Process parameters within a 750,000 litre anaerobic digester during a year of disturbed fermenter performance

A 750,000 litre fermenter was studied throughout one entire year by investigating the concentrations of volatile fatty acids (acetic, butyric, i-butyric, propionic, valeric and i-valeric acids), pH, concentrations of total C, N, S and NH4+–N, amounts of chemical and biological oxygen demand, and abundance of acetogenic microorganisms. Additionally several process parameters such as temperature, retention time, dry weight and input of substrate and liquids, and the concentrations and amounts of CH4, H2, CO2 and H2S within the biogas were monitored continuously.Various volatile fatty acids and the ratio of acetic to propionic acid were shown to allow a rough indication on the fermentation but were not sufficiently precise to describe the fermenter performance. Nutrient compounds and special fractions, such as easily extractable carbohydrates or the concentration of total fats were more strongly correlated to the gas production of the fermenter. Results of an MPN-method for the determination of acetogenic microorganisms point to an important role of these microorganisms during the phase of restoration of the fermenter performance.