**6. Scale-Up Reactor Designing**

Researchers have been trying to successfully construct large-scale reactors, so that wastewater treatment and value-added byproduct production become easier. These products can be hydrogen gas, methane, and so on. The theoretical conclusion for perfect up-scaling of the process through works of literature and data is difficult because of the differences in substrate type and other parameters such as operation mode [79]. Most of the reactors, as concluded from reviews, are operational in a continuously fed mode. The first scale-up of the MEC system was performed by Cusick et al. [80]. Based on the previous demonstration at laboratory scale, a single-chambered 1000 L MEC reactor was designed for H2 production from winery wastewater with high volatile fatty acid (VFA) content [81]. Along with hydrogen gas production, the production of methane was also observed at a gradually increasing level despite the MEC being functional for a short time. These observations, along with cases of cathode contamination, led to the development of a two-chamber system with a total capacity of 100 L by Heidrich [82,83]. In the tests conducted with wastewater produced domestically, methane production was inhibited successfully with the introduction of a polymeric membrane. This membrane separated the cathode from the rest of the reactor and helped in avoiding microbial crossover to the catholyte [79].

In batch mode, COD removal was high with higher H2 production. However, when the operational mode was switched to continuous mode, the COD level decreased along with the Coulombic efficiency. The probable reason behind the performance degradation may have been inefficient substrate flows into the reaction chamber and slow transportation of produced gas to the collection chamber. Hence, the usefulness of the transportation phenomenon in a functional scale-up reactor was highlighted [79]. Cotterill explained the cassette design module with Baeza using three different cassettes [84]. They observed material decay and degradation due to the applied voltage in a MEC. The development of a hermetic system is a challenge for reactors as the up-scaled reactors have shown leaking problems that might create balance errors. The existing systems are the perfect basis for up-scaled reactors; however, more work has to be done on the reactor design to make it fit for commercial and industrial applications [85–87]. More low-cost membranes and electrode materials need to be recognized. Attaining an optimum applied voltage and operational mode can also change the future of MECs [79]. Propylene has been the material chosen for building the body of a large-scale reactor [80,83], whereas polycarbonate is used for smaller systems [88]. Due to economic reasons, stainless-steel electrodes are used. Good electrolytic activity and proper H2 gas evolution add to its characteristics. However, nickel for the cathode and carbon materials for the anode [87] are also used.

**Figure 8.** Cost–performance ratio curves for eight scenarios of a financially competitive MEC. Scenario 0: baseline MEC model; Scenario 1—double hydrogen yield; Scenario 2—applied voltage reduced to 0.6 V; Scenario 3—energy price changes; Scenario 4—anode and current collector value returned after 20 years; Scenario 5—membrane replaced annually; Scenario 6—membrane and cathode replaced annually; Scenario 7—additional staff member required; Limit 1—capital cost of reactor minus the anode; Limit 2—capital cost of reactor minus the anode and current collector (adapted from Aiken et al. [88]).

#### **7. Optimizing Features Affecting the MEC System Design for Hydrogen Production and Wastewater Treatment**

Rapid commercialization of such a technique can result in a significant expansion in the industry, while also helping to enhance funds and boost research for continual advancements. The factors to be considered in the analysis and comparison of MECs for wastewater treatment and energy generation include several variables. This assessment highlights the main feedstock, anode and cathode materials, reactor volume, system architecture, outputs, and expenses as the most important aspects when it comes to the system design. These factors have an impact on system performance, as well as the commercialization of the technology's economic feasibility. To be economically viable, MECs must strike a stability between overall system performance optimization and the economic proficiency of entirely basic components. MEC construction in the future should include ways to cut down on anode costs, enhance organic loading rates, and develop better knowledge about component requirements and electrode life expectancy [88]. For researchers to determine the next steps in the development of this technology, they must evaluate the parameters regarding industrial use and take into consideration the economic and manufacturing benefits, while simultaneously researching how to hasten the introduction of this method into the industry.
