Energy Recovery from Garden and Park Waste by Hydrothermal Carbonization with Process Water Recycling

This study aims to obtain a carbonaceous material with suitable properties to be used as a solid biofuel by recycling process water from hydrothermal carbonization (HTC) of garden and park waste (GPW). The research is focused on maximizing mass yield and energy recovery as well as facilitating the treatment of the liquid fraction throughout reusing cycles of the liquid fraction. Process water recycling moderately improved the mass performance of the hydrochar, resulting in a higher energy recovery of almost 20 percentage points compared with that achieved (less than 79%) with conventional HTC (GPW + freshwater feed). An improvement in char fuel quality was observed, showing more suitable morphological, physical, and chemical characteristics, higher reactivity and combustion temperature, and lower probability of ash sintering. Successive process water reuse cycles allowed some increase in energy yield but, at the same time, degraded the quality of hydrochar as a biofuel. Process water composition showed an increase in chemical oxygen demand and total organic carbon, which almost doubled after three successive reuse cycles. The concentration of volatile fatty acids increased around 5-fold (up to 20 g L-1), with acetic acid accounting for 85% of the total. Subsequent anaerobic digestion of the process water removed up to 75% of the COD and yielded a biogas with high methane content (225-302 N mL CH4 g(-1) CODadded). Recycling of the process water significantly improved the total energy recovery (hydrochar + methane) to 90% after a single recycling, compared to 84% achieved with conventional HTC and subsequent anaerobic treatment of the resulting process water.