Abstract

A global thermodynamic analysis, normally used for pure cultures, has been performed for steady-state data sets from acidogenic mixed cultures. This analysis is a combination of two different thermodynamic approaches, based on tabulated standard Gibbs energy of formation, global stoichiometry and medium compositions. It takes into account the energy transfer efficiency, ε, together with the Gibbs free energy dissipation, ΔGo, analysis of the different data. The objective is to describe these systems thermodynamically without any heat measurement. The results show that ε is influenced by environmental conditions, where increasing hydraulic retention time increases its value all cases. The pH effect on ε is related to metabolic shifts and osmoregulation. Within the environmental conditions analyzed, ε ranges from 0.23 for a hydraulic retention time of 20 h and pH 4, to 0.42 for a hydraulic retention time of 8 h and a pH ranging from 7-8.5. The estimated values of ΔGo are comparable to standard Gibbs energy of dissipation reported in the literature. For the data sets analyzed, ΔGo ranges from -1210 kJ/molx, corresponding to a stirring velocity of 300 rpm, pH 6 and a hydraulic retention time of 6 h, to -20744 kJ/molx for pH 4 and a hydraulic retention time of 20 h. For average conclusions, the combined approach based on standard Gibbs energy of formation and global stoichiometry, used in this thermodynamic analysis, allows for the estimation of Gibbs energy dissipation values from the extracellular medium compositions in acidogenic mixed cultures. Such estimated values are comparable to the standard Gibbs energy dissipation values reported in the literature. It is demonstrated that ε is affected by the environmental conditions, i.e., stirring velocity, hydraulic retention time and pH. However, a relationship that relates this parameter to environmental conditions was not found and will be the focus of further research. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA.