Results of batch anaerobic digestion test – effect of enzyme addition

Abstract

The hydrolysis of lignocellulose is assumed to be the rate-limiting step in the anaerobic fermentation process. A fungal hydrolytic enzyme mixture was used to assess the enzymatic impact on different feedstocks for biogas production. The optimal conditions for enzymatic hydrolysis of rye grain silage, maize silage, grass silage, feed residues and solid cattle manure were determined in lab-scale experiments. Finally, the effects of enhanced hydrolysis on anaerobic digestion were investigated in batch digestion tests. Enzyme treatment of substrate showed Michaelis-Menten-like behavior and reached maximum values after 3 hours for reduced sugars as a product of hydrolysis. Methane production potential was determined for specific feedstock mixtures without enzyme, with inactivated enzyme and with active enzyme (with and without buffer). The results obtained show a clear increase in methane production after enzyme application for solid cattle manure (165 L_N CH₄∙kg_ODM^-1  to 340 L_N CH₄∙kg_ODM^-1 ), grass silage (307 L_N CH₄∙kg_ODM^-1 to 388 L_N CH₄∙kg_ODM^-1; enzyme plus buffer), feed residue (303 L_N CH₄∙kg_ODM^-1 to 467 L_N CH₄∙kg_ODM^-1), maize silage (370 L_N CH₄∙kg_ODM^-1 to 480 L_N CH₄∙kg_ODM^- 1)and a lower increase for rye grain silage (355 L_N CH₄∙kg_ODM^-1 to 413 L_N CH₄∙kg_ODM^-1). The ratios of heating values from methane yields to heating values from the dry materials ranged between 0.3 and 0.7 for the untreated feedstock and increased to levels between 0.6 and 0.9 after the different forms of enzyme application.