Influence of inoculum–substrate ratio on the anaerobic digestion of sunflower oil cake in batch mode: Process stability and kinetic evaluation

A study of the anaerobic digestion of the solid waste generated in the extraction process of sunflower oil (sunflower oil cake, SuOC) was conducted at mesophilic temperature (35 °C) in batch mode. A laboratory-scale multi-reactor system was used to compare the volatile solids (VS) degradation and methane production (G) at inoculum–substrate ratios (ISRs) of 3.0, 2.0, 1.5, 1.0, 0.8 and 0.5 (expressed as VS basis). All tests were carried out against controls of inoculum without substrate. The stability and progress of the reaction from solid substrate to methane as an end product was monitored by measuring the pH, the soluble chemical oxygen demand, and the total volatile fatty acids-total alkalinity (TVFA/TA) ratio. The results obtained demonstrated that in the ISR range from 3.0 to 0.8, the pH ranged from 7.1 to 7.6 and this parameter was always stable during the anaerobic digestion process. In addition, within the above ISR range the TVFA/TA ratios were always lower than the failure limit values (0.3–0.4), which demonstrated the high stability of the anaerobic digestion process of this substrate at mesophilic temperature. Two kinetic models for substrate (VS) degradation and methane production were proposed and evaluated. The apparent kinetic constants for volatile solids degradation (K1) and methane production (K2) decreased from 0.54 ± 0.09 to 0.32 ± 0.03 d−1 and from 0.36 ± 0.04 to 0.16 ± 0.03 d−1, respectively, when the ISR decreased from 3.0 to 0.5, showing the occurrence of an inhibition phenomenon by substrate concentration. The kinetic equations obtained were used to simulate the anaerobic digestion process of SuOC and to obtain the theoretical VS and methane production values. The low deviations obtained (equal to or lower than 10%) between the theoretical and experimental values suggest that the proposed models predict the behaviour of the reactors very accurately.
The authors wish to express their gratitude to the European Union (Cropgen project, reference: SES-CT-2004-502824- 6th Frame work programme, Sustainable Energy Systems) and to the “Comisión Interministerial de Ciencia y Tecnología, CICYT” of the Ministry of “Educación y Ciencia” of the Spanish Government (Project CTM 2005-01260/TECNO) for providing financial support.