Digestion of grass silage in a semi-continuously fed reactor at increasing loading rates - Process stability and kinetics evaluatio

Grass is a popular substrate in agricultural biogas plants in Central and Northern Europe. There is currently limited information about the impact of the organic loading rates (OLR) on methane yields, process stability and kinetics for continuous systems digesting grass silage. Anaerobic mono-digestion of grass silage was evaluated in semi-continuously fed laboratory continuously stirred tank reactors (CSTRs) at increasing loading rates between 1.9 and 4.7gVS/l/d. Results show that digestion of grass silage in one-stage CSTR is feasible and do not present any loading-induced inhibition for the tested OLR range. While the volumetric methane production experienced an increase of 61%, the methane yield only decreased by 13% for the highest OLR. On the other hand, the estimated first-order hydrolysis constant decreased by 24% when increasing the loading from 1.9 to 4.7gVS/l/d, which related with an accumulation of the organic material in the reactor. It was also found that the composition of the grass silage affected the digester performance significantly.