Life cycle assessment of food waste to energy and resources: Centralized and decentralized anaerobic digestion with different downstream biogas utilization

Anaerobic digestion (AD) has been identified as an efficient food waste disposal technology by many researchers. However, a holistic environmental investigation of different AD configurations integrated with different downstream biogas utilization has never been reported. This study, taking Singapore as an example, compared the sustainability of business-as-usual technology, i.e., incineration, to centralized and decentralized AD with different biogas applications: electricity generation, cooking fuel, and transportation fuel. The results showed that AD scenarios were generally preferable compared to incineration since environmental benefits were observed in seven to fourteen categories for AD scenarios but only four categories for incineration. The decentralized AD with biogas as cooking fuel (Sc_DADcook) achieved the highest environmental benefit in global warming potential (GWP, -238.4 kg CO2 eq./FU) and fossil fuel depletion potential (FFP, -93.5 kg oil eq./FU) among all the analysed scenarios, followed by centralized AD with biogas as transportation fuel (Sc_CADtran). The sensitivity analysis of the different electricity substitution showed that Sc_DADcook and Sc_CADtran lost their advantages when coal-dominated electricity was used, while biogas for electricity generation became favourable with higher environmental savings up to 280% and 150% in GWP and FFP, respectively. Besides, maximizing the methane yield and minimizing the FW collection radius were also identified as important measures to improve the overall environmental performance. Finally, the scenario analysis of different biogas upgrading technologies demonstrated the GWP advantage by membrane separation and benefits in other categories by high pressure water scrubbing. However, biological upgrading needs further optimization particularly of the H2 source associated emissions.