| Summary: | Microbial Electrolysis Cells (MECs) have the potential to transform wastewater treatment, but many studies have been carried out at a very small scale with implausible temperatures and synthetic substrates. The value of laboratory-scale controlled experiments is not questioned, but these studies do not inform us of the realities and challenges that occur when operating MEC in the real world at realistic scales. Addressing this issue led to the installation and operation of a pilot scale MEC which failed within 6 months. It was consequently dissected and analysed, to systematically understand failure, through fault tree analysis (FTA). This process identified areas for further development to move towards a more robust MEC prototype. Meta-analyses and experiments were used to asses some of the challenges still to be overcome, before the commercialisation of MEC is a realistic prospect. With this knowledge, a re-design led to the successful operation of a second pilot, which moved from the L to the m3 scale, thanks to a 16-fold increase in electrode surface area (1 m2 each) and a 5-hour hydraulic retention time (HRT). After nine months, 0.8 L of H2/d (0.003 L-H2/L-MEC/d) was produced from primary treated domestic wastewater where the wastewater temperature was as low as 5.3 ̊C. The European Urban Wastewater Treatment Directive consent of 125 mg/L was achieved 55% of the time, with 64% of the chemical oxygen demand (COD) removed. To break-even energetically each module would need to produce 4 L-H2/day. This is possible, if hydrogen loss through scavenging can be addressed and improvements to the current density can be achieved. Recommendations for both are proposed. A cost benefit analysis (CBA) and multi criteria assessment (MCA) is used to compare four potential MEC products. The model is based on current and realistic projections of MEC performance, to assess the net present value (NPV) of the technology and the potential savings that could be gained in wastewater treatment.
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