Performance and kinetic evaluation of the anaerobic digestion of two-phase olive mill effluents in reactors with suspended and immobilized biomass

A lab-scale study was conducted on the mesophilic anaerobic digestion of two-phase olive mill effluents constituted by the mixture of the wash waters derived from the initial cleansing of the olives and those obtained in the washing and purification of virgin olive oil. The digestion was conducted in two continuously stirred tank reactors, one with biomass immobilized on Bentonite (reactor B) and other with suspended biomass used as control (reactor C). The reactors B and C operated satisfactorily between hydraulic retention times of 25.0 and 4.0 days and 25.0 and 5.0 days, respectively. Total chemical oxygen demand (TCOD) efficiencies in the ranges of 88.8–72.1% and 87.9–71.2% were achieved in the reactors with immobilized and suspended biomass, respectively, at organic loading rates of between 0.86 and 5.38 g TCOD/l d and 0.86–4.30 g TCOD/l d, respectively. On comparing both reactors for the same OLRs applied, it was observed that the reactor with support was always more efficient and stable showing higher TCOD, SCOD removal efficiencies and lower VFA/alkalinity ratio values than those found in the control reactor. A mass (TCOD) balance around the reactors allowed the methane yield coefficient, YG/S, to be obtained, which gave values of 0.31 and 0.30 l CH4/g TCODremoved for reactors B and C, respectively. The cell maintenance coefficients, km, obtained by means of this balance were found to be 0.0024 and 0.0036 g TCODremoved/g VSS d, respectively. The volumetric methane production rates correlated with the biodegradable TCOD concentration through an equation of the Michaelis–Menten type for the two reactors studied. This proposed model predicted the behavior of the reactors very accurately showing deviations lower than 10% between the experimental and theoretical values of methane production rates.