https://orcid.org/0000-0001-6417-1683
ABSTRACT
As global industrialization accelerates, the treatment of nitrogenous wastewater has become a pressing environmental challenge. In response to this challenge, this study explores the potential of constructed wetland coupled microbial fuel cell (CW-MFC) technology for the treatment of nitrogenous wastewater. It systematically presents the fundamental principles and characteristics of the CW-MFC, analyzing the metabolic processes and denitrification mechanisms of nitrogen pollutants within the system. This research not only summarizes the key factors that influence the denitrification performance of the CW-MFC system but also discusses its future development trends and potential applications. The objective is to refine the field of nitrogenous wastewater treatment using CW-MFC, enhancing the denitrification efficiency, and to provide a foundation for further advancing the practical application and scientific research of this technology.
HIGHLIGHTS
We found that integrated bioelectrochemical technology shows broad practical application potential in nitrogen-rich wastewater management.
The constructed wetland coupled microbial fuel cell (CW-MFC) mechanism in the treatment of nitrogenous wastewater and the optimal control of key factors in system operation were studied.
This study aims to steer future research and provide reference for further exploring the application of CW-MFC in the field of nitrogen-containing wastewater treatment.
NOMENCLATURE
- AOA
ammonia oxidizing archaea
- AOB
ammonia oxidizing bacteria
- COD
chemical oxygen demand
- CW
constructed wetlands
- CW-MFC
constructed wetland coupled microbial fuel cell
- DO
dissolved oxygen
- EAB
electroactive bacteria
- HRT
hydraulic retention time
- MFC
microbial fuel cells
- NH3
ammonia
- N2
molecular nitrogen
- NH4+
ammonium
- NO2−
nitrite ions
- NO3−
nitrate ions
- NOB
nitrite-oxidizing bacteria
INTRODUCTION
Nitrogen is a crucial biological element abundant in both natural and human industrial processes. Excessive nitrogen emissions can result in the eutrophication of water bodies, leading to environmental issues, such as cyanobacterial blooms, thus jeopardizing both the ecological environment and human health (Hashemi et al. 2016). Consequently, research on nitrogen-containing wastewater treatment is of paramount importance for preserving water ecosystem equilibrium and human health. In recent years, constructed wetlands (CWs) have gained prominence in wastewater treatment as an ecologically efficient technology, characterized by low energy consumption and investment. However, traditionally CWs exhibit limitations in the treatment of nitrogen-containing wastewater, particularly in terms of their relatively low nitrogen removal efficiency. Microbial fuel cells (MFC) are a novel energy technology that has garnered substantial attention and have applications in wastewater treatment (Xiong et al. 2022; Zheng et al. 2022). The CW coupled MFC (CW-MFC) was developed by integrating CWs and MFC technology (Liu et al. 2014). By harnessing the synergistic interplay between the ecological purification of CW and the electrochemical reactions of MFC, organic matter, nitrogen, and phosphorus can be efficiently removed from wastewater while enabling energy recovery and utilization. This system offers significant environmental and economic advantages and has promising application potential (Yadav et al. 2012; Oon et al. 2015; Doherty et al. 2015b). In contrast to traditional water treatment methods, the CW-MFC system does not require the addition of chemical agents for manual intervention but requires minimal technical expertise and incurs relatively low maintenance costs (Fang et al. 2013).
Integrated CW-MFC technology has been developed for more than 10 years, and its application is mainly for the treatment of refractory pollutants, such as dye wastewater, heavy metal pollution, antibiotics, and emerging pollutants (Cheng et al. 2023; Liu et al. 2023; Ji et al. 2023; Teoh et al. 2024). The existing literature on CW-MFC has primarily examined the system configuration structures, electrode materials, and traditional pollutant removal capabilities; analyzed the electron transfer mechanism and microbial community structure in the CW-MFC systems; and explored the potential applications of bioelectric MFC technology. These studies propose coupling a mature CW design with MFC technology to achieve higher pollutant removal and power generation capabilities. However, there are few comprehensive summaries regarding the treatment of nitrogen-containing wastewater using CW-MFCs. Only a few studies have focused on the removal of nitrogen from traditional pollutants in specific environments. For instance, Wang et al. (2022a) achieved an 89% ammonia conversion efficiency in CW-MFCs by coupling anaerobic granular sludge with artificial aeration or iron-based substrates, yet the balance and efficiency of nitrogen transformation remain unclear. In another study, Zhang et al. (2022a) successfully removed 83.4% of total nitrogen (TN) by altering the REDOX conditions of CW-MFCs using multiple anodes and siphon aeration. In addition, the addition of cellulose carbon sources in CW-MFCs has been shown to effectively promote the growth of autotrophic and heterotrophic denitrifying bacteria, thereby facilitating the removal of ammonia nitrogen and TN (Zhang et al. 2022b). Although these studies offer valuable insights, a systematic summary of the advantages of CW-MFCs in treating nitrogen-containing wastewater and enhancing denitrification efficiency has yet to be provided, and the specific denitrification processes and mechanisms require further discussion and refinement. Therefore, based on the research results of CW-MFC over the past 10 years, this study analyzed the metabolic process and denitrification mechanism of nitrogen-containing pollutants in CW-MFC by comparing mixed treatment technologies of nitrogen-containing wastewater. In addition, the influencing factors and future development trends of nitrogen removal in CW-MFC systems are discussed in depth to provide a reference for their subsequent application and related research.
METHODS AND SCOPE
This paper reviews the mechanism of the CW-MFC treatment of nitrogen-containing wastewater and summarizes the characteristics of the mixed technology for nitrogen-containing wastewater treatment. In addition, the effects of the CW-MFC on nitrogen removal under different operating conditions were investigated. The articles selected for in this review were peer-reviewed journal articles published in the past decade. The main sources of these articles are from two databases: Elsevier ScienceDirect (https://www.sciencedirect.com/) and Web of Science (https://www.webofscience.com/). The keywords of the journal articles were as follows: wastewater denitrification technology, CW-MFC, CW-MFC denitrification treatment, CW-MFC operation condition optimization, CW-MFC treatment performance, and CW-MFC wastewater treatment mechanism.
DIFFERENT NITROGEN-CONTAINING WASTEWATER TREATMENT TECHNOLOGIES
Extensive efforts have been made to develop efficient and cost-effective wastewater treatment methods to mitigate the pollution caused by the release of nitrogen-containing wastewater into the environment. Numerous denitrification processes have been employed, and Table 1 summarizes recent research on wastewater denitrification treatment technologies. Currently, treatment technologies for nitrogen-containing wastewater are mainly divided into two categories traditional denitrification technologies and emerging biological denitrification technologies. Traditional denitrification technologies are classified into physical, chemical, and biological methods. Physical methods, such as reverse osmosis and ultrafiltration, have a good effect on the treatment of organic matter, but the cost is high and secondary pollution occurs. Chemical precipitation has a wide range of applications; however, but it has certain requirements for wastewater pH, dissolved oxygen (DO), and other conditions that must be adjusted appropriately. Biotechnology is currently the most widely used method for wastewater denitrification. It has a high pollutant degradation rate and does not produce secondary pollution; however, it has high requirements for functional microorganisms and requires an additional carbon source and energy supply. Therefore, current research focuses on more economical emerging biological nitrogen removal technologies such as the application of simultaneous nitrification and denitrification, short-cut denitrification biofilm reactors, reduced external carbon sources, low oxygen demand, and abundant microbial populations (Li et al. 2023). However, the microorganisms in a biofilm reactor are sensitive to temperature and pH, the operating conditions are high, and short-cut denitrification requires additional pretreatment (Di Capua et al. 2022). Biological filters using anaerobic ammonium oxidation (Anammox) technology also have the above advantages, but the reaction rate is slow, and the adaptation range of the ammonia nitrogen concentration is limited; therefore, they cannot be used for mainstream wastewater treatment. Compared with traditional aeration biological treatment systems, the bioelectrochemical technology represented by CW-MFC has the characteristics of a natural redox potential gradient of CW and enhanced utilization of microorganisms. It does not require a continuous external oxygen supply, reduces treatment costs, and does not require an external carbon source. Microorganisms use organic matter in wastewater for metabolic activity to avoid secondary by-products. In addition, a small amount of electrical energy can be generated while efficiently treating nitrogen-containing wastewater, realizing energy recovery and utilization, which has incomparable advantages over traditional technologies. In addition, the CW-MFC system can be designed as a modular structure through a series–parallel connection (Tamta et al. 2020), which is convenient for system expansion and adaptation to different scales of nitrogen removal treatment.
Different technologies for treating nitrogenous wastewater
| Reference | Types of technologies | Range of application | Merits and demerits | Max pollutant removal (%) | |
|---|---|---|---|---|---|
| Quan et al. (2018) | Physics | Reverse osmosis | High concentration nitrides | High efficiency, stable operation; energy consumption | 60–79 |
| Lujian et al. (2020) | Ultrafiltration | Low concentration nitrogen wastewater | High efficiency, stable operation; membrane fouling, more filtrate production | 95–98 | |
| Yan et al. (2013) | Chemistry | Chemical precipitation | Ammonium nitrogen | Wide application; secondary pollution, low efficiency | 37–90 |
| Shidong et al. (2021) | Redox | Treatment of organic matter, nitrides | High efficiency, relatively simple operation; secondary pollution | 76–94 | |
| Lu et al. (2019) | Biology | Artificial wetland | Secondary treatment | Low cost and simple maintenance; limited processing efficiency covers a large area | 59–78 |
| Miyake et al. (2023) | Activated sludge process | Urban sewage | Mature technology, low cost; high requirements for operation control | 92–96 | |
| Pan et al. (2022) | Biofilm reactor | Secondary treatment or industrial wastewater | High efficiency and strong controllability; high cost | 59–99 | |
| Karla et al. (2022) | Biofilters | Agricultural wastewater | High stability; high requirements for water quality | 95–99 | |
| Bashir et al. (2023) | Physical chemistry | Advanced oxidation process | Industrial wastewater | High efficiency, simple process; high cost, high energy consumption | 90–100 |
| Reference | Types of technologies | Range of application | Merits and demerits | Max pollutant removal (%) | |
|---|---|---|---|---|---|
| Quan et al. (2018) | Physics | Reverse osmosis | High concentration nitrides | High efficiency, stable operation; energy consumption | 60–79 |
| Lujian et al. (2020) | Ultrafiltration | Low concentration nitrogen wastewater | High efficiency, stable operation; membrane fouling, more filtrate production | 95–98 | |
| Yan et al. (2013) | Chemistry | Chemical precipitation | Ammonium nitrogen | Wide application; secondary pollution, low efficiency | 37–90 |
| Shidong et al. (2021) | Redox | Treatment of organic matter, nitrides | High efficiency, relatively simple operation; secondary pollution | 76–94 | |
| Lu et al. (2019) | Biology | Artificial wetland | Secondary treatment | Low cost and simple maintenance; limited processing efficiency covers a large area | 59–78 |
| Miyake et al. (2023) | Activated sludge process | Urban sewage | Mature technology, low cost; high requirements for operation control | 92–96 | |
| Pan et al. (2022) | Biofilm reactor | Secondary treatment or industrial wastewater | High efficiency and strong controllability; high cost | 59–99 | |
| Karla et al. (2022) | Biofilters | Agricultural wastewater | High stability; high requirements for water quality | 95–99 | |
| Bashir et al. (2023) | Physical chemistry | Advanced oxidation process | Industrial wastewater | High efficiency, simple process; high cost, high energy consumption | 90–100 |
MECHANISM OF NITROGENOUS WASTEWATER TREATMENT BY THE CW-MFC SYSTEM
The construction and basic principle of the CW-MFC system
, electrons, and protons. Electrons flowed to the cathode through an external circuit. There are several types of CW-MFC systems, mainly based on their structural differences and water flow methods. In a vertical flow CW-MFC, water flows vertically through multiple layers of a substrate (Gupta et al. 2021b), facilitating oxygen distribution within the wetland and air movement into and out of the wetland, aiding in redox condition changes. In a horizontal flow CW-MFC, water moves horizontally along a plane. This type typically has better anaerobic conditions and is suitable for organic matter decomposition and the removal of certain types of nitrogen (Shen et al. 2018). A composite flow CW-MFC combines the features of both horizontal and vertical flows to optimize the wastewater treatment process. This design allows the simultaneous use of the advantages of aerobic and anaerobic conditions within the same system. Figure 1 shows the fundamental structure of a CW-MFC. 
CW-MFC structure diagram: (a) vertical flow CW-MFC, (b) horizontal flow CW-MFC, and (c) composite flow CW-MFC; the wire in the figure connects the external resistor.
CW-MFC structure diagram: (a) vertical flow CW-MFC, (b) horizontal flow CW-MFC, and (c) composite flow CW-MFC; the wire in the figure connects the external resistor.
CW and MFC systems jointly exploit the redox gradient between the anaerobic anode and aerobic cathode, facilitating efficient electron and proton transfer, thus enhancing the system's removal efficiency (Srivastava et al. 2015).
The form of nitrogen in wastewater and its metabolic process in CW-MFC
Nitrogen exists in six forms in aquatic environments: molecular nitrogen (N2), ammonia nitrogen, ammonium 
, nitrite ions 
, nitrate ions 
, and organic nitrogen from the atmosphere. Microorganisms can catalyze the conversion of organic nitrogen into inorganic pollutants. Consequently, the primary focus is on inorganic nitrogen in wastewater treatment centers (Mai et al. 2021). Inorganic nitrogen in wastewater predominantly takes the form of ammonia nitrogen, nitrite nitrogen, and nitrate nitrogen (Odedishemi Ajibade et al. 2021). Ammoniacal nitrogen primarily originates from the decomposition of proteins, urea, ammonia, and other substances present in wastewater. Nitrate and nitrite nitrogen are generated by the oxidation of ammonia nitrogen during the wastewater treatment.
(−3 oxidation state) to 
(+5 oxidation state). This process is primarily facilitated by chemolithoautotrophic bacteria that utilize oxygen as an electron acceptor, ammonia nitrogen as an energy source, and carbon dioxide as a carbon source. In addition to the nitrification process, specialized bacteria, known as denitrifying bacteria, can employ nitrate as an external electron acceptor within the aerobic zone of the CW-MFC, reducing it to nitrogen (N2) or nitrogen oxides. This process is termed nitrite denitrification, and denitrifying bacteria play a crucial role in CW-MFC. They utilize electrons from the anode to reduce nitrates in wastewater to nitrogen or nitrogen oxides, which are subsequently released into the atmosphere. The reaction formula is as follows (Semedo et al. 2018):
(+5 oxidation state) to N2 (0 oxidation state) through intermediary products such as 
, NO, and N2O. Denitrification occurs under anaerobic conditions and relies on the presence of effective organic substrates. Facultative bacteria utilize nitrogen oxides as alternative electron acceptors during cell respiration (Li et al. 2021b). Under specific anaerobic conditions, anaerobic ammonia oxidation reactions take place, with ammonia serving as an electron donor and nitrate or nitrite as the electron acceptor. Anaerobic AOB directly convert ammonia to nitrogen. The reaction formula is as follows:
In their study, Srivastava et al. (2020b) employed electrode-dependent Anammox for wastewater treatment within a hybrid continuous-flow CW-MFC, incorporating both horizontal and vertical flows. The removal rates of ammonia nitrogen and chemical oxygen demand (COD) surpassed those of the conventional CW and open-circuit CW-MFC systems. In addition, plant-based nitrogen fixation and partial nitrification–denitrification processes also contribute to nitrogen removal in CW-MFC.
Mechanism of CW-MFC for nitrogenous wastewater treatment
Treatment of nitrogen-containing wastewater in CW-MFC primarily depends on microbial metabolism. This process capitalizes on the synergistic interplay between MFCs and CWs, significantly enhancing treatment efficacy (López et al. 2016). Throughout the treatment process, microorganisms absorb and break down nitrogen compounds in wastewater, including ammonia, nitrate, and nitrite. The existence of both negative and positive electrodes within a CW-MFC system facilitates the proliferation of nitrifying bacteria, such as AOB and NOB, as well as electrochemically active bacteria in wastewater. The compatibility and surface roughness of the electrode promote the conditions necessary for bacterial colonization and growth, particularly when activated carbon or graphite felt (González et al. 2021).
adsorption, and organic nitrogen burial (Vymazal 2007; Lee et al. 2009). Among these six denitrification mechanisms, nitrification–denitrification is the most crucial mechanism in CW-MFC. Nitrogen removal in MFC is primarily attributed to bioelectrochemical effects. Leveraging the EAB, the CW-MFC system integrates wastewater treatment and power generation by leveraging the electroactivity of the EAB. EAB generate electrons during the decomposition of organic matter in the anode region. This electron production supplies the requisite energy for microbial nitrogen conversion and various reactions, resulting in the reduction of 
and 
to N2, 
, and NO compounds, as illustrated in Figure 2. This process effectively eliminates nitrogen-containing pollutants from wastewater (Hong et al. 2015; Wang et al. 2016). MFCs expedite ammonia oxidation in wastewater while concurrently transferring electrons to the anode. This dual function augments the denitrification efficiency. Within an MFC, nitrite and nitrate can serve as electron acceptors at the cathode and undergo bioelectrochemical reduction or autotrophic denitrification. The corresponding reactions are as follows (Oon et al. 2018):
Mechanism of CW-MFC treatment of nitrogenous wastewater: This figure illustrates the different processes involved in the nitrogen cycle in CW-MFC. The blue part indicates the nitrification process, the green part denitrification, the red part anaerobic ammonia oxidation, and the yellow part nitrate decomposition and reduction to ammonia.
Mechanism of CW-MFC treatment of nitrogenous wastewater: This figure illustrates the different processes involved in the nitrogen cycle in CW-MFC. The blue part indicates the nitrification process, the green part denitrification, the red part anaerobic ammonia oxidation, and the yellow part nitrate decomposition and reduction to ammonia.
Factors affecting CW-MFC treatment of nitrogen-containing wastewater
Numerous factors, including environmental and internal variables, influence the treatment efficiencies of CW-MFCs. This paper provides an overview of the factors that affect the treatment of nitrogen-containing wastewater by CW-MFCs, incorporating the influent C/N ratio (COD/TN), pH, temperature, DO, electrode materials, vegetation, and hydraulic retention time (HRT).
Influent C/N ratio
The influent C/N ratio is a pivotal factor affecting the treatment of nitrogen-containing wastewater in CW-MFC systems. In addition, it serves as a crucial parameter that influences the activity of AOB and NOB (Li et al. 2017). Within a specific range, augmenting the influent C/N ratio enhanced the treatment efficiency of the CW-MFC. For instance, Xu et al. (2017) found in their study on bioelectrochemically assisted CWs that when the C/N ratio increased from 2 to 5, the TN removal efficiency increased from 31.9 to 68.8%. The effluent NO removal rate also increased from 58.9 to 91.3%. Table 2 (Wang et al. 2019b) provides data on electricity generation and nitrogen removal in CW-MFC systems at six distinct wastewater C/N ratios. When the influent C/N ratio was ≤3, the TN removal rate exhibited an incremental trend with higher ratios, with the highest removal rate reaching approximately 90.30%. Conversely, when the influent C/N ratio was ≥3, the average TN removal rate exhibited marginal growth and plateaued at 90.30–91.46%. Beyond a certain threshold, exceeding the C/N ratio results in diminished treatment efficiency. Excessive concentration of carbon and nitrogen concentrations adversely affect the growth and metabolism of microorganisms. Hence, regulating the appropriate C/N ratio is crucial for enhancing the efficiency of CW-MFC in nitrogen-containing wastewater treatment.
Power production performance and nitrogen removal of CW-MFC system compared with different C/N
| C/V ratio | Output voltage (mV) | Power density (mW m−3) | Pollutant removal (%) |
|---|---|---|---|
| TN | |||
| 1.23 | 70 | 0.09 | 75.11 |
| 2 | 110 | 0.25 | 85.42 |
| 3 | 165 | 0.73 | 90.30 |
| 5 | 247 | 1.21 | 90.37 |
| 7 | 312 | 1.82 | 91.30 |
| 9 | 399 | 2.86 | 91.24 |
| C/V ratio | Output voltage (mV) | Power density (mW m−3) | Pollutant removal (%) |
|---|---|---|---|
| TN | |||
| 1.23 | 70 | 0.09 | 75.11 |
| 2 | 110 | 0.25 | 85.42 |
| 3 | 165 | 0.73 | 90.30 |
| 5 | 247 | 1.21 | 90.37 |
| 7 | 312 | 1.82 | 91.30 |
| 9 | 399 | 2.86 | 91.24 |
Output voltage refers to the voltage generated by the MFC, and power density refers to the ratio of power to the area of the MFC.
pH value
pH profoundly influences the growth and metabolic activity of microorganisms. Various microorganisms exhibit distinct pH adaptation ranges. Tidal Flow Constructed Wetland-Microbial Fuel Cell (TFCW-MFC) research study by Wang et al. (2020) revealed that pH primarily shapes the niche of the device, as it significantly influences the diversity and composition of 35 microbial communities. Microbial species thrive in unique environments. For example, AOB thrive at pH 8.1, while NOB favor pH 7.9. Optimal pH conditions foster a conducive environment for microbial growth and sustain the diversity and stability of microbial communities. This, in turn, enhances the degradation of organic matter and nitrogen conversion efficiency in wastewater. In ceramsite-filled CW-MFC systems, the notable removal efficiency of 
can be partly attributed to the elevated pH (above 8.0) of pore water and concurrent nitrification and denitrification processes (Zhong et al. 2020). Moreover, pH levels affect the electrode reactions and electron transport processes. Within the CW-MFC system, the pH values in the anode and cathode regions influenced the electrochemical reaction rates and electrode surface efficiency. Yang et al. (2021) emphasized that maintaining the pH gradient around 7.5 in magnetite CW-MFCs is crucial for completing the entire wastewater treatment process and electricity generation, achieving cathodic potentials ranging from −98 ± 52 mV to approximately −175 ± 60 mV. As indicated in Table 3, the CW-MFC exhibits notable bioelectricity generation under slightly neutral and alkaline influent conditions. In a slightly neutral environment, the removal efficiency for COD and NO3–N increased by 8.3 and 40.2%, respectively. Favorable pH conditions play a pivotal role in enhancing electron transport efficiency, leading to a significant increase in the abundance of EAB, nitrifying bacteria, and denitrifying bacterial communities. This, in turn, bolsters the power generation and wastewater treatment efficiency of the system.
Effects of different pH values on power production performance and contaminant removal of CW-MFC system
| pH | Output voltage (mV) | Maximum power density (mW m−2) | Maximum current density(mA m−2) | Pollutant removal (%) | |
|---|---|---|---|---|---|
| COD | NO3–N | ||||
| 5.2 | −42 to 83 | 0.11 | 3.98 | 27 ± 3.5% | 69 ± 4.5% |
| 7.3 | 2 to 122 | 8.08 | 53.74 | 44 ± 3.3% | 80.7 ± 1.5% |
| 8.8 | 3 to 177 | 4.19 | 47.77 | 57 ± 4.6% | 70.0 ± 2.9% |
| pH | Output voltage (mV) | Maximum power density (mW m−2) | Maximum current density(mA m−2) | Pollutant removal (%) | |
|---|---|---|---|---|---|
| COD | NO3–N | ||||
| 5.2 | −42 to 83 | 0.11 | 3.98 | 27 ± 3.5% | 69 ± 4.5% |
| 7.3 | 2 to 122 | 8.08 | 53.74 | 44 ± 3.3% | 80.7 ± 1.5% |
| 8.8 | 3 to 177 | 4.19 | 47.77 | 57 ± 4.6% | 70.0 ± 2.9% |
Current density refers to the ratio of the current generated by CW-MFC to the electrode surface area. Higher current density generally indicates better electrochemical activity and higher power conversion efficiency of the battery.
Temperature
Within a specific temperature range, increasing the temperature enhances the performance of CW-MFCs, boosting the removal rates of COD and ammonia nitrogen, as well as the cell's output potential. For instance, the research study by Wang et al. (2018) identified the optimal temperature for pollutant removal and power generation within the 25–45 °C range as 35 °C. At this point, the removal rates of COD and 
reached 73.98 and 99.24%, respectively. In addition, the Coulombic efficiency of the MFC increased to 8.12%. Temperature also influences the composition and abundance of microbial communities in CW-MFC systems. Research indicates that increasing the temperature can elevate the proportion of denitrifying bacteria (Nitrosomonas europaea) within the system, leading to an increase in TN removal rate from 63 to 74% (Ahn & Logan 2010). However, excessively high temperatures can adversely affect microbial growth and reduce the removal efficiency of the system (Fdz-Polanco et al. 1994). Temperature also influences the electrode reaction rate and stability. At lower temperatures, the oxygen reduction reaction rate decreases, resulting in a lower battery output potential. Following the pilot-scale operation of CW-MFC, a phenomenon of voltage drop to 58.20 mV occurred due to an increase in the temperature difference between day and night (Yang et al. 2022). Conversely, elevated temperatures can induce corrosion and oxidation of electrode materials, affecting the reaction rate and electrode stability. Hence, judicious temperature control enhanced the removal efficiency and battery performance of the system. However, it is crucial to acknowledge that various operating temperatures may have different impacts on the system efficiency, necessitating the optimization of specific conditions.
Dissolved oxygen
DO is a vital oxygen source for microbial respiration and metabolism. Sufficient DO offers an appropriate oxygen supply, stimulating microbial growth and activity, and thereby enhancing pollutant degradation in wastewater. In recent years, several studies have investigated the impact of DO on the performance of CW-MFC systems for treating nitrogen-containing wastewater. Oon et al. (2017) investigated the impact of extensive vegetation and supplemental aeration on an upflow CW-MFC system. The findings demonstrated that incorporating large plants and aeration substantially enhanced the COD and nitrogen removal efficiency. The aeration system exhibited a high DO concentration (4–5 mg/L), which stimulated the activity of aerobic microorganisms and facilitated the oxidation and nitrification of organic matter. As a result, the removal rates of COD and ammonium reached 98 and 81%, respectively. In a different study, Gupta et al. (2021a) devised an algae-assisted CW-MFC system integrated with sand filters, aiming at efficient wastewater treatment and electricity generation. The results revealed that the algae-assisted CW-MFC system presented elevated DO levels due to oxygen release during photosynthesis, leading to increased microbial activity and improved degradation and nitrification of organic matter, resulting in the effective removal of 85.14 ± 10.73% of 
. Wang et al. (2019a) demonstrated the significance of DO as a factor influencing the performance of a CW-MFC system. The optimal DO concentration for denitrification was 1.5 mg/L. The relatively high DO concentration in the aerobic zone stimulates the activity of nitrifying bacteria and improves the nitrification process. DO plays a pivotal role in the performance of CW-MFC systems for treating nitrogen-containing wastewater. Elevated DO concentrations enhance the activity of aerobic microorganisms, improve the oxidation of organic matter, and promote nitrification. Under optimal DO conditions, CW-MFC achieved a removal of 86.26% for NH3–N and 81.35% for TN. Nonetheless, excessive DO levels can negatively impact the system performance by causing undue competition for electron transport during the oxygen reduction reaction in the cathode region, thereby diminishing the denitrification efficiency. Consequently, maintaining an optimal DO concentration in the CW-MFC system is imperative for achieving efficient wastewater treatment and energy recovery.
Electrode material
The cathode and anode are crucial components of CW-MFC, directly affecting their performance and effectiveness in wastewater treatment. Table 4 presents the effects of the five emerging electrode materials on nitrogen removal and power output in the system to varying degrees. Carbon materials have gained widespread use as electrode materials owing to their excellent adsorption capacities, high specific surface area, good electrical conductivity, and cost-effectiveness. Porous carbon materials, such as graphite, exhibit strong electrical conductivity and redox reactivity, enabling enhanced microbial attachment and expanding the electrode reaction surface area, thus improving system performance. The graphite felt electrode employed by Wang et al. (2022b) demonstrated excellent performance in treating nitrogen-containing wastewater, achieving a 91.1% removal efficiency for ammonia nitrogen and a power output of 0.628 W/m3. Metallic materials have garnered significant attention. Iron-based nanoparticle electrodes promote microbial attachment and electron transfer, thereby enhancing the system performance (Liu et al. 2022b). Xu et al. (2018) reported the use of a titanium mesh-coated electrode, which exhibited strong performance in treating nitrogen-containing wastewater, achieving a TN removal efficiency of 82.46% and a power output of 3,714.08 mW/m2. Biomaterials materials have also emerged as potential electrode materials. Ramírez-Vargas et al. (2019) introduced an innovative CW design based on microbial electrochemistry. This design operates without solid electrodes or external circuits; instead, a conductive biofilter functions as a single electrode, achieving 90% removal efficiency for COD and 46% removal efficiency for ammonia nitrogen. Consequently, the nitrogen removal efficiency and power output of the system can be enhanced by selecting appropriate electrode materials. However, further research is required to determine the optimal electrode materials and design parameters.
Effects of different electrode materials on power generation and nitrogen removal of CW-MFC system
| Reference | Electrode material | Resistance (Ω) | Output voltage (mV) | Maximum power density (mW m−2) | Pollutant removal (%) | |
|---|---|---|---|---|---|---|
 | TN | |||||
| Oon et al. (2015) | Carbon felt | 1,000 | 421 | 6.1 | 91 | – |
| Doherty et al. (2015a) | Graphite rod | 300 | 434 | 2.8 | 75 | 58 |
| Li et al. (2019) | Manganese ore particles | 1,000 | 511 | 9.7 | 87 | – |
| Yang et al. (2021) | Pyrrhotite particles | 983 | 134 | 15 | 88 | – |
| Dai et al. (2020) | Zero-valent iron | 1,000 | 230 | 2.1 | 84 | 82 |
| Reference | Electrode material | Resistance (Ω) | Output voltage (mV) | Maximum power density (mW m−2) | Pollutant removal (%) | |
|---|---|---|---|---|---|---|
| TN | |||||
| Oon et al. (2015) | Carbon felt | 1,000 | 421 | 6.1 | 91 | – |
| Doherty et al. (2015a) | Graphite rod | 300 | 434 | 2.8 | 75 | 58 |
| Li et al. (2019) | Manganese ore particles | 1,000 | 511 | 9.7 | 87 | – |
| Yang et al. (2021) | Pyrrhotite particles | 983 | 134 | 15 | 88 | – |
| Dai et al. (2020) | Zero-valent iron | 1,000 | 230 | 2.1 | 84 | 82 |
Plants
Effects of different plant types on power production performance and nitrogen removal of CW-MFC system
| Reference | Plant types | Output voltage (mV) | Maximum power density (mW m−2) | Maximum current density (mA m−2) | Coulombic efficiency (%) | Pollutant removal (%) | ||
|---|---|---|---|---|---|---|---|---|
 | NO3–N | TN | ||||||
| Wang et al. (2017) | Unplanted | 235 | 6.48 | 50.96 | 0.23 | 23 | 78 | 62 |
| Typha orientalis | 416 | 21.53 | 94.27 | 0.38 | 47 | 82 | 71 | |
| Scirpus validus | 345 | 14.11 | 73.88 | 0.32 | 45 | 80 | 74 | |
| Juncus effusus | 182 | 2.23 | 20.21 | 0.17 | 29 | 85 | 65 | |
| Yang et al. (2020) | Iris pseudacorus | 282 | 25.14 | 80.49 | 9.05 | 72 | 96 | - |
| Tao et al. (2020) | Water hyacinth | – | 6.09 | – | – | 95 | 85 | 87 |
| Ji et al. (2022) | Cyperus alternifolius | 600 | 24.37 | 129.92 | 0.69 | 93 | – | 86 |
| Ebrahimi et al. (2023) | Dwarf Papyrus | 426 | 6.72 | 82.6 | 1.75 | 87 | 82 | 65 |
| Xu et al. (2023) | Calamus | 207 | 1.33 | 7.28 | – | 79 | 82 | 71 |
| González et al. (2021) | Schoenoplectus californicus | 119 | 8.55 | 27.39 | 4.3 | 66 | 62 | – |
| Reference | Plant types | Output voltage (mV) | Maximum power density (mW m−2) | Maximum current density (mA m−2) | Coulombic efficiency (%) | Pollutant removal (%) | ||
|---|---|---|---|---|---|---|---|---|
| NO3–N | TN | ||||||
| Wang et al. (2017) | Unplanted | 235 | 6.48 | 50.96 | 0.23 | 23 | 78 | 62 |
| Typha orientalis | 416 | 21.53 | 94.27 | 0.38 | 47 | 82 | 71 | |
| Scirpus validus | 345 | 14.11 | 73.88 | 0.32 | 45 | 80 | 74 | |
| Juncus effusus | 182 | 2.23 | 20.21 | 0.17 | 29 | 85 | 65 | |
| Yang et al. (2020) | Iris pseudacorus | 282 | 25.14 | 80.49 | 9.05 | 72 | 96 | - |
| Tao et al. (2020) | Water hyacinth | – | 6.09 | – | – | 95 | 85 | 87 |
| Ji et al. (2022) | Cyperus alternifolius | 600 | 24.37 | 129.92 | 0.69 | 93 | – | 86 |
| Ebrahimi et al. (2023) | Dwarf Papyrus | 426 | 6.72 | 82.6 | 1.75 | 87 | 82 | 65 |
| Xu et al. (2023) | Calamus | 207 | 1.33 | 7.28 | – | 79 | 82 | 71 |
| González et al. (2021) | Schoenoplectus californicus | 119 | 8.55 | 27.39 | 4.3 | 66 | 62 | – |
Coulombic efficiency refers to the ratio between the electrons utilized in the electron transfer process and the total number of electrons in a MFC. This metric is used to evaluate the electron transfer efficiency and energy loss of CW-MFC.
Plant rhizosphere effect diagram: plants promote the biological transformation process of nitrogen through the oxygen secreted by their roots and through symbiosis with microorganisms.
Plant rhizosphere effect diagram: plants promote the biological transformation process of nitrogen through the oxygen secreted by their roots and through symbiosis with microorganisms.
Hydraulic retention time
The HRT is defined as the average duration during which water remains within the CW. The HRT significantly affected the removal efficiency of the CW-MFC. Within the CW-MFC system, an extended HRT affords microorganisms more time for the degradation and transformation of organic matter and nitrates in wastewater. A longer residence time offers microorganisms more opportunities to treat wastewater pollutants, leading to enhanced removal efficiency. Moreover, an extended HRT reduces the organic load, easing the burden on microorganisms, allowing more time for proper adaptation and growth, and further enhancing the removal efficiency. The findings of Zhong et al. (2020) demonstrated that at an HRT of 2.8 days, the removal efficiency for nitrogen, phosphorus, and organic matter reached its peak, with a denitrification efficiency as high as 93.8%. Furthermore, the composition of the microbial community was significantly influenced by HRT, resulting in different dominant species under varying HRT. Extending HRT augments the contact duration between the wastewater, the matrix, and microorganisms within the CW-MFC, thereby enhancing the timeframe for pollutant adsorption and degradation reactions (Wu et al. 2015). Nevertheless, if the HRT surpasses a critical threshold, the rate of improvement in the system processing efficiency diminishes, and the processing capacity of the system sharply decreases. Furthermore, an extended HRT intensifies the anaerobic conditions in CW-MFC, adversely affecting the removal efficiency of organic matter and nitrogen (Saeed & Sun 2012). Therefore, maintaining an appropriate HRT is essential to ensure optimal pollutant removal efficiency and wastewater treatment capacity within CW-MFC.
CHALLENGES AND PROSPECTS
CW-MFC provides a potential path for nitrogen-containing wastewater treatment; however, they are limited by the efficiency of the microbial degradation of organic matter, electron transfer efficiency, and anode reaction rate. When the concentrations of nitrogen and organic matter in wastewater fluctuate, the treatment and power output of the CW-MFC system is affected (Kong et al. 2023). Therefore, improving the nitrogen removal efficiency and power generation capacity of such systems is a major challenge. In addition, the CW-MFC system faces problems such as decreased microbial activity and electrode blockage during long-term operation (Guadarrama-Pérez et al. 2019), which may affect the stability and operational reliability of the system. To overcome these problems, future research should first focus on optimizing the design and operating parameters of CW-MFC systems. At present, research on the factors affecting the operation of CW-MFC systems, such as external resistance, electrode material, aeration, influent flow rate control, and reactor structure, is not sufficiently deep and needs to be further strengthened. For example, this study shows that the innovation of electrode materials is key to improving the performance of CW-MFC, but the current research mainly focuses on the conductivity and corrosion resistance of electrode materials (Srivastava et al. 2020b). In the future, additional electrode material optimizations and electrode surface modifications should be performed to improve the stability and operational reliability of the system. Simultaneously, attention should be paid to the biocompatibility and cost-effectiveness of these materials, and more efficient conductive polymers or biodegradable materials should be developed. These materials can not only improve the output of electric energy but also reduce the cost and environmental impact of system operation. The serial-parallel modular design of the CW-MFC system structure is also an effective way to improve nitrogen removal efficiency (Srivastava et al. 2020a). Simultaneous nitrification–denitrification and anaerobic ammonia oxidation were achieved within the system to achieve complete nitrogen removal and a high power output. Second, theoretical research on the CW-MFC system in nitrogen removal mechanism, electricity generation mechanism, internal interaction, and antagonistic mechanism of the system was conducted. An in-depth exploration of the competitive mechanisms between microorganisms, such as the competition between electrochemically and non-electrochemically active bacteria for the organic substances needed for power generation (Zhang et al. 2020), the mechanism of extracellular electron transfer, and how they interact and antagonize different components of the system, can more effectively remove target pollutants and generate energy. Finally, the use of microbial remediation technology to repair or restore the activity of microorganisms in a system by adding new, efficient microbial strains or growth factors is another direction for improving the stability of the system.
Future application potential.
CONCLUSION
This study confirms the effectiveness of the CW-MFC as an integrated bioelectrochemical technology in treating nitrogenous wastewater. However, the denitrification efficiency of CW-MFC is influenced by multiple factors during operation, particularly carbon–nitrogen loading, pH, temperature, HRT, and DO concentration. Precise control of these parameters is crucial for significantly enhancing denitrification efficiency. Choosing corrosion-resistant, highly conductive electrode materials, as well as implementing reasonable planting strategies for vegetation, also contributes to improving the overall system performance. It should be noted that although CW-MFC has demonstrated excellent treatment results in laboratory-scale experiments, it still faces challenges such as fluctuations in environmental factors and maintaining long-term stability during scaled-up applications in real wastewater treatment scenarios. Therefore, based on the optimized conditions proposed in this paper, it is recommended to further explore the denitrification applications of CW-MFC in industrial and environmental management and to investigate its integration with other wastewater treatment technologies. This will enhance its application potential in a wider range of environments and comprehensively address the challenges of treating nitrogenous wastewater.
AUTHOR CONTRIBUTIONS
YX wrote the main manuscript text. YS put forward ideas and determined the framework. SY and TG checked the manuscript. All authors reviewed the manuscript.
FUNDING
This research was supported by the Natural Science Foundation of China (51868010, 52170154), the Guangxi Natural Science Foundation Program (2021GXNSFBA196023), the Guilin Science and Technology Development Program (20190219-3), and the Project of Improving the Basic Scientific Research Ability of Young and Middle-Aged College Teachers in Guangxi (2021KY0275).
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.
