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Oil extraction and biodiesel production process produce a massive amount of by-products like Jatropha press cake (JPC) and crude glycerol (CG), which could be used as a potential substrate for methane production. However, the higher lignocellulosic and nitrogen content in the JPC act as a recalcitrant and inhibitor, respectivly, for microbes that are involved in the anaerobic digestion (AD) process. Therefore, the present study aimed to enhance the methane yield of JPC by optimizing the alkaline pretreatment and co-digestion process conditions. The effects of NaOH concentration, incubation temperature, and retention time on methane and soluble chemical oxygen demand (sCOD) yields were evaluated and modeled by employing a response surface methodology coupled with central composite design (RSM-CCD). Moreover, a series of batch experiments with various feedstock concentrations (FCs) were tested to investigate the methane yield of JPC when co-digested with CG at different levels. The methane yields of all pretreated samples were significantly higher when compared with these of the untreated JPC. Pretreating the JPC using 7.32% NaOH at 35.86 °C for 54.05 h was the optimum conditions for maximum methane increment of 40.23% (353.90 mL g−1 VS), while co-digesting 2% CG with JPC at 2 g VS L−1 FC enhanced the methane yield by 28.9% (325.47 mL g−1 VS). Thus, the methane yield of JPC was effectively increased by alkaline pretreatment and co-digesting with CG. However, the alkaline pretreatment was relatively more effective compared with the co-digestion process.

Biodiesel production from Jatropha curcas generates a considerable amount of Jatropha press cake (JPC) and crude-glycerol (CG) biowastes with intense biogas production potential. However, JPC contains a larger amount of lignocellulosic materials that potentially affect the hydrolysis stage of the anaerobic digestion process, while CG significantly lacks nitrogen needed for microbial biomass growth. Therefore, the present study sought to explore the optimal steam explosion (SE) pretreatment and co-digestion conditions that can improve the methane yields of JPC with inhibitor formation reduction. The effects of different temperature-time combinations during SE on soluble chemical oxygen demand (sCOD) and methane yield of JPC were evaluated using response surface methodology coupled with central composite design (RSM-CCD). JPC was also co-digested with CG, and the methane yield of the mixture was investigated by varying the total organic loading (TOL) and CG levels. The RSM-CCD model predicated that the maximum methane yield (330.14 ml g−1 VS) could be achieved after exploding the JPC at 202 °C for 9.39 min, while relatively high temperature (209 °C) and retention time (13.68 min) were needed to obtain a higher predicted sCOD yield (94.48 g L−1). During the co-digestion processes, the methane yields of the mixture were significantly varied, and co-digesting 2% CG with JPC at 2 g VS L−1 TOL was the optimum condition to obtain a maximum methane yield of 325.25 ml g−1 VS. Thus, considering the environmental and economic advantage of biowaste utilization, co-digesting JPC with CG was the best option for improving the methane yield of the mixture compared to SE pretreatment.