20% TOPO in kerosene by mixing equal volumes (10 ml) of the aqueous (0.1±0.65 M propionic acid or 0.08±0.45 M acetic acid) and organic phases in a shaker bath at 110 rpm and 258C for 1 h. Partition coecients for the mixed propionic/acetic acid solutions, with initial propionic : acetic acid weight ratio of 1 : 1 were determined as mentioned above. The amount of acid in the aqueous phase was determined by titration using a Markham apparatus [7], while the organic phase concentration was calculated by mass balance. The fermentation broth was ®rstly centrifuged to separate microbial cells and solid particles. Then various volume ratios of the clari®ed liquid phase and 20% TOPO in kerosene as a mixed solvent were used at constant temperature (308C) and at 120 rpm for 2 h to show the eect of solvent : feed ratio on the percent recovery of VFAs. The clari®ed liquid was extracted using a 1 : 1 solvent : feed ratio at 308C for 2 h and at dierent agitation rates (50, 80, 100 and 120 rpm) to show the eect of agitation rate on the per cent recovery of VFAs. Also, two dierent temperatures (25 and 308C) were tested. The aqueous phase from the ®rst step of extraction (with mixed solvent) was extracted using a pure
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solvent (kerosene) at constant temperature (308C) and constant rpm (120) for 2 h at two dierent solvent : feed ratios (1 : 1 and 1 : 2). 3. Results and discussion 3.1. Production of VFAs in batch fermentations Data for the eect of temperature on the production of VFAs from akalona is presented in Fig. 1. The results indicate that after 1 day of fermentation, the per cent VFAs was maximum at 378C followed by that at 408C while lower values were obtained at both 25 and 358C. Then after 2 days of fermentation, the per cent VFAs at 408C was increased thus approaching the values of that at 358C during the period from 3 to 5 days. After 6 days of fermentation, the per cent VFAs at 358C was increased to become nearly the same as that obtained at 378C. Consequently, the production of VFAs from akalona at 378C is recommended for both higher per cent yield of VFAs and a shorter time (nearly after 4 days). Fig. 2 shows the eect of temperature on the production of VFAs from akalona hydrolyzate. As shown in the ®gure, the maximum per cent VFAs was obtained at 378C followed by 40 and 358C. These latter temperatures gave almost the same per cent VFAs at the end of the fermentation period. On the other hand, the lower per cent VFAs was obtained at 258C. Fig. 3 shows the production of VFAs from akalona, akalona hydrolyzate and whey at 378C. As shown in the ®gure, the maximum per cent VFAs was obtained from akalona (28%)
Fig. 1. Eect of temperature on the production of VFAs from akalona.
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Fig. 2. Eect of temperature on the production of VFAs from akalona hydrolyzate.
Fig. 3. Eect of type of substrate on the production of VFAs.
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followed by whey (16.3%) and then by akalona hydrolyzate (13%). This observation can be explained by the fact that solid akalona as a substrate was free from toxic and undesirable substances such as the toxic compound in akalona hydrolyzate (due to acid hydrolysis) and the salt contained in whey which inhibit the growth of microorganisms and then reduce the rate and yield of VFAs production. This leads to the conclusion that solid akalona is the suitable substrate for VFAs production compared with whey and akalona hydrolyzate. 3.2. Recovery of VFAs Fig. 4 illustrates the eect of TOPO concentration in kerosene on the partitioning behaviour of aqueous propionic acid solutions ranging from 0.1 to 0.65 M initial concentration. As shown in the ®gure, the amount of propionic acid distributed to the organic phase increased by increasing the concentration of TOPO in the organic phase at the dierent initial concentrations. The maximum value (0.41 M) of propionic acid was obtained in the organic phase containing 20% TOPO in kerosene and these results con®rm with the literature [5, 6]. This leads to the conclusion that 20% TOPO in kerosene is the optimum concentration of this mixed solvent for higher partitioning of aqueous propionic acid solutions. Figs. 5 and 6 represent the results of using 20% TOPO in kerosene for partitioning each of propionic acid and acetic acid, respectively. As shown in the ®gures, the partition coecients for propionic acid are higher than for acetic acid at all initial concentrations. The partition coecients for propionic acid are about three to ®ve times the partition coecients for acetic acid. The TOPO loading increases by increasing the initial acid concentration for both
Fig. 4. Equilibrium distribution curves for propionic acid for a series of concentrations of TOPO in kerosene.
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Fig. 5. Equilibrium distribution curve (), partition coecients (D), and loading (q) for propionic acid solutions into 20% TOPO in kerosene.
propionic and acetic acid and becomes constant for acetic acid concentrations of 0.4 and 0.45 M, as proved by extending the experimental work. Also, the TOPO loading for propionic acid is higher than that for acetic acid, which can be explained by the fact that the per cent removal of propionic acid in TOPO was greater than that of acetic acid. Fig. 7 shows that the partition coecients for mixed acetic/propionic acid were lower than
Fig. 6. Equilibrium distribution curve (), partition coecients (D), and loading (q) for acetic acid solutions into 20% TOPO in kerosene.
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Fig. 7. Equilibrium distribution curve (w), partition coecients (D), and loading (q) for mixed acetic/propionic acid solutions into 20% TOPO in kerosene.
for propionic acid alone and this is due to the lower partition coecient for acetic acid. Also, the TOPO loading was lower than that for propionic acid only. Fig. 8 illustrates the eect of solvent to feed ratio on the per cent removal of VFAs from the fermentation broth into 20% TOPO in kerosene. It is clear that the per cent removal of VFAs
Fig. 8. Eect of solvent to feed ratio on extraction of VFAs from fermentation broth into 20% TOPO in kerosene.
N.A. Mostafa / Energy Conversion & Management 40 (1999) 1543±1553
Fig. 9. Eect of agitation rate on extraction of VFAs from fermentation broth into 20% TOPO in kerosene.
Fig. 10. Eect of temperature on extraction of VFAs from fermentation broth into 20% TOPO in kerosene.
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Fig. 11. Eect of pure solvent (kerosene) on extraction of VFAs from fermentation broth.
increased by increasing the solvent to feed ratio. On the other hand, the per cent removal at a S=F ratio of 2 : 1 was almost the same as at the S : F ratio of 1 : 1. Therefore, the optimum S : F ratio of 1:1 is most suitable from an economic point of view. In addition, separation of the two phases is more dicult in the case of a S : F ratio of 2 : 1. The results of using dierent agitation rates (50, 80, 100 and 120 rpm) for extraction are represented in Fig. 9. As shown in the ®gure, the per cent removal of VFAs was increased by increasing the agitation rate from 50 to 120 rpm. Fig. 10 illustrates the eect of temperature on extraction. As shown in the ®gure, the per cent removal of VFAs increases by increasing the temperature from 25 to 308C. Therefore, extraction at a temperature of 308C is more preferable. From Figs. 8±10, the per cent removal of VFAs becomes constant after 2 h extraction. This leads to the conclusion that 2 h were sucient for maximum per cent removal during the extraction. Fig. 11 shows the eect of kerosene only for complete extraction of VFAs. As shown in the ®gure, the extraction with kerosene at a S : F ratio of 2 : 1 tends to increase the per cent removal of VFAs. This increase may be attributed to transferring more VFAs by kerosene than in the case of mixed solvent due to the separation of higher VFAs than the propionic acid. 4. Conclusion Akalona, a wheat milling waste residue, provides a higher per cent yield of VFAs at a
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relatively short fermentation time of 4 days. It may be a favorable substrate for production of VFAs compared with whey and akalona hydrolyzate. Acid prehydrolysis of cellulosic residues is not a recommended route for VFAs production. Whey can be used for such purpose. The optimum operating conditions for VFAs extraction from fermentation broth using a mixed solvent (20% TOPO in kerosene) are: S : F : : 1 : 1, 308C, 120 rpm for 2 h. The two successive extraction processes using a mixed solvent followed by a pure solvent tend to increase the per cent removal of VFAs and good separation of lighter VFAs from higher VFAs. References [1] Miller TL, Wolin MJ. Bioconversion of cellulose to acetate with pure cultures of Ruminococcus albus and a hydrogen-using acetogen. J Applied and Environmental Microbiology 1995;61(11):3832±5. [2] Kusel K, Drake HL. Acetate synthesis in soil from a Bavarian beech forest. J Applied and Environmental Microbiology 1994;60(4):1370±3. [3] Sans C, Mata Alvarez J, Cecchi F, Pavan P. Modelling of a plug-¯ow pilot reactor producing VFA by anaerobic fermentation of municipal solid wastes. Water Science Technology 1994;30(12):125±32. [4] Nomura Y, Iwahara M, Hongo M. Acetic acid production by an electrodialysis fermentation method with a computerized control system. J Applied and Environmental Microbiology 1988;54(1):137±42. [5] Solichien MS, Brien OD, Hammond EG, Glatz CE. Membrane based extractive fermentation to produce propionic and acetic acid: toxicity and mass transfer consideration. J Enzyme and Microbial Technology 1995;17:23±31. [6] Bulock J, Kristiansen B. In: Basic biotechnology. New York: Academic Press, 1989. p. 372±4. [7] Warner AC. Production of volatile fatty acids in the rumen: methods of measurement. The Commonwealth Bureau of Animal Nutrition [special issue]. Nutrition Abstracts and Reviews 1964;34(2):339±51.
Production and recovery of volatile fatty acids from fermentation broth N.A. Mostafa Department of Chemical Engineering, Minia University, El-Minia, Egypt Received 19 June 1998; accepted 22 January 1999
Abstract The production of volatile fatty acids (VFAs) by anaerobic fermentation of wheat milling waste residues (akalona) either as a solid or hydrolyzate and whey was studied using fresh rumen ¯uid as a mixed culture for anaerobic digestion of the organic residues. Maximum VFAs production was obtained from akalona followed by whey after 8 days. The lower VFAs production was obtained from akalona hydrolyzate after 7 days. VFAs recovery from fermentation broth by liquid±liquid extraction using a mixed solvent (tri-n-octylphosphine oxide in kerosene) followed by a pure solvent was studied. Experiments were run covering the extraction kinetics of tri-n-octylphosphine in the diluent kerosene under the optimum conditions for extraction of VFAs with mixed solvent and with pure solvent only, and the per cent recovery of VFAs was calculated in both steps. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Volatile fatty acids; Whey; Akalona; Extraction; Anaerobic fermentation
1. Introduction Much of the present world output of acetic acid comes from petroleum, while microbial oxidation of ethanol to acetic acid in the so-called quick vinegar process has long been known. Recently, newer processes have been under development for the manufacture of acetic acid and other volatile aliphatic acids (VFAs) by anaerobic fermentation of cellulosic and other wastes. Bioconversion of cellulose to acetate was accomplished with cocultures of two organisms. One was the cellulolytic species Ruminococcus albus. The other was a hydrogen using acetogen (HA). The major product of the fermentation by R. albus and HA coculture is acetate. High concentrations of acetate (333 mM) were obtained when batch cocultures grown on 5% cellulose were neutralized with Ca(OH)2. Continuous cocultures grown at retention times of 2 0196-8904/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 1 9 6 - 8 9 0 4 ( 9 9 ) 0 0 0 4 3 - 6
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and 3.1 days produced 109 and 102 mM acetate, respectively, when fed 1% cellulose with utilization of 84% of the substrate [1]. Soil obtained from a beech forest formed signi®cant amounts of acetate when incubated in a bicarbonate-buered, mineral salt solution under anaerobic conditions at both 5 and 208C (21 and 38 g of acetate per kg (dry weight) of soil, respectively) [2]. The production of volatile fatty acids (VFAs) by anaerobic fermentation of municipal solid wastes was studied at pilot-plant level. A plug-¯ow reactor (80 l total volume) without solid or liquid recirculation was employed to digest a mixture of two types of organic fraction of the municipal solid waste (OFMSW): OFMSW mechanically selected and OFMSW coming from a market of fruit and vegetables. The acidogenic process was studied at dierent retention times (between 2 and 6 days) in the mesophilic (378C 2 28C) range of temperature. The VFA concentration obtained in the ®rst valve of the tubular reactor ranged from 9.1 to 13.4 g lÿ1 and in the outlet sludge, it oscillated between 11.8 and 23.1 g lÿ1, increasing when increasing the retention time from 2 to 6 days [3]. In acetic acid fermentation by Acetoboeter aceti, the acetic acid produced inhibits the production of acetic acid by this microorganism. To alleviate this inhibitory eect, an electrodialysis fermentation method was developed such that acetic acid is continuously removed from the broth. The fermentation unit has a computerized system for control of the pH and the concentration of ethanol in the fermentation broth. The electrodialysis fermentation system resulted in improved cell growth and higher productivity over an extended period. The maximum productivity of the electrodialysis fermentation was 2.13 g hÿ1, a rate which was 1.35 times higher than that of the non-pH controlled fermentation (1.58 g hÿ1) [4]. A series of microporous membranes and polymer ®lms was examined for mass transfer performance in the membrane based extraction of propionic and acetic acids. A range of organic solvents and acid-complexing carriers was screened for toxic eects on a strain of Propionibacterium acidipropionici. Based on these results, more extensive studies were made of the extraction kinetics of tri-n-octylphosphine oxide in the diluents decane and kerosene in a celgarde X-20 microporous membrane system which was chosen for use in an extractive fermentation with the membrane between the fermentation broth and the bulk extractant in a hollow ®ber module [5]. In the anaerobic digestion of sewage, two types of organisms are present, namely those converting organics to VFAs and those converting the VFAs to CH4 and CO2. The action of heat on cultures from normal sewage sludge (808C for 15 min) usually kills the methane producing organisms, and alternatively, the addition of 2-bromoethane sulphonic acid selectively inhibits the methanogenic organisms in whole sewage sludge [6]. The objective of the present study was to investigate an alternative means for the success of acidogenic fermentation of cellulosic and non-cellulosic organic residues and eliminating methanogensis. As a result, volatile fatty acids, such as acetic, propionic, etc., have been accumulated as the end products. The second part of this study was conducted to investigate the optimum conditions for the extraction of VFAs with mixed solvent and with a pure solvent.
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2. Materials and methods Akalona was obtained from the Middle Egypt Milling Company in El-Minia. The akalona was hydrolyzed by autoclaving (1188C, 20 min) with (10% H2SO4). The hydrolyzate was then neutralized with calcium hydroxide, and the calcium sulfate formed was removed. Whey, which is a waste product from the dairy and cheese industry (containing 5% lactose), was used as a substrate. 2.1. Microorganism All experiments were conducted with a fresh rumen ¯uid of cow which was obtained from the slaughter house. Kerosene, tri-n-octylphosphine oxide, glacial acetic acid, propionic acid, HCl (A.R) and sodium hydroxide were used. 2.2. Fermentation Fermentation runs were conducted in a 1.5 l ¯ask (700 ml working volume) each substrate was inoculated with 200 ml fresh rumen ¯uid which was pre-®ltered and preheated at 808C for 20 min to inhibit the methanogenic bacteria. The fermentation process was maintained at the desired temperature and pH 7 for 7±10 days. Four dierent temperatures (25, 35, 37 and 408C) were tested to show the eect of temperature on the rate and yield of VFAs production. 2.3. Recovery of VFAs The mixed solvent (tri-n-octylphoshine oxide in kerosene) was prepared by dissolving the desired amount of tri-n-octylphosphine oxide (TOPO) in kerosene using a magnetic stirrer for complete mixing. The dierent concentrations of mixed solvent used in this study were 5, 10, 15 and 20% TOPO in kerosene. Partition coecients for each of propionic acid and acetic acid were determined for extractant containing 20% TOPO in kerosene by mixing equal volumes (10 ml) of the aqueous (0.1±0.65 M propionic acid or 0.08±0.45 M acetic acid) and organic phases in a shaker bath at 110 rpm and 258C for 1 h. Partition coecients for the mixed propionic/acetic acid solutions, with initial propionic : acetic acid weight ratio of 1 : 1 were determined as mentioned above. The amount of acid in the aqueous phase was determined by titration using a Markham apparatus [7], while the organic phase concentration was calculated by mass balance. The fermentation broth was ®rstly centrifuged to separate microbial cells and solid particles. Then various volume ratios of the clari®ed liquid phase and 20% TOPO in kerosene as a mixed solvent were used at constant temperature (308C) and at 120 rpm for 2 h to show the eect of solvent : feed ratio on the percent recovery of VFAs. The clari®ed liquid was extracted using a 1 : 1 solvent : feed ratio at 308C for 2 h and at dierent agitation rates (50, 80, 100 and 120 rpm) to show the eect of agitation rate on the per cent recovery of VFAs. Also, two dierent temperatures (25 and 308C) were tested. The aqueous phase from the ®rst step of extraction (with mixed solvent) was extracted using a pure
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solvent (kerosene) at constant temperature (308C) and constant rpm (120) for 2 h at two dierent solvent : feed ratios (1 : 1 and 1 : 2). 3. Results and discussion 3.1. Production of VFAs in batch fermentations Data for the eect of temperature on the production of VFAs from akalona is presented in Fig. 1. The results indicate that after 1 day of fermentation, the per cent VFAs was maximum at 378C followed by that at 408C while lower values were obtained at both 25 and 358C. Then after 2 days of fermentation, the per cent VFAs at 408C was increased thus approaching the values of that at 358C during the period from 3 to 5 days. After 6 days of fermentation, the per cent VFAs at 358C was increased to become nearly the same as that obtained at 378C. Consequently, the production of VFAs from akalona at 378C is recommended for both higher per cent yield of VFAs and a shorter time (nearly after 4 days). Fig. 2 shows the eect of temperature on the production of VFAs from akalona hydrolyzate. As shown in the ®gure, the maximum per cent VFAs was obtained at 378C followed by 40 and 358C. These latter temperatures gave almost the same per cent VFAs at the end of the fermentation period. On the other hand, the lower per cent VFAs was obtained at 258C. Fig. 3 shows the production of VFAs from akalona, akalona hydrolyzate and whey at 378C. As shown in the ®gure, the maximum per cent VFAs was obtained from akalona (28%)
Fig. 1. Eect of temperature on the production of VFAs from akalona.
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Fig. 2. Eect of temperature on the production of VFAs from akalona hydrolyzate.
Fig. 3. Eect of type of substrate on the production of VFAs.
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followed by whey (16.3%) and then by akalona hydrolyzate (13%). This observation can be explained by the fact that solid akalona as a substrate was free from toxic and undesirable substances such as the toxic compound in akalona hydrolyzate (due to acid hydrolysis) and the salt contained in whey which inhibit the growth of microorganisms and then reduce the rate and yield of VFAs production. This leads to the conclusion that solid akalona is the suitable substrate for VFAs production compared with whey and akalona hydrolyzate. 3.2. Recovery of VFAs Fig. 4 illustrates the eect of TOPO concentration in kerosene on the partitioning behaviour of aqueous propionic acid solutions ranging from 0.1 to 0.65 M initial concentration. As shown in the ®gure, the amount of propionic acid distributed to the organic phase increased by increasing the concentration of TOPO in the organic phase at the dierent initial concentrations. The maximum value (0.41 M) of propionic acid was obtained in the organic phase containing 20% TOPO in kerosene and these results con®rm with the literature [5, 6]. This leads to the conclusion that 20% TOPO in kerosene is the optimum concentration of this mixed solvent for higher partitioning of aqueous propionic acid solutions. Figs. 5 and 6 represent the results of using 20% TOPO in kerosene for partitioning each of propionic acid and acetic acid, respectively. As shown in the ®gures, the partition coecients for propionic acid are higher than for acetic acid at all initial concentrations. The partition coecients for propionic acid are about three to ®ve times the partition coecients for acetic acid. The TOPO loading increases by increasing the initial acid concentration for both
Fig. 4. Equilibrium distribution curves for propionic acid for a series of concentrations of TOPO in kerosene.
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Fig. 5. Equilibrium distribution curve (), partition coecients (D), and loading (q) for propionic acid solutions into 20% TOPO in kerosene.
propionic and acetic acid and becomes constant for acetic acid concentrations of 0.4 and 0.45 M, as proved by extending the experimental work. Also, the TOPO loading for propionic acid is higher than that for acetic acid, which can be explained by the fact that the per cent removal of propionic acid in TOPO was greater than that of acetic acid. Fig. 7 shows that the partition coecients for mixed acetic/propionic acid were lower than
Fig. 6. Equilibrium distribution curve (), partition coecients (D), and loading (q) for acetic acid solutions into 20% TOPO in kerosene.
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Fig. 7. Equilibrium distribution curve (w), partition coecients (D), and loading (q) for mixed acetic/propionic acid solutions into 20% TOPO in kerosene.
for propionic acid alone and this is due to the lower partition coecient for acetic acid. Also, the TOPO loading was lower than that for propionic acid only. Fig. 8 illustrates the eect of solvent to feed ratio on the per cent removal of VFAs from the fermentation broth into 20% TOPO in kerosene. It is clear that the per cent removal of VFAs
Fig. 8. Eect of solvent to feed ratio on extraction of VFAs from fermentation broth into 20% TOPO in kerosene.
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Fig. 9. Eect of agitation rate on extraction of VFAs from fermentation broth into 20% TOPO in kerosene.
Fig. 10. Eect of temperature on extraction of VFAs from fermentation broth into 20% TOPO in kerosene.
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Fig. 11. Eect of pure solvent (kerosene) on extraction of VFAs from fermentation broth.
increased by increasing the solvent to feed ratio. On the other hand, the per cent removal at a S=F ratio of 2 : 1 was almost the same as at the S : F ratio of 1 : 1. Therefore, the optimum S : F ratio of 1:1 is most suitable from an economic point of view. In addition, separation of the two phases is more dicult in the case of a S : F ratio of 2 : 1. The results of using dierent agitation rates (50, 80, 100 and 120 rpm) for extraction are represented in Fig. 9. As shown in the ®gure, the per cent removal of VFAs was increased by increasing the agitation rate from 50 to 120 rpm. Fig. 10 illustrates the eect of temperature on extraction. As shown in the ®gure, the per cent removal of VFAs increases by increasing the temperature from 25 to 308C. Therefore, extraction at a temperature of 308C is more preferable. From Figs. 8±10, the per cent removal of VFAs becomes constant after 2 h extraction. This leads to the conclusion that 2 h were sucient for maximum per cent removal during the extraction. Fig. 11 shows the eect of kerosene only for complete extraction of VFAs. As shown in the ®gure, the extraction with kerosene at a S : F ratio of 2 : 1 tends to increase the per cent removal of VFAs. This increase may be attributed to transferring more VFAs by kerosene than in the case of mixed solvent due to the separation of higher VFAs than the propionic acid. 4. Conclusion Akalona, a wheat milling waste residue, provides a higher per cent yield of VFAs at a
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relatively short fermentation time of 4 days. It may be a favorable substrate for production of VFAs compared with whey and akalona hydrolyzate. Acid prehydrolysis of cellulosic residues is not a recommended route for VFAs production. Whey can be used for such purpose. The optimum operating conditions for VFAs extraction from fermentation broth using a mixed solvent (20% TOPO in kerosene) are: S : F : : 1 : 1, 308C, 120 rpm for 2 h. The two successive extraction processes using a mixed solvent followed by a pure solvent tend to increase the per cent removal of VFAs and good separation of lighter VFAs from higher VFAs. References [1] Miller TL, Wolin MJ. Bioconversion of cellulose to acetate with pure cultures of Ruminococcus albus and a hydrogen-using acetogen. J Applied and Environmental Microbiology 1995;61(11):3832±5. [2] Kusel K, Drake HL. Acetate synthesis in soil from a Bavarian beech forest. J Applied and Environmental Microbiology 1994;60(4):1370±3. [3] Sans C, Mata Alvarez J, Cecchi F, Pavan P. Modelling of a plug-¯ow pilot reactor producing VFA by anaerobic fermentation of municipal solid wastes. Water Science Technology 1994;30(12):125±32. [4] Nomura Y, Iwahara M, Hongo M. Acetic acid production by an electrodialysis fermentation method with a computerized control system. J Applied and Environmental Microbiology 1988;54(1):137±42. [5] Solichien MS, Brien OD, Hammond EG, Glatz CE. Membrane based extractive fermentation to produce propionic and acetic acid: toxicity and mass transfer consideration. J Enzyme and Microbial Technology 1995;17:23±31. [6] Bulock J, Kristiansen B. In: Basic biotechnology. New York: Academic Press, 1989. p. 372±4. [7] Warner AC. Production of volatile fatty acids in the rumen: methods of measurement. The Commonwealth Bureau of Animal Nutrition [special issue]. Nutrition Abstracts and Reviews 1964;34(2):339±51.