Liquid co-substrates repower sewage microbiomes

A range of parameters are known to shape the methanogenic communities of biogas-producing digesters and to strongly influence the amount of biogas produced. In this work, liquid and solid fractions of grass biomass were used separately for semicontinuous batch methanation using sewage sludge as seed sludge. During 6 months of incubation, the amount of input COD was increased gradually, and the underlying methanogenic microbiome was assessed by means of microscopy-based automated cell counting and full-length 16S rRNA high-throughput sequencing. In this sense, we prove for the first time the suitability of the ONT TM MinION platform as a monitoring tool for anaerobic digestion systems. According to our results, solid-fed batches were highly unstable at higher COD input concentrations, and kept Methanosaeta spp. -typically associated to sewage sludge- as the majoritary methanogenic archaea. In contrast, liquid-fed batches developed a more stable microbiome, proved enriched in Methanosarcina spp, and resulted in higher methanogenic yield. This work demonstrates the high repowering potential of microbiomes from sewage sludge digesters, and highlight the effectiveness of liquefied substrates for increasing biogas productivity in anaerobic digestions.