Upgrading Biodegradability of Orange Waste Using Various Pre-Treatment Methods to Release Its Energy Potential

Orange waste (OW) is an example of biomass that has a potential for nutrients and energy recovery. However, its use in biological processes, e.g., anaerobic digestion (AD), still poses a significant technological challenge due to the presence of hardly biodegradable lignin and compounds toxic to the microorganisms. To release its energetic potential, the use of an adequate pre-treatment method might be a solution. This research evaluated the efficacy of various pre-treatment strategies, i.e., acoustics (AC) and hydrodynamic cavitation (HC), microwave radiation (MR), and the impact of solidified carbon dioxide (SCD) in improving the properties of OW, including organic compound content, fiber components, morphological structure, and generation of toxic intermediates. The obtained results showed that HC might be considered as the most efficient pre-treatment strategy prior to its further AD decomposition. For this strategy, an over 2-fold increase in biodegradability index, expressed as the DOC/TOC (dissolved to total organic carbon) ratio, was found. In turn, the other analyzed parameters, i.e., removal of organic compounds and delignification degree, were established at the favorable level of 63 and 56%, respectively. This effect was achieved with the lowest energy demand of 0.35 MJ/kgVS. However, HC favored the generation of AD inhibitors. However, to fully assess the impact of selected methods on AD performance, further experiments should be conducted.