**6. Comparison of Related Works**

In comparison to related studies, the quantity of various important factors, including CE, COD removal, and maximum energy generated, was recorded using different agricultural residues, agro-industrial wastewater, and other by-products generated from the agricultural industry. These wastes can be used as substrates in MFC technology development. Among the agricultural waste that contributes to the highest power generation in MFC includes wheat straw effluent, rice straw hydrolysate (without pretreatment), and corn stover along with the application of glucose as a substrate; although, the percentage rate of COD removal was seen to be very effective in MFCs when rice straw was used. Similarly, the use of vegetable waste extract produces a high-power output in U-shaped MFCs than dual-chambered MFCs. Overall, some convincing results were obtained regarding power generation using slaughterhouse and animal carcass debris containing

wastewater. In particular, cattle manure and manure wash wastewater were considered good substrates for bioelectricity production, with a high percentage of COD removal achieved. Other substrates such as disposed of poultry waste and swine wastewater can be used in MFCs, indicating a 65% COD removal, with plant, fruit, and cassava processing effluent achieving COD removal of up to 88%; although with low power production. A substantial amount of power density was observed with swine wastewater using an MFC made up of an air cathode in a single-chambered cell than in an algal bio-cathode in a photosynthetic MFC. Considering chicken manure treatment, 82% of BOD can be successfully removed with high energy production using a horizontal flow continuous MFC. Similarly, the treatment of mustard tuber wastewater in the dual-MFC has shown a good result with high energy recovery and a high percentage of COD removal. Other agro-industrial wastewaters containing agricultural activity by-products showed a poor performance in terms of power generation; this includes POME, brewery, and dairy wastewater, though it has proven that a high percentage of COD can be removed from these wastewaters with a moderate quantity of CE. For winery wastewater, only white wine wastewater with the less organic matter has shown a good result during power production with a high reduction of BOD and CE. Thus, the application of agricultural waste and its effluents to generate bioelectricity was demonstrated, with some adequate energy recovery. This can thus be considered as an alternative source of renewable energy technology, supported by different microbial communities largely found in the anodic solution of most MFCs; as these types of microbial communities confer characteristics of electrogens for efficient electron transfer, most especially to support redox reactions; therefore, this ultimately characterize the ability of these organisms to support AD.

It has further been shown that sugar beet processing wastewater at a concentration of 256 g COD/L could generate a power of 1.41 W/m<sup>2</sup> . Even for coconut husk retting wastewater containing phenol, 91% of COD was successfully removed when employed a dual-chamber MFC. Similarly, the use of crude starch extract from potato processing wastewater has shown promising results when used in a dual-chambered MFC, with a substantial amount of energy recovered. In another comparative analysis, it has been observed that OMW is not usually considered a promising substrate for MFCs compared to sugar beet processing wastewater and coconut husk retting wastewater in terms of energy production. Thus, it is not usually considered an alternative substrate source in MFC technology unless combined with another substrate source. Conclusively, when various comparative analyses of related MFC-substrate studies were conducted, it is clear that substrates such as wheat straw effluent, rice straw hydrolysate, and corn stover are good substrate sources that can be utilized in MFCs for energy generation. Others include slaughterhouse and animal carcass debris containing wastewater, which provides a large quantity of energy. In contrast, substrates such as POME, brewery, plant and yard waste, and dairy wastewaters showed a poor performance in energy recovery using MFCs. To this end, other studies have shown better applicability of some substrates such as swine wastewater, livestock compost wastewater, fruit waste, vegetable waste, and cassava mill effluent to facilitate the generation of electricity using MFCs. However, all these studies have elucidated the fundamental MFC design approach in increasing power generation quantity.

However, among the physical factors affecting the performance of MFC include the type of electrode materials used (graphite rod, graphite fiber brush, carbon cloth, carbon mesh, carbon paper), the surface area of the electrode, and electrode-spacing, and characteristics of the catholyte. On the other hand, biological factors are considered as another key component that governs the overall MFC performance, which includes biocatalyst (mixed culture, monoculture) proliferation and activity, including their biofilm-forming ability and the complex organic matter degradation efficiency, whereas the operational factors affecting the working principle of MFC in terms of power generation include pH conditions, the nature and the type of anolyte and load configuration.
