ABSTRACT
This work evaluates the potential of hydrothermal carbonization (HTC) to valorize the digestate derived from the anaerobic digestion of sewage sludge into useful materials for P and energy recovery. The hydrothermal treatment of digestate at 180-240 °C did not lead to high-rank hydrochars. On the other hand, inorganic P concentration did not change with the temperature, while as the carbonization temperature increased, the organic P retention yield in hydrochar became lower, increasing the total P in the process water obtained at the highest temperature, up to 25.3%. P recovery from acid leaching of the hydrochar obtained at 180 °C, via precipitation with CaO at pH up to 9, led to a brown solid precipitate with total P content close to 42 mg g-1, in the range of low grade phosphorus ores. Moreover, acid leaching reduced by 50% the ash content, yielding lignite-like upgraded hydrochars with higher heating values in the range of 20.5-23.1 MJ kg-1, fairly interesting as solid fuels. Anaerobic digestion of the process water enabled additional energy recovery in form of biogas (325 and 279 mL CH4 g-1 VS -at standard temperature and pressure; STP- from the process water resulting at 180 and 210 °C, respectively).
Subject(s)
Carbon , Sewage , Biofuels , Phosphorus , TemperatureABSTRACT
The potential of hydrothermal carbonization (HTC) as a novel choice for treating microalgal biomass (MAB) was assessed. The hydrochar obtained at 210⯰C had a carbon content and a higher heating value (HHV) 1.09 and 1.1 times greater, respectively, than that of the feedstock. Also, washing the hydrochar with HCl efficiently removed ash and increased its carbon content 1.40-fold. Energy recovery in the liquid fraction from the hydrothermal treatment (LF) by anaerobic digestion (AD) allowed methane yields of 188-356â¯mL STP CH4 g-1 VSadded, to be obtained. As a result, the amount of energy recovered from MAB was increased from about 4â¯MJâ¯kg-1 (20% in terms of HHV) to 15.4, 12.1 and 10.4â¯MJâ¯kg-1 by combining HTC at 180, 210 and 240⯰C, respectively, with AD. Therefore, HTC at 180⯰C in combination with AD seemingly provides an effective method for valorizing MAB.