Kinetic study of the methanogenic step of a two-stage anaerobic digestion process treating olive mill solid residue

Abstract

A kinetic study of the methanogenic step of a two-stage anaerobic digestion process treating two-phase olive oil mill solid residue (OMSR) was conducted at mesophilic temperature (35 °C). The anaerobic digestion of OMSR was carried out in two different steps. After a hydrolytic–acidogenic stage, working at an organic loading rate (OLR) of 12.9 g COD L −1 day −1 (COD: chemical oxygen demand), the effluents or acidified OMSR obtained were employed for feeding a second or methanogenic step. For the methanogenic step, OLRs of between 0.8 and 22.0 g COD L −1 day −1 were studied (corresponding to hydraulic retention times (HRTs) of between 142.9 and 4.6 days). The substrate treated in the second phase (acidified OMSR) had a high total concentration in volatile fatty acids (14.5 g CH 3COOH L −1) and a high percentage of acetic acid as the main methane precursor (57.5% of the total concentration). As a consequence of the first step a high stability in the methanogenic stage was achieved. A total chemical oxygen demand balance was developed over the methanogenic step. For this model two considerations were taken in account: (1) volumetric flow constant during the experiments (the volume of effluent that was taken from the methanogenic reactor every day was equal to the volume of acidified OMSR fed). (2) Constant concentration of methanogenic microorganisms during the experiments (the slow growing rate of the methanogenic microorganisms makes it possible for the concentration of microorganisms over the process to remain constant). The cellular maintenance coefficient ( m) and methane yield coefficient ( Y G/ S ) were found to be 0.016 g COD removed g −1 VSS day −1 and 0.261 L CH 4 g −1 COD removed, respectively.