Resource-Efficient and Wastewater-Mediated Hydrothermal Carbonization of Lignocellulosic Residue: Energy Potential and Environmental Trade-Offs in Integrated Bioelectrochemical Systems

With the global community grappling with surging pressure on energy resources, freshwater availability, and lignocellulosic waste management, hydrothermal carbonization (HTC) presents a promising low-severity thermal pathway for converting biomass into energy-dense hydrochar (HC). However, HTC of dry feedstocks requires an external aqueous medium, often placing further strain on freshwater resources. This investigation employs wastewater streams─fresh sewage (FS) and compost leachate (CL)─as alternative hydrothermal media for sugarcane bagasse, coupled with microbial fuel cells (MFCs) for post-HTC process water valorization. The findings revealed a promising potential of up to 103% reduction in water footprint and about 25% lower freshwater eutrophication by substituting double-distilled water (DDW) with FS, with comparable environmental risks for the majorly affected midpoint impact categories. In contrast, the CL-based HC (HC-CL) pathway exhibited elevated environmental burdens due to the upstream emissions associated with composting. Combustion analysis showed FS-based HC achieved a comprehensive combustion index of 9.85 × 10–7 wt %2 min–2 °C–3, significantly exceeding the minimum threshold for solid fuels by about five times and falling between the performance of DDW-based HC and HC-CL. These findings highlight FS as a viable and sustainable substitute for freshwater in HTC, offering enhanced water-use efficiency without any substantial compromise in fuel quality.