Mechanical Stirring Restructures Mass Transfer and Microbial Metabolism in Pilot-Scale Solid-State Anaerobic Digesters

Mechanical stirring is the most efficient method for enhancing solid-state anaerobic digestion (SS-AD). However, the current understanding of its mass and heat transfer is limited due to experimental constraints. Here, two 100 L SS-AD reactors were established: one with mechanical stirring and the other without. Temperature distributions were conducted to study heat transfer; computational fluid dynamics (CFD) was combined with the effective diffusion coefficient (Deff) to validate mass transfer. Environmental parameters were incorporated to determine the influence of heat and mass transfer on the microenvironment. The results revealed that the cumulative CH4 yield with mechanical stirring was increased by 32.21%. Mass transfer had a greater impact on the microenvironment and microbial communities’ distribution than heat transfer. During the start-up stage of AD, mechanical stirring facilitated the homogeneous dispersion of microorganisms. It promoted substrate hydrolysis, while reducing methanogenic potential on the peak CH4 production phase, indicating a lower intensity of mechanical stirring is required in the following methanogenesis stage. For this case, metagenome analysis confirmed that mechanical stirring enhanced microbial mobility and environmental adaptability. However, it limited microbial DNA synthesis, ribosome, and functions related to microbial reproduction, resulting in a reduction in the CH4 production potential of the reactor.