Energy Recovery from Organic Wastes Using Microbial Fuel Cells: Traditional and Nonconventional Organic Substrates

Abstract

Microbial fuel cells (MFCs) are environmentally friendly energy converters that use electrochemically active bacteria (EAB) as catalysts to break down organic matter while producing bioelectricity. Traditionally, MFC research has relied on simple organic substrates, such as acetate, glucose, sucrose, butyrate, and glutamate, the production of which involves energy-intensive, CO₂-dependent processes and chemically aggressive methods. In contrast, nonconventional waste streams offer a more sustainable alternative as feedstocks, aligning with zero-waste and regenerative agricultural principles. This review highlights the potential of nonconventional organic wastes, such as fruit and vegetable wastes, raw human and livestock urine, and farm manure, as globally available and low-cost substrates for MFCs, particularly in household and farming applications at small-scale waste levels. Furthermore, complex waste sources, including hydrocarbon-contaminated effluents and lignin-rich industrial wood waste, which present unique challenges and opportunities for their integration into MFC systems, were examined in depth. The findings of this review reveal that MFCs utilizing nonconventional substrates can achieve power outputs comparable to traditional substrates (e.g., 8314 mW m⁻²–25,195 mW m⁻² for crude sugarcane effluent and raw distillery effluent, respectively) and even superior to them, reaching up to 88,990 mW m⁻² in MFCs utilizing vegetable waste. Additionally, MFCs utilizing hydrocarbon-containing petroleum sediment achieved one of the highest reported maximum power densities of 50,570 mW m⁻². By integrating diverse organic waste streams, MFCs can contribute to carbon-neutral energy generation and sustainable waste management practices.

1. Introduction

The increasing demand for electricity and the reliance on conventional fossil fuel-based power production are at odds with current goals to reduce the high carbon footprint of energy sources. Conventional energy production accounts for approximately 26% of greenhouse gas emissions worldwide, and global energy consumption is projected to increase by 15% by 2050 [1]. Exploring renewable energy sources is a potential avenue for addressing these issues. Microbial fuel cells (MFCs) present potential breakthroughs in renewable technology because of their ability to generate energy directly from biodegradable waste streams, operate under low-energy conditions, and contribute to waste management [2].

The MFC industry has made significant progress since 1911, when Potter discovered the fundamental possibility of generating bioelectricity by using bacteria [3], and one of the first MFCs, operating with glucose solution, was developed [4]. Despite the promising prospects of MFCs, the implementation of this technology faces several considerable challenges, including high system inner resistance and membrane resistance, which results in low power output [5,6], high construction material costs [7], and current instability [8]. Additionally, other issues, such as insufficient scalability [9], mean that most MFCs operate with small volumes, ranging from a few milliliters to 1 L. Currently, hundreds of MFC experiments are being conducted worldwide, exploring various configurations [10,11,12] and different operational parameters [13,14,15], nanomaterials [16,17,18], and pure cultures of electroactive bacteria (EAB), including metabolic engineering microorganisms [19,20,21] and even plants [2,22], aimed at achieving enhanced energy efficiency.

Along with multiple other factors, the substrate used is one of the key components affecting the efficiency of MFCs. It provides microorganisms with the necessary nutrients and electron donors, which they metabolize to generate electrons, resulting in the production of electricity [23]. Waste with complex polymers, toxic substances, or variable chemical compositions poses a challenge for effective integration into any MFC system. Therefore, the selection of the optimal substrate is important for the effective operation of MFC technology.

For decades, MFCs have traditionally relied on various sugar-based wastes, including large-scale industrial wastes, such as molasses-containing brewery wastes, distillery wastes, or crude sugarcane effluent, which have demonstrated high system performance [24,25,26]. For example, a 100 mL MFC operated on crude sugarcane effluent reached 8314 mW m⁻² [27], and a 2.6 L MFC utilizing raw distillery effluent produced up to 25,194.8 mW m⁻² [28]. These high performances highlight the effectiveness of sugar-containing substrates, which are optimal carbon sources for most of the microorganisms essential for MFC operation. The high organic loading rate of complex waste substrates facilitates the development of diverse electrochemically active microbial communities, resulting in an increase in power output [23]. Thus, traditionally, the use of organic waste for MFC feeding is a justified strategy.

However, in this review, we aim to highlight an additional concern: the environmental impact of MFCs throughout their entire life cycle. While MFCs are widely recognized as environmentally friendly bioelectrochemical systems (BESs), offering new avenues for power generation [29,30,31], it is important to consider whether MFC technology remains sustainable from cradle to energy production when various substrates are utilized. Nevertheless, substrate preparation or electrode production can also be energy-intensive processes. The production of commonly used organic substrates in MFCs, such as glucose [32], sucrose [33], cellulose [34], acetate [35], and glutamate [36], inherently involves significant energy consumption.

The hydrolysis of starch to produce glucose demands a substantial process involving heat. Moreover, sucrose production similarly requires significant energy consumption for raw material grinding, sap extraction, evaporation, and other mechanical processes. The extraction of cellulose from wood is energy-intensive, especially during the sulfate pulping and bleaching stages, necessitating substantial chemical inputs and consequent treatment. Although the chemical synthesis of acetate and glutamate is somewhat less energy-demanding, it involves the use of environmentally aggressive compounds, such as acetic acid and sodium hydroxide, which require careful disposal [37]. While simple substrates are more easily degraded by biofilms and increase the power output of the system [23], their contribution to CO₂ emissions throughout the life cycle warrants the consideration of alternative feedstocks for MFC technology.

In addition, large-scale production processes generate substantial quantities of waste, which current MFC technologies struggle to manage effectively, thereby complicating the scaling of the technology. Conversely, nontraditional wastes, which are produced in relatively small quantities by households or small-scale agricultural farms, present considerable interest as potential resources for MFCs. Therefore, the goal of our review is to focus on nonconventional sources for MFC technology, such as solid food waste from potatoes, avocados, bananas, livestock and raw human urine, farm manure, slaughterhouse wastewater, etc. These substrates hold untapped potential for energy recovery in emerging zero-waste industries and regenerative agriculture, as they represent widely available ready-made raw materials for MFCs which do not require energy-intensive extraction and chemical treatment. The chemical composition of these nonconventional wastes and the concentrations of their components play crucial roles in affecting the bacterial community within the anode biofilm and, consequently, the overall MFC performance. This makes nontraditional wastes an area of considerable interest for advancing MFC technology and achieving more sustainable and scalable solutions.

In previous studies, several types of organic substrates for MFCs have been critically reviewed, including the traditionally used acetate, glucose, chitin, cellulose, lignocellulosic biomass, brewery and dairy wastewater, synthetic wastewater (SWW), domestic wastewater, and dye processing wastewater [23,38], as well as inorganic nutrients, such as nitrate, phosphate, sulfate, sulfide, ammonium, carbohydrates, fatty acids, and petroleum [39]. Building on this extensive research into conventional substrates, this review focuses on the untapped potential of nonconventional organic waste as a sustainable feedstock for MFCs. By addressing gaps in the literature, this review paper provides a comprehensive analysis of the impact of unconventional substrates on MFC performance and their scalability while emphasizing their environmental and practical advantages over traditional alternatives. This work contributes to the broader discourse on carbon-neutral energy technologies and aims to serve as a foundation for future research and practical advancements, specifically in the integration of nonconventional waste streams into scalable and sustainable MFCs.

2. Factors Affecting MFC Performance

The configuration and design of MFCs play a pivotal role in determining their performance, with recent advancements highlighting the potential of innovative materials, biofilm optimization strategies, and substrate selection to increase power generation and waste treatment capabilities. The efficiency of MFCs is closely linked to their design configurations, with single-chamber MFCs (SC-MFCs) and dual-chamber MFCs (DC-MFCs) being the most studied [7,23,40].

Table 1. Performance of MFCs using traditional organic substrates and wastes.

Type of MFC	MFC Prototype/Working Volume, Electrodes	Type of Substrate	Substrate Concentration	Inoculum Source	Maximum Performance	References
Traditionally Used Organic Substrates and Wastes in MFC
SC-MFC	28 mL MFC with Toray carbon paper as the anode and carbon paper containing 0.35 mg cm−2 Pt as the cathode	Acetate	141 mg L−1	DWW	506 mW m−2	[46]
DC-MFC	250/300 mL MFC with Toray carbon paper as the anode and cathode (42 cm2)	Acetate, glucose, and butyrate	1000 mg L−1 (COD)	DWW	52–72 mW m−2	[47]
DC-MFC	300 mL MFC with carbon paper sheets as the anode and cathode (7.1 cm2)	Acetate	1.28 g L−1	DWW	362 mW m−2	[48]
DC-MFC	4 cm3 two reactors with carbon felts as electrodes	Acetate	5000 mg L−1 (COD)	Activated sludge from a wastewater treatment plant	500 mW m−2	[49]
SC-MFC	2 L Fa-MFC with brush anodes floating in a beaker	Acetate	465–1029 mg L−1 d−1	Artificial wastewater	152 mW m−2 cathode area	[50]
SC-MFC	2 L Fa-MFC with brush anodes floating in a beaker	Acetate	45–119 mg L−1 d−1	Livestock wastewater	95 mW m−2 cathode area	[50]
SC-MFC	50 mL MFC with Teflon-treated carbon paper as the cathode and carbon paper as the anode (0.3 mg cm−2 Pt/C loaded); electrode area of 25 cm2)	Acetate	0.3 mg cm2	Mixed culture (aerobic activated sludge)	86.1 mW m−2	[51]
SC-MFC	One-chamber air cathode with carbon fibers as an anode	Beer brewery wastewater	600 mg L−1	Anaerobic mixed consortia	264 mW m−2	[52]
SC-MFC	28 mL MFC with Toray carbon paper as the anode electrode and carbon paper containing 0.35 mg/cm2 Pt as the cathode electrode	Butyrate	93 mg L−1	DWW	305 mW m−2	[46]
DC-MFC	100 mL MFC with mild steel coated with Fe2TiO5 and stainless-steel electrodes	Crude sugarcane effluent	4538 mg L−1	Anaerobic sludge	8314 mW m−2	[27]
DC-MFC	300 mL MFC with a silver anode and cathode (54.57 cm2)	Domestic wastewater	100 mmol L−1	Bacterial consortia	117–209 mW m−2	[53]
DC-MFC	250 mL MFC with a graphite felt anode and cathode electrodes (effective area of 25 cm2)	Glucose	1 g L−1	Clostridium beijerinckii M13	79.2 mW m−2	[54]
DC-MFC	500 mL bottle-MFC with a carbon brush (5.0 cm in diameter, 5.0 cm in length) as the anode and cathode	Glucose and mannitol	43.65 g L−1 (glucose) and 14 g L−1 (mannitol)	L. digitata 300 mL of DWW	0.5 V	[55]
SC-MFC	28 mL MFC with a carbon brush as an anode and carbon cloth as an air cathode (7 cm2)	Lactate	500 mg L−1	Aerobic sludge	5.79 W m−3	[56]
SC-MFC	28 mL MFC with a carbon brush as an anode and carbon cloth as an air cathode (7 cm2)	Lactate	500 mg L−1	Anaerobic sludge	3.66 W m−3	[56]
SC-MFC	28 mL MFC with a carbon brush anode and carbon cloth cathode	Lactate	500 mg L−1	Pretreated sludge	1.65 W m−3	[57]
SC-MFC	400 mL MFC with Pt-coated carbon paper (2 × 2 cm2, 2 mg cm−2) as the cathode and plain carbon paper (3 × 3 cm2) as the anode	Lignocellulosic biomass	4 g L−1	Rumen microorganisms	0.405 W m−3	[58]
SC-MFC	12 mL reactor coated with a carbon, polytetrafluoroethylene, and platinum cathode (7.0 cm2) and anode (2.0 cm2)	Lignocellulosic biomass	480 mg L−1 (6.7 mM glucuronic acid)	Mixed bacterial culture	1410–2760 mW m−2	[59]
DC-MFC	400 mL MFC with graphite plates as electrodes (84 cm2)	Microcrystalline cellulose	7.5 g L−1	Rumen microorganisms	55 mW m−2	[60]
DC-MFC	1.8 L MFC with an anode and cathode chamber (10 cm × 10 cm × 18 cm)	Municipal solid waste	0.2 mg L−1	Anaerobic sludge	37.808 mW m−2	[61]
SC-MFC	350 mL MFC with carbon cloth electrodes (84 cm2)	Municipal wastewater	250 mg L−1	Microorganisms	0.2–0.3 mA	[62]
DC-MFC	50 mL MFC with graphite rods as the anode and cathode (20 cm2)	Municipal wastewater	279 mg O2 L−1	Gluconobacter oxydans	65 mW m−2	[63]
DC-MFC	25 mL MFC with graphite felt as electrodes (24 cm2)	Nitrilotriacetic acid	48.5 mg L−1	Oligotrophic consortium enriched with river water	0.0005 mA cm−2	[64]
DC-MFC	Two 250 mL MFC reactors with graphite felt and activated carbon cloth electrodes, coated with NiO/MnO2	Wastewater treatment plant	0 mg L−1 (norfloxacin)	n/a	1696.56 mW m−2	[65]
DC-MFC	Two 250 mL MFC reactors with graphite felt and activated carbon cloth electrodes, coated with NiO/MnO2	Wastewater treatment plant	20 mg L−1 (norfloxacin)	n/a	1295.91 mW m−2	[63]
SC-MFC	2600 mL MFC with graphite plates electrodes (166.81 cm2)	Raw distillery effluent	1, 53, 846 mg L−1 (COD)	Microbial community of distillery effluent	25,194.8 mW m−2	[28]
DC-MFC	240 mL MFC with carbon felt as the anode and cathode (30 cm2)	Synthetic wastewater with glucose	100 mg L−1 (COD) 0.5–10 g L−1 (glucose)	Anaerobic sludge	50.7 mW m−2	[66]
SC-MFC	150 mL reactor with carbon felt as the anode (16 cm2) and cathode (31 cm2)	Synthetic wastewater	989.5 mg L−1 (COD)	1% anaerobic sludge	995.73 mW m−3	[67]
DC-MFC	500 mL MFC with graphite rods as the anode and cathode	Synthetic wastewater	50 mg L−1	Fruit wastes	87.71 mA m−2	[68]
SC-MFC	800 mL MFC with a carbon cloth cathode (144 cm2 = 12 × 12 cm2), containing 0.35 mg cm−2 Pt, and graphite fiber brushes (6 cm in outer diameter and 7 cm long) as the anode electrode, total surface area of 4.2 m2	Xylose	20 mM	Mixed bacterial culture	673 mW m−2	[69]
DC-MFC	500 mL with two chambers with a titanium rod anode and a carbon cloth cathode	Vegetable oil industrial wastewater	n/a	100 mL of sewage sludge	6119 mW m−2	[70]
SC-MFC	28 mL reactor with a graphite fiber anode and a platinum catalyst–carbon cloth cathode	Wastewater	1 g L−1 (glucose)	14 mL of rumen inoculum	824.5 mW m−2	[71]

SC-MFC, single-chamber microbial fuel cell; DC-MFC, dual-chamber microbial fuel cell.

provides an overview of the power generation performance of MFCs using different traditional substrates and waste materials. As shown in this table, the highest power density (up to 25,195 mW m⁻²) was reached by a 2.6 L SC-MFC with graphite plate electrodes operating on raw distillery effluent, i.e., a traditionally used waste in MFC technology [28]. The recent advancements in improving the power generation efficiency of MFCs exploiting nonconventional substrates are summarized in

Table 2. Performance of MFCs using nonconventional substrates.

Type of MFC	MFC Prototype/Working Volume, Electrodes	Type of Substrate	Substrate Concentration	Inoculum Source	Maximum Performance	References
Nonconventional Organic Sources for MFCs
SC-MFC	150 mL MFC with a zinc anode and copper cathode	Avocado waste	100% decomposing avocado	N/A	5736.112 mW cm−2	[72]
SC-MFC	150 mL MFC with a zinc anode and copper cathode (both with an area of 80 cm2)	Banana waste	100% decomposing banana	N/A	566.80 mW cm−2	[73]
DC-MFC	15 L MFC with carbon brushes as electrodes	Cattle dung/ acetate and butyrate	2059.7 and 369.1 mg L−1	Biogas slurry	0.220 W m−3	[74]
SC-MFC	700 mL MFC with zinc–carbon electrodes	Cattle dung	100% cattle dung slurry	Biogas slurry	1465 mW m−2 1858 mA m−2	[75]
DC-MFC	1.2 L MFC with graphite felt as the electrode material with a surface area of 0.0108 m2	Effluent wastewater with sodium benzoate C7H5NaO2	1% sucrose 5 mM C7H5NaO2	Novel consortium	18.15 mW m−2	[76]
DC-MFC	60 cm × 30 cm × 30 cm MFC with 12 L cathode and 36 L anode chambers with PEM and carbon fiber felt electrodes	HPAM-containing oilfield wastewater	300 mg⋅L−1 crude oil, 508 mg⋅L−1 HPAM	Activated anoxic sludge	2420 mW m−2	[77]
SC-MFC	19.2 L cascade of 4 modules. Individual SSM-MFC module (400 mm × 300 mm × 170 mm) with an AC-PTFE cathode and carbon fiber veil anode	Human urine	100% urine	Activated sludge	9.9 W m−3	[78]
DC-MFC	10 L scale MFC stack with individual 0.5 L module (17.5 × 14.5 × 2 cm3), CEM and stainless-steel mesh electrodes	Human urine	100% urine	Anaerobic digestate	14.5 mW m−2	[79]
SC-MFC	2211 mL stack. Each 435 mL c-MFC module consists of 8 individual earthenware ceramic cylinders (50 mm × 21 mm × 28 mm) enclosed in a cylindrical (140 mm) PVC vessel with a parallel-connected carbon veil cathode and anode, coated AC/PTFE, and stapled stainless-steel mesh	Human urine	100% urine	Activated sludge	32.2 mW m−3, 19.36 mW per single module	[80]
SC-MFC	Each 525 mL s-MFC module consist of 28 parallel-connected carbon veil cathode–anode pairs coated AC/PTFE and assembled on stainless-steel mesh and enclosed in cylindrical PVC vessel	Human urine	100% urine	Activated sludge	69.7 mW m−3, 23.43 mW per single module	[80]
SC-MFC	Cascade of 3 modules. Each 435 mL c-MFC module consist of 8 individual earthenware ceramic cylinders (50 mm × 21 mm × 28 mm) enclosed in a cylindrical (140 mm) PVC vessel with a parallel-connected carbon veil cathode and anode, coated AC/PTFE, and stapled stainless-steel mesh	Human urine	100% urine	Activated sludge	6.45 mW per single module, 17.99 mW per cascade of 3 modules	[41]
SC-MFC	Cascade of 3 modules. Each 525 mL s-MFC module consist of 28 parallel connected carbon veil cathode–anode pairs coated AC/PTFE and assembled on stainless-steel mesh and enclosed in a cylindrical PVC vessel	Human urine	100% urine	Activated sludge	7.80 mW per single module, 26.45 mW per cascade of 3 modules	[41]
DC-MFC	2.5 L cathodic plastic bucket chamber and 0.4 L inner clayware pot anodic chamber	Livestock urine	3 kg COD m−3-diluted cow’s urine	Anaerobic sludge	5230 mW m−3	[81]
SC-MFC	643 mL P-MFC with a graphite felt anode and a stainless-steel mesh cathode	Livestock urine	100% cow urine	N/A	42.79 mW m−2	[82]
SC-MFC	643 mL P-MFC with a graphite felt anode and a stainless-steel mesh cathode	Livestock urine	100% goat urine	N/A	46.97 mW m−2	[82]
SC-MFC	643 mL P-MFC with a graphite felt anode and a stainless-steel mesh cathode	Livestock urine	100% sheep urine	N/A	19.28 mW m−2	[82]
DC-MFC	Two 7 mL reactors with a carbon-felt based iron/magnesium/zirconium polycrystalline catalytic cathode and bioanode	Nuclear industry wastewater	60Co, 90Sr, 137Cs, 138La, and 144Ce 1000 mg L−1	Mixed bacteria containing Shewanella oneidensis MR1	1400 mW m−2	[83]
DC-MFC	2.5 L MFC (15 cm × 8 cm × 22 cm) with copper and zinc electrodes	Paddy straw	10 g	Rumen fluid	8490 mW m−2	[43]
SC-MFC	80.5 mL MFC with a graphite and GAC anode and a porous graphite plate cathode	PAHs polluted groundwater	1546 mg⋅L−1 light PAHs	Bacterial community	7.8 mA m−2	[84]
DC-MFC	1 L MFC with a graphite plate anode and cathode	Petroleum refinery wastewater	350 mg⋅L−1 TPH	Mixed anaerobic bacteria (Bacillus sp.-dominant)	552 mW m−3	[85]
SC-MFC	5 L MFC with graphite carbon electrodes	TPH contaminated sediment	26,000 mg kg−1 TPH	N/A	50,570 mW m−2	[42]
DC-MFC	1.5 L with two chambers with graphite carbon electrodes	TPH contaminated sediment	26,000 mg kg−1 TPH	N/A	5760 mW m−2	[42]
SC-MFC	500/530 mL MFC with a Pt-coated carbon cloth cathode and bamboo charcoal as the anode	Potato waste	1000 mg L−1	Anaerobic mixed bacterial community	576 mW m−2	[86]
SC-MFC	28 mL MFC with a carbon fiber brush anode and a carbon paper with PTFE and Pt cathode	Raw WHTW and MWW	3343 mg⋅L−1 COD	Microbial community of WHTW and MWW	360 mW m−2	[87]
SC-MFC	28 mL MFC with a carbon fiber brush anode and a carbon paper with PTFE and Pt cathode	Raw WHTW	280 mg⋅L−1 cellulose, 250 mg⋅L−1 lignin, and other	Pretreatedmicrobial consortium of WHTW at 45 °C	334 mW m−2	[88]
DC-MFC	200 mL MFC with carbon cloth as the anode and cathode (1.5 × 1.5 cm)	Sago hampas	20 g L−1	Clostridium beijerinckii SR1	73.8 mW cm−2	[89]
DC-MFC	200 mL MFC with carbon cloth as the anode and cathode (1.5 × 1.5 cm)	Sago hampas	5.04 g L−1	Clostridium beijerinckii SR1	61.5 mW m−2	[89]
DC-MFC	1 L MFC with graphite and copper electrodes (31.4 cm2)	Slaughterhouse wastewater	1:10 waste–rumen microbes	Rumen microbes	700 mW m−2	[90]
DC-MFC	120 mL MFC with copper–graphite electrodes (17.6 cm2)	Slaughterhouse wastewater	10:2.4 waste–ruminal liquor	Ruminal liquor	568 mW m−3	[91]
DC-MFC	U-shaped MFC with graphite rods as the anode and cathode electrodes (0.0015 m2)	Vegetable waste	N/A	Sewage wastewater	88,990 mW m−2	[44]

SC-MFC, single-chamber microbial fuel cell; DC-MFC, dual-chamber microbial fuel cell; AC-MFC, air-cathode microbial fuel cell; m-MFC, mediator-less microbial fuel cell; c-MFC, ceramic cylinder-based microbial fuel cell; SSM-MFC, self-stratifying membrane-less microbial fuel cell; Fa-MFC, floating all-in-one type of microbial fuel cell; AC, activated carbon; CEM, cationic exchange membrane; PEM, proton exchange membrane; DWW, domestic wastewater; HPAM, hydrolyzed polyacrylamide; MWW, municipal wastewater; PTFE, polytetrafluoroethylene; РАН, polycyclic aromatic hydrocarbon; TPH, total petroleum hydrocarbon; WHTW, wastewater from the hydrothermal treatment of wood.

. Research findings indicate that the power density in SC-MFCs ranges from 7.80 to 50,570 mW m⁻² when exploiting nonconventional substrates [41,42]. However, in DC-MFCs, the range is higher, with power densities ranging from 14.5 to 88,990 mW m⁻² [43,44]. Furthermore, Permana et al. [42] demonstrated a tenfold increase in power generation in SC-MFCs compared to DC-MFCs (50,570 and 5760 mW m⁻²) while operating on petroleum hydrocarbon-contaminated sediment. The coulombic efficiency of SC-MFCs can be comparable to or even exceed that of two-chamber systems, as shown in Figure 1. Nevertheless, recent studies suggest that design optimization can enable DC-MFCs to achieve power densities exceeding 8314 mW m⁻² [27], thus narrowing the performance gap. While configuration plays an important role, other factors, such as the membrane type, cathode/anode surface area and electrode materials, biofilm formation, system volume, substrate type, and operating conditions, can significantly enhance MFC performance. For a comprehensive discussion on this topic, readers are encouraged to consult recent review articles focusing on the use of MFCs for sustainable electricity production [7,45]. These reviews discuss the latest advancements in MFC designs, configurations, and operational strategies.

Biofilm formation on the anode surface is another crucial parameter for the efficient performance of MFCs because of their electrochemical metabolic activity [92]. The biofilm allows for the adequate transfer of electrons between the microorganisms and the anode surface as well as for the efficient biological transfer of electrons from the anode to the cathode compartment [93] (as shown in Figure 2). Both electroactive biofilms and extracellular electron transport in MFCs are highly dependent on the physicochemical properties of the operational anode surface [11] and the type of material and configuration used.

However, certain factors, such as substrate concentration, environmental conditions, temperature, pH, type of membrane, scale-up of MFCs, and electrode materials, can negatively affect biofilm formation [94]. Modifications of the anode are essential for promoting the growth of electrogenic biofilms on the electrode surface, as illustrated in a study by Hemdan et al. [95]. Another way to improve electron transfer and MFC performance is the use of a cyanobacterial consortium as a potential feedstock or inoculum. Hence, extensive investigations have been conducted on cyanobacteria-based MFCs for their promising potential in generating bioelectricity and breaking down environmental waste contaminants [96,97,98]. The incorporation of cyanobacteria in MFCs has been found to greatly increase biomass yield, nutrient removal, organic waste degradation, and power production [99]. These proposed studies are crucial for the future commercialization of MFCs and for making this renewable energy source accessible to everyone.

Figure 2. Schematic of electroactive biofilm formation on a typical DC-MFC with acetate as the carbon source. Carbon source conversion occurs in the anode chamber by EAB with mediator involvement. Adapted from [100].

Figure 2. Schematic of electroactive biofilm formation on a typical DC-MFC with acetate as the carbon source. Carbon source conversion occurs in the anode chamber by EAB with mediator involvement. Adapted from [100].

In summary, we can argue that the complex community of EAB that attaches to the anode surface plays an important role in the electrochemical reactions that occur within MFCs. The anode biofilm serves as the primary source of electricity generation in MFCs, as it facilitates the efficient transfer of electrons from microorganisms to the anode, making the biofilm’s composition and physicochemical properties crucial. Therefore, the performance of an MFC depends on the diversity and composition of the microbial community in the anode biofilm. Additionally, the performance of MFCs is intricately tied to their configuration, materials, and biological processes. SC-MFC and DC-MFC designs each have distinct advantages, with SC-MFCs typically achieving higher power densities due to their simplified structure and efficient substrate utilization. However, DC-MFCs are catching up as optimization efforts narrow the performance gap. Advanced strategies, such as modifying anode materials and incorporating cyanobacterial consortia, offer promising avenues to enhance efficiency and expand applications. As research progresses, the potential of MFCs as a sustainable energy source for waste treatment and electricity generation becomes increasingly feasible. However, further research on optimizing MFC performance, including anode surface modification, microbial community modification, and operating conditions, is needed.

3. Substrates in MFC Technology: Availability and Potential Environmental Benefits

For decades, MFCs have predominantly utilized various simple organic substrates and organic-based wastes, e.g., large-scale brewery wastes containing molasses, industrial wastewater from distilleries, untreated sugarcane effluent, etc. Despite advancements in MFC technology involving the consumption of simple organic substrates, such as glucose, sucrose, acetate, butyrate, and glutamate, their production demands substantial energy input and results in notable CO₂ emissions. Specifically, the production processes involve energy-intensive operations, e.g., glucose and sucrose production, including filtration under a pressure of 0.3–0.4 MPa at temperatures of 80–90 °C, as well as juice evaporation at 126 °C and a pressure of 14.6–21.2 kPa. Substrate processing necessitates considerable energy, which leads to a significant overall environmental and economic impact. Glutamate production is particularly notable because of its high energy consumption and substantial pollutant discharge, which raise serious environmental concerns.

The carbon footprint associated with the production of 1 ton of glutamate is 3140 kg CO₂ eq, with the extraction and refinement processes contributing 32.92% of this footprint [101]. In the case of glucose production, nonrenewable energy use (NREU) ranges from 6.8 to 9.3 MJ/kg glucose dry solids (ds), accompanied by greenhouse gas (GHG) emissions ranging from 0.7 to 1.1 kg CO₂ eq./kg glucose (ds) depending on the technological approach [102]. For cellulose production, 15–20 kg of CO₂ is released per 1 kg of cellulose [37]. Moreover, sugar production results in high CO₂ emissions, with 241 kg of CO₂ eq emitted per ton of sugar produced and 2406 kg of CO₂ eq per hectare of cultivated area [103]. Additionally, the production processes often involve aggressive chemical compounds requiring proper disposal, which mitigates the ecological benefits of MFCs.

Although glucose, glutamate, and other purified substrates are not practical for large-scale use owing to their cost, they remain essential for laboratory experiments that allow standardized evaluation of MFC performance. These substrates have also found applications in niche areas, such as robotics (e.g., EcoBot) and medical devices [104], where their high metabolic efficiency and availability offer distinct advantages. However, for broader applications, the key challenge remains the development of technologies capable of utilizing renewable or waste-based substrates that are more environmentally and economically sustainable. Consequently, a new approach to utilize plant biomass without the need for extensive physical and chemical processing, thus avoiding the use of high temperatures, pressures, and chemical reagents, can be considered environmentally sustainable.

Therefore, waste materials serve as crucial alternative feedstocks for the development of MFCs. As byproducts, they reduce CO₂ emissions throughout the entire technology lifecycle, from cradle to gate, i.e., power generation. Additionally, waste materials are more readily available than organic substrates, which require extraction or chemical synthesis. Moreover, the World Bank predicts a 70% increase in global waste generation by 2050, which underscores the growing importance of utilizing waste as a feedstock for MFCs. Thus, the integration of waste into MFC technology not only aligns with the goal of carbon-neutral technologies but also addresses practical concerns related to feedstock availability and sustainability. In this context, raw waste, such as human urine, livestock urine, manure, and fruit and vegetable residues, including potato, banana, and avocado wastes, offers promising alternatives as substrates for MFCs. These types of waste are not only widely available but are also cost-effective, making them viable and sustainable prospects for resource-intensive substrates.

Recent studies have highlighted the potential of using nonconventional sources, such as human urine, livestock urine, and farm manure [41,75], as well as solid food waste from potatoes, mangoes, bananas, and oranges, in MFCs [86,105]. Moreover, they attract significant interest not only through their wide availability and cost-effectiveness but also due to their low-volume presence, particularly for households with nonindustrial levels of waste. For example, the average annual generation of nontraditional waste per person includes 365–730 L of human urine, 11–15 kg of potato waste, and 3.3–4 kg of avocado waste. For an average household with ten chickens and one cow, the annual manure production is approximately 1825–3650 kg and 1000–5000 L, respectively.

Relatively small quantities of waste are appropriate for effectively managing small-scale MFC systems. The current MFC technology is not yet capable of handling the large volumes of waste generated by large industrial processes. The operational volume of most MFC prototypes typically ranges from 0.1 to 1 L. A 1 L MFC can process approximately 4380 L of waste per year with an average flow rate of 0.5 L/h and an HRT of 2 h under continuous operation. However, a typical brewery that produces approximately 100,000 L of beer annually generates between 300,000 and 500,000 L of wastewater daily.

Similarly, dairy products that generate 50,000 L of milk daily can produce significantly larger wastewater streams, with approximately 50,000–150,000 L of wastewater per day [106,107]. Nevertheless, the use of small quantities of household waste in compact MFC systems holds promise for effective waste management and energy production at a local scale suitable for emerging zero-waste industries and regenerative agriculture aimed at energy recovery. Therefore, developing MFCs for small-scale farms and households is a critical consideration at the current stage of technological development and represents an initial step toward broader scalability. To assess the environmental impact of MFC technology comprehensively, it is crucial to consider the CO₂ emissions associated with the entire lifecycle and to evaluate the carbon footprint from substrate production to energy generation.

3.1. Acetate, Butyrate, Glucose, Glutamate, Etc., Are Traditionally Employed in MFCs as Feedstock

The selection of feedstock plays a pivotal role in determining the performance and efficiency of MFCs, with traditional substrates, such as acetate, butyrate, glucose, and glutamate, demonstrating diverse impacts on bioelectricity generation and system stability. Acetate is a commonly used substrate in MFC performance studies because of its effective impact on the EAB [108] and its inertness to alternative microbial conversions, such as methanogenesis and fermentation [23,38]. In 2005, Liu et al. [46] inspired researchers to use acetate in MFCs. The addition of acetate to the MFC resulted in a higher power output (66% higher) than the addition of butyrate did. The acetate-fed MFCs presented the highest power density of 506 mW m⁻² (with a concentration of 800 mg L⁻¹), whereas the butyrate-fed MFCs presented a power density of 305 mW m⁻² (with a concentration of 1000 mg L⁻¹).

Sun et al. [48] reported that acetate consistently delivered higher power densities and coulombic efficiencies than xylose or bioethanol effluent. Acetate not only improves power stability during wastewater treatment [109] but also plays a crucial role in enhancing energy generation in MFCs [110]. Asensio et al. [49] demonstrated that carboxylic acids, such as acetate and propionate, are advantageous over alcohols or fructose for improving power output.

Ullah and Zeshan [111] compared glucose, acetate, and sucrose in DC-MFCs. Acetate at 2000 mg L⁻¹ achieved a power density of 114 mW m⁻², outperforming glucose and sucrose. This value was significantly greater than that of glucose-fed MFCs, which have a power density of 52 mW m⁻² [51]. Acetate is an essential source for electron transport systems and is the final electron acceptor in various metabolic pathways that metabolize higher-order carbon sources [112].

One of the first studies that examined the use of glutamate as a power source for MFCs appeared in the early 2000s. MFCs operating with glutamate and glucose solutions generated a stable current of 2 mA for 4 weeks [101]. The advantage of glutamate is its flexibility as a source of both nitrogen and carbon in the electrochemical processes of MFCs. However, the environmental and economic aspects of its production reduce its attractiveness as a resource for the MFC. Consequently, acetate outperforms glucose, butyrate, or sucrose in increasing the power generation efficiency in MFC technologies.

Based on the above, we can argue that the integration of substrates into the MFC system presents an advantageous pathway for advancing sustainable energy generation and effective waste management. While traditional substrates, such as glucose and glutamate, serve as standardized benchmarks in controlled environments, their high energy consumption and environmental costs hinder large-scale implementation. Using waste materials as substrates—such as human urine, livestock manure, and fruit residues—marks a significant advancement in MFC applications. These alternatives not only address sustainability issues but also cater to the increasing demand for effective waste valorization as global waste production continues to rise. Small-scale MFC systems, particularly in households and small farms, offer a practical starting point for incorporating waste into energy recovery processes. However, scaling this technology to manage industrial waste volumes presents a notable challenge. Thorough life cycle assessments (LCAs) are crucial to ensure that MFCs achieve genuine carbon neutrality while balancing operational efficiency with environmental benefit footprints.

3.2. Conventional and Nonconventional Sources for MFCs: Industrial and Household Wastes

3.2.1. Sugar and Meat Food Industry Wastewater as Promising Substrates for MFC Technology

Industrial wastewater, particularly from the sugar, distillery, brewery, and meat processing industries, presents significant environmental challenges because of its high organic load and pollutant concentrations, but recent advancements in MFC technologies offer promising solutions for sustainable treatment and bioelectricity generation. According to research conducted by Amjad et al. [113], the sugar industry generates several byproducts, such as molasses, bagasse, bagasse fly ash, and press mud. Zhong and co-workers noted that the distillery industry also produces substantial amounts of molasses wastewater when sugarcane molasses is used as a raw material [114]. Both sugar industrial wastewater (SIWW) and distillery industrial wastewater (DIWW) are highly polluted, with high levels of organic, nitrogen, phosphate, and sulfate compounds, as observed by Kushwaha et al. [115]. Despite containing sugars that could be a good food source for microorganisms, disposing of molasses wastewater is challenging because of its high levels of chemical oxygen demand (COD) (65,000–130,000 mg L⁻¹) and biochemical oxygen demand (BOD) (30,000–96,000 mg L⁻¹), as highlighted previously [115].

Over the years, numerous methods for treating wastewater contaminated with molasses have been developed, including biological, physical, and chemical treatments. Recently, electrobiochemical treatment methods have emerged as promising treatment solutions. In 2009, an MFC system integrating an upflow anaerobic sludge blanket reactor (UASB) and a biological aerated filter (BAF) was used to treat molasses wastewater [116]. The UASB reactor was mainly responsible for reducing COD and removing sulfate, with removal efficiencies of 53.2% and 52.7%, respectively. The BAF unit effectively degraded phenol derivatives, whereas the MFC was used to oxidize the sulfide that is generated and to generate electricity. When highly concentrated molasses wastewater with CODs of 127 and 500 mg L⁻¹ was used as the influent, the system achieved a maximum power density of 1410.2 mW m⁻² [116]. Overall, this was a successful attempt to combine MFC technology with conventional anaerobic–aerobic processes for the treatment of actual molasses wastewater.

Since then, researchers have developed more than four dozen configurations of MFCs to explore SIWW and DIWW treatment. Various types of MFCs have been studied, including SC- and DC MFCs, aerobic and anaerobic systems, and stacked MFCs of different volumes ranging from 19 to 2600 mL, exploiting mostly 500 or 900 mL working volume units. Carbon- or graphite-based electrodes, such as carbon cloth or felt and graphite rods, were primarily used [117]. The highest recorded power output of 25,194.8 mW m⁻² was achieved on distillery effluent with a 2600 mL SC-MFC using graphite plate electrodes [28]. However, one of the highest maximal power density units operating on crude sugarcane effluent collected from SIWW was reported in 2020, with a 100 mL DC-MFC using carbon-free materials, such as mild steel coated with Fe₂TiO₅ and stainless-steel electrodes, which achieved 8314 mW m⁻² [27]. Membrane electrode assemblies (HEMs) incorporated with MFCs (called HEM-MFCs) and MFCs exhibit high COD removal efficiencies of 95–95.6% at 500 mg L⁻¹ influent COD [118,119].

In recent years, researchers have investigated effective ways to optimize molasses-based waste treatment. To this end, modified electrodes and membranes are considered important tools for improving performance [120]. A recent study revealed that a novel exopolysaccharide-producing Bacillus sp. strain has demonstrated promise in improving wastewater treatment in the molasses sugar industry, particularly in MFCs. This microbe effectively reduces the COD of molasses while also possessing electrochemical activity that enhances electron charge transfer [121]. Additionally, researchers have reported that MFC-treated molasses wastewater can be reused through microbially induced carbonate precipitation (MICP) technology, which has proven effective in dust suppression [122].

Brewery wastewater is also a highly abundant substrate that has attracted the attention of several researchers because of its unique characteristics. It contains organic matter from different types of food and low concentrations of harmful compounds (inhibitors) [123]. In fact, the concentration of organic materials in brewery wastewater (3000–5000 mg COD L⁻¹) is approximately 10 times greater than that of organic materials in domestic sewage [124]. Additionally, this substrate may be the best option for improving MFC efficiency because of its low ammonium concentration and high carbohydrate content [23]. Gao et al. [24] previously proposed and developed a novel air-cathode MFC to produce electrical energy from brewery wastewater. They achieved a maximum power density of 0.27 W m⁻² with approximately 60% COD removal. This performance was 81.12% greater for energy generation than that reported when dye wastewater was used as a substrate [125].

The global demand for meat has led to an increase in the amount of waste produced by slaughterhouses, making their operations more complex. While industrial meat waste is still a commonly used substrate in MFCs, only a few successful studies have been conducted. Given the results obtained and the ever-increasing meat consumption worldwide, further exploration of slaughterhouse wastes used in MFCs is of considerable interest. For example, a study by Meignanalakshmi and Kumar [43] revealed that a DC-MFC, which operates on caprine rumen fluid collected from slaughterhouses and paddy straw with zinc–copper electrodes, was effective in bioelectricity production, reaching 8490 mW m⁻² from a single MFC. Another study by Christwardana et al. [90] achieved a power density of 700 mW m⁻² with almost 70% COD removal by treating slaughterhouse wastewater with different doses of rumen liquor in a DC-MFC with graphite electrodes. Furthermore, researchers reported almost complete COD removal efficiency (99%) of slaughterhouse wastewater biotreatment [126,127]. A DC-MFC with a copper–graphite arrangement generated 568 W m⁻³, with 81.33% COD removal from slaughterhouse wastewater [91]. However, the study also revealed that power generation and COD removal were affected by various factors, including the high dose of ruminal liquor, copper electrode material, and optimal electrode surface area (up to 17.6 cm²). Moreover, combining an MFC with an electro-Fenton (EF) system was recently shown to enhance slaughterhouse wastewater treatment (WWT), achieving COD reductions of 69.9% and 81.6%, respectively. In contrast, an integrated MFC–EF system exhibited comprehensive performance due to a synergistic removal of 95.5% for COD [128]. In view of the results obtained and the ever-increasing meat consumption worldwide, further exploration of the use of slaughterhouse wastes in MFCs is of considerable interest.

Finally, MFC is a reliable technique for treating molasses-containing sugar industrial effluent, distillery industrial effluent, and meat industry wastewater. Substrates, such as SIWW and DIWW, are also promising for electricity production in MFCs. The modification of electrode materials can enhance the adhesion of electroactive microorganisms and stimulate biofilm formation, creating favorable conditions for electrogenesis. Furthermore, modifications to membrane materials can facilitate electron transport by increasing the water absorption capacity and proton conductivity, thereby improving the overall efficiency of proton exchange and enhancing the power generation performance. Additionally, increasing the surface area of materials and applying appropriate microbial strains can lead to a significant increase in power generation, as well as the successful removal of pollutants and the enhancement of slaughterhouse WWT.

3.2.2. Synthetic Wastewater and Dye Wastewater as Sources for MFCs

Synthetic wastewater (SWW) has been frequently used in MFCs over the last decade. The pH, conductivity, and charge strength of these substrates are easy to control [23]. El Moussaoui et al. [129] evaluated biomass growth and SWW (urban) treatment via an activated sludge reactor. The activated sludge in the reactor contained 90.6% suspended solids and had 91.5% turbidity, 83.6% biological oxygen demand (BOD), and 90.8% COD. The pilot-activated sludge reactor used in this study was highly efficient in treating SWW. The biodegradation of styrene-containing SWW was subsequently assessed via an MFC [130]. The impact of microbial populations on biodegradation was also evaluated. Here, activated sludge was used as an inoculum in MFCs for degradation efficiency, degradation rate, and power generation. The activated sludge system showed an initial styrene biodegradation rate of 15.6 mg L⁻¹ h⁻¹ with 100% styrene removal at a maximum power density of 13.6 mW m⁻². This was attributed to the activity of the microbial communities involved in the process. However, the power density obtained in this study was 80.57%, which was lower than the power densities previously reported for membrane- and mediator-less SWW-MFCs [131]. This difference in power density was attributed to the fact that both systems had different characteristics, even though they were fed SWW. In summary, SWW exhibited a lower performance than the other substrates due to its substrate concentration, conductivity, pH, degradation, organic matter (OM), and COD.

Synthetic dyes are extensively used in the textile industry to produce large amounts of cytotoxic aromatic compounds and genotoxins [132]. In addition, 10–15% of dyes are lost in the effluent during the process [133]. Apart from the textile industry, azo dyes are also widely used in pulp and paper production, leading to the presence of colorants that affect the growth of aquatic flora and fauna and the solubility of the gas found in the aquatic ecosystem [134]. The release of high dye concentrations results in severe environmental issues, such as blocking light and O₂ transfer into water [38,125]. Various techniques and methods have been used to treat these pollutants in wastewater and other polluted environments. These include physical, chemical, and biological interventions [135]. In one study, a constructed wetland MFC (CW-MFC) was used for removing methyl orange dye [136]. The CW-MFC was able to remove 68% of the dye without a power booster. However, when rice husks were added to the CW-MFC, the dye removal efficiency increased by 98%, and the COD removal efficiency increased by 85.29%. The maximum power density generated by the system was 102.08 mW m⁻², which is greater than that reported in a previous study by Khalid et al. [125], where the degradation of azo dyes was linked to the reduction in COD. A previous study evaluated the simultaneous degradation of dyes and bioelectricity production via an MFC coated with a graphene oxide-modified anode. To improve bioenergy production and dye degradation in MFCs, two key factors to consider are visible light and anode surface modification, as indicated by Ahmadpour et al. [137]. In addition, competition between the anode and azo dyes for capturing electrons from the carbon source is a significant challenge. However, if this issue can be resolved, up to 100% degradation of azo dyes from any source of contamination, such as wastewater or textiles, can be achieved.

A DC-MFC system that operates with a novel type of waste for MFC technology—nuclear plant wastewater containing radionuclides, such as ⁶⁰Co, ⁹⁰Sr, ¹³⁷Cs, ¹³⁸La, and ¹⁴⁴Ce at a concentration of 1000 mg L⁻¹, and inoculated with a mixed bacterial culture including Shewanella oneidensis MR1—achieves a power density of 1400 mW m⁻² [83]. The system effectively removed over 80% of the radioactive ions, with removal efficiencies exceeding 98% for ¹³⁸La and ¹⁴⁴Ce. This finding reveals new possibilities for removing radioactive ions from nuclear industry wastewater while simultaneously enabling energy recovery in MFCs.

In summary, MFC technology, which utilizes both industrial and household waste, offers a transformative approach to tackling environmental challenges while simultaneously producing bioelectricity. Traditional substrates, such as sugar and wastewater from the meat industry, have demonstrated significant potential due to their organic load and distinct biochemical properties that foster microbial activity. Recent advancements indicate that optimizing electrode and membrane materials can greatly enhance microbial adhesion and electron transport, thereby improving power generation and pollutant removal. For example, capitalizing on the high carbohydrate content found in brewery waste and developing tailored approaches for the complex organics present in distillery waste has led to innovative applications of MFCs. This dual functionality of MFCs—effective waste management and renewable energy generation—highlights their promise as a vital component of circular economy strategies. Nonetheless, challenges related to scalability, cost efficiency, and adaptability to various waste streams still necessitate focused research to fully unlock the potential of this technology.

3.2.3. Industrial Waste from Oil and Wood as Sources of Complex and Stable Compounds for Nonconventional MFC Feeding

Over the past decade, interest in the academic community regarding the use of MFCs to treat and thereby exploit oil waste has increased. Despite the hydrophobic and decomposition-resistant nature of oil compounds, hydrocarbon-degrading microorganisms can still break them down [138,139]. MFC technology, which utilizes electrical stimulation, can further increase the activity of these microorganisms. A study by Guo et al. revealed that compared with traditional methods, MFC technology can increase the level of hydrocarbon decomposition by up to 10.1 times [140]. This makes MFCs an effective tool for decontaminating oil waste. Moreover, MFCs can be operated onsite and generate long-term electricity, providing the added benefit of sustainability [141]. In a recent application, MFCs with granular activated carbon (GAC) were used to treat groundwater contaminated with hydrocarbons from gasworks. After three weeks, the MFCs achieved almost complete (99%) removal of total petroleum hydrocarbons (TPH) [84]. Hybrid MFCs also demonstrated excellent treatment efficiency for petroleum waste, achieving 98% coulombic efficiency, a 96.5% reduction in COD, and a 99% reduction in phenanthrene [142].

Recently, Zhao and coworkers investigated the biodegradation of hydrolyzed polyacrylamide (HPAM) [77] via MFC. This investigation demonstrated that increased crude oil input favored HPAM biodegradation and carbon (C)/nitrogen (N) bioconversion. In addition, as the crude oil content increased, HPAM biodegradation and total nitrogen (TN) removal increased, reaching maxima of 90% and 92.1%, respectively. These findings have significant implications for promoting ecologically responsible management practices regarding oil-contaminated wastewater via MFCs and the development of innovative, cost-effective wastewater management techniques. This would contribute to reducing the environmental impact of industrial activities, particularly those in the oil and gas industry, and help preserve and safeguard the environment.

Wood waste, similar to oil products, is composed of various hydrocarbon-based inert polymer compounds, including lignin (which accounts for approximately 25% of the waste), as well as complex carbohydrates, such as cellulose (45%) and hemicelluloses (25%) [143]. Lignocellulosic biomass (LCB) is an abundant substrate, with a global annual yield of 1300 million tons. Among these compounds, lignin is the most crucial component because of its stability and unique properties. It has a complex, highly branched, and amorphous structure that endows it with thermoplastic qualities. Softening the wood demands special effort, i.e., elevating the temperature to approximately 100 °C; indeed, lignin decomposition requires temperatures of over 200–500 °C [144].

Catal et al. [59] demonstrated that all 12 monosaccharides and their derivatives extracted from lignocellulosic biomass are valuable resources for MFCs, leading to high power outputs. The power density ranged from 1240 to 2770 mW m⁻², with glucuronic acid yielding the highest values and mannose yielding the lowest values. In 2018, researchers in Warsaw [87], Poland, explored the use of industrial raw wastewater from the hydrothermal treatment of wood (WHTW) as a substrate for MFCs. They discovered that WHTW was a highly effective substrate, significantly increasing the power output of MFCs fed municipal wastewater (MWW) from 70 mW m⁻² to 360 mW m⁻² while also exhibiting a high chemical oxygen demand (COD) removal efficiency of 90%. Metagenomic analysis revealed differences in the microbial composition of the two wastewater sources: Thermoanaerobacterium clostridia and Gammaproteobacteria were dominant in WHTW, whereas Paenibacillus bacilli and Actinobacteria were the main genera in municipal wastewater.

Enhancing power generation and treating wood wastewater can be achieved through the utilization of the appropriate microbial consortium, substrate, and inoculum pretreatment. Before the implementation of MFCs, wood industry wastewater underwent thermal treatment at 45 °C, which resulted in a significant increase of approximately 0.33 W m⁻² or 1 A m⁻² current density, thus stimulating power production [88]. The temperature effect on a consortium, which adjusts growth conditions and regulates microbial composition, facilitates electricity generation and cellulose decomposition. The mixed fungal–bacteria syntrophic consortium Trichocomaceae sp.–Achromobacter insolitus–Geobacter sulfurreducens could play a crucial role in MFC performance [88].

Cellulose and chitin are low-cost, renewable substrates suitable for MFC applications. In the first report of cellulose degradation associated with energy production, Rezaei et al. [145] reported maximum power densities of 4.9 ± 0.01 and 5.4 ± 0.3 mW m⁻². The energy production of chitin and cellulose is influenced by particle size; smaller particles (0.28 mm) produced a power density of 272 mW m⁻², whereas larger particles (0.78 mm) achieved 176 mW m⁻². Takeuchi et al. [146] identified Cellulomonas fimi as the most effective microorganism for cellulose degradation, achieving a maximum power density of 38.7 mW m⁻², which was superior to that of Shewanella oneidensis MR-1 by 63%. The SC-MFC with a high-performance cellulose-based MEA electrode developed by Mashkour et al. [10] achieved a power density of 1790 mW m⁻², and thus approximately twice outperformed (920 mW m⁻²) a poly-tetrafluoroethylene-based gas diffusion electrode (GDE).

Thus, lignin-rich wood industrial waste and some hydrocarbon types, such as polyacrylamide, still remain insufficiently explored, but they hold great potential as substrates for generating chemical energy from wastewater via MFC technology. The complex and stable compounds present in oil and wood industrial wastewater, such as hydrocarbon-contaminated wastewater (i.e., polycyclic aromatic hydrocarbons (PAHs), crude oil, and wastewater from the hydrothermal treatment of wood), are also excellent substrates for MFC technology. These substrates could be effectively remediated while sustainably generating electricity and treating wastewater. There is a mutual relationship where the substrates can increase the power output of the MFC, and electrical stimulation can improve substrate conversion, with the optimal syntrophic microbial consortium being crucial for further enhancing MFC performance. However, the energy-intensive processes required for the hydrolysis of lignocellulosic biomass can diminish the overall value of the obtained results, as these processes contribute to higher costs and environmental impacts.

3.3. Industrial and Domestic Food Wastes: Potato, Mango, Banana, and Orange Wastes as Nonconventional Substrates for MFCs

Food waste has gained attention as an effective substrate for enhancing the performance of MFCs, contributing to renewable energy generation and wastewater treatment. In recent years, studies have shown that potato wastewater, which contains a high content of biodegradable OM, can serve as an important renewable energy source [147,148,149]. In addition to other substrates, such as glucose and butyrate, potato waste is a promising alternative for improving the performance of MFCs. Din et al. [150] conducted experiments to evaluate the power generation performance from potato waste. For that purpose, an SC-MFC was built with copper as the anode electrode and zinc as the cathode electrode. The potato wastewater was treated in terms of COD reduction, and a maximum voltage of 1120 mV was reached in the SC-MFC, with a COD removal of 40%. Later, Radeef and Ismail [151] used potato waste in a novel DC-MFC and reported higher volumetric power densities of 612.5–1012.5 mW m⁻³ when it was operated for 120 days. The DC-MFC was inoculated with 50 mL of anaerobic aged sludge, and the removal efficiency (p ≤ 0.05) of COD reached 99% during the study. More recently, researchers have evaluated the feasibility and synergistic effects of glucoamylase hydrolysis and MFCs on electricity generation from potato waste [152]. They constructed a DC-MFC with carbon paper as the anode and cathode electrodes (surface area of 25 cm²), separated by a PEM (42180 Nafion N-117 membrane, 0.180 mm thick). Both chambers had a total working volume of 1000 mL. The maximum power density increased by 64.51%, from 33 mW m⁻³ to 93 mW m⁻³, in the treated potato waste-feed MFC. These results demonstrated that potato waste hydrolysate can be used as feed waste to improve the performance of MFCs in terms of bioelectricity generation.

On the other hand, two separate studies were conducted to assess the potential of untreated potato waste as a substrate in MFCs. The first study, conducted by Sato et al. [86], involved the use of an SC-MFC with an air cathode, four bamboo charcoal (BC) plates as the anode, and Pt-coated carbon cloth as the cathode. The SC-MFCs were inoculated with potato-processing wastewater, and a maximum current density of 1140 mA m⁻² (at a maximum power density of 520 mW m⁻²) was achieved in one of the single MFCs in fed-batch mode. This performance surpassed that of previous studies, indicating that potato waste can serve as a viable alternative substrate to improve MFC performance in terms of power generation and wastewater remediation. However, further research should be conducted to explore the potential of potato waste as a substrate in MFC technology.

In the second study, conducted by Yaqoob et al. [68], potato waste was used in a benthic MFC (BMFC), which yielded a maximum current density of 36.84 mA m⁻² within 20 days. However, this performance was lower (82.28%; 208 mA m⁻²) than that achieved in a DC-MFC operated with potato waste as the substrate, as demonstrated previously [148]. Nonetheless, the BMFC effectively removed up to 84% of the OM and COD. The concentrations of OM and the bacterial community were found to be crucial factors in enhancing MFC performance in both studies.

There are limited data available on electricity generation in MFCs using food wastes, such as mangoes, bananas, and oranges, as the main drivers. Some studies have been conducted to improve the performance of MFCs using these substrates as the primary source of electron acceptance or organic substrates. For example, Rahman et al. [66] conducted a study in which glucose was used as the primary source of electron acceptance. The results showed that orange fruit waste was the best option to promote MFC technology, with a maximum output voltage of 357 mV (0.357 V), which was higher than the output voltage of the MFCs operated with banana and mango wastes. The MFC treatment of apple waste by employing bacterial and fungal cultures demonstrated the feasibility of using solid fruit waste in bioelectrical systems for energy recovery and organic removal in a sustainable way [105]. Another study by Rojas-Flores et al. [72] used avocado as a substrate for MFCs and achieved a maximum output voltage of 740 mV (0.74 V), which was 51.75% higher than the output voltage reported in Rahman and coworkers’ study [66]. Although these findings suggest that avocado is the best driver for improving the performance of MFCs compared with other fruit wastes, further research is needed to evaluate the practical feasibility of these fruit wastes for MFC applications.

3.4. Human Urine Domestic Waste as a Successfully Applied Real-World MFC Substrate

Human urine represents an attractive substrate for bioelectricity generation in MFCs because of (1) its abundant availability as a natural waste in human communities worldwide and (2) its unique properties. Owing to its high conductivity and diverse chemical composition, urine contains organic compounds, such as urea, uric acid, creatinine, and urobilinogen that can be utilized for energy output. Additionally, inorganic compounds at high concentrations, such as nitrogen and phosphorous, can be concentrated and recovered for further use [35,153]. The abundance of urine as a natural waste in human communities worldwide makes it a sustainable and accessible substrate for MFCs [154]. In 2012, Ieropoulos et al. and Kuntke et al. [155] reported the use of human urine as a feedstock for electricity production and nutrient recovery in MFCs, respectively.

Over the last decade, stacked urine-based MFCs have shown remarkable progress and improved performance in real-world applications. Researchers were even able to power a TI Chronos digital wristwatch by connecting two small-scale (6.25 mL) MFCs in series, replacing the need for 3 V batteries [156]. In fact, a complex platform, EcoBot-IV, which consists of twenty-four ceramic membrane-less MFC units supplied with urine, became the first device to charge a mobile phone [157]. By connecting MFC units in series and parallel, researchers were able to achieve a stable system with no cell polarity reversal, and the collective open-circuit voltage was 7.2 V, enough to charge a 3.7 V 1500 mAh Samsung GT-E2121B lithium-ion cell phone battery. After a 24 h charge cycle, the phone operated for 25 min, including a 6 min call and several messages.

In 2016, Ieropoulos et al. presented the latest discovery of Pee-Power-stacked MFC modules that utilized urine waste to power indoor lighting [158]. The modules were initially tested at the Bristol campus in the UK, where university lecturers and students conducted successful four-month trials. The urinal at the campus featured 288 MFCs (12 modules of 24 EcoBot-IVs) that generated an average of 75 mW and a maximum of 160 mW. When connected directly without supercapacitors, the lights reached 400 mW. A larger trial was conducted at the Glastonbury Music Festival at Worthy Farm, Pilton (UK), where the urine MFCs reliably provided electricity for indoor lighting for a large audience of approximately 1000 daily users. With 432 MFCs stacked in 36 MFC modules, each MFC produced nearly 1–2 mW of power output, leading to almost proportional scaling of energy output, with maximum values of up to 400 mW and 800 mW when the lights were connected directly. The COD removal rate was 95% for the campus urinal and, on average, 30% for the festival urinal, depending on the temperature conditions and the number of users. A year later, the Pee-Power system was reinstalled at that year’s Glastonbury Music Festival and achieved 37% greater COD removal [78]. PEP systems have also been successfully incorporated into the boarding schools of Kisoro (Uganda) and Nairobi (Kenya), providing autonomous power for lighting in toilet blocks [153].

In 2018, an improved version of the Urinal MFC was presented [78]. This upgrade generated up to 30% more energy while reducing the total system volume by a third. The new and improved version was capable of producing a maximum power of 600 mW and 48% COD removal. A subsequent study indicated that the unit’s membranes, known as self-stratifying MFCs (s-MFCs), could be scaled down even further to only 2 cm without any loss in performance [159]. Furthermore, a cascade of s-MFCs displayed better electrical performance when neat (100%) urine was used, surpassing ceramic cylinder-based MFCs (c-MFCs) [41]. Additionally, the cascades did not display a significant decrease in performance for up to one week, even when fed with reduced feedstock concentrations of up to 75%. Recent progress in urine-based MFCs is summarized in Table 2.

Therefore, autonomous sanitation MFCs that convert organic urine into electricity to power lights while simultaneously providing urine treatment and nutrient recovery are highly valuable in modern society. They are especially valuable in remote locations, campsites, national parks, and developing countries without sewage systems. MFCs play a crucial role in advancing toward circular and zero-waste economies.

The exploration of nonconventional substrates for MFC technology, as discussed in the present chapter, reveals significant potential for both energy generation and wastewater treatment. For example, utilizing industrial waste from oil and wood, along with food waste and human urine, showcases the versatility and effectiveness of MFC technology in tackling environmental challenges while simultaneously producing eco-friendly and renewable energy. Our review paper’s findings indicate that hydrocarbon-contaminated wastewater and lignocellulosic biomass, such as wood waste, can serve as valuable substrates for MFCs. The capacity of hydrocarbon-degrading microorganisms to break down complex and stable compounds, combined with the electrical stimulation provided by MFCs, enhances the efficiency of hydrocarbon decomposition and energy recovery. Additionally, the rich composition of wood waste—especially lignin and cellulose—presents opportunities to optimize power generation through carefully tailored microbial consortia and substrate pretreatment.

Moreover, food waste, particularly from such sources as potatoes, mangoes, bananas, and oranges, has emerged as a promising alternative substrate for MFCs. The successful application of urine-based MFCs in various settings—from powering small electronic devices to providing lighting in public facilities—demonstrates the practicality and scalability of this technology. The ability to recover nutrients while generating electricity positions urine MFCs as a valuable solution for addressing sanitation and energy needs, especially in resource-limited environments.

In summary, integrating these nonconventional substrates into MFC technology not only contributes to renewable energy generation but also fosters sustainable waste management practices. Nonetheless, future research should concentrate on scaling these technologies, enhancing their efficiency, and exploring additional substrates to further improve the viability and applicability of MFCs across diverse contexts.

3.5. Agricultural Wastes

3.5.1. Livestock Urine Waste as an Essential Nonconventional Renewable Substrate for MFCs

Farming waste, including livestock urine from cows, goats, sheep, and pigs, may not appear to have much value, but it can actually be a valuable bioresource for MFC technology. While human urine has been studied extensively in MFCs, cow urine has not received as much attention. Studies from 2012–2020 on salt-bridge MFCs using cow urine reported high internal resistance, making it challenging to achieve maximum urine treatment efficiency [160,161,162]. However, in 2014, researchers successfully generated power from elephant urine, demonstrating the potential of using animal urine in MFCs [163]. In 2016, cow urine was tested in a clayware MFC for the first time, and the results revealed a significantly higher power output (5.23 W m⁻³) than MFCs fed raw urine [81].

As the livestock industry is a significant consumer of antibiotics, researchers are also interested in how typical animal antibiotics, such as salinomycin, affect bioelectricity generation. In 2020, Cheng et al. demonstrated that salinomycin increased power and current density by 63.9% and 28.1%, respectively, in Rhodococcus pyridinivorans MFC with a phosphate buffer solution simulating livestock sewage [164]. This increase in power-output performance is likely due to the facilitation of channel protein activity by antibiotics, which promotes electron transfer. However, further research on actual sewage waste is necessary for the practical application of urine-based MFCs.

The use of antibiotics in animal husbandry can result in the removal of antibiotic resistance genes (ARGs) and mobile genetic elements (MGEs) from livestock sewage. Livestock wastewater contains high levels of resistance genes (i.e., 137 ARGs and 9 MGEs) and poses a risk to health and the environment [165]. The treatment of livestock wastewater with MFCs is an effective way to reduce horizontal gene transfer and decrease antibiotic toxicity in the environment. Compared with traditional biological treatment, bioelectrical stimulation can result in more extensive metabolism and antibiotic removal [165,166].

The development of a plant (P)-MFC with a self-sustainable autofeed from livestock urine waste was reported for the first time by Apollon et al. [82] (Figure 3). The P-MFC was constructed with a clay cup embedded with a perennial plant, Stevia rebaudiana, and has been successful in recovering nutrients associated with energy generation. The P-MFC system powered by goat urine waste reached a maximum power output of 46.97 ± 0.67 mW m⁻². Moreover, the P-MFCs inoculated with cow urine were more successful at efficiently recovering NH₄⁺–N (94%), PO₄³⁻ (98%), and K⁺ (33%) than the P-MFCs inoculated with goat or sheep urine. This study demonstrates the potential for using livestock urine to generate bioelectricity [81] and promote sustainable development in agriculture [167].

In the study by Apollon and coworkers, they built an MFC using graphite felt as the anode and a zinc sheet as the cathode, which was separated with a novel vertical plug-in ceramic stick as the PEM. The plant-based MFCs were operated with a concentration of 150 mg/L ammonium nitrate (NH₄NO₃) per week for 30 days. The application of NH₄NO₃ resulted in a 94% increase in bioelectricity production and a 51.97% increase in plant growth [169]. Plant species and microbial communities play crucial roles in MFC performance [170]. Furthermore, microorganisms are paramount in the root environment when plant species are used in BES technology [171]. Therefore, plants, including root exudates, as well as the root system, which can be hydrolyzed to nutritious compounds, can be considered a significant source for feeding electroactive microorganisms [22]. Compared with plants in less productive systems, those in highly efficient P-MFCs possess a denser and more developed root system that results in greater levels of root exudation and oxygen release [172,173,174].

3.5.2. Farm Manures

Manure is an organic substrate that has been studied for its potential to enhance the performance of MFC technology. This substrate is made up of a variety of sources, including animal feces, wasted feed, water, and livestock bedding [175]. It is a carbon-rich source that contains essential macronutrients, such as nitrogen, phosphorus, and potassium (NPK). In MFCs, manure is utilized to generate electrical energy and recover nutrients simultaneously. In a recent study, Syed et al. [176] developed a membrane-less 100 mL SC-MFC using two types of animal manures, namely cow and buffalo dung. The results showed that the pretreated buffalo dung generated a maximum power density of 12.75 mW m⁻², with maximum COD and BOD removals of 80% and 87%, respectively. The MFC utilizing cow dung compost demonstrated a similar power output of 17.84 mW m⁻² [177]. However, this value was lower than that reported for a novel single-chamber zinc–carbon electrode MFC inoculated with cattle dung, which achieved the highest bioelectricity production, with a power density of 1465 mW m⁻² at a current density of 1858 mA m⁻² [75]. An MFC treating animal manure wastewater generated 160 W/m³, which was sufficient to power hydrogen production at a rate of 0.45 ± 0.03 m³ H₂/m³·day in a solar-driven photoelectrochemical cell [178]. Adegunloye et al. [179] from Nigeria developed a DC-MFC using cow dung, demonstrating an average daily power output of 3.11 mW. This power was sufficient to illuminate LED bulbs requiring a minimum of 2 mW for operation, with visibility maintained at 30 m even during daylight hours. The authors propose that this system could be applied to power road traffic lights, leveraging renewable energy from available manure sources. Overall, manure-fed MFCs provide a cost-effective alternative for exploring the low power efficiency of future MFC technologies.

In summary, considering the above discussion, we highlight the innovative potential of agricultural wastes, especially livestock urine and manure, as valuable nonconventional substrates for MFC systems. We also emphasize the underexplored utility of cow urine in MFC technology, contrasting it with the more extensively studied human urine and showcasing significant advancements in power generation from various animal urines, as described in Table 2. Additionally, we point out the dual benefits of using manure as a carbon-rich substrate, which not only generates bioelectricity but also facilitates the removal of organic pollutants and the recovery of essential nutrients for sustainable agriculture. Our review paper’s findings suggest that integrating MFC technology with agricultural practices can promote sustainable development, reduce antibiotic resistance in livestock wastewater, and provide renewable energy solutions, such as powering remote devices and charging smartphones. However, further research on practical applications of these technologies to enhance agricultural sustainability and energy efficiency is still lacking.

4. Application of Power Management Systems in MFCs

The application of power management systems (PMSs) has become increasingly important in advancing MFCs. To date, only a limited number of studies have been conducted on the practical implementation of MFCs in real-world scenarios, as described in the literature [39,180,181,182]. Hence, MFCs have attracted the attention of researchers worldwide because of their potential to generate energy. However, voltage reversal can occur during the operation of MFCs, which significantly affects electricity production by causing bioanode corrosion [183]. To resolve this issue, researchers have implemented/designed PMSs, which serve as boosters to improve MFC performance and prevent voltage reversal [184]. Figure 4 displays the highest power density reached in earlier studies from both conventional and unconventional sources.

Recently, various researchers have examined the practical use of MFCs by designing different PMSs to power remote devices, such as biosensors, the Internet of Things (IoT), and wireless devices [185,186]. Additionally, our previous study [187] evaluated a PMS by connecting a 12-MFC in series. The MFCs were operated with Opuntia ficus-indica plants in a semidesert environment, and the test lasted for one week. Throughout the week, the implementation of the PMS increased the voltage from 0.5 V to 3 V, successfully powering an LED and a digital clock. Notably, as the output voltage decreased by 5%, the devices continued functioning. In this study, external energy sources were not used to improve the performance of MFCs compared with other studies. Twelve MFCs were connected only in series to power the above remote devices. In another study, a similar strategy was explored. Modular stacking through serial connections enhanced the open-circuit voltage (OCV) by 1.88–2.90 times when 2–3 MFC units were connected. As a result, parallel-series combinations with two and three modules of the MFC operating with H. soleirolii and O. basilicum provided power for a 1.5 V, 4.3–105.2 mkA thermometer–hygrometer and a 3 V, 10.1–36.4 mkA weather station during the year of the experiment [188].

On the other hand, Mukherjee et al. [189] studied the effectiveness of PMSs in stacked MFCs for onsite applications. A stacked MFC has the following characteristics: series or parallel connections of several electrodes or MFC units. This type of arrangement aims to improve the current or output wattage of the MFC, depending on the applications that one wants to carry out to produce bioelectricity. In this case, the PMS plays the role of a booster, aiding the MFC in harvesting energy from the waste while improving its degradation. A previous study reported that the modularization of 130 MFC units with a series and parallel arrangement of 13 MFC units could treat 144 L of domestic wastewater per day [190].

In summary, on the basis of the results discussed above, we concluded that the use of PMSs in MFCs is a reliable alternative for enhancement and practical applications. These systems play crucial roles in enhancing energy output and ensuring stable performance, making MFCs more viable for applied usage. However, more studies are still needed to improve this technology. This section aims to emphasize the use of PMS as the best alternative to enhance MFCs in terms of efficiency and practical use. Furthermore, to achieve this objective, various factors, such as design, electrode materials, and membrane type, must be considered before evaluating MFC performance.

5. Challenges and Future Perspectives

Table 1 and Table 2 show that significant efforts have been made to advance the practical applications of MFC systems in the last two decades, leading to a notable extension in their efficiency range. However, since their implementation, MFC technologies have faced several challenges. To overcome or minimize these challenges, researchers have conducted numerous studies to optimize these technologies. The performance of MFCs was found to depend on various biological, biochemical, and design factors, including electrode materials; the type of membrane; operating conditions and parameters; and the substrate, microorganisms, and their metabolic pathways; electron transport mechanisms; and electron acceptors. For a comprehensive understanding of the topic, readers can refer to [2].

5.1. Sustainable Substrates and Adaptation to Small-Scale Applications

The advancement of MFC technology for the utilization of sustainable organic waste substrates is a significant future direction, addressing the limitations of traditional substrates and waste streams. Key challenges include the following.

Environmental impact and energy costs: The processing of conventional organic substrates and wastes often demands energy-intensive methods and the use of aggressive chemicals, contributing to a carbon footprint of up to 3140 kg CO₂ eq. [101]. This undermines the environmental sustainability of MFC technology.

Complex pretreatment requirements: Many waste streams require intricate pretreatment before integration into MFCs, such as the hydrolysis of wood hydrolysates, fermentation of brewery byproducts, or heavy metal sorption from dye-containing wastewater. These steps complicate scalability and economic feasibility. For example, the environmental and economic impacts associated with monosaccharides extracted from lignocellulosic biomass reduce their attractiveness as a resource for the MFC, notwithstanding the high power outputs of up to 2770 mW m⁻² [59].

High volumes of waste from industrial sources: Typical industrial processes generate substantial waste quantities that far exceed the processing capacity of current MFC prototypes (typically 0.1–1 L in volume). Indeed, the daily outputs of dairy and brewery waste can range from 50,000 to 500,000 L [106,107], necessitating alternative solutions for effective waste management.

In contrast, small-scale farms and households offer a more feasible entry point for MFC implementation, which aligns with current technological capabilities. For example, potato farms with an annual yield of 10,000 tons produce approximately 2000 tons of waste, which is suitable for MFC processing. Avocado farms that produce 1000 tons of fruit per year generate 200–300 tons of waste. Small households with 1 cow, 1 pig, and 20 chickens generate approximately 31.6–48.9 tons of manure annually, representing manageable waste volumes for small-scale MFC systems. The development of MFCs tailored to such small-scale applications is essential to bridge the gap between prototype designs and broader, more impactful utilization in diverse contexts.

5.2. Avocado, Petroleum, Urine, and Manure Waste as Valuable Bioresources

The pursuit of sustainable and renewable energy sources has intensified research into the utilization of nontraditional organic wastes as substrates for MFCs. This approach not only supports circular economy principles but also opens avenues for innovative energy systems driven by resource recovery.

Fruit and vegetable waste is increasingly recognized as a valuable resource for MFCs because of its high organic content, biodegradability, and widespread availability. These substrates not only facilitate sustainable waste management but also offer the potential for green energy generation with minimal energy-intensive preprocessing. Nontraditional substrates, such as potato and avocado waste, have shown remarkable promise. Notably, an MFC exploiting potato waste achieved no less power (576 mW m⁻²) [86] than acetate and xylose, which have demonstrated some of the highest MFC power outputs among organic substrates, with values reaching 506 mW m⁻² and 673 mW m⁻² [46,69], respectively, but are accompanied by energy-consuming processing and chemical use, which weaken their benefits.

However, the power yield of the avocado-MFC is among the highest reported values of energy output, delivering a record-breaking energy output of 5736 mW cm⁻² ([72], which makes it one of the most promising nontraditional waste substrates yet tried. Additionally, an MFC exploiting vegetable wastes, including cabbage leaves, potato peels, cucumber peels, and carrot waste, surpasses all previously reported values of energy output, reaching 88,990 mW m⁻² [44]. This intriguing finding underscores the untapped potential of solid fruit waste as an alternative MFC substrate. Owing to their environmentally friendly nature and low-energy requirements, vegetable and fruit wastes offer a sustainable, accessible, and efficient solution for bioelectricity generation, emphasizing their importance as future bioresources for energy systems.

Farming and household waste, such as livestock urine and manure, as well as human urine, can be another promising bioresource for MFC technology. This is essentially yellow gold for low-cost MFC bioelectricity generation, as aptly noted by Jadhav et al. [81] regarding cow urine. The MFC inoculated with cattle dung reached high bioelectricity production with a power density of 1465 mW m⁻² [75]. The MFC utilizing livestock urine achieved a power output of up to 5230 mW m⁻³ [81]. Long-term applications of MFCs utilizing human urine in Pee-Power systems have been successfully integrated into the lighting of toilet blocks and are excellent examples of the practicality and potential for urine waste in MFCs. Their utilization can address waste management challenges while generating energy. By integrating such resources, MFCs contribute significantly to the development of circular and zero-waste economies, fostering sustainable energy solutions worldwide.

Petroleum refinery wastewater is another encouraging nonconventional substrate for MFCs. MFCs utilizing hydrocarbon-containing petroleum sediment achieved one of the highest reported maximum power densities of 50,570 mW m⁻² [42]. Compared with traditional disposal methods, stable and hard-to-degrade compounds in hydrocarbon-containing wastewater are excellent substrates for MFC technology and can be combined in a more environmentally friendly way with refinery wastewater.

Thus, the exploration of nonconventional substrates in MFC operation is still in its early stages, and further research is needed to assess their potential for large-scale and practical applications. Nevertheless, the progress made and the high bioelectricity outputs achieved, comparable to the highest power yields from traditional waste (8314 mW m⁻²–25,195 mW m⁻² from crude sugarcane effluent and raw distillery effluent), suggest promising advancements and future potential in the field of sustainable energy.

5.3. Metabolic-Engineered Microorganisms

Biofilm formation on the anode surface is another crucial parameter for the efficient performance of MFCs because of their electrogenesis and substrate bioconversion metabolic activity. The optimization of microbial communities in MFCs is an essential focus area, as the activity and diversity of microorganisms directly influence electron transfer efficiency and overall system performance.

The application of metabolic-engineered microorganisms is another intriguing way to improve the performance of MFCs. The power density of MFCs with engineered E. coli has markedly increased from 127 to 806 mW m⁻² [21]. The peak voltage output was increased significantly by 361% through the use of genetically modified microbes [140]. Using metabolic engineering, Saccharomyces cerevisiae resulted in a 17.3% increase in energy generation [191]. These advancements highlight the potential of genetic and metabolic engineering in tailoring microorganisms to maximize both energy production and substrate utilization in MFCs.

Another strategy involves the use of a variety of microbial sources of natural origin for inoculating MFC reactors. For example, the addition of a highly metabolically active consortium from rumen microbes from slaughterhouse wastewater has led to significant improvements and reached 8490 mW m⁻² in MFCs utilizing paddy straw [43]. The integration of both engineered and naturally sourced microbial communities represents a promising approach, paving the way for innovative and sustainable advancements in MFC technology.

5.4. Incorporation of Green Electrodes, Nanomaterials, and Plants into MFC Technology

The most significant challenge for the development of MFCs is their low electricity production, which may be a result of the high internal resistance of the system. To overcome the low-power issue of MFCs, appropriate electrode materials must be chosen to maintain stable system operation [192]. The development of novel, low-cost, and sustainable electrode materials with high stabilities, conductivities, and advanced power management systems should also be the focus of future MFC research.

Another promising approach is the development of efficient and environmentally friendly electrodes. Biobased or so-called “green” electrodes manufactured from biomass waste, such as sugarcane bagasse [193], onion peels [194], pomegranate peels [18], coconut husks [195], and coffee waste [196], are gaining attention as sustainable alternatives to conventional electrode materials. These materials not only offer an eco-friendly solution but have also been reported to enhance MFC performance.

The incorporation of nanomaterials into MFC technology is an intriguing future direction for the field. Nanomaterials increase the surface area of the anode, promote biocompatibility and biofilm formation, increase proton transfer rates via improved chemical and physical properties of the membranes, and facilitate oxygen reduction reactions (ORRs) on the cathode, contributing to both electricity generation and wastewater treatment [197]. As reported, nanoscale chitosan enhances BES performance, resulting in a threefold increase in power generation [17].

The use of plant species in MFCs is another crucial strategy that benefits continuous electricity production [198]. Additional and continuous MFC feeding with photosynthesis-derived substrates through root excretion, as well as radial oxygen loss and evapotranspiration, maintains an active bacterial community on the anode surface, reducing the internal resistance of MFCs and affecting the redox potential, thereby increasing power generation [199].

Therefore, the integration of novel biobased green electrodes, nanomaterials, and plants into MFC technology offers transformative potential for addressing the low power output and environmental challenges in MFCs. These advancements not only improve system efficiency and sustainability but also pave the way for innovative applications in energy generation and wastewater treatment.

6. Conclusions

MFCs are eco-friendly power generation devices that decelerate the overexploitation of fossil fuel reserves and do not require any additional energy. However, the use of substrates or wastes requiring energy-intensive processing and aggressive chemicals, which contribute to greenhouse gas emissions, can undermine the advantages of MFCs as a green technology. Therefore, the environmental impacts of MFC technologies must be considered crucial factors in the exploration of next-generation MFCs. Hence, expanding the range of feedstock sources for MFCs to include globally available, cost-effective resources with low carbon footprint processing, such as nonconventional fruit and vegetable waste, slaughterhouse wastewater, livestock and human urine, and manure, as well as reducing the energy consumption associated with traditional substrates, holds great promise for increasing the sustainability, scalability, and efficiency of bioelectricity generation while addressing waste management challenges. MFCs utilizing nonconventional substrates have demonstrated comparable or even superior power outputs to those of conventional substrates, achieving the highest power density of 88,990 mW m⁻² when household vegetable wastes are used as substrates. The integration of MFCs into small-scale farms and households, where human and livestock urine, manure, and food residues constitute the primary waste streams, is particularly essential at this stage of development of technology, which is capable of effectively operating with waste produced in low volume. The incorporation of metabolic-engineered microorganisms, plants, nanomaterials, and green electrodes is a critical priority and offers significant potential for improving the efficiency and sustainability of MFC technologies throughout the whole life cycle from cradle to gate.