Characteristics of instability and suitable early-warning indicators for cornstalk-fed anaerobic digestion subjected to various sudden changes

In the process of anaerobic digestion (AD), the poor buffer capacity of the system could lead to the imbalance of the supply and demand of methanogens for organic matters, which may subsequently result in the decrease of biogas production, and even the instability of the whole AD system. In this study, key factors of sudden change were simulated based on the frequent instability problems in biogas projects, and subsequent instability properties, early-warning indicators, and microbial community characteristics were investigated. On the whole, the stable early-warning indicators were biogas production rate and content, pH, total alkalinity, bicarbonate alkalinity, and the sum of volatile fatty acids (VFAs). The threshold value of VFAs for AD instability was 10,000 mg/L, under which the biogas production rate reduced from 0.377 to 0.038 L/L/d, CH4 content decreased from 62.8% to 33.6%, and the proportion of Methanosaeta dropped from 74% to 51.6%. Furthermore, Methanosaeta abundance was greatly sensitive to the sudden variation of VFAs, therefore this kind of archaea could be considered as a warning microbial indicator for acid inhibition. There has been an obvious difference in the functional abundances between bacteria and archaea. This study could provide theoretical guidance for stable operation of biogas projects.