Anaerobic digestion of Jatropha curcas L. press cake and effects of an iron-additive

Abstract

Oil production from Jatropha curcas L. seeds generates large amounts of Jatropha press cake (JPC) which can be utilized as a substrate for biogas production. The objective of this work was to investigate anaerobic mono-digestion of JPC and the effects of an iron additive (IA) on gas quality and process stability during the increase of the organic loading rate (OLR). With the increase of the OLR from 1.3 to 3.2 g_VS L⁻¹ day⁻¹, the biogas yield in the reference reactor (RR) without IA decreased from 512 to 194 L_N kg_VS⁻¹ and the CH₄ concentration decreased from 69.3 to 44.4%. In the iron additive reactor (IAR), the biogas yield decreased from 530 to 462 L_N kg_VS⁻¹ and the CH₄ concentration decreased from 69.4 to 61.1%. The H₂S concentration in the biogas was reduced by addition of the IA to values below 258 ppm in the IAR while H₂S concentration in the RR increased and exceeded the detection limit of 5000 ppm. The acid capacity (AC) in the RR increased to more than 20 g L⁻¹, indicating an accumulation of organic acids caused by process instability. AC values in the IAR remained stable at values below 5 g L⁻¹. The results demonstrate that JPC can be used as sole substrate for anaerobic digestion up to an OLR of 2.4 g_VS l⁻¹ day⁻¹. The addition of IA has effectively decreased the H₂S content in the biogas and has improved the stability of the anaerobic process and the biogas quality.