Unlocking the energy potential of orange wastes using various pre-treatments and co-digestion strategy: Technological and energy analysis and polycyclic aromatic hydrocarbons distribution.

This study investigated the impact of using orange waste (OW) pre-treated by various methods on anaerobic co-digestion (AcoD) with sewage sludge (MSS). The analyses concerned the methane production and its kinetics, organic matter degradation, including polycyclic aromatic hydrocarbons (PAHs), as well as energy balance assessment. Moreover, the study employed the Fourier transform infrared spectroscopy - attenuated total reflectance and thermogravimetric and derivative thermogravimetry analyses to describe the transformation occurring within the anaerobic digestion process (AD). The following methods were chosen: both acoustics (AC) and hydrodynamic cavitation (HC), microwave irradiation (MR), and impact of solidified carbon dioxide (SCD). The AD experiments were conducted under mesophilic conditions in a batch mode. Two reference trials were provided, i.e., the mono-digestion of MSS (A0) and the co-digestion of MSS and untreated OW (A1). In other trials, the co-digestion of MSS and pre-treated OW was performed (A2-A5). The most beneficial results were achieved in the case of the OW pre-treated using HC. Therein, improvements of 7.5 and 15.3% in methane production were found compared to A0 and A1, respectively. Importantly, the results of energy balance showed that for HC, the energy profit was 54 and 43%, compared to A0 and A1, respectively. This finding was also supported by improved organic removal efficiency and effective destruction of 3-, 5-, and 6-ring PAHs, which indicated a high performance of the AcoD process.