Effect of Substrate Concentration on Methane Fermentation of Cattle Dung

Methane fermentation of cattle manure (CM) with different total solid contents were conducted in the laboratory-scale batch reactors (2.5 L volume) in order to determine the substrate concentration performance and potential energy recovery at the optimal mesophilic (35 oC) and thermophilic (55 oC) temperatures. The three reactors containing CM and water in the ratios (w/w) of 1:1 (wet fermentation), 2:1 and 1:0 (undiluted manure, dry fermentation) were utilized for the both conditions. The work was performed for a period of 63 days to compare the volatile solids and dissolved organic carbon degradation, COD removal, and VFAs. The results showed that the specific biogas yields, in terms of per kilogram of CM, were 41.68, 43.72 and 44.98 L/kg with 22.61, 23.69 and 24.96 L/kg methane contents respectively for the ratios of 1:1, 2:1 and 1:0 at 35 oC, while 43.03, 44.89 and 45.71 L/kg biogas with 23.45, 24.15 and 25.57 L/kg methane contents respectively for the ratios 1:1, 2:1 and 1:0 at 55 oC. The volume of the digester was increased in the wet fermentation for the same loading rate. Though water and thermophilic condition could promote the startup process and the biodegradability of the substrate, the total biogas and methane yields in the dry fermentation were found comparable to the conventional process (wet fermentation).