Effect of Substrate Mixing Ratio on Anaerobic Co-digestion of Cattle Dung and Vegetable Waste with and Without the Addition of Biochar

The present study investigated the effect of various substrate mixing ratios of cattle dung (CD) and vegetable waste (VW) on biogas yield, both with and without biochar addition. Two sets of batch-type anaerobic biochemical methane potential experiments were conducted at different percentages of CD:VW mixing ratio viz. 70:30, 50:50, and 30:70% on a mass basis simultaneously in triplicate, maintaining substrates/water ratio 1:1 using the 1000 ml glass reagent bottles with a working volume of 700 ml for the retention time of 45 days. The set-I experiment comprised a mixer of CD and VW only, whereas set-II comprised CD and VW with the addition of 15 g/l biochar. The biochar was prepared from the water hyacinth using a pyrolyser at a pyrolysis temperature of 550 °C. The set-II digesters with CD:VW mixing ratios of 70:30, 50:50, and 30:70% were obtained to be 28.52, 17.91, and 18.29% higher volume of methane yield than the digesters with respective mixing ratios for set-I. The higher methane generation rate was observed for set-II as the biochar can provide an adequate surface area for colonisation of the microbial flora, which was detected in the analysis of its morphology and pore surface. Improvement in pH value was also observed with the addition of biochar due to the alkaline nature of biochar. Comparing cumulative methane yield, the digester with CD and VW mixing ratio of 70:30% was detected to be the best for all sets of experiments. The maximum specific cumulative methane yields for these digesters without and with biochar addition were 132.57 mlCH4/gVS and 183.07 mlCH4/gVS, respectively. A kinetic study was also carried out for all sets of experiments using a modified Gompertz model. The correlation coefficient (R2) value was above 0.9, indicating that the modified Gompertz model and experimental data agreed reasonably well. The present study introduces a sustainable waste management method that can be applied to rural areas for clean energy generation.