Biogas Production from Waste Activated Sludge Using Cattle Dung Inoculums: Effect of Total Solid Contents and Kinetics Study

Thermophilic anaerobic digestion of waste activated sludge (WAS) was experimentally studied in this research. WAS using cattle dung inoculums with total solids (TS) concentrations of 12.02, 17.58, 23.28, 26.75, and 35.2 g L-1 were digested anaerobically in a batch digester at thermophilic temperatures (55 C°) for a retention period of 15days. Effect of TS concentration on the quality and quantity of the produced gas, pH variation, and the kinetics of biogas production were investigated. The results showed that biogas production potential and biogas production rate increased with an increasing TS concentration. The maximum biogas yields from TS concentration 12.02, 17.58, 23.28, 26.75, and 35.2 g L-1 were 0.186, 0.189, 0.93, 0.213, and 0.231 L/(g VS)-1, respectively. Modified Gompertz equation was employed to model the biogas production at different substrate concentrations. The equation gave a good approximation of the maximum biogas production (Rm) and the biogas yield potential (P) with correlation coefficient (R2) over 0.996. The digestion at TS concentration 35, 8 g L-1 gave the best results. The maximum biogas production reaches 0.856 L day-1, and the biogas yield was 6.650 L at the end of the 13th day of the experiment. This amount of biogas with composition 72.59% of CH4, and 23.6% of CO2 is equivalent to 190 KWh of electricity. These results show that WAS mixed with cow dung is an effective feedstock for biogas production, giving a high cumulative biogas yield.