Kinetic modelling of the hydrolysis, acidogenic and methanogenic steps in the anaerobic digestion of two-phase olive pomace (TPOP)

A simplified kinetic model for studying the hydrolysis, acidogenic and methanogenic steps of the anaerobic digestion process of two-phase olive pomace (TPOP) was proposed on the basis of the experimental results obtained. The process was conducted in a laboratory-scale completely stirred tank reactor at mesophilic temperature (35 °C) operating at hydraulic retention times (HRT) in the range of 10-50 days. Hydrolysis and acidogenic steps prevailed at HRTs lower than 20 days, while the methanogenic step prevailed at higher HRT values. Four differential equations described the overall process. A first-order kinetics for hydrolysis of non-soluble organic matter and a Michaelis-Menten equation type for soluble organic matter decomposition, total volatile acids consumption and methane production. The following kinetic constants with their standard deviations were obtained for the above-mentioned anaerobic stages: (a) hydrolysis and solubilization of organic matter: k1 (kinetic constant for non-soluble organic matter degradation): 0.054 ± 0.003 day−1; k2 (maximum rate of soluble organic matter production): 4.2 ± 0.3 g soluble chemical oxygen demand (SCOD)/l day; k3 (saturation constant): 9.8 ± 0.5 g SCOD/l; (b) acidogenesis: k4 (maximum rate of soluble organic matter degradation): 3.6 ± 0.2 g SCOD/l day; k5 (saturation constant): 10.2 ± 0.5 g SCOD/l; and (c) methanogenesis: k6 (maximum rate of total volatile acids (TVA) consumption): 4.3 ± 0.2 g TVA-COD/l day; and k7 (saturation constant): 3.1 ± 0.2 g TVA-COD/l. The kinetic constants obtained and the proposed equations were used to simulate the different steps of the anaerobic digestion process of TPOP and to obtain the theoretical values of non-soluble and soluble CODs, TVA and methane production. The small deviations obtained (equal or lower than 10%) between the theoretical and experimental values suggest that the parameters obtained represent and predict the activity of the different microorganism types involved in the overall anaerobic digestion process of this waste.