Kinetic of Biogas Production in Co-Digestion of Vegetable Waste, Horse Dung, and Sludge by Batch Reactors

Batch experiments were performed firstly to evaluate co-digestion of vegetable waste (VW), horse dung (HD), and sludge (S). All reactors were set at a temperature of 37°C, pH of 6.7, and total solid 2.5%. Each single-substrate in the mixture played a significant role. In which, VW contributed mainly to the formation of biogas yield, S and HD played nutrient balance role. The biogas yield was in the range of 168-554 Nml/g-TS. Especially, the biogas yield could be estimated from the proportion of the substrates by equation G1_Nml/g-TS = 53.7 + 7.448 × VW(%) + 1.922 × HD(%) or from nutrient ratio (C/N) by equation G2_Nml/g-TS = 1341 - 48.46 × C/N. Further, the experimental data was applied to evaluate the kinetic equations of biogas production including the Gompertz (G) and Logistic (L) models. Constants in both models were found out by using the least squares fitting method. Both models showed high potential, in which, G model was completely better than L model. However, both models failed at time t=0 day. Moreover, the constant λ in models did not reflect the right definition itself, it was merely a mathematical constant.