Mono fermentation of grass silage by means of loop reactors

A loop reactor was operated for mono fermentation of grass silage without manure addition under mesophilic conditions (38 °C). An averaged specific biogas production of 0.50 m N 3 per kg volatile solids (VS) with a methane concentration of 52% at an organic loading rate of up to 3.5 kgVS/(m3 d) was obtained. The retention time varied from 440 days at 1.0 kgVS/(m3 d) to 50 days at 3.5 kgVS/(m3 d). The degradation level was more than 60% based on VS and 75% based on COD. The first-order hydrolysis rate constant of the process was estimated to be 0.6 d−1. Despite the relative high ammonium concentration of up to 4 g/l, the system worked stable for an operation period of 310 days. In particular the TS content in the fermenter was found to be a key parameter and should not exceed 12% in order to avoid instabilities.