Optimizing mixing strategy to improve the performance of an anaerobic digestion waste-to-energy system for energy recovery from food waste

Abstract

This study investigates the effect of different mixing strategies on anaerobic digestion of food waste in order to make the anaerobic digestion waste-to-energy process more energy efficient. Results showed that intermittent mixing is an alternative strategy to continuous mixing or unmixing for high efficient biogas production and energy saving. Through computational fluid dynamics modelling of fluid flow in anaerobic digesters, the mixing time was optimized to 2 mins/hr, at which point the reaction mixture is almost entirely homogeneous. The results of the model was validated with the experimental data. At an organic loading rate of 2.4 g VSFW/L/day, the semi-continuously mixed reactor maintains a higher specific methane yield of 437 mL CH4/g VSFW in comparison with the other controls. In addition, the semi-continuously mixed reactor has a larger proportion of Bacteroides and Methanocuelles. Based on the results of the bench scale experiment, the energy balance of the hybrid AD waste-to-energy system was simulated and evaluated. The calculated AD efficiency of the semi-continuously mixed reactor is 74.4%, which is higher than the AD efficiencies of the continuously mixed (66.9%) and unmixed reactors (14.9%). The energy balance investigated the electricity generated and the net heat output generated, in addition to self-sustaining and meeting the energy requirements of the various AD processes investigated. Based on the analysis, it was found the semi-continuous mixing is more energy efficient and sustainable to generate sufficient biogas output for the energy system to provide a net positive heat and electricity output.