Pig urine-induced ternary buffering complex and microbial community for mitigating acid inhibition in high-solid anaerobic digestion of rice straw

Abstract

Acid inhibition is frequently recognized as the primary challenge in high-solid anaerobic digestion, often leading to severe process inhibition or even system failure. Maintaining microbial pH homeostasis is becoming increasingly critical. In this study, a pH ternary buffering complex was developed in the straw digestion system by integrating carbonate and ammonium derived from pig urine with volatile fatty acids present in the reactor. Results showed that the pH ternary buffering complex alleviated acid inhibition and promoted methane production significantly. The methane production of 342.66 mL·gVS_removal⁻¹ was achieved in the reactor with ternary buffering complex, which corresponding to 29-fold of that without buffering. Abundances of key enzyme genes in hydrolytic and acidogenic stages was improved by the ternary buffer complex. Besides, the buffering complex optimized acidogenic metabolic pathway and NADH/NAD⁺ balance during methane production. To alleviate acid inhibition, the ternary buffering complex was supposed to work with extracellular excess H⁺ in a timely manner and promoted proton pump that transferred intracellular H⁺ out of the cell to prevent cytoplasmic acidification. Simultaneously, NH₄⁺ was transported into the cell to replenish the significant loss of intracellular cations (H⁺) and maintained cellular osmotic pressure. Thus this study opens a door for alleviation of acid inhibition and maintenance of microbial cellular homeostasis.