Optimization of hydrothermal pretreatment conditions for mesophilic and thermophilic anaerobic digestion of high-solid sludge.

• Mesophilic (MAD) and thermophilic (TAD) digestion of 10–11% sludge were studied. • 160 °C for 210 min was the optimal hydrothermal pretreatment (HTP) condition. • TAD exhibited higher methane production than MAD under stable conditions. • 2800 mg/L of NH 4 +-N resulted in an inhibition of methane production in TAD. • The energy balance of MAD with HTP was the highest, 5.25 kJ/g VS. Hydrothermal pretreatment (HTP) conditions were optimized for continuous mesophilic (MAD) and thermophilic (TAD) anaerobic digestion of high-solid sludge (10–11% total solids). COD solubilization increased with prolonged HTP durations, and became not significant after 210 min. According to the methane production rate and energy consumption, the optimal HTP temperature was determined at 160 °C. Regarding continuous operation without HTP, TAD achieved higher methane yield and volatile solids (VS) reduction, at 0.12 L/g VS added and 23.9%, respectively. After HTP, methane yield and VS reduction in MAD and TAD were increased by 400% and 191% (MAD), 67% and 72% (TAD), respectively. TAD was limited due to the inhibition from about 2800 mg/L of NH 4 +-N concentration. The methanogenic activity of MAD was enhanced, whereas TAD displayed a reduced value owing to ammonia inhibition. Ultimately, MAD with HTP and TAD without HTP achieved the higher energy balance, 5.25 and 3.27 kJ/g VS, respectively. [ABSTRACT FROM AUTHOR]