Studying the impact of HRT on the anaerobic digester microbiome shows divergence between standard model predictions and experimental evidence

The hydraulic retention time (HRT) is important for anaerobic digestion because it determines the efficiency of digesters and the necessary duration of conclusive experiments. The prediction of its impact on the microbial community is therefore crucial. We run two lab-scale anaerobic digesters with HRTs of 5.5 and 14 days, and tested three models for their predictive power on biomass concentration depending on HRT. A thermodynamic black box approach and dynamic flux balance analysis strongly overestimated the increase in biomass concentration at the lower HRT. Only the anaerobic digestion model 1 (ADM1) with default parameters for microbial decay resulted in good predictions, but only with low, potentially overfitted growth yields. Furthermore, the microbial community analyses (using T-RFLP fingerprinting for methanogenic archaea, 16S rRNA amplicon sequencing and qPCR) revealed surprising results. Methanosaeta were the dominant acetoclastic methanogens even at a HRT of 5.5 d although Methanosarcina are thought to dominate at low HRTs. A higher richness for methanogens and bacteria was found at the lower HRT, contradicting the assumption that fewer microorganisms can sustain high growth rates. Although only VFAs were fed, less than 50% of the bacteria could be connected with known syntrophic VFA oxidizers.