Combined mechanical enzymatic pretreatment for an improved substrate conversion when fermenting biogenic resources

Anaerobic digestion of lignocellulosic fractions of biogenic resources in biogas plants performs slowly and incompletely leaving fibrous residues. Thus, this work focused on a combined mechanical and enzymatic pretreatment of maize and grass silage to achieve a partial hydrolysis of the substrates and a higher biogas yield. To get an intensive disintegration, the refiner technology was used. For subsequent enzymatic hydrolysis, several commercially available enzyme preparations were tested, and their effect on solid substrate degradation, viscosity and pumpability of the suspensions was investigated. Experiments were performed in laboratory fermenters as well as in pilot scale (1 m(3)) at a biogas plant. The most promising enzymatic pretreatments with respect to their effect on the fermentation process were tested in 20-L semicontinuously operated laboratory scale digesters. On maize silage, a dry mass (DM) reduction up to 40% with an amylase-based preparation was achieved. Equal results were obtained with a cellulase preparation on grass silage. Yet, due to the high demand of enzyme, the economical feasibility was doubtful. Laboratory semicontinuous fermentation experiments, therefore, were performed on maize silage. At a substrate load of 1.5-2.0 kg organic DM (oDM)/(m(3) d), a good biogas yield of about 0.7 L(N) (liter norm)/ g oDM and methane yield of about 0.4 L(N)/g oDM were obtained. However, at the chosen fermentation conditions at 37 degrees C and 25 days hydraulic retention time, no positive effect of the enzymatic prehydrolysis on the biogas yield of fine grinded substrate could be established.