Steady-state and transient behavior in microbial methanification: I. Experimental results

Area of Science:

• Environmental microbiology
• Biochemical engineering
• Anaerobic digestion

Background:

• Volatile fatty acids (VFA) are key intermediates in anaerobic digestion.
• Understanding methanification kinetics is crucial for optimizing biogas production.
• Previous assumptions linked methane production primarily to microbial growth.

Purpose of the Study:

• To investigate the kinetics of volatile fatty acid methanification.
• To elucidate the role of microbial community structure in VFA consumption.
• To determine the relationship between methane production and microbial physiology.

Main Methods:

• Experiments conducted using an upflow anaerobic sludge bed (UASB) reactor.
• Analysis of acetic, propionic, and butyric acid consumption at varying residence times.
• Monitoring of methane production in response to changes in inlet conditions.
• Assessment of reactor stability and VFA conversion efficiency at high organic loading rates.

Main Results:

• Hysteresis observed in acetic and propionic acid consumption at residence times < 2.5 h, attributed to cross-inhibition within microbial consortia.
• Butyric acid consumption showed no hysteresis.
• Methane production responded almost instantaneously to inlet condition changes, suggesting a link to maintenance energy requirements rather than growth.
• Stable reactor operation achieved with simultaneous 50% VFA concentration changes.
• High VFA conversion efficiency (82%) observed even at organic throughput rates of 35 kg COD/day m(3)-reactor.

Conclusions:

• Microbial community structure and inter-species interactions significantly influence VFA methanification kinetics.
• Methane production is primarily linked to microbial maintenance energy demands, not solely growth-associated.
• UASB reactors demonstrate robust performance and high efficiency in VFA conversion under dynamic loading conditions.