Valorization of olive mill wastewater as a process medium in co-hydrothermal carbonization with Sicilian agro-wastes: effects of interaction on product yield and properties

This work explores the valorization of residual olive mill wastewater as a process aqueous medium for co-hydrothermal carbonization (co-HTC) of typical Sicilian agro-wastes (tangerine and orange peel wastes). Co-HTC experiments were carried out at 180 and 220 °C to assess the interaction between the feedstocks. Synergistic effects increased the yield of hydrochar and gas phases while antagonism altered the formation of aqueous products. Compared to the expected value, hydrochar yield was increased by 37 wt%, on average, while the interaction effect on gas phase was weaker (+23 wt%, on average) and increased with temperature. Both the retention of unhydrolyzed primary char and the recapture of secondary char phases from process water enhanced the hydrochar recovery in different ways according to feedstock nature and co-HTC conditions. On the basis of hydrochars characterization through elemental analysis and surface functionality, the degree of carbonization was significantly improved after co-HTC due to promoted dehydration and decarboxylation reactions. As result, co-hydrochars exhibited a moderately higher energy content and a greater thermal stability compared to samples obtained from HTC of individual substrates. Co-hydrochars also retained relatively high amounts of nutrients such as phosphorus, potassium and calcium, which could enable their use as soil improvers.