| Mathieu Brulé |
| Institut für Agrartechnik, Universität Hohenheim |
| August, 2014 |
| Volltext (externe URL) |
Abstract |
| The mechanisms governing the efficiency of commercial fiber degrading enzyme additives at improving the anaerobic digestion of energy crops were investigated. Enzymatic hydrolysis of fresh maize straw was performed for 24 h with commercial cellulases, xylanases and pectinases at an enzyme dosage of 30 g/kg of substrate VS and a substrate:water ratio of 1:6. A maximal yield of 35% reducing sugars, compared with 23% without enzyme addition, was achieved. Enzyme products from acidophilic fungi were active only at low pH (<7.0). Standard batch digestion trials (BMP assays) were performed using the Hohenheim Biogas Test (HBT) on maize straw, maize corn, and rye silage with different inocula. These BMP assays showed no significant effect of enzyme additives (including commercial cellulase, xylanase, pectinase, laccase) on the methane production rate. However, batch digestion trials performed under suboptimal conditions with inoculum of weak bacterial activity from a digester that had been left in dormancy for several months, revealed a temporary increase of the methane production rate from grass silage between day 10 and day 25 of the experiment. In another batch digestion trial, a standard inoculum was diluted 10-fold to reduce its bacterial activity, and a 40% increase of the final methane yield was reached after 60 days of digestion of grass silage. Alternately, in batch digestion trials performed directly on samples of reactor content from an agricultural biogas plant, enzyme additives at a very high dosage of 2 g/kg substrate FW had no significant effect on the methane yield. Even a mixture of cellulase and xylanase specifically designed to function under slightly alkaline pH conditions was ineffective. In semi continuous acidogenic fermentation performed in laboratory digesters with maize silage and water added for dilution at OLR 4 kg VS/(m3 × d), HRT 5 days, with the medium kept in the pH range 5 5.5 through quicklime addition, enzyme additive at a dosage of 10 g/kg substrate VS significantly increased VFA release (+10%) as well as gas production, including H2 production (+20%). Combining the results of the assays with scientific knowledge gathered from the literature, assumptions were drawn about the optimal conditions to foster the performance of enzyme additives in anaerobic digestion processes. In agricultural biogas plants effects of enzyme additives may be most expected in reactors that are heavily loaded (high OLR >4 kg VS/(m³ × d) and short HRT) with a substrate mix comprising a maximal share of energy crops and a minimal share of manure. Nevertheless, the experimental results of this thesis show that a very high enzyme dosage was required to observe effects of enzyme additives on anaerobic digestion. Hence, the profitability of enzyme addition into the biogas process should be examined carefully. |
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