The one-stage autothermal thermophilic aerobic digestion for sewage sludge treatment: Stabilization process and mechanism

► VFA would produce due to oxygen-deficiency as SCOD achieved rapid increase. ► Propionic, iso-valeric, and iso-butyric acids were also the major VFA components besides acetic acid. ► ATAD system regulates metabolic pathway to produce VFA to meet with the demand for NAD+. ► The ORP affected the VFA production and depletion as well as sulfate levels. ► The pH depends on the relative equilibrium between ammonia and VFA. Batch experiment was carried out in a simulated thermophilic aerobic digester to investigate the digestion process of one-stage autothermal thermophilic aerobic digester and to explore the sludge stabilization mechanism. Volatile solids removal was 38.4% at 408h and 45.0% at 552h. Chemical oxidation demand, total nitrogen, and ammonia nitrogen in supernatant increased rapidly up to 168h, and all of them fluctuated moderately after 360h. Volatile fatty acid (VFA) accumulated rapidly up to 24 to 168h, then declined sharply, reaching a low concentration after 312h. Propionic, iso-valeric, and iso-butyric acids, in addition to acetic acids, were also the major components of VFA. As the biochemical metabolic process was inhibited under oxygen-deficiency condition, the digestion system can produce acetic, propionic, butyric acids and other VFA constituents to meet the demand for NAD+ and maximize ATP generation. The ORP affected the VFA production and depletion as well as sulfate levels.