Postliminary treatment of food-waste digestate via combined hydrothermal carbonization and microbial fuel cell for bio-energy recovery: a comparative life cycle impact assessment

Abstract

Anaerobic digestion (AD) is the predominant technique for transforming food-waste into biomethane, yet the dewatering and valorisation of the resultant digestate present a significant downstream technical challenge. This investigation provides an advanced digestate management approach for resource recovery through the synergistic integration of hydrothermal carbonization (HTC) with anaerobic digestion. The integrated system resulted in biomethane (∼466 mL g⁻¹ VS) and biocoal (hydrochar) with a high calorific value (∼22 MJ kg⁻¹). The effect of HTC operating conditions including reaction temperature and time on the coalification degree has been investigated. Additionally, the by-product of HTC, i.e., HTC process water was treated using the microbial fuel cell (MFC) with organic abatement efficiency of 76.0 ± 4.6% and power recovery of ∼4.41 W m⁻³. Further, the metagenomic analysis was conducted to affirm the high proliferation of specific electrogens (Clostridia) in the MFC. Distinctively, in this work, the impacts on the environment in eighteen different categories using life cycle assessment for the technologies AD, AD + HTC, and AD + HTC + MFC were also compared. The single score results demonstrated the least impact of the integrated AD + HTC + MFC on human health, ecosystem, and resource depletion. This highlights the potential of the integrated system for real-field applicability, sustainable digestate management, and bioenergy recovery.