Performance and inhibitory characteristics of solubilized digestate returned to anaerobic digestion

Anaerobic digestion (AD) is an efficient sludge management method but is limited by slow hydrolysis rates and solid accumulation during digestate recirculation. This study compared digestate solubilization and methane production after thermal hydrolysis (TH, 80-280 °C) and ultrasonication (US, 0.5-10 h). TH achieved a maximum soluble COD of 9,205 ± 216 mg/L with lower energy input, whereas US required much higher energy. Biochemical methane potential tests showed that TH (160-280 °C) yielded 270-288 mL CH₄/g VS, while US treatments produced only 145-166 mL CH₄/g VS. Liquid chromatography-organic carbon detection analysis indicated that TH generated readily biodegradable low-molecular-weight compounds, whereas US mainly released recalcitrant biopolymers. At temperatures above 240 °C, TH formed of melanoidin-like compounds, extending lag phases. Microbial analysis indicated that methane yield was governed by substrate bioavailability rather than gene abundance. TH enhanced hydrolytic and acetogenic bacteria, whereas US showed limited impact. These findings provide valuable insights for optimizing digestate recirculation systems.