A Study of Ammonia Shock on Anaerobic Digestion Performance and the Methane Inhibition Mechanism

Abstract

Anaerobic digestion (AD) performance may be affected by ammonia released from N-rich organics, where the specific microbial mechanism remains unclear. To provide better insight into AD performance, the mass and energy flow, thermodynamics, microbial communities, and metabolic activities in digestors were analyzed under stepwise-increased ammonia levels (2000 to 5000 mg NH4+-N/L). The results showed that the AD process was temporally inhibited at 2000 to 4000 mg NH4+-N/L but rapidly recovered, while it collapsed at 5000 mg NH4+-N/L. The energy and mass flow into biogas were negatively correlated with TAN concentration. The change in the Gibbs free energy of VFA degradation could be evaluated well for propionic and butryic acid but not for methanogenesis of acetic acid, under every ammonia level. Moreover, microbial analysis results showed a very high ratio between acetogen and methanogen at 5000 mg NH4+-N/L, causing VFA accumulation and low methane production. The metagenomic analysis indicated that ammonia stress facilitated the synthesis of osmoprotectant and maintained the intracellular homeostatic state, which benefited the microbial tolerance against ammonia, while high ammonia stress (5000 mg NH4+-N/L) exerted a fatal inhibitory effect on the microbial electron transfer activity and prevented the efficiency necessary to maintain microbial methanogenesis activity. This work comprehensively provides a detailed analysis of the mechanism of how ammonia affects AD performance, and offers theoretical knowledge for developing strategies to alleviate ammonia inhibition of the AD process.

Keywords: Anaerobic Digestion, Ammonia inhibition, Mass and energy flow, Gibbs free energy, Electron transfer