Effect of increasing organic loading rate on hydrolytic enzyme activities and microbial communities during anaerobic digestion of energy crops
Efficiency and stability of anaerobic digestion (AD) processes strongly depend on maintaining a balance between microbial conversion steps. Operational stress can disrupt this balance, leading to shifts in the microbial community, accumulation of intermediate metabolites, and changes in overall process performance. To systematically investigate the response of AD to operational stress, a stepwise increase in organic loading rate (OLR) until 15 g VS/(L d) along with stepwise decrease of hydraulic retention time was applied in two parallel operated lab-scale anaerobic digesters. The systems were comprehensively monitored, including key process parameters such as biogas production, pH, buffer capacity, and volatile fatty acid (VFA) concentrations. In addition, selected hydrolytic enzyme activities were measured, and microbial community dynamics were assessed via 16S rRNA and mcrA amplicon sequencing to allow for detailed insights into functional and microbial shifts. Until OLR was 2.5-fold increased, both reactors showed similar trends in process performance, characterized by increasing hydrolytic enzyme activities and minor VFA accumulation. With increasing OLR, VFA accumulation increased, and the process performance and microbial communities of the two reactors diverged. While methane production in one reactor collapsed due to over-acidification and was replaced by chain elongation as the predominant microbial process, the other reactor maintained methanogenic activity until 3-fold increase in OLR. These changes were also reflected in declining enzyme activities, where protease activity decreased earliest. Bacterial diversity in the reactor shifting to chain elongation decreased dramatically, indicating a loss of AD functionality under elevated OLR.
Efficiency and stability of anaerobic digestion (AD) processes strongly depend on maintaining a balance between microbial conversion steps. Operational stress can disrupt this balance, leading to shifts in the microbial community, accumulation of intermediate metabolites, and changes in overall process performance. To systematically investigate the response of AD to operational stress, a stepwise increase in organic loading rate (OLR) until 15 g VS/(L d) along with stepwise decrease of hydraulic retention time was applied in two parallel operated lab-scale anaerobic digesters. The systems were comprehensively monitored, including key process parameters such as biogas production, pH, buffer capacity, and volatile fatty acid (VFA) concentrations. In addition, selected hydrolytic enzyme activities were measured, and microbial community dynamics were assessed via 16S rRNA and mcrA amplicon sequencing to allow for detailed insights into functional and microbial shifts. Until OLR was 2.5-fold increased, both reactors showed similar trends in process performance, characterized by increasing hydrolytic enzyme activities and minor VFA accumulation. With increasing OLR, VFA accumulation increased, and the process performance and microbial communities of the two reactors diverged. While methane production in one reactor collapsed due to over-acidification and was replaced by chain elongation as the predominant microbial process, the other reactor maintained methanogenic activity until 3-fold increase in OLR. These changes were also reflected in declining enzyme activities, where protease activity decreased earliest. Bacterial diversity in the reactor shifting to chain elongation decreased dramatically, indicating a loss of AD functionality under elevated OLR.
Helmholtz Environmental Research Center Leipzig