Cattle rumen inoculated Gliricidia sepium with poultry manure: Pretreatment, biogas production, and optimization

Gliricidia sepium is a stubborn plant constituting environmental nuisance in several tropical regions. However, its richness in nutrient makes it a suitable candidate for the biotechnological production of bioenergy. This study assessed the potential of Gliricidia sepium for biogas production in a batch anaerobic digestion process, co-digested with poultry manure under mesophilic condition. A combination of mechanical and thermo-alkaline methods was applied to pretreat Gliricidia sepium in the main experiment, while another experiment without pretreatment was also conducted. The physicochemical properties of the substrates were determined using standard procedures while the Central Composite Design (CCD) of response surface methodology (RSM) was employed for experimental design and optimization of the process parameters. Results showed biogas yields of 1.3186 L/kg VS and 1.2535 L/kg VS for the pretreated and untreated substrates, respectively. The methane and carbon dioxide contents of biogas from both experiments were 60.43%; 35.08% and 38.62%; 11.72%, respectively. This showed 36.1% increase in methane yield in the pretreated experiment over the untreated. Also, the RSM predicted biogas yield was 1.32 L/kg VS in the inoculated (addition of bovine rumen content) pretreated experiment, with the model's coefficient of determination of 0.9776, indicate strong modeling and prediction accuracy for the inoculated experiment. The economic feasibility assessment of the pretreatment application also confirms that alkaline pretreatment is profitable in the pretreatment of Gliricidia sepium shoots and the investment is economically feasible. The study recommends further use of Gliricidia sepium co-digested with poultry manure especially as these two bioresources are abundant in most developing regions of the world where they serve no important purpose. Their usage on the commercial scale is therefore advocated.