An Anaerobic Digestion Waste-to-Energy System for Food Waste Treatment and Fertilizer Production

Authors: 
Zhang, J., Shanghai Jiao Tong University
Loh, K. C., National University of Singapore
Tong, Y. W., National University of Singapore

Optimizing mixing time to reduce energy consumption would create a more energy efficient anaerobic digestion (AD) process with higher biogas yield. This study investigates the effect of different mixing strategies on anaerobic digestion of food waste to make the AD 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 modeling 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. 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. 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. The original digestate from this 1000L anaerobic digester, heat treated digestate and chemical fertilizer are used for cultivation of vegetables. The results showed that the interaction between heat treatment and digestate concentration is significant, so the trend-lines for the different types of digestate are different. The treatment with the highest yield was also compared with the control treatment (chemical fertilizer, 15N:15P:15K, 1.2 g per application) using t-test. The yield of the best performing digestate is not significantly different from the control. It means that the performance of digestate is almost same to the use of chemical fertilizer.