State indicators of anaerobic digestion: A critical review on process monitoring and diagnosis

Anaerobic reactors in full-scale biogas plants often encounter process instability; thus, their operational efficiency and further promotion remain limited. Process monitoring can be used to ensure better process stability during anaerobic digestion, the key to which depends on the selection and quantitation of appropriate state indicators. The present review focuses on the recent progress related to state indicators in the field of process monitoring and discusses in detail the ability to identify process status and especially, provide early warning information on process instability, in addition to the current monitoring status at the industrial level. Results show that the explorations of effective state indicators have experienced vigorous development. Most of the proposed state indicators have mainly focused on the gas and liquid phases, and those commonly used are sufficient to achieve effective monitoring in different systems; however, their suitability for use as potential early warning indicators varies. A comprehensive interpretation of the process state requires a combination of different state indicators to supply complementary information. Moreover, the current mode based on setting threshold values to diagnose process stability remains primitive. To ensure that the early warning is more feasible and universal, improvements can be made in terms of establishing a plant-specific database and focusing on the response trends to process disturbance as a basis for diagnosis. On the other hand, monitoring of most key state indicators is limited to external laboratory tests and accordingly, the consequent additional monitoring costs can be reduced by creating a suitable schedule regarding their frequency.