Fluid Phase Equilib~a~ 70 (1991) 141-163 Elsevier Science Publishers B.V., Amsterdam 141 MICROBIAL GROWTH THERMODYNAMICS Stanley I. Sandler Department of Chemical Engineering, University of Delaware, Newark, DE 19716 (U.S.A.) ABSTRACT In this paper we consider the application of thermodynamics to microbial growth processes including fermentations and photosynthesis. We show how thermodynamic analysis can lead to accurate energy balance (heat load) calculations; indeed accurate enough for measured heat loads to be a probe of fermenter operations. We also show how a second law analysis can be used to test the consistency of fe~entation data, and to provide limits on cell mass production and on the cell mass~mduct mix in a fe~en~tion. INTRODUCTION Here we consider the thermodynamic analysis of microbial growth processes, which includes fermentation, photosynthesis and wastewater treatment. The thermodynamics of microbial processes has been the subject of a number of other studies [see, for example, Roels (1980 and 1983), Payne (1970), Luong and Volesky (1983), and Nagai (1979)]; perhaps the best review is given by Roels (1983). Nonetheless these works have a number of sho~com~gs. In this work it is shown with data for real systems that accurate energy balance (heat load) and second law constraint calculations for microbial processes are possible. The thermodynamic analysis here is not based on any organisms, metabolic pathways, or anabolic and catabolic processes. Consequently, as with any thermodynamic analysis, the second law limitations found are applicable to all microbial growth processes, but may not be attainable in practice because the appropriate mechanism (organism or pathway) may not exist, or because energy (or free energy) released in one part of a metabolic cycle cannot be efficiently used in another. MASS BALANCES Before proceeding with the analysis we will introduce the stoichiometric notation to be used. While it is possible to do an elemental analysis of bacteria, cells, etc., it is not possible to do a complete molecular analysis. Consequently, in the biochemical literature it is common to base all calculations on a carbon mole