Clarification of the Dynamic Autothermal Thermophilic Aerobic Digestion Process Using Metagenomic Analysis.

This study details a unique process of autothermal thermophilic aerobic digestion (ATAD) of human excreta useful in producing nitrogen-rich and pathogen-free organic fertilizer. The process was divided into initial, middle, and final phases, based on changes in temperature, dissolved oxygen (DO), and bacterial community structure. The aim of this study was to determine bacterial factors that would affect liquid fertilizer production in the process, using shotgun metagenomic analysis of each phase. Although the abundances of all 28 gene categories include 4 categories in SEED subsystems level 1 were similar to those in another type of wastewater treatment system, the abundances of 4 gene categories changed remarkably. Among them, a decrease in the abundance of the phage-related gene category and the presence of antibacterial substances in secondary metabolism may explain the change in bacterial community structure from the material to the initial phase. Increases in the abundances of two gene categories, phage-related and secondary metabolism, coincided with a decrease in alpha diversity from the material to the initial phase. A potential increase in the abundance of genes in the category of sporulation from the middle to the final phase was correlated with deterioration of growth conditions and stabilization processes. In addition, prompt consumption of short-chain fatty acids in the initial phase and unusually stable ammonia accumulation throughout the process could be explained by the presence/absence of related metabolic genes. In conclusion, the relationships between bacterial function and unique characteristics of ATAD were revealed; our findings support the enhancement of liquid fertilizer production from wastewater.