*7.6. Applied Potentials*

As explained in the above sections, a minimum of 0.2 V is required to break the thermodynamic barrier for feasibly producing hydrogen at the MEC cathode. Large cathodic overpotentials reduce the efficiency of the overall process. Even though hydrogen evolution increases with increasing applied potential [140], optimum potential ranging from 0.2–0.8 V must be applied for achieving process scalability [141]. Researchers have reported that varying applied potential can decrease cell metabolism and increase cell lysis [142].

#### **8. Bottlenecks in Commercialization of MECs for Biogas Production during Wastewater Treatment**

It is essential to understand the functioning of MECs on a large scale for wastewater treatment. This also poses a barrier in adopting MECs owing to the little understanding of these systems. More modest frameworks produce high energy yield per volume when standardized. For metropolitan wastewater, the benchtop investigation produced standardized net energy of 25.96 kWh·m<sup>−</sup>3·day−<sup>1</sup> [88]. On the other hand, a pilot-scale framework produced standardized net energy of 0.11 kWh·m<sup>−</sup>3·day−1. This demonstrates that energy creation cannot be accurately scaled, and that proficiency decreases as the size of the system increases. The net energy of a 1000 L pilot-scale framework with a cathodic surface area of 18.1 <sup>m</sup>2·m<sup>−</sup><sup>3</sup> was determined to be 2.11 kWh·m<sup>−</sup>3·day−<sup>1</sup> for vineyard wastewater [80]. One obstruction is the trouble of precisely contrasting the various arrangements in the examination. The reactor size is just a single boundary and does not give a genuine portrayal of the framework's versatility.

**Table 3.** Different cathode materials along with catalysts used in MECs.


#### *8.1. Economic and Cost Analysis*

The confirmation that innovation is financially viable for a large scope and has productivity that is virtually identical to or better than other arrangements accessible is critical to the passage of innovation from the lab to the field. Performing techno-economic and life-cycle assessments is important for progressing innovation used in the financial analysis of energy advancements to identify and evaluate operating and capital expenditures (OPEX and CAPEX) over their entire life cycle. The yield considers financial aid evaluators and levelized cost of energy objectives, such as investment return and net present value. To analyze the natural influence, the LCCA appraisal must be conducted similarly to the life-cycle assessment, with the same assumptions. MECs will benefit from using these projections since they will be better able to comprehend future costs, execution procedures, and risk factors [156]. When we say CAPEX, we are referring to the expenditure of performance; however, when we say OPEX, we are referring to the operational expense throughout the performance. Even though simple arrangements have a low CAPEX, they may be wasteful and require additional support, resulting in a higher OPEX than more complex arrangements. As a result, while evaluating the complete LCCA of innovations, it is necessary to consider both the CAPEX and the OPEX costs. Furthermore, when considering the profit from the venture from the energy produced, a high CAPEX might be acceptable in innovation such as MEC-AD for wastewater treatment, where energy is given [157].

In the ebb and flow UK market, 1 kWh of electrical expense is equivalent to £0.144, and, as a result, the price of treating 1 m<sup>3</sup> of metropolitan wastewater is equivalent to £0.072 per kWh of electricity. As a result, for every 1 kg of COD neutralized, activated sludge treatment produces 0.4 kg of sludge, necessitating extra processing, which is often done using AD in order to recover a fraction of the energy [157]. MECs, on the other hand, can both reduce COD and generate energy. MECs currently require a significant amount of CAPEX to be implemented and are several times more costly (248-fold) than activated sludge frameworks [88].

According to the results of an analysis, the overall cost of the MEC framework was around £2344·m<sup>−</sup>3. In aspects of toxicity, MEC was evaluated by comparing to activated sludge. Although the MEC CAPEX was twice that of the active sludge, the consumption of energy was 10-fold lower [88]. Because of the potential for MECs to generate bioenergy, they can be either energy-neutral or energy-positive, whereas activated sludge is the most energy-concentrated of the currently available wastewater treatment techniques, according to the EPA [82,83]. Generally speaking, the expenses of MECs need to decrease by 84% to £375·m<sup>−</sup><sup>3</sup> [158].

The positive cash flows generated at 2020 pricing (£15,000·year<sup>−</sup>1) (Figure 8) had a small influence on the net present value (NPV) under scenario 1 (a doubling of current yields to 30 <sup>L</sup>·m<sup>−</sup>3) due to the cost of materials (assuming current performances). MEC capital expenses are projected to exceed their revenue from hydrogen production, as a result. For the baseline scenario (0) to be economically viable, either hydrogen prices must be raised to £5.09·kg−<sup>1</sup> (higher than the EU's target of £3.55 by 2020 and £2.66 by 2030) or yields must be increased from 15 L·m<sup>−</sup><sup>3</sup> to 21.5 L·m<sup>−</sup><sup>3</sup> by 2020 and 28.7 L·m<sup>−</sup><sup>3</sup> by 2030 (increases of 43% and 91%, respectively). Due to the relatively high cathodic efficiencies, it will be necessary to raise Coulombic efficiencies and the organic loading rate (OLR) in order to improve yields. Although hydrogen must be sold at a loss in order to be competitive with other sources of energy, power costs in MECs are negligible when compared to those of activated sludge (AS) [88].
