Progress in Nitrogen Removal in Bioelectrochemical Systems

Round 1

Reviewer 1 Report

I have several comments:

• Table 1 is not cited in the text of the article,
• All abbreviations should be explained when they appear for the first time in the article (MFC, NOB, AOB, CEM, ORR, HRT),
• The captions and diagrams in Figure 1 are illegible,
• The method of recording units should be standardized - see line 203,
• I my opinion latin species names should be in italics,
• I think English should be checked and improved. There are several errors, for example - lines 11, 319, 355.

Author Response

• 本文的正文未引用表1，

谢谢你的建议。表1引用该文章

• 所有缩写词在文章中首次出现时都应进行解释（MFC，NOB，AOB，CEM，ORR，HRT），

谢谢你的建议。所有缩写均已说明。

• 图1中的标题和图表难以辨认，

谢谢你的建议。图1中的标题和图表已得到纠正。

• 记录单位的方法应标准化-参见第203行，

谢谢你的建议。记录单位标准化

• 我认为拉丁文物种名称应使用斜体，

谢谢你的建议。拉丁文物种名称已用斜体显示。

• 我认为应该检查和提高英语。有几个错误，例如-第11、319、355行。

谢谢你的建议。错误已得到纠正。

 

Author Response File: Author Response.docx

Reviewer 2 Report

Generalities

The manuscript entitled “Progress in nitrogen removal in bioelectrochemical systems” aims to review nitrogen removal in bioelectrochemical systems. However, the review is, in general, very vague and after reading it, it is difficult to know where nitrogen removal in BES is and where it is going. It is lacking more information about the fundamental processes occurring, the bacteria involed, the role of thermodynamics, the operation methods that could be applied, etc.

Specifications

• Line 28. Change “And” for “An”.
• Reference [14] This is not the first article where nitrate removal from groundwater using bioelectrochemical denitrification was demonstrated. It was Park et al., (2005) Nitrate reduction using an electrode as direct electron donor in a biofilm-electrode reactor. Process Biochem 40: 3383–3388.
• In general, section 2 is missing of a comparison with benchmark technologies. It is difficult to understand which is the benefit of BES compared to them. It is not clear how a BES should be operated to get one results or another. How are the different nitrification or denitrification steps performed, etc.
• Section 2.1. Denitrification process. The key of denitrification in BES are autotrophic processes. There is a lack of explanation of this process in the section.
• Table 1. Please, standardize the units of Concentration/loading rate and Removal efficiency/rate. Moreover, nitrate removal rates higher than those showed in the Table have been reported in the literature.
• Section 2.1. How are the rates observed compared to conventional denitrification? Which are the costs related to BES denitrification? Comparison with benchmark technologies are needed.
• Table 2. Please, standardize the units used in the table.
• Section 2.2. How are the rates observed compared to conventional SND? Which are the costs related to BES SND? Comparison with benchmark technologies are needed.
• Section 2.3. Rates reported at very low (0.0125-0.075 kg/m3d). How can them compete with, partial-nitritation anammox (>1 kg/m3d), for example?
• Section 2.4. There are some article suggesting that anammox-kind or other nitrifying bacteria could oxidize ammonium using the electrode as electron acceptor. These articles are missing.
• Section 3. Paragraph 1. The authors describe the structure of a biofilm for SND, but the section label is “Structure of cathode biofilm”. A section explaining how a denitrifying cathodic biofilm is structured, which is the role of thermodynamics, etc. would be of a high interest, but unfortunately, it is not the case with the current section.
• Section 4 Influencing factors. The description of the influencing factors are vague, and there is a lack of explanation about how they can affect depending on the way the BES is operated. For example, the authors explain about the effect of the external resistance, which can be relevant for MFC operation, but they do not even mention the effect of the cathode potential, which is extremely relevant when operating a BES for nitrate removal under potentiostatic conditions.

Comments for author File: Comments.docx

Author Response

• Line 28. Change “And” for “An”.

Thank you for your suggestions. "And" is right.

• Reference [14] This is not the first article where nitrate removal from groundwater using bioelectrochemical denitrification was demonstrated. It was Park et al., (2005) Nitrate reduction using an electrode as direct electron donor in a biofilm-electrode reactor. Process Biochem 40: 3383–3388.

Thank you for your suggestions. The Reference [14] has been corrected. Literature is “Cathodic groundwater denitrification with a bioelectrochemical system”.

3. In general, section 2 is missing of a comparison with benchmark technologies. It is difficult to understand which is the benefit of BES compared to them. It is not clear how a BES should be operated to get one results or another. How are the different nitrification or denitrification steps performed, etc.

Thank you for your suggestions. Benchmark technologies has been added.

Removal process	Wastewater	Concentration （mg/L）		Removal efficiency	nitrogen loading rate (NLR)	Ref.
denitrification and partial nitrification-Anammox	mature landfill leachate		2230 mg/L NH4+-N	96.7%,	0.45 kg/(m3·d)	[27]
partial nitrification and integrated fermentation-denitritation	mature landfill leachate and waste activated sludge		2023 ± 75 mg/L TN	95.0%	an average nitrogen removal rate (NRR) of 0.63 kg/m3·d	[28]
SND	nutrient-rich abattoir		0.43 g N/ L-1·d	86%	0.37 kg/m³·d-1	[29]
SND	ynthetic petrochemical wastewater		127 mg/L NH4+-N	87.81%	0.1115 kg/m³·d-1	[30]
SND	synthetic wastewater		246.1mg/L TN	84.6%	119.2±22.1 (mg N/L·d) TN removal loading rate	[31]
Shortcut SND	domestic sewage		84.9mg/L TN	63.2%	0.02 (kg N/m3·d)	[32]
Anammox Partial Denitrification	sewage taken from the primary clarifier		46mg/L TN	88.38%	0.4kg N/m3·d	[33]
Anammox	Synthetic wastewater		227.60mg/L TN	94.68%	25.86 kg N/(m3·d),	[34]

 

• Section 2.1. Denitrification process. The key of denitrification in BES is autotrophic processes. There is a lack of explanation of this process in the section.

Thank you for your suggestions. The explanation of autotrophic processes has been added.  Autotrophic denitrification plays an important role in BES. Autotrophic denitrification bacteria utilize hydrogen, iron, or sulfur chemical compounds as sources of carbon dioxide and power, or bicarbonate as the carbon source. The biotic process involving ferrous ions (Fe²⁺) decreases nitrate to nitrite autotrophically in low-iron surroundings

• Table 1. Please, standardize the units of Concentration/loading rate and Removal efficiency/rate. Moreover, nitrate removal rates higher than those showed in the Table have been reported in the literature.

The units of Concentration/loading rate and Removal efficiency/rate and nitrate removal rates have been updated.

• Section 2.1. How are the rates observed compared to conventional denitrification? Which are the costs related to BES denitrification? Comparison with benchmark technologies are needed.
• Thank you for your suggestions. The costs related to BES denitrification added to the paper.

 Ding at el. found that MFC and flocculation process were combined to achieved an ammonia removal rate of 269.2 ± 0.5 g m^-3d^-1, a maximum power density of 37.5 W m^-3 and a CE of 21.6% by treating swine wastewater. The effluent COD concentration were reduced to 90 ± 1 mg L^-1 and 3.663 mg L^-1. An overall ammonia removal efficiency of 99.1 ± 0.1%. In this study, polyaluminum chloride (PAC) and polyacrylamide (PAM) was the only energy-consuming process and cost about $ 0.034 m^-3 according to Chinese reagent costs. MFCs achieved 0.664 kWh m^-3 With an external resistance of 10 Ω, for an economic benefit of $ 0.06 m^-3 according to the electricity pricing system and policy in China. Therefore, resulted in a net economic gain of $ 0.026 m^-3[27].

• Table 2. Please, standardize the units used in the table.

Thank you for your suggestions. The units in Table 2 have been unified.

• Section 2.2. How are the rates observed compared to conventional SND? Which are the costs related to BES SND? Comparison with benchmark technologies are needed.

Thank you for your suggestions. The costs of BES SND has been Compared with benchmark technologies. Yang at el. observed an up-flow bioelectrochemical filter reactor (UBEF) was designed without positive aeration in-priority for treating real domestic wastewater. The removal of ammonia and total nitrogen was attained at a high efficiency of 99% and 99% respectively, when HRT was set at ∼2.53 d. During the experiment, the average removal rates of nitrogen achieved to 0.2 kg TN/(m³d). A maximum power density of 89.51 ± 13.29 mW/m³ at the stage of (HRT of 0.23–0.28 day). the UBEF spend less 33.69% (capital cost per tons CCPT _COD) and 60.71% (CCPT _nitrogen) compared with the SND sludge process. Moreover, due to aeration free, the operation cost per day of the UBEF was also decreased 18.70%. This SND reactor is mainly composed of reaction tank, aeration pump, automatic controller, airflow meter and air distributor, the capital cost of these items is 69.2 USD. Based on its pollutant removal abilities of 1.5 kg COD/(m³d) and 0.32 kgNH4⁺-N/(m³d), the capital cost per tons of this SND reactor could reach to 46.6 USD (COD) and 216.2 USD (nitrogen). Consequently, it can be easily concluded that the UBEF spend less 33.69% (CCPT_COD) and 60.71% (CCPT_nitrogen) compared with the SND sludge process. Moreover, due to aeration free, the operation cost per day of the UBEF was also decreased 18.70%[55].

9. Section 2.3. Rates reported at very low (0.0125-0.075 kg/m3d). How can them compete with, partial-nitritation anammox (>1 kg/m3d), for example?

Thank you for your suggestions. The integrated SNAD-MFC for the first time achieved energy-positive nitrogen removal, and exhibited several advantages over conventional BNR processes. First, by maintaining low DO concentration (1–3.5 mg/L) in the cathode chamber of MFCs, short-cut nitrification was achieved by inhibiting nitrate production and energy consumption was reduced. The pH impact was minimized so that the operational stability enhanced. Second, the ratio of nitrite and nitrate in the short-cut nitrification MFC reached as high as 3.0, which could be a good nitrite source for anammox as electron acceptor [32,45]. Third, autotrophic bioelectrochemical denitrification does not require additional carbon source (e.g. COD) and saves the treatment chemical dosage and reduces operational complexity. Finally, electricity was harvested from the chemical energy stored in nitrogen, through which power production and wastewater treatment were achieved simultaneously. The novel integrated SNAD-MFC system was examined in the batch-mode tests. Aeration energy consumption and carbon sources were saved, since ammonium was oxidized to nitrite in short-cut nitrification MFC, and nitrite was reduced to nitrogen gas by using the electrons produced from anodes in the autotrophic denitrification MFC. The nitrogen removal efficiency was 99.9%, the total removal rate was 0.0125 kg N/m3d, and the power density was 294.9 mW/m²at DO 3.5 mg/L. Compared with the complete nitrification/denitrification MFCs and conventional BNR processes, the SNAD-MFCs simplified operation, shortened operational duration, lowered aeration cost, saved carbon sources, and achieved energy-positive status.

• Section 2.4. There are some article suggesting that anammox-kind or other nitrifying bacteria could oxidize ammonium using the electrode as electron acceptor. These articles are missing.

Thank you for your suggestions. Section 2.4 did not mention that anammox-kind or other nitrifying bacteria could oxidize ammonium using the electrode as electron acceptor.

• Section 3. Paragraph 1. The authors describe the structure of a biofilm for SND, but the section label is “Structure of cathode biofilm”. A section explaining how a denitrifying cathodic biofilm is structured, which is the role of thermodynamics, etc. would be of a high interest, but unfortunately, it is not the case with the current section.

Thank you for your suggestions. The anode generates electrons from organic matter and flows into the cathode after flowing through the external circuit. Nitrate or nitrite react as corresponding electron receptors on the biofilm. The cathode electrode can directly transfer electrons to the active denitrifying bacteria to remove pollution.

Deng at el. found that a sponge MFC can be aerobic outside the cathode and anaerobic inside, so that the reaction can be performed better. The ammonia output of the anode is similar, but the effect of the whole sponge MFC is significantly better than that of ordinary MFC, 0.166 ± 0.032 kgNm³d, and the nitrate is lower in the effluent, while the MFC effluent is higher, the TN removal rate reached 90.73 ± 1.87% higher than 68.85 ± 2.98%. TN removal efficiencies in the Sponge-MFC were 90.73 ± 1.87%, higher than those for the MFC(C) at 68.85 ± 2.98% at removal rates of 0.165 ± 0.018 and 0.131 ± 0.029 kg N/m³ ·d, respectively. The power densities of the Sponge-MFC reached 30.50 ± 4.06 mW/m²[66].

It suggested that abolishing oxygen or inhibiting nitrite oxidizing bacteria would favor nitrogen removal. Heterotrophic denitrification is more abundant than autotrophic denitrification, The genus Thauera and Pseudomonas were predominant in autotrophic denitrification MFC while genus Klebsiella and Alkaliphilus were abundant in heterotrophic denitrification MFC[67].

• Section 4 Influencing factors. The description of the influencing factors are vague, and there is a lack of explanation about how they can affect depending on the way the BES is operated. For example, the authors explain about the effect of the external resistance, which can be relevant for MFC operation, but they do not even mention the effect of the cathode potential, which is extremely relevant when operating a BES for nitrate removal under potentiostatic conditions.

Thank you for your suggestions. Influencing factors have been improved.

Author Response File: Author Response.docx

Reviewer 3 Report

The manuscript “Progress in Nitrogen Removal in Bioelectrochemical Systems” by sun et al. deals with the bioelectrochemical reduction/conversion of nitrogenous compounds.  The review has been conducted in an admirable way, and it can greatly increase the knowledge on the matter. Nevertheless, some improvements and modifications are necessary before the manuscript can be accepted for publication.

 

• The novelty of the present review is not clear. As there are several reviews on this facet, therefore the novelty of the manuscript should be highlighted by pointing out the missing/uncovered attributes of other reviews
• The health hazard concentration of the nitrogenous compounds should be included in the introduction and the nitrite is more toxic in comparison to nitrate. So, therefore it can also be included.
• The redox potential of nitrate can be included along with the redox potentials of O₂, to suffice the use of nitrate as a terminal electron acceptor
• The integrated systems of the nitrification and denitrification and operation of bioelectrochemical systems with high strength nitrogenous wastewaters can be included in the manuscript as a separate section
• There were few studies, which have dealt with the use of nitrates and nitrites in the same cathode chamber. Those studies can be discussed to increase the knowledge of readership
• A detailed explanation of the shortcut SND process should be included. As it was abruptly included as a section 2.3 without any prior explanation
• At section 2.5, there were errors in pointing the subscript and superscript of the nitrogenous compounds and their removal rates
• Conclusion and future perspective can be separated and in a future perspective, some views on the field application of bioelectrochemical systems can be included
• Continuous mode operation of BES in relation to HRT should be discussed more
• Influence of applied voltage/current in relation to nitrate removal can be included
• A literature survey can be extended by including more tables.

 

Author Response

• The novelty of the present review is not clear. As there are several reviews on this facet, therefore the novelty of the manuscript should be highlighted by pointing out the missing/uncovered attributes of other reviews.

Thank you for your suggestions. I try my best to increase novelty.

• The health hazard concentration of the nitrogenous compounds should be included in the introduction and the nitrite is more toxic in comparison to nitrate. So, therefore it can also be included.

Thank you for your suggestions. Excessive intake of nitrite can cause methemoglobinemia and may produce carcinogenic nitrosamines in the body.

• The redox potential of nitrate can be included along with the redox potentials of O₂, to suffice the use of nitrate as a terminal electron acceptor

Thank you for your suggestions. O2 ＋ 2H2O ＋ 4e-＝4OH-，E=0.401V.

• The integrated systems of the nitrification and denitrification and operation of bioelectrochemical systems with high strength nitrogenous wastewaters can be included in the manuscript as a separate section

Thank you for your suggestions. 

2.5 high strength nitrogenous wastewaters

The nitrogen removal process can treat high-nitrogen wastewater. It has a large application in industry, because high nitrogen wastewater exists in many existing industries, such as Swine wastewater, soybean protein wastewater, chemical wastewater and so on.

In electrochemistry, some people use the microbial electrolytic cell (MEC) process to treat synthetic wastewater with higher ammonia nitrogen content, such as the treatment of pig wastewater with high ammonia nitrogen content. Lim et al. Found that when the total influent nitrogen concentration was 1992.7 ± 86.2 mg / L. The nitrogen loading rates (NLR) (kg NH³–N/m³/d) was 0 V :0.4 ± 0.0; 1 V :0.3 ± 0.0 and 2 V :0.3 ± 0.0. The average total nitrogen removal efficiencies at the applied voltage of 0, 1, and 2 V were 39.8%, 49.5%, and 58.7%, respectively.

Sevda et al. observed that with the complex industrial was as substrate the ammonia loading rates of 0.43 kg NH₃^- m^-3 d^-1 at 68 h HRT achieved maximum power production along with 40% ammonia reduction in the cathodic chamber. A stable power density and volumetric power density of 23.56 mW m^-2 and 112.50 mW m^-3, respectively.

Huang et al. found that exoelectrogens in single-chamber air cathode microbial fuel cells enhanced denitrification activity. A high denitrification rate (12.2 ±0.6 kg NO₃^--N m^-3d^-1) at a high nitrate concentration of 2000 mg NO₃^--N L^-1. The maximum power densities of the MFCs was 25 W m^-3. The results of the kinetic tests provided evidences of the feasibility of single-chamber MFCs in treating wastes containing high-strength nitrate (over 1000 mg NO₃^—N L^-1), such as wastewater produced in explosives, fertilizer, nuclear and metals finishing industries. Therefore, the electrochemical process for treating high-concentration nitrogen-containing wastewater will attract more and more attention.

5 There were few studies, which have dealt with the use of nitrates and nitrites in the same cathode chamber. Those studies can be discussed to increase the knowledge of readership

Thank you for your suggestions. We did not find relevant studies. Only few studies have involved the use of nitrates or nitrites in the same cathode chamber.

6. A detailed explanation of the shortcut SND process should be included. As it was abruptly included as a section 2.3 without any prior explanation

Thank you for your suggestions. Shortcut SND is to control the nitrification process of ammonia in the nitrite stage, followed by denitrification, that is, the nitrogen removal process is NH4 + → NO2- → N2.  SND can reduce the amount of aeration and the addition of carbon sources. So combining Shortcut SND with electrochemical technology is a good choice.

• At section 2.5, there were errors in pointing the subscript and superscript of the nitrogenous compounds and their removal rates

Thank you for your suggestions. All errors have been corrected.

• Conclusion and future perspective can be separated and in a future perspective, some views on the field application of bioelectrochemical systems can be included

Thank you for your suggestions. 6. Future perspective

In the present, one challenge facing to BESs is the scaling up for practical application to treat real wastewaters in terms of optimization of reactor configuration and operation parameters, long-term stability, investment and operation cost. The operation of large-scale BESs for nitrogen removal is beneficial to discover the key factors to nitrogen removal efficiency, and evaluate the energy budget and economic benefits for practical application of these biological systems.

Bioelectrochemical wastewater is usually energy-intensive, which has great potential, and traditional water treatment processes have energy potential. In the treatment of actual wastewater, the economics of the experimental equipment and the power efficiency produced are also the key to BES and obtain a higher CE. The addition of an electrochemical system not only facilitates the removal of contaminants, but also avoids the addition of an additional electron donor in the cathode compartment. BES can either use organic matter as an electron source, or use an autotrophic microorganism to remove ammonia nitrogen at a low C / N ratio. The combination of BES and traditional nitrogen removal process provides a new direction for the removal of nitrogen-containing wastewater. The bioelectrochemical system has a wider carbon-nitrogen ratio and can flexibly treat a variety of wastewater. BES nitrogen removal has been used to treat actual wastewater, wastewater containing nitrate and nitrite, and related pilot experiments have also been studied. Pig wastewater Wastewater from landfill leachate, groundwater, wetlands, urban domestic sewage, raw coking wastewater, kitchen waste effluent and other wastewater have been used for nitrogen removal. By analyzing the cost of the electrochemical system, the superior economic benefits of the electrochemical system are calculated.

• Continuous mode operation of BES in relation to HRT should be discussed more
• Influence of applied voltage/current in relation to nitrate removal can be included

Thank you for your suggestions. HRT has been discussed.

• A literature survey can be extended by including more tables.

Thank you for your suggestions. Tables have been added

 

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Review of the manuscript ID Processes-748029, entitled "Progress in nitrogen removal in bioelectrochemical systems"

Generalities

The manuscript entitled “Progress in nitrogen removal in bioelectrochemical systems” aims to review nitrogen removal in bioelectrochemical systems. Despite some changes have been included, the core of the review has not changed and, moreover, most of initial questions has not been answered. The review is still very vague and it adds no novelty to other reviews present in literature. It is still difficult to know where nitrogen removal in BES is and where it is going. It is still lacking more information about the fundamental processes occurring, the bacteria involed, the role of thermodynamics, the operation methods that could be applied, etc.

Specifications

• Table 2 has improved, but still it is difficult to understand which is the benefit of BES compared to them. Which is the operational cost of the systems? How much space do you need for the system according to its HRT, for example? Still it is not clear how a BES should be operated to get one results or another. How are the different nitrification or denitrification steps performed, at which cathode potential is the denitrification working, etc. Concentration removal alone are not enough. Moreover, the concentration removal is not relevant, because it will depend on the HRT. Nitrogen removal rates should be presented.
• Section 2.1. Denitrification process. Some information about autotrophic denitrification has been included, but still all articles reported about BES are articles where organic matter is present and thus, conventional heterotrophic denitrification is the main pathway. The main advantage of BES in denitrification field is on autotrophic denitrifiers. Heterotrophic denitrifiers are not electroactive and thus, they should not be the scope of a BES review on nitrogen removal.
• Section 2.1. How are the rates observed compared to conventional denitrification? Which are the costs related to BES denitrification? Comparison with benchmark technologies are needed.
• Section 2.3. Rates reported at very low (0.0125-0.075 kg/m3d). They cannot compete with partial-nitritation anammox (>1 kg/m3d). The inclusion of a MFC system in any WWTP increases the complexity of the treatmen. If the removal rates are not much higher than conventional technologies, they are out of the market. Partial-nitritiation anammox gives higher removal rates and it is an easier to operate and easier to scale process.
• Section 2.4. There are some article suggesting that anammox-kind or other nitrifying bacteria could oxidize ammonium using the electrode as electron acceptor. These articles are missing. The authors should add these article in the current review.
• Section 2.4. There are some article suggesting that anammox-kind or other nitrifying bacteria could oxidize ammonium using the electrode as electron acceptor. These articles are missing.
• Section 3. Paragraph 1. A section explaining how a denitrifying cathodic biofilm is structured, which is the role of thermodynamics, etc. would be of a high interest, but unfortunately, it is still not the case.
• Section 4 Influencing factors. The description of the influencing factors is still vague. Moreover, it is missing one of the main influencing factor for an electrochemical process, which is the anode and the cathode potential.

Comments for author File: Comments.docx

Author Response

• Point 1: Table 2 has improved, but still it is difficult to understand which is the benefit of BES compared to them. Which is the operational cost of the systems? How much space do you need for the system according to its HRT, for example? Still it is not clear how a BES should be operated to get one results or another. How are the different nitrification or denitrification steps performed, at which cathode potential is the denitrification working, etc. Concentration removal alone are not enough. Moreover, the concentration removal is not relevant, because it will depend on the HRT. Nitrogen removal rates should be presented.

Response 1: Thank you for your suggestions. HRT and Volume has been added in table2. Page 4 lines 113.

• Point 2: Section 2.1. Denitrification process. Some information about autotrophic denitrification has been included, but still all articles reported about BES are articles where organic matter is present and thus, conventional heterotrophic denitrification is the main pathway. The main advantage of BES in denitrification field is on autotrophic denitrifiers. Heterotrophic denitrifiers are not electroactive and thus, they should not be the scope of a BES review on nitrogen removal.

Response 2: Thank you for your suggestions. Section 2.1 proposes a method of denitrification, under less C / N, there will be simultaneous accumulation of heterotrophic denitrifying bacteria and autotrophic denitrifying bacteria.

• Point 3: Section 2.1. How are the rates observed compared to conventional denitrification? Which are the costs related to BES denitrification? Comparison with benchmark technologies are needed.

Response 3: Thank you for your suggestions. Sorry, no related costs were found

• Point 4: Section 2.3. Rates reported at very low (0.0125-0.075 kg/m3d). They cannot compete with partial-nitritation anammox (>1 kg/m3d). The inclusion of a MFC system in any WWTP increases the complexity of the treatmen. If the removal rates are not much higher than conventional technologies, they are out of the market. Partial-nitritiation anammox gives higher removal rates and it is an easier to operate and easier to scale process.

Response 4: Thank you for your suggestions. Shortcut SND has a lower nitrogen removal rate, and electrochemical and ammonia oxidation have a higher nitrogen removal rate.

Zhang at el. found that ANAMMOX sludge can be used as an anode microbial catalyst to promote the efficiency of ANAMMOX removal of total nitrogen (TN). The operation of the system with a nitrogen loading rate (NLR) of 1.74 kg · N / m³ · d showed a TN removal rate of 96.3%, and the highest nitrogen removal rate (NRR, 1.69 kg · N / m³ · d) was obtained. Compared with the open circuit (control group), increased by 14.9% and 0.30 kg · N / m³ · d, respectively. The maximum voltage (39.8 mV) and power density (21.20 ± 0.05 mW / m³, standardized to anode surface area) were also observed[1]. Page 7 lines 246-252.

• Point 5: Section 2.4. There are some article suggesting that anammox-kind or other nitrifying bacteria could oxidize ammonium using the electrode as electron acceptor. These articles are missing. The authors should add these article in the current review.
• Point 5: Section 2.4. There are some article suggesting that anammox-kind or other nitrifying bacteria could oxidize ammonium using the electrode as electron acceptor. These articles are missing.

Response 5: Thank you for your suggestions. Article has added to review.

In an MFC, organic substrates are oxidised by exoelectrogenic bacteria, which produce electrons that are transferred to an anode electrode and then flow to a cathode. The anode and cathode are linked by a conductive material containing a resistor. Protons produced at the anode migrate through the solution across a cation exchange membrane to the cathode chamber where they combine with a reducible compound and electrons. Bioelectrochemical denitrification is carried out by autotrophic denitrifying bacteria that are capable of accepting electrons from a solid electron donor (e.g., a cathode electrode)[2]. Page 7 lines 211-217.

• Point 6: Section 3. Paragraph 1. A section explaining how a denitrifying cathodic biofilm is structured, which is the role of thermodynamics, etc. would be of a high interest, but unfortunately, it is still not the case.

Response 6: Thank you for your suggestions.

The applied cathode potential is theoretically lower than 150mV, which is sufficient for autotrophic denitrification, but due to the overpotential, the potential should be more negative[94].

Pous et al. Found that when the cathode potential decreased from +597 to -403 mV, the nitrate removal rate increased from 1.05 to 5.44 mg N-NO^3-L_NCC ^-1h^-1. Using water as the anode electron donor, the highest conversion rate of nitrate to N₂ (2.59 mg N-NO^{3 -}L_NCC ^-1h^-1, 93.9%) appeared at -123 mV. When providing infinite electron source instead of current, it can reduce the competition of denitrifying bacteria and reduce the accumulation of by-products (nitrite and nitrous oxide)[19,95]. Page 14 lines 457-464.

Point 7: Section 4 Influencing factors. The description of the influencing factors is still vague. Moreover, it is missing one of the main influencing factor for an electrochemical process, which is the anode and the cathode potential.

Response 7: Thank you for your suggestions.

The extension of HRT will cause the biofilm and organic matter to contact more fully, which will lead to an increase in voltage, affecting biofilm absorption and organic matter degradation.

Increased HRT will increase the removal rate of COD and nitrogen, but it can not just extend the HRT just for the removal rate, so it should be combined with other factors to select the best HRT to obtain higher removal efficiency and economical economy. Factors are combined to select the best HRT to obtain higher removal efficiency and economical economy[6]. Page 13 lines 405-410.

Author Response File: Author Response.docx

Reviewer 3 Report

The manuscript “Progress in Nitrogen Removal in Bioelectrochemical Systems” by sun et al. deals with the bioelectrochemical reduction/conversion of nitrogenous compounds. The authors should re-revise the manuscript by including the changes pointed by the reviewer rather than simply answering them in the review response.  For instance, the author should include the health hazards of nitrate and redox potentials of oxygen in the manuscript rather than simply answering them in the response.

• The difference of redox potential of nitrate and O2 is less than 100 mV, authors should highlight this point to elevate the nitrate as a terminal e- acceptor in bioelectrochemical systems.
• In table 2 include the applied voltages for MEC along with influent concentration and HRT if these studies are carried out in continuous mode

The manuscript can be accepted after carrying out the minor revisions suggested by the reviewers.

 

Author Response

Point 1: The difference of redox potential of nitrate and O2 is less than 100 mV, authors should highlight this point to elevate the nitrate as a terminal e- acceptor in bioelectrochemical systems.

Response 1: Thank you for your suggestions. O₂(g) + 2H₂O + 4e⁻ → 4OH⁻(aq) (Neutral to alkaline), E(O₂/OH⁻) = 0.40V. The difference of redox potential of nitrate and O₂ is less than 100 mV, E^○’(O₂/OH⁻) = 0.40V. Page 1 lines 45.

Point 2: In table 2 include the applied voltages for MEC along with influent concentration and HRT if these studies are carried out in continuous mode

The manuscript can be accepted after carrying out the minor revisions suggested by the reviewers.

Response 2: Thank you for your suggestions. The applied voltages for MEC along with influent concentration and HRT have been added to Table 2. Page 4 lines 113.

 

Author Response File: Author Response.docx