Combination of alkaline and enzymatic pre-treatment to increase bio-methane production potential of sorghum and wheat straw

Sorghum and wheat straw represent suitable substrates for anaerobic digestion in agricultural biogas plants. As for their lignocellulosic nature, anaerobic biodegradability depends on cellulose, hemicellulose and lignin content. Pretreatments are then needed to improve holocelluloses accessibility and thereafter increase the methane potential of these substrates. The purpose of this study was to increase methane production by enzymatic and combined alkaline-enzymatic pretreatments on ensiled sorghum forage and wheat straw. These kind of biomasses, used for animal feed, were collected from a farm near Cremona (Lombardy region, Italy), dried and finally stored in air-tight containers prior to use. 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 (10 g NaOH/100 gTS). Enzymatic pretreatments were comparatively performed employing the following commercial preparations: Agazym BGL, Agazym Ultra L (Garzanti Specialties), Pulpzyme HC (Novo Nordisk) and Primafast 200 (Genencor Inc.). Enzyme preparations were first characterized for their xylanase, CMCase (endoglucanase) and avicelase activities: BGL contains 235.7 ± 24.3 IU/mL endoglucanase activity and 126.5 ± 10.6 IU/mL xylanase. Agazym Ultra L is composed mainly by endoglucanase 613.2 ± 42.9 IU/mL, while Pulpzyme by xylanase 106.8 ± 1.9 IU/mL. Primafast is a highly concentrated preparation containing 2063.4 ± 0.8 IU/mL endoglucanase and 282.8 ± 5.7 IU/mL xylanase. Avicelase activity was always found in traces (max 3.5 IU/mL). Enzymes were added to untreated or treated (alkaline) substrates; distilled water was used to reach a concentration of 3 ml/g dried substrate, and pH corrected at each appropriate enzyme-specific value. Sugars were determined by HPLC using a Refractive Index (RI) detector. Trials were performed in order to identify the best biotechnological conditions, in terms of type and enzyme concentrations, contact time and incubation temperature. To assess the effect of the applied pre-treatment on the methane production, methanisation (BMP) tests, before and after enzymatic and combined alkaline-enzymatic pretreatment, were performed. The combined use of BGL and Primafast (0.12 and 0.2 ml/g dried substrate, respectively), an incubation time of 72 h at 50 °C increased the amount of soluble total and reducing sugars released by the hydrolytic enzymatic activities. Glucose was the most important monosaccharide released, but also xylose, mannose and galactose, as well as arabinose and glucuronic acid were found. Combined alkaline-enzymatic pre-treatment (combination of BGL and Primafast) resulted in the highest methane yield increase (+35% and +71% for ensiled sorghum and wheat straw respectively).