Propionic to acetic acid ratios in overloaded anaerobic digestion

The aim of this study was to examine the ratio of propionic to acetic acids as a viable and sensitive indicator for upsets caused by organic overloading in an anaerobic digestion system. An anaerobic bacterial population, adapted to glucose (anaerobically digested cow manure fed daily with glucose for 27 days) was used. Biogas production and composition, pH, volatile fatty acids and the CH4:CO2 ratio were also examined in digesters subjected to a steady, slow organic overload and rapidly overloaded concentrations of glucose. During the process of overload, it was observed that, after raising the feed concentration, the ratio of propionic to acetic acid increased immediately, indicating an overload effect prior to changes being observed in the other parameters examined. Where pH was not controlled, a drop in pH resulted from the accumulation of fatty acids some days after this stage. Instability and processes leading to digestion failure can be seen more clearly by means of the propionic: acetic acid ratio when the anaerobic digestion system is well buffered, but the ratio also indicates bacterial stress in overloaded digesters that are not buffered. It is therefore concluded that variations in propionic: acetic acid ratio provide a reliable indicator for impending failure.