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.