Reversibility of hydrolysis inhibition at high hydrogen partial pressure in dry anaerobic digestion processes fed with wheat straw and inoculated with anaerobic granular sludge

Abstract

In dry anaerobic digestion (AD), methanogenic performances are lowered by high solid contents. Low performances are often caused by a decrease of the gas-liquid transfer kinetics leading to local accumulation of inhibitory by-products. Hydrogen was previously identified as an inhibitor of hydrolytic and acetogenic microbial activities in dry AD. CO₂ is also generated but its impact on the microbial activity remains unknown. In this study, the reversibility of dry AD inhibition at high H₂ partial pressure (P_H2 of 1 bar) was investigated by adding CO₂ (400 mbars) after 11 and 18 days of methanogenesis inhibition, in an AD process operated at 25% TS, using wheat straw as substrate and inoculated with anaerobic granular sludge. As soon as CO₂ was added, the methanogenic activity rapidly recovered within 3 days, from 0.41 ± 0.1 to 3.77 ± 0.8 and then 2.25 ± 0.3, likely through the hydrogenotrophic pathway followed by the acetoclastic pathway, respectively. This result was confirmed by the high abundance of Methanomicrobiales (83%) and the emergence of Methanosarcinales sp (up to 17%) within the methanogenic community. Furthermore, the recovery kinetics were impacted by the duration of the inhibition period suggesting a different impact of the high P_H2 on hydrogenotrophic and acetoclastic methanogens.

Keywords: Acidogenesis; Carbon dioxide; Gas transfer; Hydrogen; Solid-state anaerobic digestion.