Assessment of kinetic parameters for the mesophilic anaerobic biodegradation of two-phase olive pomace

A kinetic study of the anaerobic biodegradation of two-phase olive pomace (TPOP) was carried out using a laboratory-scale stirred tank reactor. The reactor was operated at 35°C. The influent contained between 20% and 100% TPOP, representing in terms of total chemical oxygen demand (TCOD) 34.5–187.9 g TCOD l−1. The hydraulic retention times (HRTs) were set between 40.0 and 8.3 d. It was found that the increase of influent substrate concentration favoured the process failure reducing the pH and increasing the ratio of the total volatile fatty acid (TVFA) to alkalinity. This ratio was found to be proportional to the substrate concentration (S), as follows: TVFA/alkalinity=0.04(S). The kinetic model of Andrews was used to describe the relation between anaerobic biodegradation of TCOD and Volatile Solids (VS) and the formation of methane. The values of the kinetic constants for TCOD removal were determined to be 28 g TCOD l−1 d−1, 27 g TCOD l−1 and 352 g TCOD l−1, respectively, for maximum substrate utilization rates (RSmax), saturation constant (KS) and inhibition constant (Ki). Process inhibition started at substrate concentrations of around 20 g TCOD l−1. For VS biodegradation the kinetic constant values, RSmax, KS and Ki, were 45g VS l−1d−1, 37 and 36 g VS l−1, respectively. Inhibition started at VS concentration of around 18 g l−1. The rates of TCOD removal were lower than those observed for VS removal and inhibition of VS removal occurred at a lower concentration compared to that for TCOD. The QM(max), KS and Ki constants for methane production were approximately 3.1 l CH4 l−1 reactor d−1, 8.7 g TCOD l−1 and 272 g TCOD l−1, respectively. Inhibition of methane formation started at a substrate concentration of around 17 g TCOD l−1. TCOD and VS removal rates were higher than the rate of methane formation and these differences increased when the substrate concentration increased. This fact was underlined by the decrease of pH, the increase of TVFA/alkalinity ratio and the reduction of methane production rate.