Performance and kinetic evaluation of the semi-continuous anaerobic digestion of sunflower oil cake pretreated with ultrasound

Abstract

A study of the semi-continuous anaerobic digestion of sunflower oil cake previously sonicated (at a specific energy of 24,000 kJ/kg TS, constant sonication frequency of 20 kHz and ultrasonic power of 120 W) was carried out in laboratory-scale completely stirred tank reactors at mesophilic temperature (35°C). Two anaerobic inocula were used: a mixture of flocculant biomass (I) from a full-scale anaerobic reactor treating waste activated sludge and a granular inoculum (II) from an industrial UASB reactor treating brewery wastewater. Soluble COD (CODs) removal efficiencies ranged between 67.7% and 70.1% and between 61.3% and 67.7% at hydraulic retention times (HRTs) of between 24–10 days for inoculum I and 24–8 days for inoculum II. However, for HRTs lower than 8 days and 6.7 days, equivalent to organic loading rates (OLRs) higher than 2.62 and 3.15 g COD/(L·d), respectively, a sudden decrease in the CODs removal efficiency was observed in both cases. In any case, inoculum II allowed for a more stable and efficient operation for a wider range of both OLRs and HRTs, permitting an appropriate and reliable operation for OLRs as high as 3.15 g COD/(L·d) and HRTs as low as 6.7 days. The methane production rates achieved with inoculum II were always higher than those reached with inoculum I. The overall methane yield obtained with inoculum II was 13% higher than that achieved with inoculum I. In addition, this value was 1.9 times higher than the methane yield obtained with untreated (non-sonicated) SuOC. A second-order kinetic model was found to be adequate to fit the experimental results obtained for the two inocula used. The kinetic constant obtained with inoculum I was 3.5 times higher than that achieved with inoculum II.