Kinetic Model for Anaerobic Digestion of Livestock Manure

Abstract

Two unstructured segregated kinetic models to describe the anaerobic digestion of livestock manure are developed and experimental batch data obtained from beef cattle in a 2.0-l work volume digestor fitted to both proposed kinetic models to obtain the values of the parameters. The results obtained by fitting show that the second model proposed has both statistical and physical meaning in the parameter values obtained. The model takes into account a simplified reaction scheme formed by six reactions. Several simplifications have been made (lumping, pseudo-steady state for one compound, first order kinetics, etc.) yielding four key compounds to be analysed and fitted to the model as production-rates expressions (total biomass, chemical oxygen demand (COD), volatile acids, and methane). The model considers three main stages in the process: enzymatic hydrolysis of the waste, growth of ‘acetogenic’ microorganisms (production of acids nongrowth associated), and growth of ‘methanogenic’ microorganisms associated with methane production; the two last processes are accompanied by substrate consumption for maintenance. A non-linear multiple-response regression technique coupled to a fourth-order Runge–Kutta algorithm has been employed to obtain the values of the ten parameters. The model is able to reproduce the experimental data obtained for beef manure anaerobic digestion with more accuracy than experimental error.