Experimental Analysis and Model Validation on the Performance of Impregnated Activated Carbons for the Removal of Hydrogen Sulfide (H₂S) from Sewage Biogas

Round 1

Reviewer 1 Report

Dear Authors

It was a pleasure for me to read Your paper, because it presents a good level of scientific, but I have critical comments on the editorial side:

- Figure 1. Illustration should be corrected - the text is hard to read,

- line 126, 136, 137, 145 and more - Error! Reference source not found,

- Figure 2. - the chart for me is not clear,

- Figure 4. it looks bad,

- Figure 5. - there are too small photos,

- table headers are incorrect,

- Figure 6. - it looks bad,

- the numbering of equations is not correct,

- Figure 7, Figure 8, Figure 9, Figure 10, Figure 11, Figure 12, 13, 14, 16, 17  - does not look professional,

- Figure 15 - there are too small fonts.

Author Response

Reviewer #1

Comment #1 - Figure 1. Illustration should be corrected - the text is hard to read,

Response: for Figure 1 the quality was improved.

Comment #2- line 126, 136, 137, 145 and more - Error! Reference source not found,

Response: The cross-references have been updated throughout the manuscript.

 Comment #3 - Figure 2. - the chart for me is not clear,

Response: Figure 2 was clarified in the manuscript.

Comment #4 - Figure 4. it looks bad

Thank you for the comment. Figure 4 the quality was improved.

 Comment #5 - Figure 5. - there are too small photos,

Response: Components in Figure 5 were enlarged.

 Comment #6- table headers are incorrect,

Response: The Table headers have been corrected in the revised manuscript.

 Comment #7- Figure 6. - it looks bad,

Response: Figure 6 was revised.

 Comment #8 - the numbering of equations is not correct,

Response: Equation numbering has been revised.

 Comment #9 - Figure 7, Figure 8, Figure 9, Figure 10, Figure 11, Figure 12, 13, 14, 16, 17  - does not look professional,

Response: Figures were revised.

 Comment #10 - Figure 15 - there are too small fonts.

Response: Figure 15 was revised.

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper deals with a very interesting topic but it suffer some major deficiencies.

The first is that it is not clear in the title and in the introduction that the paper deals with the removal of H2S only.

The main issue is however that the tested H2S concentrations are in the lower end of the values found in literature (0.5-1%). The authors justify this fact mentioning a monitoring activity in a specific plant.

This however highly reduce the general interest for the paper. In fact the highest tested concentration is 750 ppm while a test with H2S concentration in the order of at least 3000-5000 ppm would have been necessary. Please comment on this in the manuscript.

To improve readability explain abbreviations and acronyms at first occurrence

In many cases a problems with reference is signalized (reference source not found)

Introduction

Erase lines 39-40 digestate is unrelated to biogas use,

Please summarize available technologies for biogas cleaning focusing specifically on H2S

Clarify the objectives of the paper

Theoretical approach

Try to make it informative but more concise

Comment on the assumptions: are they usual in the field, do they reduce the validity of the results….???

Line 85 report at least a reference on Fuller’s eq.

Materials and methods

The material of sterile gauze must be reported

Results and discussion

The 0.1% oxygen has a high impact. Comment on this, do the plant manager should add some oxygen to improve treatment or what???

In general try to improve discussion making it more concise

Conclusions

Please better summarize the specific findings of this research

Author Response

Reviewer #2

Comment #1 - The paper deals with a very interesting topic but it suffer some major deficiencies.

The first is that it is not clear in the title and in the introduction that the paper deals with the removal of H2S only.

Response: Dear reviewer the title of the work was revised according to your suggestions. New proposed title is the following:

Experimental analysis and model validation on the performance of impregnated activated carbons for the removal of hydrogen sulfide (H₂S) from biogas

 Comment #2 - The main issue is however that the tested H2S concentrations are in the lower end of the values found in literature (0.5-1%). The authors justify this fact mentioning a monitoring activity in a specific plant.

This however highly reduce the general interest for the paper. In fact the highest tested concentration is 750 ppm while a test with H2S concentration in the order of at least 3000-5000 ppm would have been necessary. Please comment on this in the manuscript.

Response: it is true that we refer a specific plant configuration, which is the biogas-fed fuel cell plant. However, we do not refer to just a specific plant or case study. The use of biogas in fuel cell plants (typically molten carbonate or solid oxide fuel cell plants, of which many installations already exists worldwide and mostly based on the molten carbonate fuel cell) requires an ultra-purification of the biogas stream and any sulfur trace should be removed. Hence, it is relevant to study the performance of activated carbons in the removal of ultra-low levels (<1 ppm) of sulfur. Sulfur that is generally found in biogas in the form of H₂S.

Furthermore, it can be stated that sewage biogas from WWTPs is generally ‘cleaner’ than other biogas from agricultural wastes, crops, food waste or landfill. Siloxanes are always present but H₂S level is usually in the range 50-200 ppm(v). The reason for this is found in a typical practice in WWTPs, which is the iron salts (usually ferric chloride) dosing in the water line to precipitate phosphor. Even if the main goal is the removal of phosphor, ferric chloride also generated a precipitation of sulphur which is later translated in a low H₂S content. Other plants are also directly inject iron salts inside the digester to specifically reduce sulphur content in the outlet biogas stream.

The authors have published other works were the analysis on literature confirmed this trend of low-sulphur content in sewage biogas [1], [2] and the practice of ferric chloride dosing to control – as a sub-goal – the sulphur level, is well studied in literature [3]–[6].

If other biogas plants are considered (agricultural plants, food waste, municipal solid waste, landfill), sulphur content would be higher and a first desulphurization system (usually chemical) would be required for bulk sulphur removal. The system presented in this work could be considered as the analysis, for these second set of plants, of the second cleaning stage (after bulk sulphur removal).

Comment #3 - To improve readability explain abbreviations and acronyms at first occurrence

Response: The paper was revised according to the suggestion.

 Comment #4 - In many cases a problems with reference is signalized (reference source not found)

 Response: The reference not linked have been updated and corrected.

Comment #5- Introduction

Erase lines 39-40 digestate is unrelated to biogas use,

Response: The comment is correct, digestate is a way to exploit biogas. The sentence has been deleted.

 Comment #6 - Please summarize available technologies for biogas cleaning focusing specifically on H2S

Response: it was added in the introduction section a proper summary.

Clarify the objectives of the paper

Response: it was added in the introduction section a proper clarification of objectives.

 Theoretical approach

Try to make it informative but more concise

Response: it was revised in the specific section.

 Comment on the assumptions: are they usual in the field, do they reduce the validity of the results….???

Response: Assumptions are usual for this field.

 Line 85 report at least a reference on Fuller’s eq.

 Response: it was added the reference for the Fuller equation in the manuscript.

 Materials and methods

The material of sterile gauze must be reported

Response: it was added the composition in the MM section.

 Results and discussion

The 0.1% oxygen has a high impact. Comment on this, do the plant manager should add some oxygen to improve treatment or what???

In the majority of the plants, oxygen is available even if in very small quantities, usually not detected by . The enhancement of the sorbents performance due to oxygen are related to the following equation:

This means that oxygen is required in a maximum 3/2 (1.5) ratio respect to the hydrogen sulphide content. In WWTPs similar to the one analyzed, which performs iron chloride dosing in the water treatment line to precipitate phosphorous, and thus sulphur), the average H₂S content in sewage biogas is 50-200 ppm. Consequently, oxygen is required in a 75-300 ppm range, which is usually not measured because of detection limits of standard gas analyser, as demonstrated in many literature works [7]–[11] where O₂ content is usually < 0.1% or in the range 0-1%, depending on the plant.

Furthermore, the authors have performed – through an external certified laboratory – an analysis of the sewage biogas composition in the WWTP mentioned in the paper. Results, shown in the table below, pointed out a variable oxygen content between 0.01 and 0.33%.

From the authors’ experience with real WWTPs gas analysis and from internal discussions with sorbents sellers, the presence of the required amount of oxygen can probably be guaranteed in all the biogas plants.

 In general try to improve discussion making it more concise

Response: it was added a discussion section.

 Conclusions

Please better summarize the specific findings of this research

Response: New conclusions were added.

Reference:

[1]        A. Lanzini et al., “Dealing with fuel contaminants in biogas-fed solid oxide fuel cell (SOFC) and molten carbonate fuel cell (MCFC) plants: Degradation of catalytic and electro-catalytic active surfaces and related gas purification methods,” Prog. Energy Combust. Sci., vol. 61, pp. 150–188, 2017.

[2]        A. Lanzini, D. Ferrero, D. Papurello, and M. Santarelli, “Reporting Degradation from Different Fuel Contaminants in Ni-anode SOFCs,” Fuel Cells, vol. 17, no. 4, pp. 423–433, Aug. 2017.

[3]        D. Firer, E. Friedler, and O. Lahav, “Control of sulfide in sewer systems by dosage of iron salts: Comparison between theoretical and experimental results, and practical implications,” Sci. Total Environ., vol. 392, no. 1, pp. 145–156, 2008.

[4]        R. DeVries, “Ferric Chloride in Wastewater Treatment,” Environ. Sci., vol. 2, pp. 6–26, 2011.

[5]        S. Aslanidou, N. Lydakis-Simantiris, C. Kotsifaki, D. Pentari, and E. Katsivela, “FERRIC CHLORIDE (FeCl 3 ) AS REGULATORY AGENT FOR THE REDUCTION OF HYDROGEN SULFIDE (H 2 S) IN MUNICIPAL WASTEWATER.”

[6]        R. Devries IV, “Ferric Chloride in Wastewater Treatment,” Environ. Sci. Water Life, 2011.

[7]        M. M. Manyuchi, C. Mbohwa, and E. Muzenda, Resource Recovery from Municipal Sewage Plants : an Energy-Water-Nutrients Nexus for Developing Countries. .

[8]        R. Chicheportiche, M. Balerna, A. Lombet, G. Romey, and M. Lazdunski, Synthesis and mode of action on axonal membranes of photoactivable derivatives of tetrodotoxin, vol. 254, no. 5. 1979.

[9]        S. Rasi, J. Läntelä, and J. Rintala, “Trace compounds affecting biogas energy utilisation – A review,” Energy Convers. Manag., vol. 52, no. 12, pp. 3369–3375, 2011.

[10]      V. Paolini et al., “Characterisation and cleaning of biogas from sewage sludge for biomethane production,” J. Environ. Manage., vol. 217, no. March, pp. 288–296, 2018.

[11]      X. Y. Chen, H. Vinh-Thang, A. A. Ramirez, D. Rodrigue, and S. Kaliaguine, “Membrane gas separation technologies for biogas upgrading,” RSC Adv., vol. 5, no. 31, pp. 24399–24448, 2015.

 Round 2

Reviewer 2 Report

I am not satisfied with the changes made. Many answers were given but non incoroporated into text

Comment #2 - The main issue is however that the tested H2S concentrations are in the lower end of the values found in literature (0.5-1%). The authors justify this fact mentioning a monitoring activity in a specific plant.

This however highly reduce the general interest for the paper. In fact the highest tested concentration is 750 ppm while a test with H2S concentration in the order of at least 3000-5000 ppm would have been necessary. Please comment on this in the manuscript.

Response: it is true that we refer a specific plant configuration, which is the biogas-fed fuel cell plant. However, we do not refer to just a specific plant or case study. The use of biogas in fuel cell plants (typically molten carbonate or solid oxide fuel cell plants, of which many installations already exists worldwide and mostly based on the molten carbonate fuel cell) requires an ultra-purification of the biogas stream and any sulfur trace should be removed. Hence, it is relevant to study the performance of activated carbons in the removal of ultra-low levels (<1 ppm) of sulfur. Sulfur that is generally found in biogas in the form of H₂S.

Furthermore, it can be stated that sewage biogas from WWTPs is generally ‘cleaner’ than other biogas from agricultural wastes, crops, food waste or landfill. Siloxanes are always present but H₂S level is usually in the range 50-200 ppm(v). The reason for this is found in a typical practice in WWTPs, which is the iron salts (usually ferric chloride) dosing in the water line to precipitate phosphor. Even if the main goal is the removal of phosphor, ferric chloride also generated a precipitation of sulphur which is later translated in a low H₂S content. Other plants are also directly inject iron salts inside the digester to specifically reduce sulphur content in the outlet biogas stream.

The authors have published other works were the analysis on literature confirmed this trend of low-sulphur content in sewage biogas [1], [2] and the practice of ferric chloride dosing to control – as a sub-goal – the sulphur level, is well studied in literature [3]–[6].

If other biogas plants are considered (agricultural plants, food waste, municipal solid waste, landfill), sulphur content would be higher and a first desulphurization system (usually chemical) would be required for bulk sulphur removal. The system presented in this work could be considered as the analysis, for these second set of plants, of the second cleaning stage (after bulk sulphur removal).

THIS IS VERY IMPORTANT MUST BE INCLUDED IN THE TEXT

Comment on the assumptions: are they usual in the field, do they reduce the validity of the results….???

Response: Assumptions are usual for this field.

PLEASE INCLUDE IN THE TEXT

Materials and methods

The material of sterile gauze must be reported

Response: it was added the composition in the MM section.

IN MY OPINION THE PRESENCE OF COTTON IS A PROBLEM. DO YOU HAVE BLANKS SHOWING THAT IT DOES NOT REMOVE ANY H2S?

Results and discussion

The 0.1% oxygen has a high impact. Comment on this, do the plant manager should add some oxygen to improve treatment or what???

In the majority of the plants, oxygen is available even if in very small quantities, usually not detected by . The enhancement of the sorbents performance due to oxygen are related to the following equation:

This means that oxygen is required in a maximum 3/2 (1.5) ratio respect to the hydrogen sulphide content. In WWTPs similar to the one analyzed, which performs iron chloride dosing in the water treatment line to precipitate phosphorous, and thus sulphur), the average H₂S content in sewage biogas is 50-200 ppm. Consequently, oxygen is required in a 75-300 ppm range, which is usually not measured because of detection limits of standard gas analyser, as demonstrated in many literature works [7]–[11] where O₂ content is usually < 0.1% or in the range 0-1%, depending on the plant.

Furthermore, the authors have performed – through an external certified laboratory – an analysis of the sewage biogas composition in the WWTP mentioned in the paper. Results, shown in the table below, pointed out a variable oxygen content between 0.01 and 0.33%.

From the authors’ experience with real WWTPs gas analysis and from internal discussions with sorbents sellers, the presence of the required amount of oxygen can probably be guaranteed in all the biogas plants.

 VERY IMPORTANT INCLUDE IN THE TEXT

In general try to improve discussion making it more concise

Response: it was added a discussion section.

NO NEED FOR AN ADDITIONAL DISCUSSION SECTION BUT YOU SHOULD MAKE THE WHOLE "RESUSLTS AND DISCUSSION" MORE CONCISE

Author Response

Manuscript processes-540079

Answer to reviewers

The authors wish to thank the reviewers for their assistance and critique. The suggestions have been helpful and have improved the readability and quality of the submission. This document contains the exact reviewer’s comments and our responses to each of these comments.

Reviewers’ comments are coloured in blue pen. First review round comments are reported below for each point in black pen. Actions taken during the second review round are highlighted as ‘actions’ and are underlined in red. Changes to the manuscript are highlighted in red pen in the marked version of the manuscript.

Reviewer #2

The main issue is however that the tested H2S concentrations are in the lower end of the values found in literature (0.5-1%). The authors justify this fact mentioning a monitoring activity in a specific plant.

This however highly reduce the general interest for the paper. In fact the highest tested concentration is 750 ppm while a test with H2S concentration in the order of at least 3000-5000 ppm would have been necessary. Please comment on this in the manuscript.

Response: it is true that we refer a specific plant configuration, which is the biogas-fed fuel cell plant. However, we do not refer to just a specific plant or case study. The use of biogas in fuel cell plants (typically molten carbonate or solid oxide fuel cell plants, of which many installations already exists worldwide and mostly based on the molten carbonate fuel cell) requires an ultra-purification of the biogas stream and any sulfur trace should be removed. Hence, it is relevant to study the performance of activated carbons in the removal of ultra-low levels (<1 ppm) of sulfur. Sulfur that is generally found in biogas in the form of H₂S.

Furthermore, it can be stated that sewage biogas from WWTPs is generally ‘cleaner’ than other biogas from agricultural wastes, crops, food waste or landfill. Siloxanes are always present but H₂S level is usually in the range 50-200 ppm(v). The reason for this is found in a typical practice in WWTPs, which is the iron salts (usually ferric chloride) dosing in the water line to precipitate phosphor. Even if the main goal is the removal of phosphor, ferric chloride also generated a precipitation of sulphur which is later translated in a low H₂S content. Other plants are also directly inject iron salts inside the digester to specifically reduce sulphur content in the outlet biogas stream.

The authors have published other works were the analysis on literature confirmed this trend of low-sulphur content in sewage biogas [1], [2] and the practice of ferric chloride dosing to control – as a sub-goal – the sulphur level, is well studied in literature [3]–[6].

If other biogas plants are considered (agricultural plants, food waste, municipal solid waste, landfill), sulphur content would be higher and a first desulphurization system (usually chemical) would be required for bulk sulphur removal. The system presented in this work could be considered as the analysis, for these second set of plants, of the second cleaning stage (after bulk sulphur removal).

THIS IS VERY IMPORTANT MUST BE INCLUDED IN THE TEXT

Action: the section on the H₂S level in WWTP has been added to  the ‘Materials and Methods’ section, in order to justify the choices in terms of hydrogen sulphide level in the experimental campaign. The added section in the manuscript are also reported here below.

“The work focuses – as visible from the screening phase H₂S concentration (20 ppm(v)) – on the analysis of low-H₂S level biogases, like sewage gas from Wastewater Treatment Plants (WWTP). It can be stated that sewage biogas from WWTPs is generally ‘cleaner’ than other biogas from agricultural wastes, crops, food waste or landfill. Siloxanes are always present but H₂S level is usually in the range 50-200 ppm(v). The reason for this is found in a typical practice in WWTPs, which is the iron salts (usually ferric chloride) dosing in the water line to precipitate phosphor. Even if the main goal is the removal of phosphor, ferric chloride also generated a precipitation of sulphur which is later translated in a low H₂S content. Other plants are also directly inject iron salts inside the digester to specifically reduce sulphur content in the outlet biogas stream. The authors have published other works were the analysis on literature confirmed this trend of low-sulphur content in sewage biogas [19,39] and the practice of ferric chloride dosing to control – as a sub-goal – the sulphur level, is well studied in literature [40–43]. If other biogas plants are considered (agricultural plants, food waste, municipal solid waste, landfill), sulphur content would be higher and a first desulphurization system (usually chemical) would be required for bulk sulphur removal. The system presented in this work could be considered as the analysis, for these second set of plants, of the second cleaning stage (after bulk sulphur removal).”

Comment on the assumptions: are they usual in the field, do they reduce the validity of the results….???

Response: Assumptions are usual for this field.

PLEASE INCLUDE IN THE TEXT

Action: References have been added in the revised manuscript, which deal with studies using similar assumptions as the ones applied to our dynamic adsorption model. Specifically:

“Several assumptions have been made to build the model:

the adsorption process is isothermal [24,25]; the axial dispersion is neglected in the radial direction, being considered only longitudinally [25,26]; spherical and homogeneous in size and density adsorbent particles [25,26]; the film diffusion coefficient depicts the mass transfer across the boundary layer [25,26]; the intra-particle mass transport is characterized by the effective pore diffusion coefficient [25,26]; linear velocity independence from concentration [27]; a local equilibrium establishes between the amount of gas molecules adsorbed and the adsorbent solid particles [28]; the pressure drop between entrance and exit of the filter is considered negligible [25,28].”

[24] A. Khazraei Vizhemehr, F. Haghighat, C.S. Lee, Gas-phase filters breakthrough models at low concentration - Effectof relative humidity, Build. Environ. 75 (2014) 1–10. doi:10.1016/j.buildenv.2014.01.010.

[25] A.K. Vizhemehr, F. Haghighat, Modeling of gas-phase fi lter model for high- and low-challenge gas concentrations, Build. Environ. 80 (2014) 192–203. doi:10.1016/j.buildenv.2014.05.034.

[26] A. Khazraei Vizhemehr, F. Haghighat, C.-S. Lee, Predicting gas-phase air-cleaning system efficiency at low concentration using high concentration results: Development of a framework, Build. Environ. 68 (2013) 12–21. doi:10.1016/j.buildenv.2013.05.023.

[27] J.M.P.Q. Delgado, A critical review of dispersion in packed beds, (2006) 279–310. doi:10.1007/s00231-005-0019-0.

[28] Z. Xu, J. Cai, B. Pan, Mathematically modeling fixed-bed adsorption in aqueous systems *, 14 (2013) 155–176. doi:10.1631/jzus.A1300029.

Materials and methods. The material of sterile gauze must be reported

Response: it was added the composition in the MM section.

IN MY OPINION THE PRESENCE OF COTTON IS A PROBLEM. DO YOU HAVE BLANKS SHOWING THAT IT DOES NOT REMOVE ANY H2S?

Action: Based on your comment, we have conducted new experiments to assess the effect piping and reactor only (i.e., blank test without gauze and adsorbent material), and the same setup with the gauze (i.e., blank test without activated carbons, with gauze), on H2S adsorption. For the first blank test, a filter cartridge with an empty Teflon tube (4 mm of inner diameter), stainless steel Swagelock fittings was used – we called this the “no gauze” test. Another test was done with a Teflon tube filled with 0.48 g of sterile gauze, under the same conditions – we called this the “gauze” experiment. A simulated biogas mixture (60/40 %vol.) with 500 ppmv of H2S and 919 Nml/min was used for both experiments. The saturation condition was reached after 5.8 minutes and 6.18 minutes of test, respectively. Thus, we found a negligible effect on adsorption capacity from the used experimental setup.

In the revised manuscript, we added a specific comment on this:

“We investigated on the effect of the sterile gauze on the sulfur removal capacity. A blank cartridge (without carbon filter), and with or without sterile gauze, was used to reveal a negligible adsorption capacity from the adopted experimental setup. At saturation condition, i.e., 100% of H2S breakthrough at an inlet concentration of 500 ppm(v) and under a CH4/CO2 60/40 %vol. mixture, the adsorption capacity varies from 0.0038 mg/g (piping and reactor only, without gauze) to 0.0040 mg/g (with gauze).”

Results and discussion. The 0.1% oxygen has a high impact. Comment on this, do the plant manager should add some oxygen to improve treatment or what???

Response: In the majority of the plants, oxygen is available even if in very small quantities, usually not detected by standard analyzers. The enhancement of the sorbents performance due to oxygen are related to the following equation:

This means that oxygen is required in a maximum 3/2 (1.5) ratio respect to the hydrogen sulphide content. In WWTPs similar to the one analyzed, which performs iron chloride dosing in the water treatment line to precipitate phosphorous, and thus sulphur), the average H₂S content in sewage biogas is 50-200 ppm. Consequently, oxygen is required in a 75-300 ppm range, which is usually not measured because of detection limits of standard gas analyser, as demonstrated in many literature works [7]–[11] where O₂ content is usually < 0.1% or in the range 0-1%, depending on the plant.

Furthermore, the authors have performed – through an external certified laboratory – an analysis of the sewage biogas composition in the WWTP mentioned in the paper. Results, shown in the table below, pointed out a variable oxygen content between 0.01 and 0.33%.

From the authors’ experience with real WWTPs gas analysis and from internal discussions with sorbents sellers, the presence of the required amount of oxygen can probably be guaranteed in all the biogas plants.

VERY IMPORTANT INCLUDE IN THE TEXT

Action: the section on the O₂ content in biogas has been added in the text, in particular in the ‘Materials and Methods’ section, in order to justify the tests performed with oxygen and the level chosen. The added section in the manuscript os also reported here below.

“In these two testing phase, sorbents materials were tested in anaerobic conditions. However this is not representative of the real plant situation. In the majority of the anaerobic digestion plants, oxygen is available in the biogas mixture even if in very small quantities, usually not detected by standard analysers [7]. The enhancement of the sorbents performance due to oxygen are related to the following equation:

   (Eq. 15)

This means that oxygen is required in a maximum 3/2 (1.5) ratio respect to the hydrogen sulphide content. In WWTPs which performs iron chloride dosing in the water treatment line to precipitate phosphorous, and thus sulphur), the average H₂S content in sewage biogas is 50-200 ppm. Consequently, oxygen is required in a 75-300 ppm range, which is usually not measured because of detection limits of standard gas analyser, as demonstrated in many literature works [44–48] where O₂ content is usually < 0.1% or in the range 0-1%, depending on the plant.

Furthermore, the authors have performed – through an external certified laboratory – an analysis of the sewage biogas composition in the SMAT Collegno WWTP [49]. Results, shown in Table 1, pointed out a variable oxygen content between 0.01 and 0.33%. From the authors’ experience with real WWTPs gas analysis and from internal discussions with sorbents sellers, the presence of the required amount of oxygen can probably be guaranteed in all the biogas plants.

Table 1 –Biogas composition analysis in the SMAT Collegno WWTP.

For this reason, further investigations have been made on selected sorbents from the first tests (AirDep CKC, AirDep CKI and SulfaTrap R8G) considering the addition of 0.1% oxygen in the inlet biogas stream, with an H₂S inlet concentration of 95 ppm(v).”

In general try to improve discussion making it more concise

Response: it was added a discussion section.

NO NEED FOR AN ADDITIONAL DISCUSSION SECTION BUT YOU SHOULD MAKE THE WHOLE "RESUSLTS AND DISCUSSION" MORE CONCISE

Action: The discussion section was removed, we tried also to be more concise in the Results and discussion section by removing Figure 6 of the unrevised manuscript and several paragraphs of Section 4.1 (the detail on the removed text can be found in the annotated manuscript with track changes on.). Figure 8 has been also removed.

Section 4.4 shows simulation results from the dynamic adsorption model. It shows a sensitivity analysis around the most relevant operating conditions of a real adsorption bed. The entire section is now included as an Appendix A.

References:

[1]         A. Lanzini et al., “Dealing with fuel contaminants in biogas-fed solid oxide fuel cell (SOFC) and molten carbonate fuel cell (MCFC) plants: Degradation of catalytic and electro-catalytic active surfaces and related gas purification methods,” Prog. Energy Combust. Sci., vol. 61, pp. 150–188, 2017.

[2]         A. Lanzini, D. Ferrero, D. Papurello, and M. Santarelli, “Reporting Degradation from Different Fuel Contaminants in Ni-anode SOFCs,” Fuel Cells, vol. 17, no. 4, pp. 423–433, Aug. 2017.

[3]         D. Firer, E. Friedler, and O. Lahav, “Control of sulfide in sewer systems by dosage of iron salts: Comparison between theoretical and experimental results, and practical implications,” Sci. Total Environ., vol. 392, no. 1, pp. 145–156, 2008.

[4]         R. DeVries, “Ferric Chloride in Wastewater Treatment,” Environ. Sci., vol. 2, pp. 6–26, 2011.

[5]         S. Aslanidou, N. Lydakis-Simantiris, C. Kotsifaki, D. Pentari, and E. Katsivela, “FERRIC CHLORIDE (FeCl 3 ) AS REGULATORY AGENT FOR THE REDUCTION OF HYDROGEN SULFIDE (H 2 S) IN MUNICIPAL WASTEWATER.”

[6]         R. Devries IV, “Ferric Chloride in Wastewater Treatment,” Environ. Sci. Water Life, 2011.

[7]         M. M. Manyuchi, C. Mbohwa, and E. Muzenda, Resource Recovery from Municipal Sewage Plants : an Energy-Water-Nutrients Nexus for Developing Countries. .

[8]         R. Chicheportiche, M. Balerna, A. Lombet, G. Romey, and M. Lazdunski, Synthesis and mode of action on axonal membranes of photoactivable derivatives of tetrodotoxin, vol. 254, no. 5. 1979.

[9]         S. Rasi, J. Läntelä, and J. Rintala, “Trace compounds affecting biogas energy utilisation – A review,” Energy Convers. Manag., vol. 52, no. 12, pp. 3369–3375, 2011.

[10]      V. Paolini et al., “Characterisation and cleaning of biogas from sewage sludge for biomethane production,” J. Environ. Manage., vol. 217, no. March, pp. 288–296, 2018.

[11]      X. Y. Chen, H. Vinh-Thang, A. A. Ramirez, D. Rodrigue, and S. Kaliaguine, “Membrane gas separation technologies for biogas upgrading,” RSC Adv., vol. 5, no. 31, pp. 24399–24448, 2015.

Round 3

Reviewer 2 Report

Thanks for the changes. Please improve language in the added parts.

Author Response

Thanks for the comments. We have improved language in the added parts.