3.4. Effects of different nitrogen sources on biogas production
The influence of different nitrogen sources (urea and poultry droppings) on cumulative biogas yield and on biogas production rate is shown in Fig. 4 and Fig. 5, respectively. From the results in Fig. 4, the supplementation of the rice husk with urea reduced the biogas yield when compared with the control while addition of poultry droppings (PD) improved biogas production. The maximum values of biogas production rate were 30 and 69 mL/day for the control and poultry droppings, respectively, on day 2, while urea gave the maximum biogas yield of 8 mL/day on day four (Fig. 5). The higher biogas production rate from PD could be attributed to the fact that the poultry droppings contained some remnants of their unconsumed feed which made poultry droppings to have higher carbon content than the urea N. In biogas production, it is necessary to maintain a proper composition of the feedstock for efficient plant operation so that C:N ratio in feed remains within desired range. Thus, to meet this requirement, microbes need a 25–30:1 ratio of C to N with the largest percentage of the carbon being readily degradable [45] and [46]. Vermeullen [47] reported that high concentration of N reduced the biogas production rate and this was improved by the addition of paper, which contributed to increase in C/N ratio.