Hydrothermal carbonization of olive mill wastewater: Liquid phase product analysis

Hydrothermal carbonization (HTC) is an emerging technique for wastewater treatment. It uses water at moderate temperatures (180-250 °C) and autogenous pressures (35-55 bar) to break down organic waste. In this work, HTC was used to treat olive mill wastewater. The reaction time and the water-sludge ratio were varied, over a large set of values, to study their effect on the liquid phase products. Liquid extracts were sampled under various operating conditions, and different analytical techniques were used to analyze the acids/phenols quality and concentrations, along with the non-polar hydrocarbons in the liquid phase. Different extraction methods were developed and tested in this work to meet with GC-MS analyses requirements. Many simple biophenols such as Tyrosol, hydroxyl-Tyrosol, phenol, homo-vanillyl alcohol, in addition to various ketones, flavones, flavonols, alcohol, aldehydes, cyclic/aromatics, alkanes, alkenes and even alkynes were detected in the liquid phase product. In contrast, negligible amounts of polyaromatic hydrocarbons (PAH) were found. The hydrocarbons concentration decreased as the reaction time and water-sludge ratio were increased. A detailed ultraviolet quantification method was developed and showed that the total acids/phenols content decreased by 10 folds when the water-sludge ratio increased by a factor of 9 under a reaction time of 8 h. However, the optimum HTC conditions were found to be at a water-sludge ratio of 3.5 and a reaction time of 4 h.