The Potential of Microbial Fuel Cells as a Dual Solution for Sustainable Wastewater Treatment and Energy Generation: A Case Study

Abstract

Microbial fuel cells (MFCs) are bio-electrochemical systems that harness microorganisms to convert organic pollutants in wastewater directly into electricity, offering a dual solution for sustainable wastewater treatment and renewable energy generation. This paper presents a holistic techno-economic and environmental feasibility assessment of large-scale MFC deployment in Dhaka’s industrial zone, Bangladesh, as a relevant case study. Here, treating 100,000 cubic meters of wastewater daily would require a capital investment of approximately USD 500 million, with a total project cost ranging between USD 307.38 million and 1.711 billion, depending on system configurations. This setup has an estimated theoretical energy recovery of 478.4 MWh/day and a realistic output of 382 MWh/day, translating to a per-unit energy cost of USD 0.2–1/kWh. MFCs show great potential for treating wastewater and addressing energy challenges. However, this paper explores remaining challenges, including high capital costs, electrode and membrane inefficiencies, and scalability issues.

1. Introduction

Globally, 380 trillion liters of wastewater are generated every year [1]. In developed countries, about 70% of wastewater is treated, while this figure drops significantly to just about 8% in the least developed countries [2]. Bangladesh’s agricultural economy has recently seen significant industrial growth, particularly in sectors like textiles. This expansion has increased energy demand and exacerbated environmental impacts, including wastewater management challenges [3,4]. According to the Department of Environment (DoE), only 1773 effluent treatment plants (ETPs) were installed by 2019, and most are not fully operational [5]. In 2015, the country produced an estimated 456 million m³ of wastewater, with only 6% treated [6]. By 2021, wastewater generation reportedly reached 1733.5 million m³, yet only 0.01% was treated [7]. Improperly treated industrial wastewater pollutes major rivers, leading to drinking water contamination and other environmental problems [8].

The urgency of addressing wastewater pollution in Bangladesh is compounded by the country’s growing energy crisis. Bangladesh’s energy sector is heavily dependent on natural gas, which accounts for nearly 64% of the country’s total energy consumption [9]. However, domestic gas reserves are depleting, while the country faces increasing energy demand, projected to grow by 10% annually [10]. The current power demand of Bangladesh is approximately 25 GW, growing steadily at a rate of around 7% annually, and is expected to exceed 61 GW by 2041 [9,11]. This demand is primarily met by consuming fossil fuels, with about 39.5% coming from natural gas, 21.39% from coal, and 20.87% from oil, while only 4.47% is from renewable sources such as hydro and solar [12]. To meet rising demand and transition to a sustainable energy future, Bangladesh launched the ‘Vision 2041’ plan. This plan includes expanding renewable energy sources like solar and wind and developing waste-to-energy (WtE) technologies [13]. The target is to generate at least 10% of energy from renewables by 2041, including an estimated 2 GW from WtE [14].

Integrating renewable energy solutions into wastewater treatment processes offers an attractive opportunity to simultaneously address environmental and energy challenges. On the other hand, hydrogen production has gained significant attention for supporting various renewable energy scenarios [15,16,17]. Its high energy content (141.8 MJ/kg heating value) and potential for zero in-operation carbon traces make it suitable for large-scale industries and the transport sector [18]. This is while hydrogen production methods may not be emission-free. For example, some conventional methods, such as steam methane reforming, are energy-intensive and produce substantial carbon emissions [19,20,21].

MFCs use microorganisms to decompose the organic matter contained in the wastewater and produce electricity as a byproduct. The microbes in the anodic chamber oxidize organic compounds, releasing electrons, which are transferred to the cathode via an external circuit, thus generating electricity. This technology can transform wastewater from a disposal problem into an energy source while also offering an environmentally friendly treatment method. MFCs provide an attractive choice for the treatment of wastewater and the generation of energy in Bangladesh [22]. MFCs can also reduce the chemical oxygen demand (COD) and remove harmful contaminants [23]. Studies have demonstrated that MFCs can achieve COD removal efficiencies of up to 99% while generating power densities ranging from 0.222 W/m² to 1.089 W/m², depending on the substrate and operational conditions [24]. In Bangladesh, the textile, leather, and chemical industries produce large volumes of organic-rich wastewater that could be ideal for MFC operations. Thus, MFC technology in Bangladesh could help mitigate industrial pollution and meet national clean energy targets.

Earlier studies on MFCs have extensively explored their potential in wastewater treatment and energy generation. Khan et al. [25] reviewed microbial electrolysis cells (MECs) for hydrogen production and wastewater treatment. They emphasized the energy generation potential from wastewater treatment in Saudi Arabia. Boas et al. [26] discussed the industrial applications of MFCs, including their configuration and cost, and highlighted the challenges associated with this technology. Mohyudin et al. [27] analyzed parameters affecting the performance of MFCs and underlined the point that the technology needs to be developed further. Kouam Ida and Mandal [28] reviewed MFC fundamentals and energy generation potential, focusing on components and design principles but offered limited discussion on industrial-scale applications. Roy et al. [29] provided a detailed exploration of MFC applications in sustainable wastewater treatment and identified key process parameters influencing efficiency. The study discussed the potential of MFCs to be used in wastewater treatment plants. Bazina et al. [30] also discussed the role of microbial catabolism in electricity generation from wastewater. Their study emphasized substrate oxidation, electron transfer, and internal resistance as critical factors determining MFC performance and touched on alternative cost-effective MFC models, particularly membrane-less designs. Meylani et al. [31] focused on the biodiversity of microbial consortia in MFCs, showing how mixed microbial communities enhance electron transfer efficiency compared to pure cultures. Unuofin et al. [32] examined MFCs and argued their critical role in shaping a sustainable future. They also indicated the scalability and performance limitations of MFCs, similar to other studies mentioned so far. Apollon et al. [33] explored the advancements of MFCs in treating agro-industrial wastewater, offering insights into pollutant removal mechanisms through bioelectrochemical processes. The study comprehensively examined MFC configurations and performance limitations, advocating for further research to overcome technological barriers and enhance commercial viability. Ali et al. [34] reviewed the role of nanotechnology in enhancing MFC performance, focusing on anode material optimization, surface modifications, and electrochemical improvements. The study emphasized MFCs’ dual function in wastewater treatment and energy generation while highlighting economic feasibility and real-world applicability. However, it noted challenges in large-scale implementation, the long-term stability of nanomaterials, and the need for cost-effective fabrication techniques, suggesting further research on scalable and durable nanostructured electrodes. While previous studies [25,26,27,28,29,30,31,32,33,34] examined MFC components, configurations, and microbial mechanisms or offered general reviews, few provided detailed, quantified case studies. Specifically, the techno-economic feasibility of large-scale MFC implementation in urban–industrial settings within developing countries like Bangladesh has been largely overlooked.

Prior reviews have typically addressed technical, environmental, or economic factors in isolation and often lacked the contextual specificity needed for real-world deployment. While previous studies have examined MFC components, configurations, and microbial mechanisms or offered general overviews, few have provided detailed, quantified case studies geared toward large-scale implementation, particularly in urban-industrial settings within developing countries like Bangladesh. This study offers an innovative and methodologically distinct contribution by constructing a comprehensive, applied feasibility model specifically tailored for Dhaka, Bangladesh. It integrates critical technical, economic, and environmental dimensions into a single evaluative framework. The authors quantify energy recovery potential, estimate detailed capital and operational costs, and assess economic viability through per-unit energy cost comparisons with the regional energy market.

By focusing on a specific, high-potential industrial zone, this research offers novel insights beyond generalized discussions. It illuminates the practical challenges and opportunities associated with integrating MFCs into existing wastewater treatment infrastructure in a resource-constrained, rapidly developing urban environment. Ultimately, this paper contributes to sustainable wastewater management and renewable energy discourse, particularly for developing nations confronting similar environmental and energy challenges.

2. Materials and Methods

2.1. Overview of Industrial Wastewater and Sludge Pollution

Bangladesh’s rapidly expanding industrial sector creates significant challenges in managing industrial wastewater and sludge. The primary industries contributing to this pollution include textiles, tanneries, pharmaceuticals, chemicals, and more [35]. These industries collectively generate approximately 378 million m³ of wastewater annually. Often discharged without adequate treatment into rivers, canals, and farmlands, it poses serious risks to human health and the environment [36]. On a per-capita basis, Bangladesh generates approximately 30.9 cubic meters of wastewater annually for its population of 170 million. In comparison, developed countries, such as Germany, the UK, and France, have significantly higher total wastewater production, averaging 92 cubic meters, 92 cubic meters, and 66 cubic meters per person annually, respectively [37]. Among developing countries, India produces approximately 4.3 cubic meters of wastewater per person annually [38]. This comparison reveals Bangladesh’s per-capita wastewater generation is lower than in industrialized nations and India, suggesting relatively low per-capita industrial activity. However, it highlights the importance of proper wastewater management given the country’s increasing industrial development and environmental issues.

The increasing population and industrial development have increased the requirement for water and wastewater services. The urban population has doubled over the last three decades, placing immense strain on existing water and wastewater infrastructure. Currently, only about 20% of Bangladesh’s urban population is connected to a sewer system, while the remaining population relies on septic tanks, some of which are improperly maintained [3]. The rapid depletion of groundwater and reliance on untreated wastewater have caused serious environmental and public health issues. Moreover, the energy required for wastewater treatment is substantial, with the existing infrastructure relying primarily on fossil fuels. Energy recovery projections, using technologies like anaerobic digestion and micro hydropower plants, estimate potential outputs from 976 GWh to 1986 GWh per year [39]. This highlights the need for alternative wastewater treatment solutions that can simultaneously address energy challenges.

Figure 1 provides a summary of the wastewater generation from key industries in Bangladesh. The average COD of wastewater across industries is 3000 mg/L, which is far exceeding the safe discharge limit set by the DoE in Bangladesh [40]. The textile industry is the largest contributor as it generates approximately 288 million cubic meters of wastewater annually. The COD values of this industry range from 1200 to 1750 mg/L [41]. The leather industry, concentrated in the Hazaribagh area of Dhaka, contributes 30 million cubic meters of sludge and wastewater annually, with a COD value of 1610–1750 mg/L [42]. This wastewater is particularly hazardous due to the presence of heavy metals like chromium, cadmium, and lead, which pose severe risks to both ecosystems and human health [43]. The pharmaceutical sector, although it produces only 1 million m³ of sludge annually, lacks a comprehensive wastewater management policy, leading to uncontrolled sludge disposal in farmlands and rivers [44].

Similarly, the chemical and plastic industries contribute around 7 million cubic meters of wastewater annually, and their COD value is highest, ranging from 2912 to 5239 [45]. The pulp and paper industry produces approximately 25 million m³ of sludge every year, which is 7% of the total amount. Most of this sludge is dumped on farmlands, which causes soil degradation and fertility loss [46]. This emphasizes the need for sector-specific regulations so that proper treatment and disposal practices are ensured. Wastewater flowing from various sources and the sludge management process in Bangladesh are shown in Figure 2.

In Bangladesh, the high organic content in industrial wastewater, especially from textile, tannery, and food processing industries, offers a significant opportunity to apply MFC technology. Bangladesh’s industrial wastewater contains enough organic matter to generate electricity [47]. MFCs have demonstrated considerable improvement in energy recovery efficiency, making them a viable technology for addressing both energy shortages and wastewater treatment challenges [48]. Introducing MFC technology could reduce the country’s reliance on fossil fuels by converting wastewater into a renewable energy source.

2.2. Regulatory Framework and Standards

Bangladesh has established several policies and regulations to manage industrial wastewater and sludge. The Environment Conservation Rules of 1997, under the DoE, set effluent quality standards that industries must comply with before discharging wastewater into the environment [40]. However, enforcement of these regulations remains a challenge, and many industries continue to discharge untreated or partially treated effluents. While there are specific policies in place for major industries, like textiles and tanneries, other sectors, particularly pharmaceuticals, lack comprehensive regulations, resulting in significant environmental pollution. The quality standard limits for various parameters in industrial wastewater and sludge are illustrated in Figure 3.

There are various policies for managing industrial waste, but the policies are not properly enforced. Thus, the environment of Bangladesh is continually degrading, with added health risks to the residents. Strict enforcement of existing regulations, development of new policies for unregulated sectors, and incorporation of sustainable wastewater treatment technologies can address these challenges successfully.

2.3. Composition and Impact of Industrial Wastewater and Sludge

Wastewater and sludge from industries in Bangladesh carry a range of harmful substances, including heavy metals, organic compounds, dyes, and other toxic chemicals. The way these substances degrade and their effects on human health, ecosystems, and farmlands are discussed below:

• Heavy Metals: Tanneries and chemical plants dump heavy metals like chromium (Cr), lead (Pb), mercury (Hg), cadmium (Cd), and zinc (Zn) [43]. These are dangerous for marine life and can also build up in the food chain, creating cancer and neurological disorders in humans [49]. A study on sediment samples from the Dhaleshwari River, near a newly established tannery industrial estate in Savar, Bangladesh, revealed significant concentrations of heavy metals, with Cr levels ranging from 14.8 to 748 mg/kg and Pb concentrations between 2.38 and 21.1 mg/kg. The study also revealed that Cr and iron (Fe) had the highest concentrations, and approximately 75% of the samples were low to severely polluted by Cr [50].
• Organic Compounds: Organic pollutants, including pharmaceutical residues, pesticides, phenols, and volatile organic compounds (VOCs), are often found in wastewater from pharmaceuticals, textiles, and food industries. These compounds contribute to the depletion of dissolved oxygen in water bodies, leading to the death of aquatic organisms and the disruption of ecosystems [51].
• Dyes and Chemicals: The dyeing industry releases large quantities of synthetic dyes, including azo dyes, reactive dyes, and sulfur dyes, into water bodies, causing severe water pollution. These dyes are often non-biodegradable and can have carcinogenic and mutagenic effects on humans [52]. Studies show that effluent from Bangladesh’s industries often has elevated electrical conductivity, with values reaching up to 1.93 mS/cm, indicating high concentrations of dissolved chemicals [53].
• Nutrients and Salts: Industrial wastewater can also contain high levels of nitrogen (N), phosphorus (P), ammonia (NH₃), and salts (such as chlorides (Cl⁻) and sulfates (SO₄²⁻)), which contribute to eutrophication in water bodies, resulting in harmful algal blooms and oxygen depletion [54]. For example, total dissolved solids (TDSs) are found to be significantly higher at Bangladeshi industrial discharge points, correlating with the high nutrient and salt content in these effluents [53].
• Sludge: Sludge from various industries, when dumped on farmlands, can lead to soil contamination, reducing soil fertility and affecting crop yields [55]. The presence of toxic chemicals such as arsenic (As) and Cd, which were measured at levels as high as 0.98 mg/L and 1.90 mg/L, respectively, in some regions, poses a risk of entering the food chain, leading to long-term health issues [53,56].

3. Results of MFC Potential Analysis

3.1. Basic Components and Working Principles

MFCs are innovative bio-electrochemical systems that utilize the metabolic activity of microorganisms to convert chemical energy stored in organic substrates directly into electrical energy [57]. This dual-function process not only provides an efficient method for wastewater treatment by breaking down pollutants but also enables the simultaneous generation of electricity and hydrogen as valuable byproducts [58,59].

A typical MFC consists of two main compartments, an anode and a cathode, separated by either a proton exchange membrane (PEM) or an ion exchange membrane. This membrane allows only protons or hydroxyl ions to pass, thereby preventing compartment mixing and ensuring maximum power output [60]. A schematic representation of a simple dual-chamber MFC is illustrated in Figure 4.

The anode chamber in an MFC operates under anaerobic conditions, promoting the growth of electroactive bacteria, such as Geobacter sulfurreducens, which oxidize organic pollutants in wastewater. During this process, electrons and protons are released. The electrons travel through an external circuit to the cathode, generating an electric current, while the protons pass through the PEM to the cathode chamber, where they combine with oxygen to form water. This mechanism not only purifies wastewater but also captures energy in the form of electricity. Moreover, MFCs function similarly to traditional polymer electrolyte membrane fuel cells, where electrochemical oxidation of organic compounds takes place on two separate electrodes: the anode and the cathode. The microbial population in the anode oxidizes organic pollutants, breaking them down into protons and free electrons. The electrons travel to the cathode through the external electrical circuit, performing work, while the protons move across the membrane and combine with atmospheric oxygen in the cathode chamber to form water as a byproduct [61]. Bioelectricity generation relies on electrogenic bacteria’s redox reactions [58]. Anaerobic conditions in the anodic chamber are essential for microbial growth and efficient electron transfer. The anaerobic conditions are maintained by PEM, which allows protons to pass while blocking oxygen diffusion into the anode chamber, as it could hinder bioelectricity production [62,63]. In the cathode, O₂ or other electron acceptors, like H₂O₂, are reduced, combining electrons and protons to produce water [58]. Material choices for both the anode and cathode play a significant role in the system’s overall efficiency. Anodic chambers are usually made from glass, plexiglas, or polycarbonate, while cathodes consist of carbon-based materials such as graphite, carbon paper, or reticulated vitreous carbon (RVC) [64,65]. Materials are selected based on their ability to support the growth of active biofilms, enhancing electron transfer [66]. The cathode is also essential to power generation, with high redox potential materials, like graphite, platinum (Pt), or carbon paper, being preferred due to their ability to reduce the activation energy of the cathodic reaction [67]. Though platinum boosts initial performance, metal–carbon–nitrogen materials offer better long-term durability, making them a promising substitute [58].

3.2. Performance Metrics and Efficiency

The performance of MFCs is evaluated through several key metrics, like power output, power density, and coulombic efficiency. Power (P, in watts) is determined by Equation (1):

P = I × E_cell

(1)

where I (in amperes) represents the current, and E_cell (in volts) denotes the cell potential, calculated from the current–voltage (I–V) curve. Coulombic efficiency is defined as the ratio of the actual coulombs generated from the anodic oxidation of the substrate to the theoretical maximum, assuming complete oxidation of the fuel substrate. This is calculated using the current produced by the MFC over time, as expressed in Equation (2):

$$C_E={\displaystyle \frac{8{\int }_0^{t_b}Idt}{F{b}_{es}{V}_{Anode}∆COD}}$$

(2)

where the constant 8 is derived from the molecular weight of O₂ (32 g/mol), and the number of electrons exchanged per mole of oxygen, b_es = 4, ∆COD is the change in chemical oxygen demand over time t_b. F is Faraday’s constant (96,485 C/mole), and V_Anode is the anode chamber volume. Coulombic efficiency in MFC systems can be reduced due to alternative electron acceptors, like oxygen and other chemical species present in wastewater, which are utilized by bacteria. As a result, single-chamber MFCs typically exhibit lower coulombic efficiency compared to double-chamber MFCs, as the latter reduces bacterial exposure to oxygen via a separator.

Power density (W/m²) refers to the power output normalized to the electrode area, typically using the anode surface area where the biological oxidation of fuel occurs. However, in many instances, the cathodic ORR is the limiting factor in power generation, so power density is often expressed relative to the cathode area. For engineering purposes, to assess MFC size and cost, power can also be normalized to the reactor volume, yielding volumetric power density (P_v), as calculated by Equation (3):

P_v = (E × I)/v

(3)

where P_v is volumetric power density (W/m³), E is the cell voltage (V), I is the current (A), and v is the total reactor volume (m³).

Energy efficiency, a critical factor in evaluating MFCs’ energy recovery, accounts for the heat released during fuel oxidation, referred to as the heating value. In cathodic reactions, protons and oxygen react with electrons to produce water, which can be in vapor or liquid form. The heating value is classified as the lower heating value (LHV) if the product water is vapor, and as the higher heating value (HHV) if the product water is liquid. The latent heat of vaporization is lost if water is in vapor form but is recoverable if the water is liquid. For 1 mol of H₂ reacting with 1 mol of O₂, the HHV is 285.83 kJ/mol, and the LHV is 241.82 kJ/mol, corresponding to cell voltages of 1.23 V and 1.18 V, respectively. The overall efficiency of MFCs can be determined by the ratio of the power generated over time to the heat of combustion of the organic substrate, considering HHV [68]. This efficiency is calculated by Equation (4):

$$CE={\displaystyle \frac{{\int }_0^t{E}_{cell}Idt}{m_{added}∆H}}$$

(4)

where ∆H is the heat of combustion (J/mol), and m added is the amount of substrate added (mol).

3.3. Factors Affecting the Performance of MFCs

The performance of MFCs is influenced by several key factors that impact their efficiency and energy generation capabilities. Understanding these factors is crucial for optimizing MFC design and operation. A summary of different MFC types, materials, and operating conditions for energy extraction is stated in

Table 1. Summary of MFC types, materials, and operating conditions for energy extraction.

MFC Type	Cathode Material	Anode Material	pH Buffer and Electrolyte	Ion Exchange Membrane	Substrate Used	Operating Conditions	Types of Energy Extracted	Remarks	Ref.
Dual-chamber MFC	Copper wire with aluminum mesh	Copper wire with aluminum mesh	-	Cotton rope boiled with various salt solutions	Municipal wastewater	Operated at 18.5 °C, batch mode for 7 days	Bioelectricity	The study optimized a cost-effective MFC using a novel cotton rope separator. The highest current of 103 µA was achieved with a 1 mm thick aluminum mesh electrode, 1000 mL of wastewater, and a separator treated with 0.5 M NaCl.	[69]
Dual-chamber, H-shaped MFC	Copper bar	Carbon fibers	Synthetic medium with a pH of 7.1 and NaCl concentration of 5.23 g/L	Nafion N117	Synthetic saline wastewater	Continuous-flow mode at 22 °C. Three hydraulic retention times (HRTs) were tested: 1, 3, and 6 days	Bioelectricity	The inoculum from the fish canning industry yielded the highest performance, with a maximum voltage of 802 mV and a power density of 78 mW/m2. Shorter HRTs (1 day) improved the organic removal rate and current production across all tested inocula.	[70]
Dual-chamber MFC	Carbon cloth with 60% platinum	Carbon felt	Nutrient mineral buffer with sodium acetate	PTFE fabric-reinforced perfluorosulfonic acid	Synthetic hydroponic wastewater	Operated at 25 °C	Bioelectricity	At an optimal anodic pH of 6, the system achieved a maximum power density of 122.5 mW/m2, 93.7% COD removal, and 27.5% anode-side TN removal.	[71]
Stacked pulsating gas–liquid–solid circulating fluidized bed microbial fuel cell (SPCF-MFC)	W1S1010 carbon fiber cloth with a Pt/C catalyst	Spectral graphite rods	Operated at a pH of 6–7	-	A mixture of anaerobic sludge and self-configured sewage	Operated at 37 °C, with a central sinusoidal pulsating liquid flow and nitrogen gas injection	Bioelectricity	The novel SPCF-MFC design was optimized using response surface methodology. Introducing pulsating liquid flow and gas circulation significantly improved performance, achieving a maximum output voltage of 2.396 V and a COD removal rate of 95.2% under optimal conditions.	[72]
Single-chamber cell (SCC)	Stainless steel wire	Carbon fabric	Phosphate buffer (PB) (pH 7)	-	Dacar sludge (from poultry slaughterhouse wastewater) mixed with a sucrose solution	34 °C, single-batch operation	Electrical energy	MFCs are effective for simultaneous energy production and effluent treatment, functioning well at specific COD levels. Activated sludge is particularly effective in promoting higher current production, and ionic conductivity is crucial for energy efficiency.	[73]
Dual-chamber, H-shaped cell	Carbon fabric with Pt/C catalyst layer	Graphite	N2 gas	PEM Nafion 117	Various sludges (activated sludge, anaerobic bacteria, municipal slaughterhouse sludge)	-	Electrical energy
Dual-chamber MFC	Polyacrylonitrile carbon felt	Polyacrylonitrile carbon felt	pH adjusted to 7–7.2 using SME	Nafion 117 membrane	50% sugar mill effluent (SME)	Operated at ambient temperature (25–28 °C), batch-fed mode for 15 days	Bioelectricity	The study demonstrated bioelectricity production using SME with a maximum power density of 140 mW/m2 and achieved a 56% reduction in COD from a 50% SME substrate.	[74]
Dual-chamber MFC with a salt bridge	Zinc (Zn) and aluminum (Al)	Copper (Cu) and aluminum (Al)	-	Salt bridge used for ion exchange	Municipal, textile, and tannery wastewater	-	Bioelectricity	The Zn-Cu electrode is more efficient than the Al electrode. The study suggests MFC technology is economically feasible for countries like Bangladesh and emphasizes the need for further research for practical implementation.	[59]
Dual-chamber MFC	Carbon	Carbon	-	PEM Nafion 117	Sewage water, containing a natural consortium of electrochemically active bacteria	Batch mode, room temperature, 1000 mL volume in both anode and cathode chambers, inoculated with 1000 mL of sewage water	Bioelectricity	The study demonstrated that stacked PEM MFCs can efficiently generate electricity from sewage water and produce enhanced voltage outputs. MFCs are environmentally friendly, avoid mediators and catalysts, and have the potential for practical applications, like lighting an LED bulb.	[75]
Single-chamber MFC	Air	-	Operated under pH 7	-	Tannery wastewater (COD 1100 mg/L and TKN 431 mg/L)	Temperature: 37 °C, semi-batch operation	Electrical energy	The study demonstrated simultaneous wastewater treatment (COD and nitrogen removal) and electricity generation using air–cathode SC-MFCs. Approximately 50% of TKN and 88% of COD were removed, and the maximum power density reached 7 mW/m2.	[76]
Dual-chamber, cylindrical MFC	Graphite	Graphite	Phosphate-buffered saline (PBS) and designed synthetic wastewater (DSW) with adjusted pH (7 ± 0.1)	Cation exchange membrane	Wastewater treated from a sequencing batch reactor (PDBR) with azo dye, glucose (3 g/L), and various salts for synthetic wastewater	Room temperature, wastewater flow rate: 0.2 L/min, fed-batch mode, suspended growth configuration, anoxic–aerophilic–anoxic microenvironment with periodic aeration	Bioelectricity	Microbial electrochemical treatment (MET) showed high dye degradation and COD removal efficiency with simultaneous bioelectricity generation. Cathodic effluent could be reused as a biofertilizer due to its nutrient-rich content, supporting the potential for resource recovery and wastewater remediation.	[77]
Dual-chamber, paraboloid-shaped, membrane-less MFC	Graphite	Graphite	PB solution with neutral pH (pH 7) in the cathode chamber	None (membrane-less design)	Municipal solid wastewater	Room/ambient temperature (22 ± 2 °C), Anode chamber anaerobic conditions ensured with nitrogen gas	Bioelectricity	The novel membrane-less, truncated paraboloid MFC configuration showed feasibility for treating pharmaceutical industrial wastewater and generating bioelectricity. It demonstrated a significant reduction in COD and TDS, suggesting its potential for commercial scale-up.	[78]
Dual-chamber, cylindrical MFC	Carbon-fiber brush	Graphite felt	Synthetic swine wastewater with pH adjusted to 7.5 ± 0.1	Cation exchange membrane (CEM), CMI-7000	Synthetic swine wastewater with added sulfonamides (SMX, SDZ, and SMZ)	Room temperature: ~25 °C, flow rate: 20 mL/min, operating in batch mode with cycles of 120 h, oxygen level in cathode chamber: ~6 mg/L	Electrical energy	The study concludes that the MFC could effectively remove organic matter and sulfonamides (with a COD removal >95% and high removal rates for SMX, SDZ, and SMZ). It demonstrates a strong potential for treating swine wastewater contaminated with antibiotics while also producing electricity. The MFC system was resistant to antibiotic toxicity and may enhance microbial activity for electricity production with prolonged acclimation.	[79]
Dual-chamber MFC	Activated carbon felt	Activated carbon felt	Sodium acetate nutrient solution consisting of 50 mM phosphate buffer, vitamins, and trace minerals	PEM Nafion 117	Sodium acetate (NaAC) and oxytetracycline (OTC)	Temperature: 30 ± 0.5 °C; wastewater flow rate controlled by a peristaltic pump with a hydraulic residence time of 22 h	Electrical energy	The ES-MFC system was stable, feasible, and demonstrated a high removal efficiency (up to 98.8%) for OTC from wastewater, with the activated carbon fibers being recyclable, making the system applicable for antibiotic removal from wastewater.	[80]
Dual-chamber MFC	Carbon-fiber brush	Cylindrical graphite felt	pH was adjusted to 7.4 ± 0.1. Nitrogen gas was supplied for 10 min before feeding the anode chamber	Cation exchange membrane	Wastewater sample collected from an effluent treatment anaerobic digester plant	Temperature: 30 ± 2 °C; wastewater flow rate: 10 mL/min; self-circulated, running cycle: 100 h in batch mode; DO concentration: 6 ± 0.2 mg/L in the cathode chamber	Electrical energy	The study demonstrated that MFCs are effective for the simultaneous bioremediation of wastewater and electricity generation. MFCs achieved 89.2 ± 2.1% sulfadiazine removal efficiency after 100 h and maximum COD removal of 91.9 ± 2.3%. The electrogenic strain Bacillus subtilis EL06 was characterized by the MFC, showing adaptability and antibiotic tolerance.	[81]
Dual-chamber MFC	Carbon cloth	Carbon cloth	Catholytes with various initial pH: potassium persulfate, M9 medium, PB, NaCl, and water. Anolyte pH was maintained at 7.0 ± 0.2 using NaOH or HCl	Cationic exchange membrane (CMI-7000)	2,4-Dichlorophenol (2,4-DCP) (10 mg/L) as a pollutant for degradation, glucose (0.2%), and yeast extract in M9 medium as the carbon source	Room temperature. Air purging in the anodic chamber for bacterial growth, followed by an anoxic environment after 12 h. Aeration in the cathodic chamber	Electrical energy	The Bacillus subtilis-catalyzed MFC is feasible for both generating electricity and degrading phenol pollutants, like 2,4-DCP, making it a potential technology for industrial wastewater treatment while producing energy.	[82]
Dual-chamber MFC	Toray carbon paper	Toray carbon paper	Electrolyte: synthetic wastewater composed of glucose, fructose, NaHCO3, (NH4)2SO4, KH2PO4, MgCl2, CaCl2, and (NH4)2Fe(SO4)2	Sterion® commercial membrane	Synthetic wastewater composed of glucose (161 mg/L), fructose (161 mg/L), NaHCO3 (111 mg/L), and other components	Flow rate: 0.2 mL/min, operating duration: two days before inoculation with Geobacter-enriched mixed culture	Electrical energy	The electrical performance of the MFC was maintained, with almost unaltered nitrate concentrations of below 0.9 mg N-NO3 L−1. Above this value, a reduction in voltage was observed due to competition between denitrifiers and electrogenic microorganisms, leading to mass transfer limitations and reduced electrical performance.	[83]
Single-chamber, Twist ‘n Play design MFC	Carbon fiber	Carbon fiber	Initial pH of anolyte: 7.3. Replenished every 24 h with 1 mL of TYE (1% Tryptone, 0.5% yeast extract)	Cation exchange membrane (CMI-7000)	Human urine (non-treated)	Ambient temperature: 22 °C ± 2 °C; wastewater (urine) flow rate: 1 mL/h; hydraulic retention time: 6.8 h	Electrical energy	The study concluded that materials and conditions optimized for one type of MFC are not necessarily optimal for others. The new MFC design showed increased performance compared to the control, especially in terms of power and COD treatment. RC25 Nanocure and ABS performed better in these metrics. RC25 Nanocure was the most robust, suggesting the potential for a hybrid material to improve MFC miniaturization.	[84]
Single-chamber, cylindrical MFC	Stainless-steel	Carbon-fiber brush	Artificial wastewater containing CH3COONa, NaH2PO4(H2O)2, Na2HPO4, NH4Cl, and KCl	-	Natural wastewater was initially replaced later with artificial wastewater containing acetate as the carbon source	Inoculation periods: 7 months for R1 and R2, 5 months for R3-R6; external load: varied from 1 kΩ to 20 Ω	Electrical energy	The study concludes that the electrical behaviour of MFCs can be standardized for steady-state conditions. A mathematical model was proposed to estimate and predict the internal power losses in MFC reactors, proving useful for designing a DC/DC converter for smart sensors.	[85]
Dual-chamber Microbial Electrosynthesis (MES) reactor	Nickel foam	Ti/IrO2-Ta2O5	100 mM PB solution. Components: NaH2PO4·H2O, Na2HPO4·H2O, NH4Cl, KCl, trace minerals, and vitamins	Cation exchange membrane (CMI-7000S)	CO2 (99.999%) for methanogenesis	Temperature: 30 °C; hydraulic retention time: 5.88 h in continuous flow mode	Methane production	The study concludes that the ACV method’s success is due to the lowest cathodic charge transfer resistance and the enrichment of specific microbial communities (M. taiwanensis). The study suggests adjusting or combining electrochemical start-up methods to enhance the performance of CO2-reducing biocathodes.	[86]

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MFCs rely on electrogenic microbial consortia to harvest bioenergy through direct electron transfer (DET) and mediated electron transfer (MET) [87]. DET involves direct contact between electroactive bacteria and electrodes, while MET uses redox mediators secreted by microorganisms [88,89]. Both mechanisms are crucial for maximizing energy output. Substrate oxidation during microbial metabolism generates electrons, which travel through an external circuit to produce electricity [68]. The highest voltage reading from a single-cell MFC typically ranges between 0.3 and 0.7 V, with potential losses affecting the actual achievable voltage [90]. However, the performance of MFCs is influenced by several operational parameters, including pH, temperature, salinity, external resistance, and shear stress.

The pH of the electrolyte plays a pivotal role in microbial activity and energy efficiency. Optimal performance, with peak power density and coulombic efficiency, occurs at pH levels between 8 and 10 [91]. Similarly, temperature significantly affects microbial kinetics. Higher temperatures, such as 40 °C, enhance substrate degradation and energy production, although extreme heat can disrupt microbial communities [92]. Salinity, maintained at 10–20 g/L NaCl, is another critical factor improving ionic conductivity, which facilitates efficient bioelectricity conversion and over 90% organic removal [93]. However, excessive salinity levels, exceeding 40 g/L, lead to a sharp decline in power generation, significantly at 60–70 g/L due to microbial inhibition [94]. External resistance also governs electron transfer efficiency and power density. While lower resistance improves current output, excessively low resistance may divert microbial metabolism toward less efficient fermentation pathways [95,96]. An optimal resistance range of 2–5 kΩ balances these effects [97], ensuring stable power output.

Finally, shear stress on the anode critically influences biofilm formation, directly impacting electricity production in MFCs. Godain et al. [98] used controlled shear stress levels of 1, 5, and 10 mPa and demonstrated that higher shear stress enhances biofilm coverage on the anode surface, as illustrated in Figure 5. This increased biofilm coverage strongly correlates with improved power density, suggesting biofilm physical characteristics are as important as its microbial composition. The observed relationship indicates that electroactive biofilm growth under higher shear stress conditions is not only a result of microbial selection but also enhanced mass transport and electrochemical conditions. These factors facilitate more efficient extracellular electron transfer, which is crucial for energy generation in MFCs. Therefore, optimizing shear stress levels during MFC operation can significantly improve performance by promoting the development of dense, electroactive biofilms capable of sustained electricity production.

4. MFC for Wastewater Treatment and Energy Production in Dhaka: A Case Study

4.1. Wastewater Generation in Dhaka

Water demand in Bangladesh is driven by agriculture (86%), while industrial and domestic sectors account for the remaining 14% [99]. Bangladesh produces approximately 4874 million m³/year of domestic wastewater and 452 million m³/year of industrial wastewater [39]. In addition to industrial wastewater, the total domestic wastewater generation in Bangladesh is approximately 4874 million cubic meters per year [33]. Combining this with industrial wastewater generation gives a total wastewater generation of 5252 million cubic meters annually.

Dhaka, the capital of Bangladesh, with a population of over 20 million, generates approximately 1.5 million cubic meters of wastewater daily from about 7000 industrial units in nearby areas [100]. With most industries established in Dhaka, its wastewater primarily originates from domestic, industrial, and commercial sources; only about 20–50% is treated by conventional methods like aerobic digestion [101]. The untreated wastewater, with an average biochemical oxygen demand (BOD) concentration of 276 to 425 mg/L, significantly contributes to the pollution of the Buriganga River and other water bodies [102]. In the proposed case study, we focus on deploying MFC technology to treat 100,000 m³/day of wastewater in a densely populated industrial zone of Dhaka. This scenario aligns with previous studies that suggest MFCs can be scaled to manage large wastewater volumes while recovering energy and reducing environmental pollutants [103]. The overall wastewater treatment and energy generation process of the MFC plant is shown in Figure 6.

4.2. Energy Recovery Potential

MFCs generate electricity by microbially converting chemical energy in organic matter directly into electrical energy. In Dhaka, the typical COD concentration in wastewater ranges from 972 to 1420 mg/L [102]. For this case study, an average COD concentration of 1196 mg/L is assumed. It represents a value within the reported range for Dhaka’s industrial wastewater, and the potential energy recovery can be estimated.

Theoretical energy recovery from MFCs is estimated to be about 4 kWh/kg with COD removed [104]. Given a flow of 100,000 m³/day and an average COD concentration of 1196 mg/L, the COD load is calculated by Equation (5):

$$CODLoad=\mathrm{100,000}{\displaystyle \frac{{\mathrm{m}}^3}{\mathrm{d}\mathrm{a}\mathrm{y}}}\times 1.196{\displaystyle \frac{\mathrm{k}\mathrm{g}}{{\mathrm{m}}^3}}=\mathrm{119,600}\mathrm{k}\mathrm{g}{\displaystyle \frac{\mathrm{C}\mathrm{O}\mathrm{D}}{\mathrm{d}\mathrm{a}\mathrm{y}}}$$

(5)

At 4 kWh/kg COD, the total potential energy recovery is:

$$\mathrm{E}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{y}\mathrm{r}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{y}=\mathrm{119,600}\mathrm{k}\mathrm{g}{\displaystyle \frac{\mathrm{C}\mathrm{O}\mathrm{D}}{\mathrm{d}\mathrm{a}\mathrm{y}}}\times 4{\displaystyle \frac{\mathrm{k}\mathrm{W}\mathrm{h}}{\mathrm{k}\mathrm{g}\mathrm{C}\mathrm{O}\mathrm{D}}}=\mathrm{478,400}{\displaystyle \frac{\mathrm{k}\mathrm{W}\mathrm{h}}{\mathrm{d}\mathrm{a}\mathrm{y}}}=478.4{\displaystyle \frac{\mathrm{M}\mathrm{W}\mathrm{h}}{\mathrm{d}\mathrm{a}\mathrm{y}}}$$

This energy could provide electricity to thousands of homes in the Dhaka region. However, due to efficiency losses in real-world conditions, only a portion of this theoretical energy can be recovered. For a coulombic efficiency of 80% [105], the recoverable energy would be around 382 MWh/day. The energy balance of the MFC plant is depicted in Figure 7 with a Sankey diagram.

• Sensitivity Analysis:

The recoverable energy is sensitive to variations in COD concentration. If the COD concentration varies between 972 mg/L and 1420 mg/L, the COD load would range from 97,200 kg/day to 142,000 kg/day. This results in a theoretical energy recovery range of 388.8 MWh/day to 568 MWh/day. With an 80% columbic efficiency, the recoverable energy would range between 311 MWh/day and 454 MWh/day.

4.3. Cost Estimation: Installation and Operation

The installation cost for MFCs varies depending on the scale and materials used. Previous studies suggest that the capital cost for MFC systems ranges between USD 1500 and USD 15,000 per cubic meter of wastewater treatment capacity [106]. For the base case estimation for a 100,000 m³/day treatment plant in Dhaka, a mid-range specific capital cost of USD 5000 per m³/day of treatment capacity is assumed, based on the literature values in [106]. Thus, the total installation cost can be estimated as:

Capital Cost = 100,000 m³/day × 5000 USD/m³ = USD 500 million

This estimate includes the costs for the reactor construction, electrodes, membranes, current collectors, and other essential components.

The operational costs, typically 3–4% of the capital cost per year, would primarily cover maintenance, replacement of components, such as electrodes, and labor [107]. For the base case estimation, an operational cost at the higher end of this typical range, 4% of the capital cost per year, is assumed. The operational cost can be estimated as:

Operational Cost = USD 500 million ×0.04 = USD 20 million/year

This includes the cost of electricity for running pumps and other ancillary equipment, though much of the system’s power needs could be met through the electricity generated by the MFC itself. The breakdown structure of the capital costs and operational costs is demonstrated in Figure 8.

• Sensitivity Analysis:

Installation Cost per m³: If the installation cost varies between USD 1500/m³ (representing a lower-bound estimate from [106]) and USD 15,000/m³ (representing an upper-bound estimate from [106]), the total installation cost would range from USD 150 million to USD 1.5 billion.

Operational Cost Percentage: If the operational cost varies between 3% (lower end of the typical range cited in [107]) and 4% (assumed base value at the upper end of the typical range cited in [107]) of the capital cost, the annual operational cost would range from USD 15 million to USD 20 million with the base capital cost. The overall project cost is most sensitive to variations in the installation cost per cubic meter, with operational cost percentage having a moderate impact.

It is important to acknowledge that these cost estimates are based on global data. They are subject to significant variation based on local economic factors. This is particularly relevant in a country like Bangladesh. Region-specific cost data for large-scale MFC implementation is not yet available in the literature. This lack of data underscores the exploratory nature of this case study. Factors such as lower domestic labor costs could potentially reduce the overall capital and operational expenses. However, these savings may be offset by higher costs for importing specialized components. These include high-performance electrodes and membranes. Such components are not manufactured locally. They may also be subject to import duties and logistical challenges. Therefore, while the initial capital investment remains a major barrier, a detailed, localized cost analysis would be essential. This analysis is necessary for any practical project proposal.

4.4. Economic Assessment

MFC systems have a typical lifespan of 10 to 15 years, with the durability of components like electrodes and membranes being the key factors in their longevity [26,108]. Advances in electrode materials, such as activated carbon or semicoke, can extend the system’s life while reducing operational costs. For instance, the cost of electricity per watt (W) for semicoke and activated carbon is 2.8% and 22.7% that of graphite electrodes, respectively [109].

Assuming a life expectancy of 12 years (a mid-range value based on the 10–15 years typical lifespan reported in [26,108]), the operational cost over the system’s lifetime is discounted to present worth according to the following general equation for the present worth of a continuous cost, as described in Equation (6). However, direct benchmarks for large-scale MFC plants are not available for Bangladesh, as this technology is not yet deployed there. Similarly, the 5% discount rate was chosen as a stable, illustrative figure based on general corporate bond rates at the time of analysis. For a specific government or private infrastructure project in Bangladesh, both the expected lifetime and the discount rate would require a detailed technical and financial risk assessment:

P_o = (P_t (1 − e^−iT))/I

(6)

where

• P_o = present worth (USD);
• P_t = yearly payment (USD);
• i = discount rate (assumed to be 5% based on corporate bond rates at the time of this paper);
• T = MFC system lifetime (assumed to be 12 years).

Operational cost:

P_o = 20,000,000 (1 − e^{(−0.05 × 12)})/0.05 = USD 180.48 million

Using the previously assumed yearly operational cost of USD 20 million (derived from 4% of the USD 500 million base capital cost):

Total Cost = Base Capital Cost + Present Worth of Operational Cost = 500 million USD + 180.48 million USD = USD 680.48 million

• Sensitivity Analysis:

  ○

  System Lifetime: If the system lifetime varies between 10 (lower bound from [26,108]) and 15 years (upper bound from [26,108]), the present worth of operational costs would range from USD 157.38 million to USD 211.05 million.

  ○

  Discount Rate: If the discount rate varies between 4% and 6% (representing potential fluctuations around the assumed 5% corporate bond rate benchmark), the present worth for a 12-year lifespan would range from USD 190.6 million to USD 171.08 million.

Similarly, analyzing the impact of a shorter lifetime due to potentially harsh environmental or operational conditions is an important addition to the sensitivity analysis. The impact of a reduced 8-year lifetime on the project’s economics shows the following:

○

The present worth of operational costs over 8 years would be approximately USD 131.87 million.

○

This would bring the total project cost to USD 631.87 million.

○

The cost per unit of energy would increase from the base case of USD 0.41/kWh to approximately USD 0.57/kWh.

Both the system lifetime and the discount rate significantly influence the present worth of operational costs, with a longer lifespan increasing costs and a higher discount rate decreasing costs.

4.5. Per Unit Energy Cost

Daily recoverable energy = 382 MWh/day

Total energy produced over the plant’s lifetime = 382 MWh/day × 365 days/year × 12 years

Total Energy = 382 × 365 × 12 = 1,673,160 MWh over 12 years

• Cost Per kWh:

Total cost over the plant’s lifetime = USD 680.48 million

Cost per kWh = (Total Cost)/(Total Energy) = 680,480,000/(1,673,160×1000) = USD 0.41 per kWh

The base value of per unit energy cost for this MFC system is approximately USD 0.41 per kWh. This is calculated using the total base project cost of USD 680.48 million (derived from a USD 500 million capital cost, 4% annual operational cost, 12-year lifetime, and 5% discount rate) and the total energy produced over the lifetime. According to the sensitivity analysis, per unit energy cost can vary between USD 0.2 and 1 per kWh. This is significantly higher than the average cost of electricity generation in Bangladesh (which typically ranges from USD 0.078 to 0.10 per kWh, depending on the energy source [110]); however, it is still seen as a sustainable option by considering the added benefits of green energy and the reduction of CO₂ emissions.

4.6. Environmental Impact

MFC technology offers significant environmental benefits compared to conventional wastewater treatment plants (WWTPs), particularly in reducing GHG emissions and energy consumption. Conventional aerobic wastewater treatment plants consume about 0.3–0.6 kWh/m³, with 50–60% of this energy used for aeration [111,112]. The size of these plants ranges from small (as low as 15,000 m³/day in the USA) to extremely large (as high as 2,000,000 m³/day in China) [111]. For a 100,000 m³/day plant, the energy consumption would be 30,000–60,000 kWh. By replacing the conventional system with MFCs, this energy requirement is offset, and instead, net energy is recovered. Additionally, by reducing the energy used for aeration, MFCs could prevent 31–62 tons of CO₂ emissions annually, as 0.942 kg CO₂ per kWh of electricity is produced from fossil fuels [113]. Figure 9 shows the annual CO₂ equivalent emission comparison between an MFC plant and a conventional wastewater treatment plant with 100,000 m³/day capacity.

MFCs also reduce the environmental burden of sludge management. Conventional WWTPs produce large quantities of sludge that require energy-intensive processing, whereas MFCs significantly minimize sludge generation [114]. Xiao et al. [115] conducted a study integrating MFCs into an anoxic–oxic treatment process. They found that the MFC-integrated system reduced the cumulative production of waste-activated sludge by 24.0% and the sludge yield by 24.2% when compared to a conventional control system. This reduction is attributed to the lower cell yield of electricity-producing microorganisms (electricigens) compared to the microorganisms in a typical activated sludge process.

Applying these findings to the Dhaka case study, a conventional plant with a daily COD load of 119,600 kg and a typical sludge yield of 0.42 g SS/g COD would produce approximately 18,335 tons of sludge annually. By implementing MFC technology with a 24.2% reduction in sludge yield, the proposed plant could prevent the generation of over 4400 tons of sludge each year. This minimization of sludge leads to further reductions in GHG emissions and operational costs associated with sludge handling, transport, and disposal [114]. Moreover, MFCs can improve water quality by reducing the discharge of untreated or partially treated wastewater into rivers and other water bodies [116]. The improved wastewater treatment efficiency could restore ecosystems, particularly in the Buriganga River, which is heavily polluted with untreated effluents.

4.7. Results and Discussions

It is important to note that this analysis faced challenges common to feasibility studies of emerging technologies. The primary difficulty was the scarcity of long-term operational data. Validated costings for commercial-scale MFC plants were also lacking, especially within a developing country context. Consequently, the analysis relies on extrapolating data from pilot-scale projects. It also draws from a broad range of literature values. This approach introduces inherent uncertainties. The following discussion of the results should be interpreted with these limitations in mind.

The analysis highlights the feasibility and potential of MFC technology to address the dual challenge of wastewater treatment and renewable energy generation in Dhaka. The results reveal both the potential benefits and the significant challenges that must be addressed before widespread implementation can be considered. The results are summarized in

Table 2. Summary of key results for MFC implementation in Dhaka.

Parameter	Base Value	Range
Wastewater Treatment Capacity	100,000 m3/day	-
COD Concentration	1196 mg/L	972–1420 mg/L
Theoretical Energy Recovery	478.4 MWh/day	388.8–568 MWh/day
Recoverable Energy (80% CE)	382 MWh/day	311–454 MWh/day
Capital Cost	USD 500 million	USD 150 million–USD 1.5 billion
Annual Operational Cost	USD 20 million/year	USD 15–20 million/year
System Lifetime	12 years	10–15 years
Present Worth of Operational Cost	USD 180.48 million	USD 157.38–211.05 million
Total Project Cost	USD 680.48 million	USD 307.38 million–USD 1.711 billion
Per Unit Energy Cost	0.41 USD/kWh	0.2–1 USD/kWh
Avoided CO2 Emissions	-	31–62 tons/year

Note: The base values for the cost parameters assume a capital cost of USD 5000/m³ capacity, an annual operational cost of 4% of the capital cost, a system lifetime of 12 years, and a discount rate of 5%. The ranges are derived from sensitivity analyses with varying assumptions: installation cost (USD 1500–15,000/m³ based on [106]), operational cost (3–4% of capital cost based on [107]), system lifetime (10–15 years based on [26,108]), and discount rate (4–6%).

. The analysis reveals the substantial potential of MFC technology for simultaneous wastewater treatment and energy recovery in Dhaka. With a treatment capacity of 100,000 m³/day, the system could theoretically recover an average of 478.4 MWh of energy daily, translating to a recoverable energy of 382 MWh/day. This energy output has the potential to significantly contribute to the local energy supply, powering thousands of homes. The sensitivity analysis of COD concentration shows that the energy recovery is dependent on the effluent COD levels. So, proper characterization of the wastewater composition is essential for accurate energy predictions.

However, the implementation of MFC technology is not economically feasible. The capital cost is estimated between USD 150 million and USD 1.5 billion. This variation is due to uncertainties in per-unit installation costs, which makes cost estimation challenging. Also, the high initial investment is another obstacle to large-scale adoption. Additionally, the energy cost per unit is significantly higher than Bangladesh’s current electricity price (USD 0.078–0.10/kWh) [110].

This difference indicates that further research and development are needed to reduce MFC technology’s capital costs and increase overall system efficiency. It clearly demonstrates the economic gap that technological advancements must bridge for practical implementation in markets with lower existing energy costs. Optimizing electrode materials, refining system configurations, enhancing microbial communities, and scaling up production can make MFCs more economically feasible. Despite the economic challenges, MFCs offer undeniable environmental advantages. While traditional aerobic wastewater treatment consumes significant energy for aeration, MFCs can recover energy and reduce CO₂ emissions. By lowering greenhouse gas output, MFCs support global efforts to reduce climate change. MFCs can also reduce sludge production, which eases the burden of sludge management. This study uniquely quantifies these benefits (e.g., 31–62 tons/year with CO₂ avoidance) at a scale relevant to addressing the significant pollution load from industrial zones like Dhaka’s. This strengthens the case for their strategic deployment despite the current economic limitations. Sensitivity analyses on system lifetime and discount rates further demonstrate their role in long-term cost evaluations. Extending the operational life of an MFC system and reducing the discount rate can improve its economic feasibility. This highlights the necessity of durable and efficient components in an MFC plant.

Although the high initial and per-unit energy costs are challenging for widespread adoption now, the long-term benefits of MFCs remain promising. Their dual capability of energy recovery and environmental benefits makes them a potential solution for sustainable wastewater treatment in Dhaka. Future research should prioritize reducing costs and increasing overall system performance to enhance economic feasibility. Exploring hybrid models that integrate MFCs with other treatment technologies could also pave the way for a more cost-effective and efficient wastewater treatment system.

5. Challenges with MFCs

5.1. High Capital and Operational Costs

The initial investment required for MFCs is nearly 30 times higher than conventional wastewater-treatment technologies. This is mainly due to using expensive materials like Pt catalysts, PEMs, and high-grade electrode materials [29,117,118]. Beyond the financial burden, the production of these components carries a significant environmental cost. Manufacturing common materials like carbon-based electrodes and PEMs is energy-intensive, creating a carbon footprint from ‘embodied’ emissions that can partially offset the operational CO₂ savings from the MFC system [119]. The long-term durability of these materials is, therefore, critical, as frequent component replacement increases this lifecycle environmental cost.

Among these components, electrode costs account for a major portion of the total cost, with carbon cloth and carbon paper being common but expensive materials. Membranes also contribute significantly to overall MFC costs [120]. Although MFCs produce less sludge than conventional systems, the operation costs are still impacted by the relatively low power output of MFCs. To address this challenge, researchers are exploring alternative materials, such as biomass-derived anodes and graphene–metal oxide composites, which offer higher conductivity, biocompatibility, and reduced costs [121]. Chen et al. [122] used carbonized waste tires as anode materials, which addresses both cost and sustainability concerns.

5.2. Membrane Challenges

Membranes are another cost-intensive component of MFCs. Membrane fouling is often caused by biofilm formation, which reduces ion transfer efficiency and power output by up to 33% within 90 days of operation [123]. Biofouling further reduces MFC performance by interrupting proton migration and competing for available substrates [120]. Wang et al. [124] explored membrane-less MFC designs as a cost-effective alternative. Although their construction costs are lower than traditional two-chamber MFCs, they present challenges like substrate crossover and reduced coulombic efficiency due to the absence of ionic membranes [124]. Electrolyte mixing at the anode and cathode is caused by migration, convection, and diffusion. It affects performance by allowing oxygen and substrate crossover, which reduces power density [125].

Innovative solutions have been proposed to mitigate these limitations. Kim et al. [126] demonstrated that incorporating dual anodes in membrane-less MFCs could supply a larger reaction surface, effectively preventing organic crossover and enhancing the overall performance of MFCs. Additionally, Liu et al. [127] showed a significant fouling reduction in membrane bioreactors (MBRs) by integrating a bio-electrochemical cell with iron anodes, microbes, and conductive membranes modified with polypyrrole. This approach retained the benefits of MBR systems while generating a constant electrical potential, further reducing cathode membrane fouling. Modified membranes with catalytic coatings, such as multiwalled carbon nanotubes and the H₂O₂ production in the cathode chamber by electrodes with Pt catalysts, have also shown promise in improving long-term performance by mitigating fouling [128].

5.3. Power Density and Scale-Up Issues

Scaling up MFCs comes with significant technical challenges, including reduced power densities due to increased internal resistance and mass transfer limitations. Reported power densities for large-scale MFCs are approximately 100 mW/m², which is significantly lower than chemical fuel cells (104–105 W/m²), highlighting the limitations of scaling [120]. Stacking multiple MFC units in series or parallel configurations often results in issues such as voltage reversal, parasitic losses, and mass transfer hindrances, as noted by Janicek et al. [129]. Modular or tubular stack designs have been proposed to overcome these challenges. Also, integrating complementary technologies, like anaerobic digestion, can enhance bioelectricity production and water recovery efficiency [130]. Despite these advancements, achieving practical power densities for industrial-scale applications remains a major challenge.

5.4. Microbial Activity and System Design

Microbial activity is central to MFC performance, as it drives efficient electron transfer to electrodes. Biofilm formation, microbial kinetics, and electron transfer efficiency influence the overall performance of MFCs [29,107]. Certain microbes, such as Geobacter species and genetically engineered strains, have shown superior electron transfer capabilities [105]. Additionally, mixed microbial populations derived from real wastewater can significantly improve electrochemical reactions and organic matter breakdown. Liu et al. [131] highlighted the benefits of coupling MFCs with forward osmosis and anaerobic acidification processes to enhance microbial efficiency. Optimizing the system design by increasing electrode surface area, minimizing anode–cathode spacing, and using nanostructured materials has also enhanced power density and overall system efficiency [132,133].

5.5. Maintenance of Optimal pH Levels

The stability of microbial activity in MFCs is dependent on balancing pH levels. Variations in pH can decrease power output and reduce system efficiency [134]. Buffering agents have shown the potential to successfully regulate pH levels. Liu et al. [127] integrated bio-electrochemical cells into membrane bioreactors to stabilize pH and enhance system efficiency. Genetic engineering can improve microbial tolerance to pH variations, maintaining optimal performance under variable conditions [106].

5.6. Controlling Surface Reactions

Uncontrolled surface reactions, like O₂ diffusion into the anodic chamber, reduce MFC efficiency by competing with the electron transfer process. Modifications of electrode surfaces using nanomaterials and conductive coatings have improved microbial affinity and enhanced electron transfer rates. Zhou et al. [135] demonstrated that carbon nanotube-based composites significantly increase electrode surface area, boosting electron transfer rates. Strategies such as metabolic engineering or further surface modifications can strengthen microbial interactions with electrodes [136].

5.7. Reducing Ohmic Losses

Ohmic losses are caused by resistance in electrodes and membranes, which can significantly impact MFC efficiency [137]. Qiao et al. [138] showed that the use of advanced materials with higher conductivity, such as graphene–metal oxide composites, can minimize resistance and improve power output. Additionally, conductive coatings and nanomaterial-based modifications, such as multi-walled carbon nanotubes, can significantly reduce internal resistance. Increasing electrode surface area and refining material properties remain crucial for reducing losses and achieving higher efficiency in large-scale applications [139].

6. Next-Generation MFCs

6.1. Advancements and Prospects in Next-Generation MFCs

The development of MFC technology has been shaped by continuous innovations focused on enhancing efficiency, scalability, and cost-effectiveness. Next-generation MFCs are designed such that they can handle diverse wastewater types, including domestic and industrial effluents. MFCs have already proven effective in treating a wide array of influents beyond municipal wastewater, such as agricultural run-off and various industrial effluents from dairy, detergent, and soft drink production [58,140,141]. Their versatility extends to specialized and nutrient-rich streams, like hospital or slaughterhouse wastewater, urine, and greywater, with some systems even designed to handle multiple wastewater types simultaneously [142,143]. These systems can function efficiently across a range of conditions, like fluctuating temperatures and ionic conductivity. They can also overcome challenges like ohmic losses and microbial reaction limitations. For instance, stacked MFC systems have achieved current outputs up to 8 times higher than single units [144].

Additionally, spiral-type anodes and 3D architectures provide a significantly larger surface area. This expanded space enhances biofilm growth and increases power density to over 1 kW/m³ of reactor volume in optimized setups [144]. Integrating advanced materials and novel system designs allows these next-generation MFCs to overcome current model limitations, like ohmic losses, slow microbial activity, and poor electrode conductivity.

6.2. Introduction of Advanced Materials for Anodes and Cathodes

The anode and cathode directly influence the efficiency and power output of MFCs. Traditional materials, like Pt, have been used due to their high catalytic activity, but their high cost and gradual degradation present significant barriers to widespread industrial use. Researchers are exploring alternative materials, such as carbon-based electrodes doped with nitrogen (N-doped carbon nanotubes or carbon nanofibers) to enhance electron transfer rates [144,145]. These advanced materials provide a large surface area for microbial attachment, promote faster electron transport, and improve biocompatibility [146]. Three-dimensional electrode structures are becoming increasingly important in next-generation MFCs. These 3D architectures improve surface area, promote better biofilm growth, and allow for more efficient substrate propagation. Combined with redox enzyme coating, these materials facilitate higher power output by improving extracellular electron transfer between bacteria and electrodes [147,148]. The spiral-type anode provides a large surface area that supports bacterial attachment and enhances electrical conductivity [144]. Multiple-anode chamber MFCs can enable simultaneous treatment of different wastewater streams [144].

6.3. Membranes and Substrates for Improved Performance

Traditional membranes like Nafion^® 117 are effective but expensive and are prone to biofouling, reducing their performance over time [29]. The next generation of MFCs is expected to incorporate innovative membrane materials that are more cost-effective and resistant to fouling. For example, hybrid membranes made from polymer–polymer cross-linked materials or biopolymer sulfonated biocellulose are being explored for their enhanced durability and lower cost [149]. Recent advancements include cation exchange membranes, like Ultrex CMI-7000, which can provide similar performance at one-fifth the cost of Nafion [150]. Membrane-less designs can reduce costs and complexity in treating various types of wastewaters [144]. Another critical aspect of membrane improvement is addressing ohmic losses. The new generation of MFC membranes can reduce these losses by using conductive materials that minimize resistance and facilitate ion exchange more efficiently.

Substrate selection is also an area of focus for next-generation MFCs. Different types of wastewaters contain varying levels of organic and inorganic compounds, which can impact the efficiency of microbial processes. Adaptability to various substrates is essential for the commercial success of MFC technology. Future MFCs will be able to treat diverse waste streams more effectively by optimizing microbial populations through genomic analysis and enhancing their electrogenic capabilities [144].

6.4. Addressing Operational Challenges

Current MFC technologies suffer from their sensitivity to temperature fluctuations, wastewater flow rates, and sediment formation. These factors can significantly affect microbial activity and the overall performance of the system. Maintaining optimal pH and temperature is crucial for microbial efficiency. Future MFCs use automated control systems that regulate pH levels and temperature and ensure consistent microbial performance even in harsh environments [151]. The development of genetically modified bacteria with enhanced tolerance to extreme conditions could further improve the robustness of MFCs [146].

Conductivity within the system is another critical factor that can influence power output. The next generation of MFCs is expected to utilize substrates and materials that enhance ionic conductivity and reduce internal resistance. Sediment formation can hinder electron transfer and reduce performance. It can be addressed by designing systems that prevent the buildup of solids through optimized flow dynamics [152].

6.5. Scalability and Integration with Other Technologies

One of the primary goals of next-generation MFCs is to make the technology scalable for industrial applications. Current capital costs are approximately 30 times higher than conventional systems like activated sludge treatments [153]. Lab-scale setups often suffer scalability issues from increased internal resistance, voltage losses, and difficulty maintaining consistent microbial performance over large areas. However, advancements in MFC stacking, connecting multiple MFC units in series or parallel configurations, are helping to address these issues. Stacked MFCs have been shown to generate significantly higher power outputs, making them more viable for large-scale wastewater treatment plants [144,154].

Hybrid MFC systems, such as those integrated with anaerobic digestion and nutrient recovery, improve treatment capacity and energy recovery. Pilot-scale projects, like the 200 L MFC at Tirupur (India) and the Okinawa Prefectural Project in Japan, demonstrate the practical potential of integrating MFCs with existing wastewater treatment systems [48,155].

6.6. Future Research Directions and Commercialization Potential

While significant progress has been made in the development of next-generation MFCs, several challenges remain. The long-term stability of new materials, the optimization of microbial communities, and the reduction of capital and operational costs are all critical factors that need to be addressed before large-scale commercialization can be realized. Optimizing microbial populations and developing low-cost, high-efficiency materials are crucial for large-scale commercialization. Novel configurations, like algae-assisted MFCs, are also being explored to enhance energy recovery and wastewater treatment.

The integration of MFCs with renewable energy systems and their use in hybrid configurations can make wastewater treatment more sustainable while recovering valuable by-products, like H₂ and CH₄. Moreover, the development of hybrid MFC systems can combine energy production with chemical recovery or biohydrogen production. These hybrid systems could significantly improve the overall efficiency and sustainability of wastewater treatment by leveraging the advantages of both MFCs and other biological, physical, or chemical processes.

7. Conclusions

This study provides a comprehensive techno-economic and environmental assessment of large-scale MFC technology deployment for treating 100,000 cubic meters of wastewater daily in industrial zones like Dhaka, Bangladesh. Moving beyond general reviews or lab-scale experiments, it offers a holistic, site-specific feasibility model tailored to the context of a developing nation’s industrial infrastructure. Our analysis quantifies a significant theoretical energy recovery potential of up to 478.4 MWh/day, with a realistic output of 382 MWh/day after accounting for efficiency losses, which is enough to power thousands of homes in Dhaka city.

This study’s primary contribution lies in presenting a quantified, data-driven framework for evaluating MFC systems in real-world scenarios. It also underscores substantial environmental co-benefits, including the mitigation of over 4400 tons of sludge annually and the potential reduction of 31–62 tons of CO₂ emissions per year. These findings reinforce the alignment of MFC technology with multiple Sustainable Development Goals (SDGs).

Despite these advantages, many barriers remain before MFC technology can be widely implemented in Bangladesh. The projected capital investment remains high, and the estimated per-unit energy cost (USD 0.2–1/kWh) is not yet competitive with prevailing local energy prices. The need for cost-efficient materials and the enhancement of microbial activity pose major challenges. Advanced materials such as graphene-based electrodes and membranes designed to resist fouling could reduce costs and improve overall efficiency. Also, further research must focus on optimizing reactor design and improving stacking techniques to make MFC systems viable on a larger scale.

The successful adoption of MFC technology could bring transformative change to the country. In addition to improving wastewater management, it could also support Bangladesh’s renewable energy ambitions. If effectively scaled and integrated, MFCs could reduce reliance on fossil fuels, improve public health by enhancing wastewater treatment, and contribute to global efforts to combat climate change. Continued investment, research, and policy support are essential to unlock the full potential of MFCs and drive Bangladesh toward a more sustainable and environmentally resilient future.