Production and recovery of volatile fatty acids from fermentation broth

Production and recovery of volatile fatty acids fromfermentation 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 wasteresidues (akalona) either as a solid or hydrolyzate and whey was studied using fresh rumen ¯uid as amixed culture for anaerobic digestion of the organic residues. Maximum VFAs production was obtainedfrom akalona followed by whey after 8 days. The lower VFAs production was obtained from akalonahydrolyzate after 7 days. VFAs recovery from fermentation broth by liquid±liquid extraction using amixed 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 keroseneunder 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 rightsreserved.

Keywords: Volatile fatty acids; Whey; Akalona; Extraction; Anaerobic fermentation

1. Introduction

Much of the present world output of acetic acid comes from petroleum, while microbialoxidation 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 andother 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. Highconcentrations 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

Energy Conversion & Management 40 (1999) 1543±1553

0196-8904/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.

PII: S0196-8904(99 )00043-6

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-bu�ered, 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 di�erent 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 e�ect, 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 e�ects 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.

N.A. Mostafa / Energy Conversion & Management 40 (1999) 1543±15531544

2. Materials and methods

Akalona was obtained from the Middle Egypt Milling Company in El-Minia. The akalonawas hydrolyzed by autoclaving (1188C, 20 min) with (10% H2SO4). The hydrolyzate was thenneutralized 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 fromthe 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 substratewas inoculated with 200 ml fresh rumen ¯uid which was pre-®ltered and preheated at 808C for20 min to inhibit the methanogenic bacteria. The fermentation process was maintained at thedesired temperature and pH 7 for 7±10 days. Four di�erent temperatures (25, 35, 37 and 408C)were tested to show the e�ect 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 thedesired amount of tri-n-octylphosphine oxide (TOPO) in kerosene using a magnetic stirrer forcomplete mixing. The di�erent concentrations of mixed solvent used in this study were 5, 10,15 and 20% TOPO in kerosene.Partition coe�cients 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 at110 rpm and 258C for 1 h. Partition coe�cients 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 Markhamapparatus [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 amixed solvent were used at constant temperature (308C) and at 120 rpm for 2 h to show thee�ect 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

di�erent agitation rates (50, 80, 100 and 120 rpm) to show the e�ect of agitation rate on theper cent recovery of VFAs. Also, two di�erent temperatures (25 and 308C) were tested. Theaqueous phase from the ®rst step of extraction (with mixed solvent) was extracted using a pure

N.A. Mostafa / Energy Conversion & Management 40 (1999) 1543±1553 1545

solvent (kerosene) at constant temperature (308C) and constant rpm (120) for 2 h at twodi�erent solvent : feed ratios (1 : 1 and 1 : 2).

3. Results and discussion

3.1. Production of VFAs in batch fermentations

Data for the e�ect of temperature on the production of VFAs from akalona is presented inFig. 1. The results indicate that after 1 day of fermentation, the per cent VFAs was maximumat 378C followed by that at 408C while lower values were obtained at both 25 and 358C. Thenafter 2 days of fermentation, the per cent VFAs at 408C was increased thus approaching thevalues of that at 358C during the period from 3 to 5 days. After 6 days of fermentation, theper 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 e�ect 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 and358C. These latter temperatures gave almost the same per cent VFAs at the end of thefermentation 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. E�ect of temperature on the production of VFAs from akalona.


Fig. 2. E�ect of temperature on the production of VFAs from akalona hydrolyzate.

Fig. 3. E�ect of type of substrate on the production of VFAs.


followed by whey (16.3%) and then by akalona hydrolyzate (13%). This observation can beexplained by the fact that solid akalona as a substrate was free from toxic and undesirablesubstances such as the toxic compound in akalona hydrolyzate (due to acid hydrolysis) and thesalt contained in whey which inhibit the growth of microorganisms and then reduce the rateand 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 e�ect of TOPO concentration in kerosene on the partitioning behaviourof aqueous propionic acid solutions ranging from 0.1 to 0.65 M initial concentration. Asshown in the ®gure, the amount of propionic acid distributed to the organic phase increased byincreasing the concentration of TOPO in the organic phase at the di�erent initialconcentrations. The maximum value (0.41 M) of propionic acid was obtained in the organicphase 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 thismixed 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 coe�cientsfor propionic acid are higher than for acetic acid at all initial concentrations. The partitioncoe�cients for propionic acid are about three to ®ve times the partition coe�cients for aceticacid. 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.


propionic and acetic acid and becomes constant for acetic acid concentrations of 0.4 and 0.45M, as proved by extending the experimental work. Also, the TOPO loading for propionic acidis higher than that for acetic acid, which can be explained by the fact that the per cent removalof propionic acid in TOPO was greater than that of acetic acid.Fig. 7 shows that the partition coe�cients for mixed acetic/propionic acid were lower than

Fig. 5. Equilibrium distribution curve (�), partition coe�cients (D), and loading (q) for propionic acid solutionsinto 20% TOPO in kerosene.

Fig. 6. Equilibrium distribution curve (�), partition coe�cients (D), and loading (q) for acetic acid solutions into20% TOPO in kerosene.


for propionic acid alone and this is due to the lower partition coe�cient for acetic acid. Also,the TOPO loading was lower than that for propionic acid only.Fig. 8 illustrates the e�ect 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. 7. Equilibrium distribution curve (w), partition coe�cients (D), and loading (q) for mixed acetic/propionic acidsolutions into 20% TOPO in kerosene.

Fig. 8. E�ect of solvent to feed ratio on extraction of VFAs from fermentation broth into 20% TOPO in kerosene.


Fig. 9. E�ect of agitation rate on extraction of VFAs from fermentation broth into 20% TOPO in kerosene.

Fig. 10. E�ect of temperature on extraction of VFAs from fermentation broth into 20% TOPO in kerosene.


increased by increasing the solvent to feed ratio. On the other hand, the per cent removal at aS=F ratio of 2 : 1 was almost the same as at the S : F ratio of 1 : 1. Therefore, the optimumS : F ratio of 1:1 is most suitable from an economic point of view. In addition, separation ofthe two phases is more di�cult in the case of a S : F ratio of 2 : 1.The results of using di�erent 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 byincreasing the agitation rate from 50 to 120 rpm.Fig. 10 illustrates the e�ect 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 su�cient for maximum per cent removal during theextraction.Fig. 11 shows the e�ect 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 centremoval of VFAs. This increase may be attributed to transferring more VFAs by kerosenethan in the case of mixed solvent due to the separation of higher VFAs than the propionicacid.

4. Conclusion

Akalona, a wheat milling waste residue, provides a higher per cent yield of VFAs at a

Fig. 11. E�ect of pure solvent (kerosene) on extraction of VFAs from fermentation broth.


relatively short fermentation time of 4 days. It may be a favorable substrate for production ofVFAs compared with whey and akalona hydrolyzate. Acid prehydrolysis of cellulosic residuesis 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 fromhigher VFAs.

References

[1] Miller TL, Wolin MJ. Bioconversion of cellulose to acetate with pure cultures of Ruminococcus albus and ahydrogen-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 pro-

pionic 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.


Documents

Production and recovery of volatile fatty acids from fermentation broth