Abstract
Hydrothermal carbonization allows material valorization and energy recovery from wet biomass waste. In this study, the hydrothermal treatment of dewatered waste-activated sludge (DWAS) was evaluated at several temperatures (170–230 °C) and reaction times (5–60 min) in an acid-free medium or in media such as citric acid or HCl (0.1–0.5 mol/L). Compared with the DWAS, an increase in the fixed carbon content (>45 wt%) and heating value (18.9–22.9 MJ/kg) was observed in the hydrochar; however, their ash content remained high, which is the main drawback hindering their direct use as a biofuel. The addition of acids during hydrothermal treatment favored the solubilization of N and P in the process water, which required strict control of the reaction time to avoid the recrystallization of P in the hydrochar. Under optimum operating conditions (230 °C, 15 min, 0.5 mol/L HCl), 94 % of P (as of PO4) and almost 100 % of N (14 % as NH4-N) present in the feedstock were concentrated in the process water.
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