Isotope Fractionation in Biogas Allows Direct Microbial Community Stability Monitoring in Anaerobic Digestion.

Process monitoring of anaerobic digestion is typically based on operational parameters, such as pH and volatile fatty acid concentration, that are lagging on actual microbial community performance. In this study, 13 C isotope fractionation in CH4 and CO2 in the biogas was used to monitor process stability of anaerobic digestion in response to salt stress. A gradual and pulsed increase in salt concentration resulted in a decrease in methane production. No clear shift in δ13 CH4 was observed in response to the gradual increase in salt concentration, and δ13 CO2 of the biogas showed only a clear shift after process failure, compared with the control. In contrast, both δ13 CH4 and δ13 CO2 in the biogas changed in response to the pulsed increase in salt concentration. This change preceded the decrease in methane production. A significantly different bacterial and archaeal community profile was observed between the DNA and RNA level, which was also reflected in a different relation with the δ13 CH4 and δ13 CO2 values. This shows that isotope fractionation in the biogas can predict process stability in anaerobic digestion, as it directly reflects shifts in the total and active microbial community, yet, due to its temporal character, further validation is needed.