Incorporating hydrothermal liquefaction into wastewater treatment – Part II: Characterization, environmental impacts, and potential applications of hydrochar

Abstract

Hydrothermal liquefaction (HTL) is a promising technique for renewable biofuel (biocrude) production from municipal sludge. However, its solid byproduct, hydrochar, requires sustainable management for further resource recovery and pollution control. This study comprehensively assessed the properties, environmental influences, and possible utilizations of hydrochar generated from mixed sludge (MS). With the increase of HTL reaction temperatures (290–360 °C) and residence time (0–30 min), the dry-weight contents of ash and fuel ratio increased from 10.5% and 0.1 in MS to 48.7–68.5% and 0.4–0.7 in hydrochar, respectively. However, the dry-basis contents of volatile matter, carbon, and higher heating value sharply decreased to 18.7–35.9%, 22.9–37.3%, and 8.6–16.0 MJ/kg, respectively. The leaching risks of inorganic contaminants from hydrochar were limited and controllable in various scenarios. Hydrochar performed more stable combustion than sludge, but its high ash contents and alkali index (0.28–0.72 kg/GJ) implied high risks of slagging and fouling. Hydrochar has a good potential for carbon sequestration due to low O/C ratios (<0.2) and improved recalcitrance index (0.43–0.48). Benefiting from intrinsic metals (e.g., Ca and Fe), catalytic hydrochar graphitization was feasible at a moderate temperature (1200 °C). Although hydrochar was restricted from land application for heavy metals accumulation, it is promising for metals and nutrient recovery. It has a total phosphorus (P) of 7.2–8.5% by weight, and thus P recovery is critical and necessary for mitigating environmental challenges and global P scarcity. Overall, this study contributed to the state-of-the-art in waste-to-resource development and cleaner production.