State indicators for monitoring the anaerobic digestion process.

Anaerobic process state indicators were used to monitor a manure digester exposed to different types of disturbances, in order to find the most proper indicator(s) for monitoring the biogas process. Online indicators tested were biogas production, pH, volatile fatty acids (VFA), and dissolved hydrogen. Offline indicators tested were methane and hydrogen content in the biogas. A CSTR reactor with 7.2 L working volume was operated at a varying hydraulic loading rate (HRT 10-20 days) for 200 days. During this period, the reactor was overloaded with extra organic matter such as glucose, lipid, gelatine, and bio-fibers, in order to create dynamic changes in the process state. Biogas production increased in response to the increase in organic load with a slight decrease in methane content. pH was relatively stable and did not show clear response to hydraulic load changes. However, pH changes were observed in response to extra organic load. Individual VFA concentrations were an effective indicator, with propionate persistent for the longest time after introduction of the disturbance. Dissolved hydrogen was very sensitive to the addition of easily degradable organics. However, it responded also to other disturbances such as slight air exposure which had no impact on process performance. A combination of acetate, propionate and biogas production is an effective combination to monitor this type of digesters effectively.