Analysis of the Effect of Sludge Reduction and Energy Production from Introducing Anaerobic Digest Plant into a Sewage Treatment Plant

Round 1

Reviewer 1 Report

Anaerobic digestion is a natural biological process.  Anaerobic digesters are built systems that deliberately harness the natural process. AD systems can minimize odors and vector attraction, reduce pathogens, produce gas, produce liquid and solid digestate, and reduce waste volumes. In this study, based on the current STP data in Korea, the effect of introducing AD was analyzed and a plan for extending AD supplies to STP was presented. However, the following problems need to be improved:

1. The chapter “Introduction” is very general and does not provide a sufficient background. Some citations are from 1999 or 2003, but there are more recent works on this subject.

The problem of sewage sludge and the effects of introducing AD into sewage treatment plants are described too generally.

The text of the in lines 37-41very general. It is advisable to extend this paragraph.

In the description of the aim of the work (lines 59-60) clearly indicate that the study included STP in Korea.

2. The chapter "Materials and Methods" is hardly readable. Maybe some information should be given in the formulas.
3. Chapter "Results" - "Results and Discussion" would be more appropriate

p.5 line 169 - it is "Figure 5" but it should be Figure 4.

4. "Conclusions" - conclusion 2 have an application nature - it should be redrafted to show that it concerns solutions for STP in Korea and it is possibly that similar solutions can be applied in other countries
5. "References" - requires a thorough correction, including the unification of the entry of the works cited, supplementing item 14, deleting item 27.

Author Response

Anaerobic digestion is a natural biological process.  Anaerobic digesters are built systems that deliberately harness the natural process. AD systems can minimize odors and vector attraction, reduce pathogens, produce gas, produce liquid and solid digestate, and reduce waste volumes. In this study, based on the current STP data in Korea, the effect of introducing AD was analyzed and a plan for extending AD supplies to STP was presented. However, the following problems need to be improved:

1. The chapter “Introduction” is very general and does not provide a sufficient background. Some citations are from 1999 or 2003, but there are more recent works on this subject.

The problem of sewage sludge and the effects of introducing AD into sewage treatment plants are described too generally.

The text of the in lines 37-41very general. It is advisable to extend this paragraph.

In the description of the aim of the work (lines 59-60) clearly indicate that the study included STP in Korea.

=> While listing the limitations of composting, we added specific sewage sludge reduction effects and cost savings due to anaerobic digestion.

2. The chapter "Materials and Methods" is hardly readable. Maybe some information should be given in the formulas.

=> It is stated that the current situation in Korea has been analyzed.

      3. Chapter "Results" - "Results and Discussion" would be more appropriate

p.5 line 169 - it is "Figure 5" but it should be Figure 4.

=> The overall expression has been refined and rewritten to make it easier to understand. => The result was written as a result and discussion, and the figure number was also corrected.

4. "Conclusions" - conclusion 2 have an application nature - it should be redrafted to show that it concerns solutions for STP in Korea and it is possibly that similar solutions can be applied in other countries

=>The conclusion has been completely rewritten. In particular, conclusion 2 is presented to be applicable to other countries as well.

4. "References" - requires a thorough correction, including the unification of the entry of the works cited, supplementing item 14, deleting item 27

=> The items of the cited thesis have been unified. However, item 14 is a statistic, item 27 does not exist, so I am not sure what it means.

Reviewer 2 Report

The manuscript "Analysis of the effect of introducing anaerobic digesters into sewage treatment facilities" deals with an analysis of historical and actual implementation of Anaerobic Digestion (AD) to Sewage Treatment Plants (STPs) to reduce Sewage Sludge (SS) production and increase sustainability of STPs. Altough the topic may be of interest to the readers of Water, I regret to suggest its rejection.

The major issues revealed by the manuscript during its revisions are the following:

• The article was submitted as research article but I did not detect any "research" in the paper. It is all about data collection from statistics and data elaboration (really simple elaboration). The paper is missing the scope of elucidating how AD can improve STPs sustainability at all. To do that, an LCA approach starting from data collected and analyzed is mandatory. In this sense, the work of the authors can be considered as a preparatory work for LCA analysis. We cannot say that AD integration to STPs is sustainable only because statistics show SS reduction and/or because AD produce biogas. The paper in its present form may be considered as a mini-critical-review, but not a research article for sure. In this sense, the authors themself have stated this in the manuscript since it did not present the Discussion section which is mandatory in any Research article.
• The manuscript needs the revision from a English-native speaker. It is full of complex sentences (someone are really hard to understand) and punctuation/ortographic mistakes. All this make the comprhension of the manuscript even harder.
• The introduction is missing of recent and relevant information about AD of SS, as well as Co-AD of SS. 

Other minor comments:

• The title is vague and you should specify which kind of analysis was carried out (i.e., economic, environmental, social?)
• L23-24 need to be rephrased
• L27: how can composting cause environmental pollution? are you talking about CO2 emissions? please specify
• L32-36 need to be rephrased
• L39: WWTP need to be spelled the first time
• AD is usually intended for Anaerobic Digestion (not Digester)
• L54: 10% or 1% (please see Abstract)
• Paragraph 2.2 is really hard to read
• L90: ...without AD. compared. (?)
• L110: It is possible, ... (What?)
• L169: It should be Figure 4, not 5
• L201: what are B and C?
• Paragraph 4: Conclusions are only repetition of Results

Considering all these issues, I had to suggest rejection for the manuscript. I would like to suggest the authors to work really hard on it and try to resubmit it as mini-critical-review or to use the data collected to produce an LCA analysis.

Author Response

The manuscript "Analysis of the effect of introducing anaerobic digesters into sewage treatment facilities" deals with an analysis of historical and actual implementation of Anaerobic Digestion (AD) to Sewage Treatment Plants (STPs) to reduce Sewage Sludge (SS) production and increase sustainability of STPs. Altough the topic may be of interest to the readers of Water, I regret to suggest its rejection.

The major issues revealed by the manuscript during its revisions are the following:

• The article was submitted as research article but I did not detect any "research" in the paper. It is all about data collection from statistics and data elaboration (really simple elaboration). The paper is missing the scope of elucidating how AD can improve STPs sustainability at all. To do that, an LCA approach starting from data collected and analyzed is mandatory. In this sense, the work of the authors can be considered as a preparatory work for LCA analysis. We cannot say that AD integration to STPs is sustainable only because statistics show SS reduction and/or because AD produce biogas. The paper in its present form may be considered as a mini-critical-review, but not a research article for sure. In this sense, the authors themself have stated this in the manuscript since it did not present the Discussion section which is mandatory in any Research article.

=>This study is not an LCA analysis, so it cannot show a clear effect. However, by analyzing the operation status rather than the analysis based on theory, the operational effects in the field are presented as data, and measures to improve them are also proposed. Therefore, although it is not necessarily an academic approach, I think it can be viewed as research in terms of analyzing the current situation.

• The manuscript needs the revision from a English-native speaker. It is full of complex sentences (someone are really hard to understand) and punctuation/ortographic mistakes. All this make the comprhension of the manuscript even harder.

=> I have edited the description to make it easier to understand overall.

• The introduction is missing of recent and relevant information about AD of SS, as well as Co-AD of SS. 

=> Recently, as information on anaerobic digestion, we added that research using pre-treatment facilities and various biomass is active.

Other minor comments:

• The title is vague and you should specify which kind of analysis was carried out (i.e., economic, environmental, social?)
• L23-24 need to be rephrased
• L27: how can composting cause environmental pollution? are you talking about CO2 emissions? please specify
• L32-36 need to be rephrased
• L39: WWTP need to be spelled the first time
• AD is usually intended for Anaerobic Digestion (not Digester)
• L54: 10% or 1% (please see Abstract)
• Paragraph 2.2 is really hard to read
• L90: ...without AD. compared. (?)
• L110: It is possible, ... (What?)
• L169: It should be Figure 4, not 5
• L201: what are B and C?
• Paragraph 4: Conclusions are only repetition of Results

=> The overall expression has been changed.

=> Modified of conclusion

Considering all these issues, I had to suggest rejection for the manuscript. I would like to suggest the authors to work really hard on it and try to resubmit it as mini-critical-review or to use the data collected to produce an LCA analysis.

Reviewer 3 Report

The article is about the analysis of sewage treatment plants in Korea in terms of anaerobic digestion. This kind of analysis is needed and the topic is interesting, but the article needs language correction due to some expressions that should be clarified “reduction effect”.

Lines 27-28: “SS can cause environmental pollution through composting and incineration [3-4]. Accordingly, there is a need for a method that can reduce and treat it in an environment friendly manner.” In my opinion, composting is regarded as an environment-friendly way of SS utilization or stabilization. These sentences should be rewritten and more information should be added.

Lines 30-32: “Recently, biomass is a renewable and renewable material derived from biomass, and this biomass is recognized as a carbon-neutral material and is a subject of interest to create new value [5]. It is possible to reduce and process at the same time while obtaining additional energy by using materials that require cost in treatment [6]”. This part should be clarified. What does it mean “create new value”?

Line 55: “the reduction effect of AD installed in STP”. What kind of reduction effect? This part should be rewritten and clarified.

Lines 61-100: If the 4,281 places were analyzed, they should be grouped and the main characteristic should be shown in the table.

Line 103: The “material” should be clarified.

Lines 130-134: “It can be 129 inferred that this is because the BOD concentration increased from 120ppm in 2010 to  160ppm in 2020 and at the same time the BOD treatment load of the influent sewage increased by 36% from 2.5 million tons to 3.4 million tons as the discharge water regulations were strengthened. Considering that there is a correlation between BOD and SS, an increase in BOD may cause an increase in SS [19].”

This part should be clarified. The BOD values are not shown and the cited article is from 2014, but the values mentioned are from 2020.

Lines 150-157: “The amount of surplus gas incinerated without being used is 89,308.8㎥/day, which 150 accounts for 14.0% of the total biogas production. This means that even with an increase in energy production capacity, there are insufficient measures available [21]. Assuming that the methane content is 60% [22], it means that 53,717㎥/day of biomethane is wasted. Therefore, in order to promote the utilization of biogas, it seems necessary to conduct research related to the social foundation for the efficient use of biogas, which is the most economical, in the form of gas, and institutional support related to the commercialization of biogas.”

This part should be clarified. Why biomethane is wasted?

Lines 232-243: The conclusions part contains too much data. It should be rewritten and include only a summary.

Author Response

The article is about the analysis of sewage treatment plants in Korea in terms of anaerobic digestion. This kind of analysis is needed and the topic is interesting, but the article needs language correction due to some expressions that should be clarified “reduction effect”.

=> Changed the existing reduction effect to sludge reduction and energy production.

Lines 27-28: “SS can cause environmental pollution through composting and incineration [3-4]. Accordingly, there is a need for a method that can reduce and treat it in an environment friendly manner.” In my opinion, composting is regarded as an environment-friendly way of SS utilization or stabilization. These sentences should be rewritten and more information should be added.

=> Although composting sewage sludge is an eco-friendly method, the risk of heavy metal leaching, greenhouse gases and odors during the composting process, and restrictions on use were presented as problems.

Lines 30-32: “Recently, biomass is a renewable and renewable material derived from biomass, and this biomass is recognized as a carbon-neutral material and is a subject of interest to create new value [5]. It is possible to reduce and process at the same time while obtaining additional energy by using materials that require cost in treatment [6]”. This part should be clarified. What does it mean “create new value”?

=> New value creation means the mentioned value as a carbon-neutral material. Changed the sentence structure

Line 55: “the reduction effect of AD installed in STP”. What kind of reduction effect? This part should be rewritten and clarified.

=> It was specified as the effect of reducing sewage sludge.

Lines 61-100: If the 4,281 places were analyzed, they should be grouped and the main characteristic should be shown in the table.

=> The analysis targets are grouped and presented in a table.

Line 103: The “material” should be clarified.

=> Clarified with sewage sludge.

Lines 130-134: “It can be 129 inferred that this is because the BOD concentration increased from 120ppm in 2010 to  160ppm in 2020 and at the same time the BOD treatment load of the influent sewage increased by 36% from 2.5 million tons to 3.4 million tons as the discharge water regulations were strengthened. Considering that there is a correlation between BOD and SS, an increase in BOD may cause an increase in SS [19].”

This part should be clarified. The BOD values are not shown and the cited article is from 2014, but the values mentioned are from 2020.

=> This part has been deleted as it is judged not to be significantly related to the thesis.

Lines 150-157: “The amount of surplus gas incinerated without being used is 89,308.8㎥/day, which 150 accounts for 14.0% of the total biogas production. This means that even with an increase in energy production capacity, there are insufficient measures available [21]. Assuming that the methane content is 60% [22], it means that 53,717㎥/day of biomethane is wasted. Therefore, in order to promote the utilization of biogas, it seems necessary to conduct research related to the social foundation for the efficient use of biogas, which is the most economical, in the form of gas, and institutional support related to the commercialization of biogas.”

This part should be clarified. Why biomethane is wasted?

=> It means that 14% of biogas is wasted because it is incinerated without being used.

Lines 232-243: The conclusions part contains too much data. It should be rewritten and include only a summary.

=> We reduced the data and rewritten it in the form of a conclusion.

Round 2

Reviewer 2 Report

Dear authors,

After revisions, your paper increased in quality and merit and I believe it is now fine for publication.

Best regards