Optimization of methane output for an anaerobic waste digester

In response to the need for renewable energy resources, the replacement of fuel gas with methane produced from the anaerobic digestion of sewerage, agricultural and municipal solid wastes is considered. The utilization of methane for power generation offsets the energy requirements of the digester facility. We discuss the optimization of methane output for a model digester. The model uses Monod based kinetics of methane fermentation and does not include spatial effects. The model assumes that the solid waste acts as a substrate for acid forming bacteria which produce volatile fatty acids, which is converted to methane by a second type of bacteria. It is found that the initial concentrations of the two bacteria and biodegradable volatile solids that maximize the total methane output are independent of the temperature. However, the optimal hydraulic residence time and initial concentration of volatile fatty acids are temperature dependent. This suggests that flow rates should be adjusted, depending on the temperature, to maximize methane output. References D. T. Hill, Simplified Monod Kinetics of Methane Fermentation of Animal Wastes, Agricultural Wastes, 5:1–16 (1983). doi:10.1016/0141-4607(83)90009-4 D. T. Hill, E. W. Tollner and R. D. Holmberg, The Kinetics of Inhibition in Methane Fermentation of Swine Manure, Agricultural Wastes, 5:105–123 (1983). doi:10.1016/0141-4607(83)90089-6 D. T. Hill, Design Parameters and Operating Characteristics of Animal Waste Anaerobic Digestion Systems–-Swine and Poultry, Agricultural Wastes, 5:157–178 (1983). doi:10.1016/0141-4607(83)90081-1 Y. R. Chen and A. G. Hashimoto, The Kinetics of Methane Fermentation, Biotechnology and Bioengineering Symposium, 8:269–282 (1978). A. Husain, Mathematical Models of the Kinetics of Anaerobic Digestion, Biomass and Bioenergy, 14:561–571 (1998). doi:10.1016/S0961-9534(97)10047-2 D. J. Batstone, J. Keller, I. Angelidaki, S. V. Kalyuzzhni, S. G. Pavlostathis, A. Rozzi, W. T. M. Sanders, H. Siegrist and V. A Vavilin, Anaerobic Digestion Model No. 1 (ADM1), Water Science and Technology, 45:65–73 (2002). http://www.iwaponline.com/wst/04510/wst045100065.htm O. Levenspiel, Chemical Reaction Engineering, 3rd Ed., Wiley, New York (1999). J. F. Andrews, A Mathematical Model for the Continuous Culture of Microorganisms Utilizing Inhibitory Substrates, Biotechnology and Bioengineering, 10:707–723 (1968). doi:10.1002/bit.260100602 K. J. Beers, Numerical Methods for Chemical Engineering: Applications in Matlab, Cambridge, New York, USA (2007).