Comparative study of different pretreatments to enchance methane production of sorghum forage.

Sorghum represents a suitable substrate for anaerobic digestion in agricultural biogas plants. As lignocellulosic substrate, the anaerobic biodegradability of sorghum depends on its content of cellulose, hemicellulose and lignin. Cellulose and hemicelluloses are generally degradable under anaerobic conditions; nevertheless, lignin acts as a physical barrier, preventing their degradation. Therefore, pretreatments are needed to improve the accessibility of holocellulose and thereafter increase the methane potential of sorghum. The aim of this study was to compare the effects of different pretreatments on methane production of ensiled sorghum forage. Ensiled sorghum forage, used for animal feed, was collected from a farm near Cremona (Italy). After collection, it was dried and stored in air-tight containers prior to use. Mechanical pretreatment was performed by using a cutting mill (Retsch) with a 2, 1, 0.5 and 0.25 mm screens. Sorghum samples were ground into particles with mean diameters of 994, 471, 269 and 169 µm. Sodium hydroxide pretreatment tests were conducted by soaking samples (0.5–1.5 mm particle sizes) in a NaOH solution at 40°C for 24 h, at different NaOH dosages (1 and 10 g NaOH/100gTS). Thermal pretreatment tests were performed using a cylindrical steel tank with a volume of 6.2 L. Sample was introduced in the tank with tap water (6 g H2O/gTS), heated to reach the desired temperature (at 100°C and 160°C) and maintained at this condition for 30 min. After alkaline and thermal pretreatments, sam-70 ples were filtered through a sieve of 0.8 mm of pore size. The sieve-separated solid and liquid fractions were taken for compositional analysis. Enzymatic pretreatment was performed employing a mixture of the following commercial preparations: Agazym BGL (Garzanti Specialties) and Primafast 200 (Genencor Inc.). BGL was found to contain 120.3 ± 5.9 mg protein /ml, 235.7 ± 24.3 IU/ml endoglucanase activity and 126.5 ± 10.6 IU/ml xylanase. Primafast is a highly concentrated preparation containing 167.0 ± 9.5 mg protein/ml, with 2063.4 ± 0,8 IU/ml endoglucanase and 282.8 ± 5.7 IU/ml xylanase. Enzymes were added at a final concentration of up to 0.40 and 0.12 ml/gTS respectively, then H2O was added (3 ml/gTS), pH set at 5.0 and samples incubated at 50 °C for up to 72 h. Sugars were determined by HPLC using a Refractive Index (RI) detector. BMP tests were performed in duplicate under mesophilic conditions (35°C). The substrate to inoculum ratio was 1 gVS/gVS. NaOH pretreatment and combined NaOH and enzymatic pretreatment resulted in the highest methane yield increase. At the highest NaOH dosage, the specific methane production of sorghum increased by 26% and up to 35-36%, by a combined NaOH and enzymatic pretreatment. A reduction of particle sizes, within the range studied, didn’t improve significantly the maximum methane production of ensiled sorghum forage. No significant effect of thermal and enzymatic pretreatment were also observed.