Experimental exergy-based sustainability assessment of a novel fully submerged membrane-less microbial fuel cell for decentralized wastewater-to-energy conversion

Microbial fuel cells (MFCs) have emerged as promising wastewater-to-energy technologies capable of simultaneously treating wastewater and generating renewable electricity. However, the practical implementation of conventional MFCs is hindered by membrane-related limitations and the lack of comprehensive thermodynamic evaluations under varying operating conditions. In this study, a novel fully submerged, membrane-less, and exchangeable microbial fuel cell was designed, fabricated, and experimentally evaluated using real municipal wastewater over five distinct operational phases, with dynamic exergy-based modelling applied to Phases 2–4. A comprehensive thermodynamic framework was established by integrating exergy analysis with environmental, economic, emergy, and preliminary social sustainability considerations to quantify the overall performance of the proposed system. The fabricated 21 L prototype (12.17 kg) required an initial investment of 450 USD and exhibited a baseline life-cycle environmental impact of 1.04 Pts. Among the dynamically modelled phases (Phases 2–4), Phase IV was identified as the optimal operating condition for exergy-based sustainability, achieving the highest exergy performance and COD removal within this comparable set. Nitrobacter oxidizes nitrite to nitrate, while ammonium oxidation is mediated by indigenous ammonium-oxidizing bacteria, was excluded from the dynamic model due to its distinct microbial boundary conditions and shorter operating duration; however, it exhibited the highest maximum power density, although this may reflect both bio-augmentation and biofilm maturation. Under this condition, the environmental burden associated with electricity generation was limited to 0.16 mPts per cycle, with an emergy requirement of 1.10 × 10 11 Sej per cycle, a carbon footprint of 1.02 g CO 2 -eq per cycle, and a water footprint of 16.8 g H 2 O per cycle. Comparative analysis of the five operational phases demonstrated that improvements in thermodynamic performance were consistently accompanied by reductions in environmental burdens and enhanced resource utilization efficiency. The integrated assessment confirms that the proposed membrane-less MFC provides an efficient and promising wastewater-to-energy platform for decentralized applications at the laboratory scale. However, further long-term continuous operation studies and modular scale-up investigations are required to confirm its full scalability and practical viability. The proposed methodology offers a comprehensive framework for the thermodynamic evaluation and optimization of bioelectrochemical energy conversion systems for decentralized wastewater treatment applications.