Volumes Ratio Optimization in a Cascade Anaerobic Digestion System Producing Hydrogen and Methane

As focus of humans has turned to renewable energy, the role of anaerobic digestion has started to become economically viable. Reducing the volume of agro-wastes for the generation of gaseous and liquid fractions with energy carriers and valuable products is an enormous challenge. A two-stage anaerobic digestion process consisting of hydrogenic stage followed by methanogenic stage was studied in a laboratory scale. Five simple nonlinear models of this continuous cascade process were studied in order to determine the optimal ratio of working volumes of bioreactors, in view of maximising energy production. This ratio was reported for all adopted models. The optimal ratio (maximal energy production criterion) depends of the adopted mathematical model. Static characteristics of both bioreactors were obtained using Symbolyc toolbox of Matlab. Numerical experiments concerning dynamics of the main variables of both bioreactors for these models using Simulink of Matlab are performed for different step changes of the dilution rate of the first bioreactor, together with the influence of the substrate (acetate) inhibition for one of the models. The value of the constant of inhibition plays an important role on the admissible interval of the dilution rate. The developed idea could serve for optimally designed experiments of anaerobic digestion for production of hydrogen and methane from lignocelluloses wastes (wheat straw) in two phase process.