Biorefinery perspectives of microbial electrolysis cells (MECs) for hydrogen and valuable chemicals production through wastewater treatment

The degradation of waste organics through microbial electrolysis cell (MEC) generates hydrogen (H2) gas in an economically efficient way. MEC is known as the advanced concept of the microbial fuel cell (MFC) but requires a minor amount of supplementary electrical energy to produce H2 in the cathode microenvironment. Different bio/processes could be integrated to generate additional energy from the substrate used in MECs, which would make the whole process more sustainable. On the other hand, the energy required to drive the MEC mechanism could be harvested from renewable energy sources. These integrations could advance the efficiency and economic feasibility of the whole process. The present review critically discusses all the integrations investigated to date with MECs such as MFCs, anaerobic digestion, microbial desalination cells, membrane bioreactors, solar energy harvesting systems, etc. Energy generating non-biological and eco-friendly processes (such as dye-sensitized solar cells and thermoelectric microconverters) which could also be integrated with MECs, are also presented and reviewed. Achieving a comprehensive understanding about MEC integration could help with developing advanced biorefineries towards more sustainable energy management. Finally, the challenges related to the scaling up of these processes are also scrutinized with the aim to identify the practical hurdles faced in the MEC processes.