Hydrothermal carbonization of the filter bed  remained after filtration of olive mill wastewater on olive stones for biofuel application

In the present study, the olive stones (OS) filter bed used for the filtration of olive mill wastewater (OMWW), named OMWW/OS-FR, was converted via hydrothermal carbonization (HTC) into solid biofuel. HTC was operated at three different temperatures (180, 240, and 300 °C) and the chemical and combustible characteristics of the produced hydrochars were investigated. The hydrochars prepared at 240 and 300 °C HTC temperatures exhibited high carbon recovery (80.1–84.2%) and fixed carbon content (45.8–46.8%), reduced volatile matter concentration (49.9–52.2%), low hydrogen to carbon (H/C) and oxygen to carbon (O/C) ratios (0.91–0.98 and 0.18–0.22, respectively) and high heating value (29.7–31 MJ/kg). The characteristics of the HTC-hydrochars were compared to those of biochar prepared by low temperature pyrolysis (LTP) at 400 °C. The results highlighted the improved fuel properties of HTC-hydrochars as compared to those of LTP-biochar in term of energy yield and energy content; this indicates the adaptability of HTC for OMWW/OS-FR upgrading. The temperature of 240 °C was selected as the optimal HTC temperature for the production of hydrochar with enhanced biofuel properties and with lower energy consumption for the thermal treatment of the water contained in the OMWW/OS-FR. Consequently, the use of HTC at 240 °C instead of dry LTP allowed for an energy saving over 49%. Accordingly, it can be concluded that HTC, in term of energy consumption, is more appropriate than LTP to manage the moisture in the OMWW/OS-FR.