A THERMODYNAMIC MODEL OF MIAK BIODEGRADATION IN A BIOFILTER ABSTRACT

Transcription

1 A THERMODYNAMIC MODEL OF MIAK BIODEGRADATION IN A BIOFILTER Hyun-Keun Son Department of Environment and Health, Kosin University hkson@kosin.ac.kr ABSTRACT A thermodynamic model of MIAK biodegradation was developed. This thermodynamic model incorporated Gibbs energy dissipation values for growth of various microorganisms on various organic substrates. Even though the model is developed specifically for MIAK(methyl isoamyl ketone), it can be adapted to other types of organic compounds by changing the physical properties associated with the selected compound. An equation to estimate the heat release from the biodegradation process was developed. The heat from the biodegradation of the substrate can affect the operation of the biofilter significantly through the evaporation of the water content of the biofilter media.. INTRODUCTION When the substrate is degraded, some portions of the removed substrate are used for the biosynthesis of microorganisms and the remaining portions of the degraded substrate are mineralized to end products (such as carbon dioxide and water). Through the mineralization of substrate, energy is created and transferred to support the biosynthesis of microorganisms. According to the second law of thermodynamics, when energy is transferred, a certain amount of energy is wasted. This wasted energy is the released as heat. This heat from the biodegradation of the substrate reduces the water content of the media through evaporation, which, in turn, significantly affects the operation of the biofilter (Son et al., 2001; Son et al., 2001). Therefore, an understanding of the

2 thermodynamics of microbial degradation, related with the prediction of biomass yield in the biofilter, is very important and valuable.. Conventional Thermodynamic Model The production of biomass during the decomposition of an organic waste is well defined. The following is a summary of the thermodynamics of biological waste treatment to estimate microbial yields from free energy calculations for heterotrophic metabolism (Young, 1995). The following terms are defined herein: ΔFr = Free energy released per gram of pollutant COD converted to end products ΔFs = Free energy required to synthesize one gram of biocell COD ΔFp = Free energy change of substrate conversion to intermediate. ΔFc = Free energy in converting intermediate to one gram of biocell COD k 1 = efficiency of transferring energy from ADP (adenosine diphosphate) to form ATP (adenosine triphosphate) fs = Fraction of substrate COD that goes to biosynthesis reactions. = [(COD of cells formed)/(cod of waste converted)] fe = 1 - fs = Fraction of substrate COD that goes to energy reactions. A = fe/fs = the mass of COD converted to energy (i.e. to end products) per unit mass of biological solids COD formed. Yo = gram of biomass synthesized per gram of ethylbenzene consumed. With these definitions, an energy balance for production of new biocells can be estimated. For each gram of substrate COD utilized, the energy released by the reaction can be expressed as -fecδfr and that captured for synthesis as -k 1 CfeΔFr. The energy captured for synthesis and that used for synthesis are the same, therefore: -k 1 CfeΔFr = fscδfs (1) By the definition of A (= fe/fs), the equation (1) can then be written

3 -k 1 AΔFr = ΔFs (2) From the definition, ΔFr is the free energy of the reaction through which the substrate is converted to end-products. This is equivalent to the difference between the free energy of formation of the substrate and the sum of the free energies of formation of the end products. Free energies of formation are readily available in handbooks. A list of the free energy of formation of some selected compounds is given in Table 1. The energy transfer steps in cell synthesis from the carbon source can be expressed as follows: ΔFs = ΔFc + k 1Δ Fp (3) Equation (3) can be substituted into equation (2) to yield the following expression: -k 1 AΔFr = ΔFc. + k 1Δ Fp (4) Using the equation 4, the weight fraction A can be determined, since for a given organic material ΔFr, ΔFc, ΔFp can be found in the literature or determined easily. Since then A = fe/fs = (1 fs)/fs (5) fs = 1/(1 + A) gm cell COD/ gm substrate COD utilized (6) As the oxygen equivalent of volatile cell solids is 1.42 mg O 2 /mg biomass (Young, 1995), the above equation can be rewritten; Yo = fs/1.42 gm biomass/gm substrate COD utilized (7) Table 1. Free Energies of Formation for Various Compounds

4 . Modified Thermodynamic Model for MIAK Biodegradation Bauchop and Elsden (1960) proposed the concept of expressing the yield of biomass in terms of consumed ATP (YATP in grams of biomass dry weight/mol ATP). Their conclusion was that YATP was 10.5g/mol and constant. However, the results from more recent papers suggest YATP may not be constant (Heijnen et al., 1992; Heijnen et al., 1992). Heiijnen and van Dijken (1992) reported that biomass yield (YATP) can vary widely, with the range of 2 to 30 g/mol ATP. They said biomass yield (YATP) depends strongly on the microorganism and its growth substrates. Heiijnen et al.(1992) reported the average Gibbs energy dissipation values for growth of various microorganisms on various organic substrates (Table 2). Gibbs energy dissipation can be expressed as 1/YATP and the energy available from a mole of ATP is 12.5 kcal. Prediction of microbial yield and heat generation from the substrate degradation is demonstrated by the following example in which MIAK is utilized as the substrate. Figure 1 shows the proposed degradation pathway of MIAK in which acetone and acetate are generated as intermediates. The molecular weight of MIAK(C 7 H 14 O) is and one gram of MIAK is equivalent to 2.80 gram of chemical oxygen demand (COD). Theoretical bio-oxidation equation of MIAK is: C 7 H 14 O + 10O 2 = 7CO 2 + 7H 2 O (8) From equation (8), the free energy released per mole of MIAK to yield the end products (ΔFr) was calculated to be -964 kcal/mol and is equivalent to 3.01 kcal/g COD. Through the proposed MIAK biodegradation pathways, one mole of MIAK produces one mole of acetone (C 3 H 6 O), and two moles of acetate (C 2 H 3 O - 2 ), and the following equation was used; C 7 H 14 O + H 2 O + 1.5O 2 = C 3 H 6 O + 2C 2 H 3 O 2 - (9) The free energy released from MIAK to the intermediate products (ΔFp) of acetone and two acetate was calculated to be 62.6 kcal/mol and is equivalent to 0.20 kcal/g COD. The molecular weight of 1 C-mole of biomass was assumed to be 24.6 gram, therefore the required energy for the synthesis of 1 gram of biomass was determined from the values in Table 2. If the oxygen equivalent of

5 biocell is 1.42 mg O 2 /mg biomass, the free energy required to convert each intermediate to one gram of cell COD(ΔFc) was calculated. The calculated Fc of selected intermediates appear in Table 3. One mole of MIAK produces one mole of acetone and two mole of acetate, the free energy required for the synthesis of 3 grams of produced biomass COD (ΔFc ) is the sum of ΔFc(acetone), ΔFc(acetate), ΔFc(acetate) and was equal to kcal/gram cell COD. Equation (3) can be adjusted for the case of MIAK as follows; ΔFs = ΔFc /3 + k 1 ΔFp /3 (10) Equation (1) can also be adjusted as follows in the case of MIAK; -k 1 CfeΔFr /3 = fscδfs (11) Combining the equation (10) and (11), -k 1 CfeΔFr /3 = fsc(δfc /3 + k 1Δ Fp /3) (12) As ΔFc = [ΔFc(acetone)+ΔFc(acetate)+ΔFc(acetate)], equation (12) can be; -k 1 CfeΔFr /3 = fsc{k 1Δ Fp /3 + [ΔFc(acetone)+ΔFc(acetate)+ΔFc(acetate)]/3} (13) Equation (2) can be converted to determine the mass of COD converted to energy per unit mass of biological COD formed, A; A = -ΔFs/(k 1 *ΔFr) (14) A was calculated as 7.01 and fs was 0.13 gram cell COD/gram MIAK COD utilized. From equation (7), Yo was determined to be 0.25 gram biomass per gram of MIAK utilized. Appendix shows the detailed calculations for the procedure. Thermodynamic values for selected intermediates appear in Table 3. According to the second law of thermodynamics, there is always some waste heat; that is, it is impossible to transfer energy with 100 percent efficiency. Efficiency of ATP energy transfer (k 1 ) was assumed to be 0.6 based upon the

6 literature (Young, 1995). Therefore, an estimated forty percent of total energy converted is emitted as heat. As mentioned previously, the net portion of the substrate converted to biocell is fs and the remainder of the substrate is fe which used for the energy to sustain biosynthesis. If the total amount of consumed substrate is C, then the heat released from the portion of substrate conversion for the energy is (1-k 1 )CfeΔFr. The degradation process from substrate to intermediates is exothermic (Figure 2). Heat is released from the portion of substrate conversion to biosynthesis and that amount of heat may be predicted by the expression, (1-k 1 )fscδfp. Therefore, the total amount of heat released from the biodegradation process (H) can be expressed by the following equation; H = (1- k 1 ) C fe ΔFr + (1- k 1 ) fs C ΔFp. (15) Figure 2 also illustrates the amount of heat generated from the biodegradation process. With equation (15), heat released from the biodegradation of MIAK can be estimated to be 341 kcal/mol. Selected thermodynamic values for MIAK biodegradation appears in Table 4. Table 2. Average Gibbs Energy Dissipation for Growth of Various Microorganisms on Different Substrates. (Source: Heiijnen and van Dijken, 1992; Heiijnen et al., 1992) Figure 1. Proposed pathway for the degradation of 5-methyl-2-hexanone Table 3. ΔFc Values from Some of the Selected Intermediates to One Gram of Cell COD Figure 2. Schematic of energy transferred during the biodegradation(heterotrophic Growth) Table 4. Thermodynamic Values for MIAK Biodegradation.

7 . CONCLUSION The conventional thermodynamic model is based upon the work of Bauchop and Elsden (1960). They reported that YATP was 10.5g/mol and constant. However, the results from more recent papers suggest YATP may not be constant (Heijnen et al., 1992; Heijnen et al., 1992) and biomass yield (YATP) can vary widely, with the range of 2 to 30 g/mol ATP. Heiijnen and van Dijken (1992) said biomass yield (YATP) depend strongly on the microorganism and its growth substrates. This author improved the conventional thermodynamic model with the average Gibbs energy dissipation values for growth of various microorganisms on various organic substrates (Table 2). Gibbs energy dissipation can be expressed as 1/YATP and the energy available from one mole of ATP is 12.5 kcal. The improved thermodynamic model is very specific for each selected pollutant compound, rather than a general model for all types of pollutants. From the early 1980s, numerous biofilter models have been developed by many researchers, and more and more rate limiting factors have been included for the different types of biofilter models. This thermodynamic model may be used to predict the heat generation from the microbial degradation of pollutants. Estimation of the heat generation from the microbial degradation is essential to improve the currently available biofilter model. Heat generation from microbial degradation significantly impacts the water content fluctuations, and Y values for the estimation of the growth of microorganism in the biofilter. Therefore, this thermodynamic model makes a contribution to improve the accuracy and reliability of currently available biofilter models.. REFERENCES [1] Bauchop, T.& S.R. Elsden J. gen. Microbiol. 23 : 457 [2] Cookson, J.T Bioremediation engineering: design and application. McGraw-Hill, Inc., New York [3] CRC Handbook of Chemistry and Physics. 72nd Edition. The Chemical Rubber Co. Cleveland, OH [4] Eweis, J.B., S.J. Ergas, D.P.Y. Chang & E.D. Schroeder Bioremediation principles. WCB/McGraw-Hill, Boston

8 [5] Heijnen, J.J. & J.P. van Dijken Biotechnology and Bioengineering. 39 : 833 [6] Heijnen, J.J., M.C.M. van Loosdrecht & L. Tijhuis Biotechnology and Bioengineering. 40 : 1139 [7] Mandelstam, J.K., McQuillen & I. Dawes Biochemistry of Bacterial Growth. 3rd Edition, Halsted Press. a Division of John Wiley & Sons Inc. 605 Third Avenue, New York, NY [8] Son, H.K. & B.A. Striebig Journal of the Air & Waste Management Association. accepted for publication [9] Son, H.K. & B.A. Striebig Journal of Environmental Engineering. submitted for publication [10] Sylvia, D.M., J.J. Fuhrmann, P.G. Hartel & D.A. Zuberer Principles and Applications of Soil Microbiology. Prentice Hall Inc., Simon & Schuster/ A Viacom Company. Upper Saddle River, New Jersey [11] Young, J.C Fundamentals of Biological Wastewater Treatment. The Pennsylvania State University, University Park. PA.