Thermophilic digestion of cellulose at high‐organic loading rates

The influence of high punctual organic loadings on performance of anaerobic biocenosis was examined. Thermophilic batch series with microcrystalline cellulose at six organic loading rates (OLRs) were performed (5.7–34.3 kg VS/m 3 ). With the loading increase and exceeding of recommended substrate/inoculum ratio of 0.5 for volatile solids, an inhibitory state was targeted. Although extreme OLRs had been applied, they did not lead to the permanent acidosis. The only explicit sign of system stress was the increasing total degradation time and the extended period of higher biogas production rate with every increase of OLR. Measured fermentation parameters (pH, oxidation–reduction potential, volatile acids and acids/buffer capacity ratio) were changed simultaneously during fermentation. A clear system evaluation was not possible with any of these parameters without gas production. This confirms gas production as the first and most important control parameter for the supervision of the fermenting process. Also, propionic/acetic acid ratio as well as C 4 ‐C 5 fatty acids delivered only ambiguous results. Even significant exceeding of limits given by the literature did not result in extreme system performance. The observed signs of system stress were not proportional to the applied OLRs. They resulted from hydrolysis or acetogenesis rate delay. In addition, they prove a much higher than expected adaptation level of microbial biocenosis to increasing OLR if the loads are introduced gradually.