Microbial Community Ability to Adapt to Altered Temperature Conditions Influences Operating Stability in Anaerobic Digestion

This study examined the influence of altered operating temperature on microbial community composition and how this affects the ability of anaerobic degradation (AD) processes to cope with increasing loading rate. Four digesters fed household and slaughterhouse waste, mimicking two large-scale processes, were operated in sets of two at 37°C or 52°C, followed by a gradual increase or decrease in temperature in one digester in each set. All digesters were then subjected to step-wise increases in organic loading rate (OLR) from 3 to 7g VS/L/day, concurrently with decreased hydraulic retention time (HRT). Temporal microbial changes were monitored by Illumina MiSeq analysis of bacterial and archaeal 16S rRNA gene sequences. The digester transformed from thermophilic to mesophilic conditions failed at 6g VS/L/day, whereas the reference digesters and the mesophilic-to-thermophilic digester remained stable at all OLR investigated. The bacterial community was characterised by relatively diversified structure dominated by Bacteriodetes and Firmicutes in mesophilic conditions. Increasing temperature caused loss of complexity at phyla level and enhanced the relative abundance of Firmicutes and Thermotogae. Temperature adaptation in the thermophilic-to-mesophilic digester resulted in a bacterial community structure reflecting the communities observed in digesters with initial temperature of 37°C. However, certain populations found in the mesophilic digesters did not appear at detectable levels during operation at 37°C in the thermophilic-to-mesophilic digester. Overall, the results showed that anaerobic process stability was highly dependent on the innate resilience of the microbial community. The mesophilic microbiota formed from a thermophilic community had a slightly different structure than its mesophilic counterpart and was considerably less resistant to increased OLR in digesters examined within this study.