Anaerobic digestion of waste waters after washing olives used for oil production: Influence of harvest time on the kinetics of the process

Abstract

A kinetic study was carried out of anaerobic digestion of waste waters after washing olives used for oil production, collected at three different harvest times (December 1992, January and February 1993). A 1-litre mixed batch bioreactor operated at 35°C and containing a sepioliteimmobilized biomass was used.
Assuming that the overall anaerobic digestion process conforms to a first-order kinetics, experimental data pairs, namely methane volume yield (G) and time (t), fit Roediger's equation, from which the rate coefficient values, K₀, were determined in each of the situations studied. The rate coefficient considerably decreased with the han/est time, over the substrate concentration range studied (0.5-2.5 g COD/l). The average values obtained were: 1.67,1.13 and 0.75 days^-1 for the waste waters corresponding to the three harvest times considered. Also, the methanogenic activity decreased with the ripening of the olives; the observed differences increased when the substrate concentration in the digester increased.
The coefficients of methane yield, Yp, ranged between 0.263 (first harvest time) and 0.298 l CH₄/g COD removed (third harvest time). The elimination of COD exceeded 64% in all cases.