Bio-methane production from tomato pomace: preliminary evaluation of process intensification through ultrasound pre-treatment

The effect of ultrasound pre-treatment (for 5, 15, and 30 min) on tomato pomace methane yield was evaluated. With respect to the control (238.0 m³CH₄/tVS), no improvement was recorded for the substrates pre-treated at 80 µm (0.6 W/mL). 18.9% and 5% higher methane yields were, instead, recorded from the substrate pre-treated for 15 min with an ultrasound wave’s amplitude of 152 µm (0.9 W/mL) after 4 and 22 days of AD, respectively. However, the main achievement arising from the ultrasound application at 152 µm amplitude was the degradation kinetics speed up (89.7% faster maximum methane production rate after 15 min US). Nonetheless, the methane yield increase was not high enough as to compensate the electricity requirement of ultrasonication (between 3.3 and 19.5 MJ/kgVS) as verified through the energy assessment. The dataset that has been presented and discussed lead to preliminary considerations that may be useful in view of a possible scale-up of the process. When equipoising higher quantities of treated biomass and recovered methane, the process might turn economically convenient; however a new energy feasibility assessment and capex–opex evaluation would be required.