A full-scale anaerobic digester receiving a mixture of primary and secondary sludge was monitored for one hundred days. A chemical oxygen demand (COD), volatile solids (VS), and mass balance were conducted to evaluate the stability of the digester and its capability of producing methane gas. The COD mass balance could account for nearly 90% of the methane gas produced while the VS mass balance showed that 91% of the organic matter removed resulted in biogas formation. Other parameters monitored included: pH, alkalinity, VFA, and propionic acid. The values of these parameters showed that steady state had occurred. At mesophilic temperature and at steady state performance, the anaerobic digester stability was defined as a constant rate of methane produced per substrate of ΔVS (average rate=0.40 L/g). This constant rate can be used as stability index to determine the anaerobic digestion stability in an easy and inexpensive way.
Demitry, M. E., & McFarland, M. J. (2015). Defining full-scale Anaerobic Digestion Stability: The Case of Central Weber Sewer Improvement District. Environment and Pollution, 4(2). https://doi.org/10.5539/ep.v4n2p1