ANAEROBIC-DIGESTION OF WASTE-WATERS AFTER WASHING OLIVES USED FOR OILPRODUCTION - INFLUENCE OF HARVEST TIME ON THE KINETICS OF THE PROCESS

A kinetic study was carried out of anaerobic digestion of waste waters after washing olives used for oil production, collected at three diff erent harvest times (December 1992, January and February 1993). A 1-li tre mixed batch bioreactor operated at 35 degrees C and containing a s epiolite-immobiiized biomass was used. Assuming that the overall anaer obic digestion process conforms to a first-order. kinetics, experiment al data pairs, namely methane volume yield (G) and time (t), fit Roedi ger's equation, from which the rate coefficient values, k(0), were det ermined in each of the situations studied. The rate coefficient consid erably decreased with the harvest time, over the substrate concentrati on 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 decre ased with the ripening of the olives; the observed differences increas ed when the substrate concentration in the digester increased. The coe fficients of methane yield, Yp, ranged between 0.263 (first harvest ti me) and 0.298 l CH4/g COD removed (third harvest time). The eliminatio n of COD exceeded 64% in all cases.