**2. Agricultural Waste Usage in Microbial Fuel Cell Technology**

The technological approach of MFC in electricity generation fulfills numerous requirements. It allows the recovery of electricity from liquefied agro-waste and the removal of pollutants when wastewater is used. Therefore, MFC refers to the system of bioelectrochemical components that aids in converting organic matter to energy from a large source of complex carbon-based compounds. This has been achieved through the action of microorganisms on waste to produce electrical energy (Figure 5) [43].

**Figure 5.** A schematic representation of an experimental set-up of a dual-chambered MFC.

However, within the MFC, the bacteria facilitate oxidation processes that oxidize the organic substrates, leading to the production of electrons transferred by several different enzymes within some essential cells. At the terminal section, electrons are released in the cathode compartment leading to a reduction in oxygen. Today, MFCs have developed two emerging solutions, which are of significance, contributing to environmental concerns' mitigation, i.e., the production of an abundance of pollutant-free and hygienic water while generating the required power at some stage of wastewater treatment in the MFC. In essence, MFC is a promising technology that can achieve the simultaneous production of energy and the treatment of wastewater [6].

The COD removal efficiency was reported in MFC technology for wastewater treatment using such technology being associated with an emergence of a need for renewable energy sources. Although MFC technology still needs further improvement that can make them economically viable and attractive on the international market, it can be another system of organic matter removal from the effluent of different industries. The organic matter removal rate of the MFC compared to the other wastewater treatment systems was estimated, with the result indicating that the removal rate was up to 7 kg COD/m<sup>3</sup> ·day. In comparison, a range of 0.5–2 kg COD/m<sup>3</sup> ·day was determined for generic wastewater treatment systems, with studies reporting 8–20 kg COD/m<sup>3</sup> ·day removal being directly associated with AD [44,45].

The inadequate production of power and current cannot be the sole measure of MFC's practical application and implementation on an industrial scale for electricity generation. For example, in comparison with AD, the gain in electricity generation in using MFCs was seen to be very low with reduced capital investment and operational costs, respectively [46].

In previous studies, the use of MFC has played a vital role in wastewater treatment. It provided various alternatives as a secondary means of energy production and a promising way for technological upscaling in wastewater treatment, particularly whereby agro-waste is to be oxidized [47]. Therefore, the use of wastewater from agro-based industries, in particular, seems to be promising, as such wastewater is constituted by a high content of oxidizable organic matter with its biodegradability, i.e., BOD/COD ratio, being greater than 60% [48–50].

Overall, a typical MFC consists of two chambers, an anode and cathode for oxidationreduction reactions, respectively, with the chambers usually separated by an anion exchange membrane (IEM) (Figure 4). Electrons are usually produced after an anodic oxidation reaction which leads to the production of electric current. In contrast, protons, on the other hand, travel through an IEM as they are utilized for cathodic reduction reactions to generate water [51]. Other studies have shown that the oxidation-reduction reactions from both anodic and cathodic sides from organic matter-containing wastewater using electron acceptors can be attached bio-electrochemically in an MFC [52].

In a typical mediator-less MFC, the extracellular electrons are transferred via electroactive bacteria (EAB). These microbes are dissimilatory metal or sulphate-reducing bacteria. In the presence of an anode, they donate extracellular electrons to the anode to continue anaerobic respiration. These EABs capture electrons released by the oxidized organic matter and transport it directly to the anode. This form of direct electron transfer is further divided into three pathways: Cytochrome mediated, nanowire, and electron shuttle or soluble mediators. In electron transport, cytochrome C (CTC) plays a critical function. It is a heme-containing protein that is found in both archaebacteria and eubacteria. Electricity harvesting is aided by Cytochrome C. CymA, whose N-terminal is connected to the inner membrane. At the same time, the C-terminal is exposed to the periplasm, is a good example of CTC. Because it links the inner membrane to the periplasmic region, CymA is an essential electron route. It is important in anaerobic respiration and interacts with a variety of terminal reductases, including nitrate and fumarate reductases. Microbial nanowires are one of the most recent methods for transporting electrons. These nanowires are the bacterium's pilus, which are electrically conductive and were found by reducing iron oxide using *G. sulfurreducens* bacteria. Other bacteria also have an electrically conducting pilus, indicating the presence of bacterial appendages in the environment. The electron shuttles, also known as electron mediators, are gram-negative bacteria secretions that assist power generation in MFCs. Ideally, these mediators should be soluble, stable, reusable, and environmentally benign, with a redox potential between the bacterial membrane protein and anodic substance. Endogenously generated flavins by *Shewanella* species are a well-known electron shuttle in MFCs. Riboflavin (RF) and flavin mononucleotide (FMN) are the most common, as described in Savla et al. [53]. As previously stated, MFC uses two types of bacteria: mediator-dependent and mediator-independent. *Actinobacillus succinogenes*, *Proteus mirabilis* and *Pseudomonas fluorescens* are among the bacteria that need mediators, according

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to the National Institutes of Health. There is a growing interest in bacteria that do not need mediators, such *Shewanella putrefaciens* [54], *Rhodoferax ferrireducens*, and *D. desulfurcans* [55]. Various materials utilized in the MFC components have been illustrated in Table 1.

Liquefied agro-waste is considered one of the most promising substrates for microbial oxidation in the anodic chamber of the MFC. It contains a high amount of carbohydrates, organic matter, and other nutrients [56].


**Table 1.** Various materials utilized in MFC configuration and construction.
