In order to solve the growing need of excess sludge (ES) disposal in China, anaerobic digestion with thermal hydrolysis (THP-AD) is gaining more and more applications. However, engineering microbial ecology in THP-AD is still not clear. Accordingly, four full-scale THP-AD which were operated under similar operational parameters but with different influent composition, were studied for their microbial communities by high-throughput Illumina-MiSeq sequencing in company with the physiochemical indices. The four facilities could be divided into two groups according to their methanogenesis performance, the inferior group (InG) and the superior group (SupG). In both groups, THP promoted the solubilization of particulate polysaccharide and proteins, also increased the soluble proportion of chemical oxygen demand (COD), meanwhile raised ammonia and volatile fatty acids (VFAs) concentration. In the InG, thermal hydrolyzed ES with higher level of polysaccharides and proteins boosted the ratio of decomposing bacteria of the corresponding substances. As the degradation products, VFAs were generated and accumulated although ratio of VFAs decomposing bacteria were higher than the SupG. On the other hand, more proteins decomposing bacteria in the InG led to higher ammonia concentration, resulting in higher level of both ammonia and VFAs. Since ammonia and VFAs have higher inhibition effect on aceticlastic methanogens, which are more efficient in methanogenesis than hydrogenotrophic methanogens, hydrogenotrophic methanogens turned dominant in the InG, consequently caused the inferior methanogenesis performance. Based on the mechanisms revealed above, it is recommended that great attention should be drawn to ammonia, VFAs and COD concentration in full-scale operation, of which the high concentration may indicate poor performance. While the higher relative abundance of Proteophilum and/or lower Methanosaeta could also be potential implication of inferior methanogenesis efficiency. This study revealed that microbial community structures were shaped by operational parameters and substrate properties, and further determined the process performance in THP-AD, provided enlightenment for operational regulation in full-scale projects.
