A comparative life cycle assessment on mono- and co-digestion of food waste and sewage sludge

It is common practice for Singapore wastewater treatment plant (WWTP) to treat primary sludge and secondary active sludge by anaerobic digestion (AD) to recover energy through biogas. However, sewage sludge (SS) is low in biodegradable organic matter and biomethane potential, which result in low AD efficiency both from a processing and an economic standpoint. Co-digestion of SS together with food waste (FW) not only provides a practical alternative for FW management, but also improves the performance and energy efficiency of often under-performed anaerobic digester in WWTP, as the potential synergistic effect between FW and SS could increase the biogas production and system stability. Prior to applying co-digestion in reality, life cycle assessment (LCA) is required to understand the environmental advantages and drawbacks of the co-digestion strategy so that it can be implemented more sustainability. It was found that everyday 4400 tonne of SS and 1000 t of FW were generated in Singapore. For co-digestion, the possible outcomes may be categorized as neutral, synergistic, and antagonistic, if methane production from the mixture is equivalent, higher or lower than the sum of mono-digestion. The LCA results show that when the antagonistic situation happens, the co-digestion system becomes much less favorable, although it requires less water consumption and land foot print. Therefore, the key efforts of plant manager are suggested to be spent on the maximization of biogas output. (C) 2019 The Authors. Published by Elsevier Ltd.