The effects of C/N (10-25) on the relationship of substrates, metabolites, and microorganisms in inhibited steady-state of anaerobic digestion

The carbon/nitrogen ratio (C/N) is a key parameter that affects the performance of anaerobic digestion (AD). Recent AD research has focused on optimizing the C/N of feedstock. The so-called inhibited steady-Bstate refers to a special state of ammonia inhibition of AD that often occurs at low-C/N (below 25) when degradable nitrogen-rich substrates, such as livestock manure, are used as feedstock. However, the mechanism behind the inhibited steady-state is still unknown. In the current study, co-digestion and recirculation were used to create a C/N gradient in the influent to explore the relationship between substrates, metabolites, and microorganisms in the inhibited steady-state. Data were collected at the macro, microbial, and genetic levels. Three CSTRs were successfully made run into the inhibited steady-state using influent C/Ns of 10-12. Digestion performance levels of R10-R12 were low and stable, transitioning from an aceticlastic methane-producing pathway to a hydrogenotrophic pathway as the C/N gradually decreased. As the abundance of the hydrogenophilic methanogens increased, the abundance of syntrophic acetate-oxidizing bacteria (SAOB) also increased. The succession between populations of Methanosaeta and Methanosarcina may be used as a microbiological indicator of ammonia inhibition. Under high-C/Ns, cooperation among bacteria was high, while under low-C/Ns, competition among bacteria was high. These results clarify the processes underlying the inhibited steady-state, which is a condition often faced in actual large-scale biogas facilities that use degradable nitrogen-rich substrates. Moreover, practical guidelines for evaluating ammonia inhibition are provided, and strategies to alleviate ammonia suppression are developed. (C) 2020 Elsevier Ltd. All rights reserved.

Automated summary

This study explored the relationship between substrates, metabolites, and microorganisms in the inhibited steady-state by creating a C/N gradient in anaerobic digestion. The results showed that competition among bacteria was high under low C/N conditions, while cooperation among bacteria was high under high C/N conditions. Additionally, the study further elucidated the process of ammonia inhibition and microbiological indicators of the inhibited steady-state.