Synergistic Effect of Ultrasonic Sonification in Anaerobic Digestion of Expired Food Products

The regulations in force in the European Union require the thermal pre-treatment of substrates containing animal by-products prior to the anaerobic digestion (AD) process. Expired food products (EFP) belong to this category of substrates. Traditionally, the requirement for thermal pre-treatment has been met with waste heat from combined heat and power plants (CHP). However, due to the increasing trend towards biomethane production and the move away from CHP, alternative heating methods need to be explored. The main objective of this study was to evaluate the impact of the dual synergistic effect achieved by ultrasonic-thermal pre-treatment of EFP on the kinetics, efficiency and energy balance of AD. The combined thermal treatment and ultrasonic disintegration at 90 °C resulted in an increase in dissolved phase chemical oxygen demand (COD) by (84.4 ±3.9) % and total organic carbon (TOC) by (56.8 ±6.2) %, leading to final concentrations of (33010 ±1280) mg O₂/L and (10710 ±670) mg C_org/L, respectively. After thermal pretreatment of EFP at 70 °C, the methane yield reached (515 ±24) mL/g VS, while at 90 °C it increased to (525 ±22) mL/g VS. The application of ultrasonic thermal pretreatment resulted in a positive net energy gain, with the most efficient variant yielding (4.20 ±0.23) Wh, which was (0.37 ±0.17) Wh higher than the control sample. In contrast, a negative energy balance was observed in the variants where conventional heating methods were used.