Pilot-Scale Limitations

MEC technology is at the technological readiness level (TRL) 5, which indicates that the system is being evaluated in a particular context. Instead of focusing solely on illustrating the result, future research should seek to standardize it. According to the literature study of the various pilot projects, there are two major areas where MECs must improve in order to become a viable wastewater treatment technology. It would be critical to achieve the target volumetric treatment rates at the beginning of this process in order to determine the feasibility of this strategy. Existing research suggests that increasing the rate at which organic materials are loaded into the system can improve this volumetric technique. This can be performed by either increasing the intensity of the wastewater or increasing the flow rate of the wastewater. Any of these modifications would increase the rate at which organics are distributed into the reactor within the wastewater, which in turn would increase the rate at which organics are removed from the wastewater. By fully appreciating and optimizing this mass transfer, it is able to design the optimal reactor length for the given flow rate while also accurately predicting the costs. Furthermore, the issue of scale is a significant concern [161]. The highest MEC recorded so far was 1000 L, achieved with a hydraulic retention time of 1 day. When it comes to wastewater treatment activities, it is far from the scale that is required. Therefore, several BESs have been built with compact electrodes, numerous units of which can be mounted in any established tank. This is crucial because it allows for treatment in urban environments with a small land footprint to be achieved with these current tanks because of their depth. The design of an electrode that can stretch to a depth of 3 m would have to take into account the influence of hydrostatic pressure on the development of the biofilm, the output, and the structural integrity of the electrode itself. It would, therefore, be necessary to cope with the advancements in the kinetic and thermodynamic features of biological and electrochemical mechanisms that have occurred throughout time. More research is required to investigate the most recent reactor designs that keep the same land footprint as existing properties, which is currently lacking [162].

#### *8.3. Investigation of Methane and Hydrogen Generation and Its Implications for Industrial Application*

Purified hydrogen can be utilized as an important asset for fuel and assembling different synthetic substances. Because hydrogen has a greater energy content than methane, it has the potential to improve the efficiency of hydrogen generation in comparison to methane. MECs for the evolution of hydrogen consist of a double-chamber reactor with a film placed at the cathode to prevent hydrogenotrophic methanogenesis from occurring, thereby improving the framework's cost, and complications will only increase their expense and complexity [161]. In typical wastewaters, a rich source of microorganisms is present,

rendering the cathode susceptible to biofouling, which could require replacement of the cathode, thus increasing the costs [88]. Hydrogen likewise requires modern storage spaces and handling hardware. On account of the on-site production of energy, the worth of methane and hydrogen will decrease to the levelized cost of energy (LCOE) and levelized cost of heat (LCOH) produced and used. There are broad foundations and equipment to preserve and change methane into electrical and nuclear power. A pilot-scale framework was developed to provide methane while also generating a significant amount of net energy [80]. In comparison, the standardized net energy announced for a hydrogendelivery MEC was 76.2 kWh·m<sup>−</sup>3·day−<sup>1</sup> at 98% purity, which is higher than the previous estimate [127].

The gas storage foundation additionally assumes a critical part in the business reasonability of utilizing MECs for energy production. It has been recommended that MECs can be combined with inexhaustible resources [162]. Renewable energy provides the MEC with the ability to generate gas, which can be used as a kind of energy storage. Compared to hydrogen, methane has a greater energy thickness when it comes to barometrical pressing factors. In addition to its increased energy thickness and greater atomic size, methane's increased practicability makes it the most feasible candidate for non-compressed energy storage. Biogas generated by AD is of low quality and difficult to store, with storage energy accounting for only 10% of the total energy produced [163]. MEC-ADs generate biogas of good quality with CH4 content coming to 86% ± 6% [80], featuring the upside of MEC-ADs over AD frameworks with respect to energy storage capacity. MEC-ADs designed to generate methane can contend with AD innovation; however, this is generally neglected on a wide scale owing to expenses. In contrast to the generation of power from hydrogen, there are various innovations that can use biogas straightforwardly with responding motors, microturbines, energy units, gas turbines, steam turbines, and consolidated cycle frameworks. Moreover, biogas can be converted into biomethane, which can then be infused into the matrix [163]. Lastly, when compared to the development of methane storage and conversion innovation, the development of hydrogen storage and conversion innovation is less advanced.

#### Hydrogen Production Technology and Inventory

The production of hydrogen is primarily accomplished through two methods: steam methane reforming (SMR) of natural gas and electrolysis of water. Recently, there has been increased interest in hydrogen production from biomass resources. This is due to the large amount of biomass waste generated by various industrial and agricultural activities, which has the potential to be converted into useful energy in a very short period of time. A further interesting alternative for hydrogen production from renewable resources is high-temperature electrolysis (HTE) by solid oxide electrolysis cells (SOECs). These are the hydrogen production techniques that were investigated in this study, together with their inventories, which are summarized and provided in Table 4. In the literature, you can find a thorough overview of each of the different H2 technologies [164]. Although many key input parameters have been changed in this study, several of the underlying assumptions used in the analysis are still based on the default assumptions of the sub-models used for inventory data (the hydrogen production analysis models—H2A and the GREET model) and the Eco-invent. The emphasis of this study was on the unit-operational level, and, for the sake of simplicity, only the major unit (i.e., electrolyzer, industrial reformer, furnace) for each unit source was reconstructed utilizing generic data from the Eco-invent database for the manufacturing infrastructure. Because of the lack of data availability, several secondary systems were ruled out. It is widely acknowledged that such stages have minor consequences if they are spread out over the course of their operating lifespan [165]. Several different system configurations were simulated in order to test the sensitivity of the findings.


**Table 4.** Resources required to produce 1 kg of H2 from different production technologies and pathways [166].
