Hydrothermal carbonization of anaerobically digested sewage sludge for hydrochar production

Hydrothermal carbonization (HTC) can process anaerobic digestion effluent (ADE) from sewage sludge into hydrochar and liquor. The objective of this study was to assess the effects of HTC temperature, time, and initial ADE pH on the yields and properties of hydrochar and liquor. HTC was conducted at different ADE pH (original ~8.2 and modified ~6.8), temperatures (180 to 260 °C) and times (30 to 70 min) following a central composite design. Hydrochar yields from ADE at original and modified pH ranged from 70 to 80% and 62% to73%, respectively. Higher temperatures resulted in lower hydrochar and liquor yields. The obtained hydrochar had a low carbon content, high ash content, high H/C ratio, and low higher heating value, suggesting that it has little potential as solid fuel, and more potential as soil amendment. The liquor's composition and chemical oxygen demand indicate a potential for recirculation into anaerobic digestion.