Kinetic Characterization of Mass Transfer Limited Biodegradation of a Low Water Soluble Gas in Batch Experiments— Necessity for Multiresponse Fitting Bart De heyder, 1,3 Peter Vanrolleghem, 2 Herman Van Langenhove, 3 Willy Verstraete 1 1 Laboratory of Microbial Ecology, Faculty of Agricultural and Applied Biological Sciences, University of Gent, Coupure Links 653, B-9000 Gent, Belgium; telephone: 32-9-264.59.76; fax: 32-9-264.62.48; e-mail: willy.verstraete@rug.ac.be 2 Department of Applied Mathematics, Biometrics and Process Control, Faculty of Agricultural and Applied Biological Sciences, University of Gent, Belgium 3 Department of Organic Chemistry, Faculty of Agricultural and Applied Biological Sciences, University of Gent, Belgium Received 8 August 1996; accepted 10 January 1997 Abstract: A method was developed to characterize the kinetics of biodegradation of low water soluble gaseous compounds in batch experiments. The degradation of ethene by resting Mycobacterium E3 cells was used as a model system. The batch degradation data were re- corded as the progress curve (i.e., the time course of the ethene concentration in the headspace of the batch ves- sel). The recorded progress curves, however, suffered gas:liquid mass transfer limitation. A new multiresponse fitting method had to be developed to allow unequivocal identification of both the affinity coefficient, K aff , and the gas:liquid mass transfer coefficient, K l a, in the batch ves- sel from the mass transfer limited data. Simulation showed that the K aff estimate obtained is influenced by the dimensionless (volumetric basis) ethene gas:liquid partitioning coefficient (H). In the fitting procedure, Monod, Teissier, and Blackman biokinetics were evalu- ated for characterization of the ethene biodegradation process. The fits obtained reflected the superiority of the Blackman biokinetic function. Overall, it appears that resting Mycobacterium E3 cells metabolizing ethene at 24°C have, using Blackman biokinetics, a maximum spe- cific degradation rate, v max , of 10.2 nmol C 2 H 4 mg -1 CDW min -1 , and an affinity coefficient, K aff.g , expressed in equilibrium gas concentration units, of 61.9 ppm, when H is assumed equal to 8.309. © 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 55: 511–519, 1997. Keywords: ethene; kinetics; biodegradation; mass trans- fer; multiresponse fitting INTRODUCTION The kinetic characterization of microbial degradation pro- cesses involves several steps: (i) design of the experimental set-up to collect the biodegradation data; (ii) formulation of a biophysical model that describes the experimental set-up and incorporates a biokinetic function describing the rela- tion between the specific biodegradation rate and the con- centration of the compound; (iii) selection of a mathemati- cal technique to fit the biophysical model to the collected biodegradation data allowing estimation of the biokinetic parameters of the biokinetic function. For the design of the experimental set-up, practical fac- tors such as, for instance, the availability of reactor equip- ment or analytical equipment, play an important role. Es- pecially for gaseous compounds, which are generally more difficult to manipulate than solid or liquid compounds, the simplicity of the experimental set-up is a decisive factor. The characterization procedure for gaseous compounds is therefore, in most cases, based on batch experiments (de Bont, 1976; Robinson and Tiedje, 1982, 1983; van Ginkel and de Bont, 1986; van Ginkel et al., 1986). The batch experimental set-up involves injection of the compound in the headspace of a gas-tight batch vessel containing a mi- crobial suspension. The concentration of the compound in the headspace will decrease as a result of a combined gas:liquid mass transfer and biodegradation process. The decrease of the gas concentration as a function of time, referred to as the progress curve, can easily be collected by regular sampling of the headspace, and represents the bio- degradation data used for the fitting of a biophysical model. The biophysical model should, in principle, consider both the gas:liquid mass transfer and biodegradation process. However, in several cases, it is assumed that the gas:liquid mass transfer rate is sufficiently high so that the course of the progress curve is determined solely by the biodegrada- tion process (de Bont, 1976; Robinson and Tiedje, 1983; van Ginkel and de Bont, 1986; van Ginkel et al., 1986). Correspondence to: W. Verstraete Contract grant sponsors: Institute for Promotion of Scientifique Re- search in Industry and Agriculture; Flemish Impulse Programme for En- vironmental Technology; Belgian National Fund for Scientific Research © 1997 John Wiley & Sons, Inc. CCC 0006-3592/97/030511-09