Should we build "obese" or "lean" anaerobic digesters?

Area of Science:

• Microbial Ecology
• Anaerobic Digestion
• Bioreactor Microbiology

Background:

• Conventional anaerobic digesters (ADs) treat dairy manure, often raw or fermented, which is rich in volatile fatty acids (VFAs).
• Pre-fermented AD (PF-AD) uses a pre-fermented, fiber-rich manure fraction depleted of VFAs, akin to a lean diet.
• Previous studies indicate comparable methane yields between conventional and PF-AD systems.

Purpose of the Study:

• To characterize bacterial communities in various anaerobic digestion bioreactors using next-generation DNA sequencing.
• To compare the Firmicutes to Bacteroidetes (F/B) ratios across different digester types and substrates.
• To correlate microbial community structure with substrate composition and digester performance.

Main Methods:

• Next-generation DNA sequencing was employed to analyze bacterial communities.
• Samples were collected from pre-fermentation stage fermenters, PF-AD bioreactors, conventional single-stage AD, and full-scale AD systems.
• The relative abundance of bacterial phyla, particularly Firmicutes and Bacteroidetes, was quantified.

Main Results:

• Firmicutes and Bacteroidetes were the most abundant phyla across all analyzed samples.
• PF-AD samples showed a higher relative abundance of Bacteroidetes (26-51%).
• Fermenter and manure samples exhibited higher Firmicutes abundance (up to 76% and 66%, respectively), correlating with higher F/B ratios and labile substrates.

Conclusions:

• Primary stage fermenters displayed microbial traits associated with 'obesity' (higher F/B ratios, less fibrous substrate).
• PF-AD systems demonstrated microbial characteristics linked to 'leanness' (lower F/B ratios, fibrous substrate).
• Bacterial communities in AD shift based on substrate quality, maintaining VFA yields in PF-AD, analogous to lean mice with high-fiber diets.