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

摘要

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℃). 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/(Ld), 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.