Modeling Anaerobic Digestion of Biodegradable Solid Wastes

Anaerobic digestion (AD) is a series of biological processes that converts biodegradable organic matter into biogas. The US Air Force aims to use this process to treat the biodegradable solid waste produced on their forward operating bases (FOBs). The focus of this study was to obtain the Monod kinetic parameters of AD process intermediates (volatile fatty acids, monomers, and polymers) that are typically found in cardboard and food waste; the primary wastes produced on FOBs. These substrates were analyzed through initial rates experiments until 5% substrate was consumed to assume negligible growth. Granular sludge was used as the anaerobic inoculum. Monod kinetic parameters were obtained using experimental and computational methodologies. First, the AD of a single concentration of each of the substrates was regressed to obtain preliminary saturation constants (K_s) and maximum rates (µ_max). Next, these preliminary values were used to obtain actual experimental values. The experimental values obtained for the volatile fatty acids include: K_s = 0.237 g L^-1, µ_max = 0.000592 g COD g VSS^-1 h^-1 for acetic acid, K_s = 0.368 g L^-1, µ_max = 0.000475 g COD g VSS^-1 h^-1 for butyric acid, and K_s = 0.066 g L^-1, µ_max = 0.000155 g COD g VSS^-1 h^-1 for propionic acid. Likewise, the experimental values obtained for complex substrates were K_s= 0.0.92839 g L^-1, µ_max = 0.02679 g COD g VSS^-1 h^-1 for gelatin, K_s= 4.4157 g L^-1 and µ_max = 0.00674 g COD g VSS^-1 h^-1 for cellulose. The parameter values obtained were within the range of previously reported literature values. Once all of the kinetic parameters of the most representative fractions of cardboard and food waste are obtained, they will be incorporated into the Anaerobic Digestion Model No.1 (ADM-1). ADM-1 predictions will be compared to actual experimental batch AD performance for food waste and cardboard, and model deficiencies will be discussed.