Kinetic model for anaerobic digestion of livestock manure

Two unstructured segregated kinetic models to describe the anaerobic digest ion of livestock manure are developed and experimental batch data obtained from beef cattle in a 2.0-1 work volume digestor fitted to both proposed ki netic models to obtain the values of the parameters. The results obtained b y fitting show that the second model proposed has both statistical and phys ical meaning in the parameter values obtained. The model takes into account a simplified reaction scheme formed by six reactions. Several simplificati ons have been made (lumping, pseudo-steady state for one compound, first or der kinetics, etc.) yielding four key compounds to be analysed and fitted t o 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 'aceto genic' microorganisms (production of acids nongrowth associated), and growt h 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-o rder Runge-Kutta algorithm has been employed to obtain the values of the te n parameters. The model is able to reproduce the experimental data obtained for beef manure anaerobic digestion with more accuracy than experimental e rror. (C) 1999 Elsevier Science Inc. All rights reserved.