Volatile fatty acid anaerobic degradation: Kinetic modeling with an inoculum under controlled conditions

Due to the multiple reactions that are involved, the mathematical description of anaerobic degradation of organic matter is normally complicated. Several efforts have been made for the development of the ADM1 model, that cover the major processes involved in complex organic substrate conversion. This model application requires a large number of constants and coefficients, which were proposed, reviewed the information available at the time of their publication. The more recent published papers about ADM1 model application report a necessary revision of the kinetic parameters used for volatile fatty acid (VFA) degradation. This work presents a kinetic study of VFA anaerobic degradation performed in batch and continuous stirred tank reactor. Acetic, propionic, and butyric acids (mixed in a ratio 2:1:1 COD basis) and acetic acid only were used as substrate. The biomass for kinetics assays was previously produced in a codigestion process using pig manure mixed with sewage sludge obtained from anaerobic municipal digester. The inoculum build-up and maintenance were conducted in a laboratory stirred tank digester, under controlled conditions to avoid any variability of the resulting parameters obtained. Moreover, the black box approximation was applied in order to reduce the number of parameters for a complete description. A set of lineal relations was obtained to estimate methane, carbon dioxide, and mixed biomass production rates, from VFA degradation rates only. Finally, a good simulation of experimental data was obtained for VFAs, biomass, methane, and carbon dioxide both in continuous and batch operation modes.