Valorisation of waste from the olive oil sector via hydrothermal treatments

Abstract

Olive cake is arguably one of the most significant wastes generated during olive oil production. Currently, the predominant method of utilisation is through combustion for energy recovery, which is a low-value application of this material. This work proposes a valorisation route for olive cake based on sequential hydrothermal processes, with the advantage of using only water as a reagent. Two washing processes at 30 °C and 70 °C are followed by an autohydrolysis process at either 150 °C or 170 °C. Quantitative acid hydrolysis and chromatography techniques were used to characterise the solid and liquid phases. The solid phases were pyrolyzed, after which the char obtained was characterised in order to assess its potential as a renewable solid biofuel and CO2 adsorbent. It was demonstrated that 1-h washings were sufficient to remove soluble compounds and obtain a filtrate rich in glucose and polyphenols. Autohydrolysis at 150 °C was shown to give superior results based on increased oil yield from the pyrolysis and reduced energy requirements. Pyrolysis following hydrothermal treatments also allowed obtaining chars with higher calorific values and augmented potential for utilisation as a solid biofuel. Finally, despite their low surface area and negligible microporosity, the chars obtained from the pyrolysis of washed and autohydrolysed olive cake exhibited enhanced capacity and saturation time for adsorbing CO2. In conclusion, the valorisation pathway proposed in this work allows easy extraction of valuable compounds from olive cake, as well as the production of bioenergy, renewable fuels, and a potential CO2 adsorbent.