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Article

Effects of Carbon Source on Denitrification and Electricity Generation in Composite Packing MFC-CW for Tail Water Treatment

by
Yu Kong
1,2,
Jing Hu
3,
Xiwu Lu
1,* and
Changgen Cheng
2
1
School of Energy and Environment, Southeast University, Nanjing 210096, China
2
Nanjing Municipal Design and Research Institute Co., Ltd., Nanjing 210008, China
3
Xuzhou Water Supply and Drainage Management Center, Xuzhou 221000, China
*
Author to whom correspondence should be addressed.
Water 2023, 15(24), 4285; https://doi.org/10.3390/w15244285
Submission received: 20 November 2023 / Revised: 2 December 2023 / Accepted: 11 December 2023 / Published: 15 December 2023
(This article belongs to the Special Issue Constructed Wetlands for Water Treatment and Reuse)
Browse Figures
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Abstract

:
The tail wastewater from sewage treatment facilities usually lacks carbon sources, and its subsequent treatment for deep nitrogen removal is difficult in natural conditions. In this study, the constructed wetland (CW) was integrated with microbial fuel cell (MFC) with high-density polyethylene (HDPE) fillers as the main matrix to improve nitrogen removal under inefficient carbon source conditions. Compared with the regular MFC and CW systems, MFC-CW attained higher nitrogen removal under low-carbon source conditions. The influence of influent carbon/nitrogen ratio (C/N) on the denitrification and electricity-generation performance was explored. Although the increase of carbon source simultaneously improved chemical oxygen demand (COD), ammonia (NH4+-N), nitrate (NO3−-N) and TN removal, the power generation during the carbon source adjustment showed low relation with the variation of influent COD in the range of 40–120 mg/L. CW was more dependent on carbon sources, and the addition of bioelectrochemical systems into MFC-CW could reduce the dependence of nitrogen removal on carbon sources, especially under low carbon source conditions. These findings offer valuable insights into the potential applications of MFC-CW for tail water treatment, and its parameters for utilization in real CWs should be explored in future studies.

1. Introduction

The purpose of sewage treatment facilities is usually to meet discharge standards, but the quality of tail water is still insufficient, and there is a need for surface water protection [1]. When tail water is discharged into the surface water directly, it can cause serious pollution or eutrophication [2]. At the same time, the tail water from sewage plants usually contains limited carbon sources, which makes it more difficult for deep nitrogen removal under natural conditions [3]. In China and many countries, there are increasingly strict requirements for deep nitrogen removal from tail water. In Jiangsu province, China, the total nitrogen (TN) and ammonia (NH4+-N) in the tail wastewater of sewage treatment plants must be controlled below 10 mg/L and 1.5 mg/L, respectively, when discharged to sensitive water bodies. New methods for deep nitrogen removal from tail water should be developed.
The deep nitrogen removal is usually focused on nitrogen-composite decomposition with highly advanced oxidation such as photocatalytic technologies [4,5], and efficiency improvements with low carbon sources. In fact, it is very difficult to realize deep nitrogen removal under low carbon source conditions. Many research studies have been carried out to enhance nitrogen removal for low carbon wastewater treatment, such as biological aerated filters, zeolite adsorption, and constructed wetlands (CWs) [6]. CWs as a kind of ecological system also struggle to remove nitrogen under low carbon conditions [7]. Ge et al. [8] found that the nitrate removal was only 55.2% in CWs with the influent chemical oxygen demand/total nitrogen ratio (C/N) of 5. Some researchers combine CWs with microbial fuel cell (MFC) as MFC-CW, and utilize the comprehensive effects of biology, physics, chemistry, and electrochemical to improve the removal of pollutants such as heavy metals, dyes, antibiotics, and nitrogen [9,10]. In this system, one conductive material is embedded in the surface layer of the CW as the cathode, and another conductive material is embedded in the lower layer of the constructed wetland as the anode, coupling the constructed wetland with the microbial fuel cell, which can simultaneously treat wastewater and generate electricity.
In MFC-CW, the influent C/N is one of the important factors affecting the microbial nitrification and denitrification [11]. Influent C/N, representing the relative content of organic carbon source in the influent, plays a key role in the nitrogen removal of the biological and ecological treatment systems [12]. During the denitrification reaction, the carbon source is not only the energy source for microbial growth but also the electron donor for the denitrifying bacteria [13]. The tail water of the sewage treatment plant usually contains inefficient carbon sources [14] and also affects the power production performance of the MFC-CW system.
In this study, MFC-CW, MFC, and CW were constructed with high-density polyethylene (HDPE) fillers as the main filling substrate. Their removal performance, especially nitrogen removal and power generation under low-carbon conditions during the initiation period, was investigated. Moreover, the influence of C/N on the three kinds of CWs was studied, so as to provide a reference for MFC-CW systems in engineering applications under low carbon source conditions for tail wastewater treatment.

2. Materials and Methods

2.1. Experimental Setup and Fillers for CWs

The experimental setup was built as a plexiglas cylinder with an inner diameter of 30 cm and a height of 55 cm shown in Figure 1. The total volume of the unit was 40.25 L and the working volume was 21.5 L. The graphite electrode plate (200 mm × 100 mm × 8 mm) was placed at the gas–water interface and the anaerobic area inside the device as the cathode and anode. The electrode plates were connected by copper wire with the external resistance (ZX 21, Shanghai Dongmao Electronic Technology Company, Shanghai, China). The external resistance could be adjusted according to the internal resistance derived from the polarization curves. In a recent study, the internal resistance under low carbon source conditions was adjusted around 1000 Ω [15]. Consequently, the external resistance was set as 1000 Ω. The generated voltage was monitored in real time by a paperless voltage recorder (Khat-300AG, Xiamen Kehao Automation Co., Ltd., Xiamen, China). Water hyacinths were planted near the cathode of the system to help form aerobic conditions owing to their fast growth rate and flourishing root systems [16]. MFC had the same structure as MFC-CW, but no water hyacinths were planted. CW was the same as MFC-CW, except that there were no electrodes.
Gravel is a commonly used substrate for MFC-CW systems. The gravel packing with a particle size of 8–12 mm was used to fill the bottom layer of the system to avoid blockages and stabilize influent water. The main part was filled with 10 mm high-density polyethylene (HDPE) fillers to a height of 30 cm, and the surface layer was filled with gravel packing to prevent the HDPE from floating. The HDPE fillers had a void ratio of 95%, and a specific surface area of over 500 m2/m3, which was conducive to biofilm growth and microorganism adhesion.

2.2. System Operation

The influent water was prepared in a 200 L barrel. A peristaltic pump (Baoding Chuangrui Pump Co., LTD., BT100M, Baoding, China) was used to pump the wastewater from the barrel to MFC-CW, MFC, and CW with an inflow rate of 7.47 mL/min. The water flow inside the systems was in the form of rising flow, and the hydraulic retention time was controlled at 2 days. The COD concentration of the influent was adjusted in the range of 40–120 mg/L with TN around 15 mg/L, and the corresponding influent C/N was in the range of 2.67–8. The influent water quality is shown in Table 1.

2.3. Water Quality, Electrochemical Analysis, and Scanning Electron Microscope of the Fillers

The influent and effluent water quality in the CWs were determined. COD, TN, NH4+-N, and NO3−-N were determined via the potassium dichromate method, alkaline potassium persulfate photometry, Nessler’s reagent spectrophotometry (752N, INESA (Group) Co., Ltd., Shanghai, China), and UV spectrophotometry (752N, INESA (Group) Co., Ltd., Shanghai, China), respectively.
The output voltage (U, mV) of MFC-CW and MFC were collected and saved using a paperless voltage recorder. The effective area of the anode was used to calculate the power density according to a previously outlined method [17]. The polarization curve was drawn to obtain the internal resistance by adjusting the resistance box from 10,000 Ω to 50 Ω.
The fillers were characterized before and after the application to the system during the initiation period using a scanning electron microscope (SEM, Quanta 200, FEI, Hillsboro, OR, USA).

3. Results and Discussion

3.1. Pollutants Removal and Power Generation during Initiation Period

3.1.1. Pollutants Removal during Initiation Period

The removals of COD and TN in CW, MFC-CW, and MFC during the initiation period are shown in Figure 2. In the first stage (0–7 days), the COD removal of the three systems fluctuated from 4.13% to 67.86%, and the average removals of CW, MFC-CW, and MFC were 15.33%, 48.00%, and 46.32%, respectively. The large fluctuation indicated that the microorganisms in the system were still in the adaptation stage, while the adhesion and adsorption of the fillers in the systems might play important roles in COD removal. The TN removal of the three systems fluctuated from 19.90% to 80.95%, and the average removal efficiencies of CW, MFC-CW, and MFC systems were 51.34%, 60.53%, and 57.54%, respectively, showing a relatively low TN removal at this stage, indicating that the microbial population in the three systems was not well adapted to the environment.
In the second stage (8–17 days), the COD removals of the three systems were in a range of 62.28–93.25%, and the average removal efficiencies of CW, MFC-CW, and MFC were 84.13%, 83.42%, and 77.56%, respectively. COD removal increased significantly compared with the first stage, indicating that the population density and activity of microbes increased with the initiation time extension. The TN removals of the three systems were in a range of 56.19% to 88.99%, and the average removal efficiencies of CW, MFC-CW, and MFC were 74.73%, 82.43%, and 61.70%, respectively. At this stage, the TN removal efficiency increased significantly, but it still fluctuated within a relatively large range, indicating that the microbial population within the system had not been fully adapted to the environment.
In the third stage (18–25 days), the COD removals of the three systems appeared more stable in the range of 85.17% to 91.83%, and the average removal efficiencies of CW, MFC-CW, and MFC were 88.03%, 89.80%, and 89.80%, respectively. With the initiation time extension, the COD removal of the three systems increased slowly to a stable state. The TN removals of the three systems fluctuated between 58.82 and 78.99%, and the average removal efficiencies of CW, MFC-CW, and MFC systems were 63.88%, 73.91%, and 68.65%, respectively. As the initiation time increased further, the TN removal efficiency of the three systems was relatively stable compared with the first and second stages, indicating that the microbial population has gradually adapted to the environment and the three devices were successfully started.
In the steady state of the system, the average TN removal of the three systems was ranked as MFC-CW > MFC > CW, with average removal efficiencies of 73.91%, 68.65%, and 63.88%, respectively. The average TN removal efficiency of the MFC-CW and MFC systems were higher than that of the CW system, indicating that the bioelectricity in the closed-circuit system and the current generation promoted the richness of the denitrification microorganisms, which in turn promoted the nitrification and denitrification in MFC-CW and MFC [18]. In addition, MFC-CW attained higher TN removal than MFC. The water hyacinth planted around the cathode might have produced more oxygen to form an aerobic environment, while the anode of the system was able to form an anaerobic environment owing to its upward flow mode. The alternation of the redox state was conductive to nitrification and denitrification inside the system. Water hyacinth planting around the cathode was beneficial for nitrogen removal, because its secretions around the roots were able to promote the metabolism of microbes and enrich the nitrogen removal community [19].

3.1.2. Voltage Output during Initiation Period

The output voltage of the MFC-CW and MFC was monitored in real time using a paperless voltage recorder with an external resistance of 1000 Ω. The resulting output voltage during the initiation period is shown in Figure 3. In the first stage (0–7 days), the output voltage of the MFC-CW and MFC systems fluctuated between 20–135 mV and 1–118 mV, with average output voltages of 60 mV and 55 mV, respectively. In the second stage (8–17 days), the average output voltage of the MFC-CW and MFC systems increased to 83 mV and 89 mV, respectively. In the third stage (18–25 days), the average output voltage of the MFC-CW and MFC systems increased further to 107 mV and 123 mV, respectively. With the increase in the initiation time, the average output voltage of MFC-CW and MFC increased, indicating that the electrogenic bacteria had also gradually adapted to the environment during the initiation period. More microorganisms gathered around the anode and cathode, and the voltage production increased with the enrichment of microbes.

3.1.3. Scanning Electron Microscope (SEM) of the Fillers

In this study, gravel and HDPE composite fillers were used in the CWs. Figure 4 and Figure 5 show the SEM plots of the surface of the gravel and HDPE composite fillers, respectively, before and after the system initiation. It can be seen from the figures that the unused gravel surface had more pores with a certain adsorption capacity. After the initiation period, the gravel in the system had a more concave and convex surface structure with many lumps (see the red box in Figure 4). The unused HDPE composite filler surface was relatively smooth. After the system was initiated, the HDPE composite filler surface became very rough, indicating that biofilm grew well on the surface of the fillers in the three systems (see the red box in Figure 5). The biofilm also played an important role in pollutant removal and electricity generation. In addition, the comparison of SEM images of gravel and HDPE composite fillers shows that the surface of HDPE composite filler changed more significantly than the gravel, and the number of attached microorganisms on the filler surface was much larger. This demonstrates that the HDPE composite filler was more conducive to microbial attachment and reproduction.

3.2. The Effects of C/N on Pollutant Removal and Electricity Generation

3.2.1. Pollutant Removal

The influent C/N is a vital factor affecting the microbial nitrification and denitrification in CW, CW-MFC, and MFC, especially under low carbon source conditions. The COD concentration of the influent was adjusted from 40 mg/L to 120 mg/L with TN around 15 mg/L, resulting in C/N of 2.67–8. The pollutant removal and power generation were monitored to analyze the effect of C/N.
The pollutant removal efficiencies of CW, CW-MFC, and MFC at different C/N ratios are shown in Figure 6. With the increase in influent carbon sources and the removal of COD, NH4+-N, NO3−N, and TN all increased. As shown in Figure 6a, when C/N was increased to 8, the COD removal efficiencies of the CW, MFC-CW, and MFC systems reached maximum values of 92.66%, 99.23%, and 96.00%, respectively, and the COD removal was higher than that in the study by Xie et al. [20], in which the highest COD removal of MFC-CW reached 78.3% with the influent COD at 500 mg/L. Under different C/N conditions, the COD removal was mostly above 90%, and the effluent COD concentrations of the three systems were less than 15 mg/L, which met the requirements of class A in Chinese Surface Water Environmental Quality Standard (GB 3838-2002) [21]. In the constructed wetland, except the parts of organic pollutants that were adsorbed by the fillers and plants, most were used as nutrients to promote the growth and reproduction of microorganisms, resulting in organic pollutant reduction in wastewater. Meanwhile, in MFC-CW and MFC systems, organic pollutants in the anode region were oxidized by anaerobic microorganisms to generate electrons, which flowed to the cathode for nitrogen removal. Organic pollutants in the cathode could also be used directly by denitrifying bacteria as electron suppliers. In addition, the removal of COD by the MFC system was higher than that of the CW and MFC-CW system, because the water hyacinth planted in the CW and MFC-CW system was also able to release carbon sources [22]. However, the dissolved oxygen content in the upper layer and cathode of the system was always in the range of 1.5–2.5 mg/L, and the organic matter released from the water hyacinth could not be used directly by microorganisms for denitrification.
As shown in Figure 6c, when C/N was above 4, the NO3−-N in the effluent in all three systems was low, with a removal efficiency of nearly 100%, which was higher than 84.9 ± 8.7% in the study by Tao et al. with C/N of 5.37 [23]. The high denitrification might be due to the fillers used in this study being HDPE composite fillers, which had higher porosity and specific surface area than regular gravel and sand, resulting in more microbes such as denitrifying bacteria adhesion and growth. With the increase in C/N, the removal of NO3−-N improved first and then stabilized, indicating that carbon source increase enhanced the activity of denitrification bacteria and promoted denitrification reaction. Under low C/N conditions, the denitrification requirement could not be met, resulting in the accumulation of NO3−-N [24]. In the MFC system, organic matter was mainly oxidized in the anode, and the generated electrons flowed to the cathode through the external circuit. The microorganisms in the cathode reduced NO3−-N to N2, without the need for carbon sources as the electron donors [25]. Compared with the CW system, the NO3−-N effluent concentrations of the MFC-CW and MFC systems were lower, indicating that the bioelectrochemical system significantly enhanced the denitrification.
As shown in Figure 6b,d, with the increase in C/N, the removal efficiencies of NH4+-N and TN increased with similar variation. The removal of NH4+-N was closely related to the removal of TN, because the nitration provided electron acceptors for the denitrification, and the complete oxidation of NH4+-N was the basis for the efficient TN removal. When C/N was 8, the removal efficiencies of NH4+-N and TN in three systems were the highest. The effluent TN concentrations of the CW and MFC-CW systems were 1.30 mg/L and 1.43 mg/L, respectively, meeting the fourth-grade requirement of Surface Water Environmental Quality Standard in China.
In addition, when the inlet C/N increased from 2.67 to 5.33, the TN removal of the CW system increased from 50.08% to 85.88%, the MFC-CW system from 57.28% to 73.77%, and the MFC system from 66.48% to 70.48%. The increase range of CW was the largest, indicating that the CW system was more dependent on carbon sources in relatively low C/N conditions. The dependence on carbon sources can be reduced by the introduction of a circuit in the MFC-CW system [26]. In the MFC system, organic matter was oxidized at the anode to generate electrons, and the microorganisms on the cathode utilized the electrons flowing from the anode to reduce nitrate and nitrite to nitrogen [27,28]. In this way, the electron suppliers, such as the carbon source, could be fully used for denitrification.

3.2.2. Electricity Production Analysis

The output voltages of the MFC-CW and MFC systems under different C/N conditions are shown in Figure 7. On the whole, the output voltage of MFC-CW and MFC systems showed similar changes under different C/N conditions, and the average output voltage was about 100 mV. With the increase in influent C/N, the output voltage of the two systems changed little, indicating that the influent C/N had no significant impact on the output voltage. Wen et al. [29] explored the effect of carbon sources on the output voltage of the MFC-CW system by increasing the influent COD from 100 mg/L to 400 mg/L (C/N from 5 to 20). The results showed that the average output voltage of the MFC-CW system increased significantly from 146 mV to 412 mV. Adequate organics are crucial for the normal metabolism and electroproduction of microorganisms in MFC, thus the power output rose with the increase in the influent organic load. However, in this study, the increase in the influent C/N did not greatly affect the voltage output because this research was conducted with relatively low carbon sources, with the highest COD concentration of 120 mg/L, which did not significantly affect the power production of the MFC-CW system.
As shown in Figure 8, the MFC-CW system achieved the maximum current density in the range of 5–9 mA/m2 and maximum power density of 0.23–0.83 mW/m2 under different C/N conditions. The maximum power density of the MFC-CW system was realized at the C/N of 6.67, while the maximum current density and power density of the MFC system were in the ranges of 5–8 mA/m2 and 0.23–1 mW/m2, respectively. The MFC system attained the maximum power density at a C/N of 5.33. The internal resistance could be obtained from the linear region of polarization curve in Figure 9. The internal resistances of MFC-CW at C/N of 2.67, 4, 5.33, 6.67, and 8 were 1150.7, 1792.5, 2211.9, 1680.3, and 2027.2 Ω, respectively. In MFC, the internal resistances were 1635.6, 1623.8, 1466.8, 1578.2, and 1485.4 Ω. The slightly higher internal resistance in MFC-CW was also a factor that led to the lower output power. The highest power density in MFC-CW was obtained at a C/N of 6.67, with an internal resistance of 1680.3 Ω. While in MFC, the maximum power density was obtained at a C/N of 5.33 with an internal resistance of 1466.8 Ω. The result indicated that the higher power density was obtained with lower internal resistance. This finding was also consistent with previous research, in which the maximum power density showed a negative relationship with internal resistance [29].
Many studies have shown that wetland plant roots release oxygen in the cathode region of the MFC-CW system and increase the redox potential, which can improve the electrical performance of the system [30]. Liu et al. [31] tested the power density of MFC-CW systems that were planted and non-planted. The power density of MFC-CW was higher than that of the non-planted MFC at the outset. But the power density of the non-planted MFC was higher in the later stages of the experiment, indicating that planting might also have a negative impact on the electrical performance. When plants with vigorous roots were placed in the cathode region of the MFC-CW system, excessive organic matter secreted by the root was not conducive to the production of a higher redox potential on the cathode. At the same time, oxygen could be transferred through the plant roots and organic matter such as rhizosphere sediments and exudates, affecting the electrode potential. On the whole, the power density of MFC was larger than that of MFC-CW in this study. The roots of the water hyacinth were abundant and vigorous during the experiment, which led to relatively low output power in the MFC-CW system.

4. Conclusions

After 25 days initiation, CW, MFC-CW, and MFC all had high and stable COD and TN removal. SEM images indicated the surface of gravel and HDPE fillers became rougher and HDPE fillers were more conducive to microorganism attachment. During the whole initiation period with low carbon source conditions, MFC-CW always attained the highest TN removal, with 73.91% in the steady state. With the increase in carbon sources in the influent, the nitrogen removal capacities of the three systems were simultaneously enhanced. When C/N was above 4, nearly 100% of the nitrate was deprived, and the effluent TN of MFC-CW and MFC was below 1.5 mg/L with C/N at 8. However, at low C/N ratios, the denitrification effect of CW was more dependent on carbon sources compared with MFC-CW and MFC. The output voltage of MFC-CW and MFC showed low relativity with carbon source variation in the range of 40-120 mg/L. The integration of MFC and CW could reduce the dependence of denitrification on carbon sources, especially in low carbon source conditions. Consequently, MFC-CW can be applied in tail water treatment with low carbon levels to achieve efficient nitrogen removal. Its application in real CWs should be explored in detail in future research.

Author Contributions

Y.K.: conceptualization, resources, data curation, writing—original draft; J.H.: formal analysis, methodology; X.L.: supervision, writing—review and editing; C.C.: project administration, investigation. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

We gratefully acknowledge the support from Jiangsu Province Industry-University Research Cooperation Project and the Nanjing Science and Technology Plan for Construction Industry (Ks22420).

Conflicts of Interest

Author Yu Kong was employed by the company Nanjing Municipal Design and Research Institute Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Installation of microbial fuel cell-integrated constructed wetland.
Figure 1. Installation of microbial fuel cell-integrated constructed wetland.
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Figure 2. Contaminant removal during three system start-up periods: (a) COD; (b) TN.
Figure 2. Contaminant removal during three system start-up periods: (a) COD; (b) TN.
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Figure 3. Voltage variation of MFC-CW and MFC during startup.
Figure 3. Voltage variation of MFC-CW and MFC during startup.
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Figure 4. SEM images of gravel before and after systems start-up: (a) unused; (b) CW; (c) MFC-CW; (d) MFC.
Figure 4. SEM images of gravel before and after systems start-up: (a) unused; (b) CW; (c) MFC-CW; (d) MFC.
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Figure 5. SEM images of HDPE fillers before and after systems start-up: (a) unused; (b) CW; (c) MFC-CW; (d) MFC.
Figure 5. SEM images of HDPE fillers before and after systems start-up: (a) unused; (b) CW; (c) MFC-CW; (d) MFC.
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Figure 6. The removal rate of pollutants in three systems under different C/N conditions: (a) COD; (b) NH4+-N; (c) NO3−-N; (d) TN.
Figure 6. The removal rate of pollutants in three systems under different C/N conditions: (a) COD; (b) NH4+-N; (c) NO3−-N; (d) TN.
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Figure 7. Voltage variation of MFC-CW and MFC at different C/N ratios.
Figure 7. Voltage variation of MFC-CW and MFC at different C/N ratios.
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Figure 8. Power density curves of the systems at different C/N: (a) MFC-CW (b) MFC.
Figure 8. Power density curves of the systems at different C/N: (a) MFC-CW (b) MFC.
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Figure 9. Polarization curves of the system at different C/N: (a) MFC-CW; (b) MFC.
Figure 9. Polarization curves of the system at different C/N: (a) MFC-CW; (b) MFC.
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Table 1. Water quality of influent in the experiment.
Table 1. Water quality of influent in the experiment.
IndicesConcentration (mg/L)
COD38.67–120.16
NH4+-N6.56–7.99
NO3−-N11.61–11.74
TN12.50–15.00
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Kong, Y.; Hu, J.; Lu, X.; Cheng, C. Effects of Carbon Source on Denitrification and Electricity Generation in Composite Packing MFC-CW for Tail Water Treatment. Water 2023, 15, 4285. https://doi.org/10.3390/w15244285

AMA Style

Kong Y, Hu J, Lu X, Cheng C. Effects of Carbon Source on Denitrification and Electricity Generation in Composite Packing MFC-CW for Tail Water Treatment. Water. 2023; 15(24):4285. https://doi.org/10.3390/w15244285

Chicago/Turabian Style

Kong, Yu, Jing Hu, Xiwu Lu, and Changgen Cheng. 2023. "Effects of Carbon Source on Denitrification and Electricity Generation in Composite Packing MFC-CW for Tail Water Treatment" Water 15, no. 24: 4285. https://doi.org/10.3390/w15244285

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