Acid-mediated hydrothermal treatment of sewage sludge for nutrient recovery

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 waterThe authors greatly appreciate funding from the Spanish MICINN (Project PID2019-108445RB-I00; PDC2021-120755-I00), Madrid Regional Government (Project S2018/EMT-4344), and US National Science Foundation (NSF #CBET-1856009). A. Sarrion wishes to thank the Spanish MICINN and ESF for a research grant (BES-2017-081515