Results of batch anaerobic digestion test – effect of enzyme addition
Keywords:anaerobic digestion, energy crops, solid manure, feed residue, hydrolytic enzymes
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 LN CH4∙kgODM-1 to 340 LN CH4∙kgODM-1 ), grass silage (307 LN CH4∙kgODM-1 to 388 LN CH4∙kgODM-1; enzyme plus buffer), feed residue (303 LN CH4∙kgODM-1 to 467 LN CH4∙kgODM-1), maize silage (370 LN CH4∙kgODM-1 to 480 LN CH4∙kgODM- 1)and a lower increase for rye grain silage (355 LN CH4∙kgODM-1 to 413 LN CH4∙kgODM-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.