Replicable microbial community development and activity during long-term low temperature treatment of synthetic sewage

The process performance and reproducibility of long-term (889 days), high-rate, low-temperature anaerobic digestion of synthetic sewage was studied. The primary focus of this study was on the acclimation, development and activity of the microbial population originating from a mesophilic biomass source. Two identical hybrid anaerobic sludge bed-fixed-film reactors were used to degrade a synthetic sewage based wastewater at 12°C, at organic loading rates (OLRs) of 0.25-1.0 kg COD m-3 d-1. Chemical oxygen demand (COD) removal efficiencies were regularly above 78% for the total and soluble COD fractions. Proteins were completely hydrolysed within the systems and carbohydrate removal was >68%. Sludge biomass characterisation indicated the successful development of psychrotolerant/psychrophilic activity in the biomass of both reactors. The hydrolysis rate constant (K) had increased by 20 times from the seed inoculum, for example, after 500 days of reactor operation. Next generation sequencing demonstrated that the adaptation of the mesophilic inoculum involved a shift in community structure that involved a 1-log fold change in 25 sequences. A diverse bacterial community was present, dominated by Proteobacteria, Clostridia, Firmicutes, Bacteroidetes and unusually for anaerobic digesters, Synergistetes. Methanosaeta dominated the archaeal portion of the community. The microbial community structure (both during steady state and in periods of perturbation) and development in both reactors was reproducible at genus level, with >95% confidence for the five phases of operation.