The high-efficient utilization of sludge and its hydrothermal aqueous-phase through multi-stage hydrothermal-pyrolysis process: experiments, mechanisms, and life cycle assessment

The hydrothermal treatment is an effective method to achieve sludge dewatering. However, the problem of high chemical oxygen demand and high biological toxicity in the sludge hydrothermal aqueous-phase remains to be urgently addressed. A multi-stage hydrothermal treatment (MHT) plus pyrolysis (PY) process that achieves both harmless and resourceful utilization of sludge and its hydrothermal aqueous-phase is proposed in this research. Additionally, two conventional processes, drying-pyrolysis (D-PY) and single-stage hydrothermal treatment-pyrolysis (SHT-PY), have been introduced for comparative purposes. In multi-stage hydrothermal treatment, the hydrothermal aqueous-phase rich in proteins (25.66 %) and carbohydrates (11.23 %), about 26.70 wt% of raw sludge, can be obtained in the first low-temperature hydrothermal stage (150 °C) and reused as fertilizer. While, the solid residue of the first stage is further dehydrated during the second high-temperature hydrothermal stage (270 °C). Due to better dewatering, denitrification, and deoxygenation effects of multi-stage hydrothermal treatment, the quality of bio-oil and bio-char produced from subsequent pyrolysis are also improved with the less N and O. The energy consumption of multi-stage hydrothermal treatment is only 173.28 MJ/h, about 45.03 % of drying in D-PY and 43.19 % of single-stage hydrothermal treatment. Meanwhile, life cycle ansessment shows that the environmental impact indicators of MHT-PY, including NOx and SOx emissions, ODP, HTP, AP, EP, ADP, GWP and POCP are all lowest in comparison scenarios. Furthmore, MHT-PY obtain an additional 1438.46 kg of high-value liquid fertilizers, possessing the highest economic benefit of 67.12 $/(t dry sludge) among three systems.