The role of oxidation-reduction potential as an early warning indicator, and a microbial instability mechanism in a pilot-scale anaerobic mesophilic digestion of chicken manure

To identify an early warning indicator for the anaerobic digestion of chicken manure and reveal a microbial mechanism of instability, a bench-scale digester with a working volume of 55 L was utilized by raising the organic loading rates (OLRs). Three characteristics were identified and posited as early warning indicators, namely: one-way changes, a longer early response time, and sudden variations. The core early warning indicator set was constructed, which included the oxidation-reduction potential (ORP), individual volatile fatty acids, and CH4/CO2. Considering the convenience, economic nature, and reliability of industrial biogas plants, the ORP was recommended as a final early warning indicator with an inhibition threshold of >-540 mV. The initial instability was caused by the accumulation of ammonia, which suppressed methanogenic activity, although it stimulated the growth of Methanosarcina. The accumulation of free volatile fatty acids, which inhibited both the growth and methanogenic activity of Methanosarcina, was responsible for the final collapse. (C) 2021 Elsevier Ltd. All rights reserved.