Performance evaluation and substrate removal kinetics in the semi-continuous anaerobic digestion of thermally pretreated two-phase olive pomace or “Alperujo”

A study of the semi-continuous anaerobic digestion process of thermally pretreated two-phase olive pomace (TPOP) at 120 °C during 180 min was carried out in laboratory-scale completely stirred tank reactors at mesophilic temperature. Organic loading rates (OLRs) in the range of 2.0-7.0 g VS/(L d) were tested after the pretreatment and the breakage of complex organic matter. Total and soluble chemical oxygen demand (CODt and CODs) removal efficiencies decreased slightly between 77.9-71.7% and 92.3-85.0% when the OLR increased from 2 to 5 g VS/(L d). However, when the OLR increased to 7.0, a sudden decrease in process performance was observed. The maximum methane production rate (1.72 L CH4/(L d)) was achieved at an OLR of 4.5 g VS/(L d). This value was 39.8% higher than that obtained in the semi-continuous anaerobic digestion of untreated TPOP at the same OLR. The modified Stover-Kicannon model and the Grau second-order kinetic model were found to be adequate to fit the experimental results of CODt and CODs, respectively.