Quality of Hydrochar from Wine Sludge under Variable Conditions of Hydrothermal Carbonization: The Case of Lesvos Island

Abstract

Lesvos island has several food and beverage production industries and the valorization of their waste has been an unexplored task. The focus of this study is the valorization of wine sludge which is a very interesting waste stream due to the high phenolic content. This study identified all the operating wineries on the island and sampled local wine sludge. Hydrothermal carbonization (HTC) was utilized for the valorization of wine sludge and the production of hydrochar and liquid HTC liquor. The experiments had a residence time of 24 h and were performed at 200 °C. Except the uniqueness of wine sludge as a utilized material, this study performed HTC under different pressure regimes that were developed by different filling percentages of the reactor, i.e., 24–48%. The different pressure regimes influenced the measured parameters of both the liquid and the solid products of HTC. The Chemical Oxygen Demand (COD) ranged between 230 and 280 g/L with the maximum reduction was observed at a filling percentage of 32%. At the same time, lower filling percentages favored the total phenolic content (max value: 21 g/L) and higher filling percentages favored the Higher Heating Value (HHV) of the hydrochar (max value: 20.36 MJ/Kg) and the produced mass yield of hydrochar (max value: 234.3 mg). For all cases, low pH values were measured on the liquid fraction and this can be attributed to the presence of organic acids. Future work will focus on the characterization of the specific phenolic content of the liquid fraction.