Design and Initial Testing of Acoustically Stimulated Anaerobic Digestion Coupled with Effluent Aeration for Agricultural Wastewater Remediation

Round 1

Reviewer 1 Report (Previous Reviewer 3)

Comments and Suggestions for Authors

The article is improved now.

Author Response

Thank you for your help with this manuscript.

Reviewer 2 Report (Previous Reviewer 5)

Comments and Suggestions for Authors The authors set the stage to describe the construction of an anaerobic digester coupled with post-digestion low level aeration for agricultural wastewater treatment. The system employs sound to accelerate the speed of anaerobic digestion and as a substitute for mechanical mixing, reduce the concentration of solids in the digester effluent. The effluent then undergoes low level aeration to achieve partial nitrification to aid in the ammonia removal and is discharged to a small, constructed wetland to further reduce nitrogen and phosphorus concentration. The discharge from the wetland is send to a lagoon, serving as a reservoir for process water for the digester. It is a complex system which can improve the treatment of agricultural wastewater at low cost and in a efficient way. The authors have made some improvements in the manuscript, but there are still some issues that have to be addressed   1) In introduction the authors should mention other studies based in the construction or (technical) improvements of such anaerobic digestion systems. In addition, they should define the difference this work with their previous published work (reference [28]).   2) The authors mention in the manuscript that the sound treatment was of low intensity (lines 375-376). High intensity or treatment of longer time will improve the biogas production and quality?    

Author Response

Thank you for your help with this manuscript. We have expanded the Introduction. As for using longer times and greater intensity of sound, this manuscript is basically presenting the design of the system that we can reference as we proceed with our research. We do anticipate based on previous research that both greater intensity and duration of sound treatment will enhance biogas production.

Reviewer 3 Report (Previous Reviewer 6)

Comments and Suggestions for Authors

The topic is interesting to find a cost effective technology system for animal wastewater treatment. The abstract, the conclusion, and the context of the article should stay consistently. how much cost, how much efficiency, and so on, should be explained by data.

Comments for author File: Comments.pdf

Author Response

Thank you for your help with this manuscript. I'm afraid that adequately describing the costs and efficiency benefits of sound treatment compared to other technologies is beyond my expertise, but I have added some discussion of this to the Introduction and hope this improves the manuscript.

Round 2

Reviewer 2 Report (Previous Reviewer 5)

Comments and Suggestions for Authors

-

Reviewer 3 Report (Previous Reviewer 6)

Comments and Suggestions for Authors

Please make the figure 2 cleaner and more attractive.

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

No comments

Author Response

We thank the reviewer for their kind help with this manuscript.

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript introduces an agricultural wastewater treatment system that combines acoustically stimulated anaerobic digestion with low-level post-aeration. While I acknowledge that the manuscript describes an innovative agricultural wastewater treatment system, there are numerous unreasonable aspects that need improvement.

1. The manuscript mentions using underwater speakers to accelerate the anaerobic digestion process, but lacks detailed scientific evidence and explanations of the mechanism. Is there any data to support this?

2. Specific sound frequencies (e.g., 1, 2, 3, 4, 5 kHz) and playback modes (e.g., 2 hours on, 1 hour off) are mentioned, but the manuscript does not explain how these parameters were determined, nor does it provide a comparison of effects under different parameters. This makes the method selection appear arbitrary.

3. Line 76-77: "It employs sound to accelerate the speed of anaerobic digestion and as a substitute for mechanical mixing." I do not believe that sound can serve as an effective substitute for mechanical mixing. Is there any data on mixing efficiency?

4. The description of the experimental design in the manuscript is relatively brief, lacking specific details on the number of repetitions, the setting of control groups, etc. This affects the reliability and reproducibility of the experimental results.

5. No control group or comparative experiments were set up to verify the actual effects of acoustic stimulation and low-level aeration on anaerobic digestion and wastewater treatment. This deficiency in experimental design significantly reduces the persuasiveness of the research conclusions.

6. The data provided in the manuscript is limited to the initial few weeks of system operation, lacking long-term operational data to assess the system's stability and sustainability.

7. Some of the charts in the article are not clear enough, and lack detailed legends, making it difficult for readers to accurately understand the chart content. For example, the spectrogram in Figure 3 lacks sufficient annotations to explain the meanings of different parts. Why are there only a few tables in the manuscript and no continuous line graphs?

8. The manuscript emphasizes the low cost of the system but does not provide a specific economic analysis or cost-benefit comparison.

9.The review of anaerobic digestion technology in the introduction section of the article is not comprehensive, lacking detailed discussions and citations of recent related research progress.

Author Response

We thank you for your help in improving this manuscript.

The manuscript mentions using underwater speakers to accelerate the anaerobic digestion process, but lacks detailed scientific evidence and explanations of the mechanism. Is there any data to support this? We added a brief discussion of previous research in Section 3.1 where using sound greatly increased biogas production in two separate experiments compared to a control digester. We also discussed possible mechanisms for this increase.

2. Specific sound frequencies (e.g., 1, 2, 3, 4, 5 kHz) and playback modes (e.g., 2 hours on, 1 hour off) are mentioned, but the manuscript does not explain how these parameters were determined, nor does it provide a comparison of effects under different parameters. This makes the method selection appear arbitrary. It is somewhat arbitrary but in the case of this paper we just wished to discuss how the sounds employed affected the bubble harmonic emissions and expanded on this in Section 3.1.
3. Line 76-77: "It employs sound to accelerate the speed of anaerobic digestion and as a substitute for mechanical mixing." I do not believe that sound can serve as an effective substitute for mechanical mixing. Is there any data on mixing efficiency? We added a reference to this in Section 3.1. We also added additional references since we cannot exclude additional mechanisms that might contribute to increased biogas production nor discount that sound may increase biogas production likely due to mixed mechanisms, the relative contribution of which would be difficult to ascertain.
4. The description of the experimental design in the manuscript is relatively brief, lacking specific details on the number of repetitions, the setting of control groups, etc. This affects the reliability and reproducibility of the experimental results. There were no control groups in this system, it was more a description of the design of an anaerobic digestion system coupled with post digestion modules to treat the wastewater. We felt this was necessary to give a more in depth discussion of the system prior to more detailed operation of the system over a longer period of time. We have emphasized this more in the current version of the manuscript. Nevertheless, we have found significant increases in biogas production due to sonication (as opposed to ultrasonication) in a number of independent research studies:

Loughrin, J., Lovanh, N., Antle, S., Bryant, M., and Berry, Z. Sound enhances wastewater degradation and improves anaerobic digester performance. SN Appl. Sci. 2019;1:533.

Loughrin, J., Antle, S., Sistani, K., Lovanh, N. In situ acoustic treatment of anaerobic digesters to improve biogas yields. Environments 2020;7:11.

Loughrin, J., Antle, S., Bryant, M., Berry, Z., Lovanh, N. Evaluation of microaeration and sound to increase biogas production from poultry litter. Environments 2020;7:62.

Loughrin, J., Antle, S., Simmons, J., Sistani, K., Lovanh, N. In situ sonification of anaerobic digestion: Extended evaluation of performance in a temperate climate. Energies 2020;13:5349.

Loughrin, J., Silva, P., Lovanh, N., Sistanti, K. Acoustic stimulation of anaerobic digestion: Effects on biogas production and wastewater malodors. Environments 2022; 9: 102.

Loughrin, J.H., Parekh, R.R., Agga, G.E., Silva, P.J., Sistani, K.R. Microbiome diversity of anaerobic digesters is enhanced by microaeration and low frequency sound. Microorganisms 2023; 11: 2349.

 

5. No control group or comparative experiments were set up to verify the actual effects of acoustic stimulation and low-level aeration on anaerobic digestion and wastewater treatment. This deficiency in experimental design significantly reduces the persuasiveness of the research conclusions.  We deemphasized some of the research conclusions and added the word Initial to the manuscript title. This paper was designed more to present the design of a system and initial research as well as initial results. The emphasis is more on the system itself rather than our early findings.
6. The data provided in the manuscript is limited to the initial few weeks of system operation, lacking long-term operational data to assess the system's stability and sustainability. This paper is more a description of the system’s design with preliminary results. We have emphasized this in the revised manuscript.
7. Some of the charts in the article are not clear enough, and lack detailed legends, making it difficult for readers to accurately understand the chart content. For example, the spectrogram in Figure 3 lacks sufficient annotations to explain the meanings of different parts. Why are there only a few tables in the manuscript and no continuous line graphs? We added more detail to the legend of Figure 3 and modified Figure 2 a bit.
8. The manuscript emphasizes the low cost of the system but does not provide a specific economic analysis or cost-benefit comparison. This paper is preliminary and is not a comparison with existing technologies.

9.The review of anaerobic digestion technology in the introduction section of the article is not comprehensive, lacking detailed discussions and citations of recent related research progress. This paper is not a review of anaerobic digestion technologies. There are numerous reviews of anaerobic digestion technologies such as Khawer at al. 2022, Anaerobic digestion of sewage sludge for biogas & biohydrogen production: State-of-the-art trends and prospects, Fuel 329; and  Saravanakumar et al (2023) “Biomethane production as a green energy source from anaerobic digestion of municipal solid waste: A state-of-the-art review” in Biocatalysis and Agricultural Biotechnology 53. This is a description of a single protype Digester/Wastewater treatment system.

Reviewer 3 Report

Comments and Suggestions for Authors

The study presents an advanced pilot-scale design for treating agricultural wastewater by combining various wastewater treatment technologies. The design and analysis of the study are adequate, and the data collected are sufficient to demonstrate the treatment capacity of the technology under investigation. The results obtained in this study will be beneficial for readers working in this field.

Here are some comments and suggestions to enhance the manuscript before publication:

1- Could you provide a comparison to show the relative merits of the designed technology compared to any typical wastewater technology used for similar influent type and quality?

2- What is the tolerance of the designed system to variations in influent loads and quality?

3- The applicability of the proposed design system at the industrial scale and its feasibility in terms of capital and operational costs compared to conventional wastewater treatment technologies.

4- Could you provide a more schematic representation of the system to make it clearer and easier for readers to visualize?

5- The abstract could be revised to include only the main aims and objectives of the study, key achievements, and recommendations for further study, without the need to describe the detailed methodology of the designed system in the abstract section of the manuscript.

Author Response

The study presents an advanced pilot-scale design for treating agricultural wastewater by combining various wastewater treatment technologies. The design and analysis of the study are adequate, and the data collected are sufficient to demonstrate the treatment capacity of the technology under investigation. The results obtained in this study will be beneficial for readers working in this field.

 

Here are some comments and suggestions to enhance the manuscript before publication:

We thank the reviewer for their helpful comments.

1- Could you provide a comparison to show the relative merits of the designed technology compared to any typical wastewater technology used for similar influent type and quality?

Not at this point. We are more describing the design of the system along with some preliminary results to describe the data we intend to collect as described in the last paragraph of the Introduction.

2- What is the tolerance of the designed system to variations in influent loads and quality? This is a preliminary design and operation of the system. In the past however we had the anaerobic digesters operating outdoors with no post digestion wastewater treatment. We fed the digesters a wide variety of feedstocks including corn, poultry litter, brewewr’s spent grain, waste activated sludge and cardboard. During warm weather we were able to go from feeding the digester 22.7 kg a week to 45.4 without degradation in performance and no negative affects on digestate pH.   

 

3- The applicability of the proposed design system at the industrial scale and its feasibility in terms of capital and operational costs compared to conventional wastewater treatment technologies. This is a paper describing the setup and design of the system so whether this system would be capable of performing well at the industrial scale is unanswerable. However, the system is already much larger than is the typical research project.

 

4- Could you provide a more schematic representation of the system to make it clearer and easier for readers to visualize? We added more details to Figure 2 and hope this helps.

 

5- The abstract could be revised to include only the main aims and objectives of the study, key achievements, and recommendations for further study, without the need to describe the detailed methodology of the designed system in the abstract section of the manuscript. The objective was  basically to describe the system and describe the preliminary results of data we are planning to collect during operation of the system. So in that sense, the abstract is about describing the system. We are planning on a late paper with much more data conducted over a long period and hopefully using this data to refer to describing the system. 

Reviewer 4 Report

Comments and Suggestions for Authors

General Comment:

The manuscript explores the incorporation of acoustic stimulation in anaerobic digestion systems to improve biogas production and wastewater treatment. While this is an intriguing concept, the article lacks the structural and scientific rigor necessary for publication. The introduction fails to provide a sufficient review of the state of the art, and the results lack clear evidence of the effectiveness of the proposed technology. Additionally, the discussion is overly focused on justifying the lack of improvement in biogas production without offering insights into potential antagonistic effects or strategies to enhance the technology.

 

Specific Comments:

 

Introduction

Lines 28-29: The authors should provide quantitative data on greenhouse gas (GHG) emissions to avoid a superficial description of the problem.

Lines 31-32: Rephrase and elaborate on why large-scale animal production poses a significant issue. How does it relate to GHG emissions and environmental degradation?

Lines 36-37: Include exact figures for emissions from facultative lagoons to substantiate the discussion.

Lines 37-40: Explain why the emissions from sulfur and nitrogen compounds are relevant. What are the broader environmental and health impacts?

Lines 75-80: The scientific contribution of the manuscript is unclear. The authors must articulate the novelty of the research and the knowledge gap it addresses. Additionally, the introduction of acoustic treatment is abrupt and unsupported by a state-of-the-art discussion. A thorough review of previous studies on sound-based technologies in anaerobic digestion is necessary to justify the approach.

Lines 82-93: This section primarily describes the experimental system, which is not relevant to the introduction. Move this content to the methodology or justify its inclusion with a stronger connection to the study's objectives.

Materials and Methods

Lines 171-173: The justification for selecting the specific musical pieces and sound frequencies used in the experiment is absent. The authors must clarify whether the choice of "Neptune, The Mystic" and other audio sources was arbitrary or based on prior evidence. Could the results differ with alternative soundtracks?

Results and Discussion

Section 3.3 (Biogas Production): The results show no improvement in biogas production with acoustic stimulation. The discussion merely rationalizes this outcome without exploring potential antagonistic effects between the acoustic treatment and the anaerobic digestion process. The authors should provide a more comprehensive discussion on the implications of these results and suggest modifications to the experimental setup.

Figures and Tables

The quality and readability of the figures are inconsistent. For example, Figure 3 is challenging to interpret, and the spectrogram lacks clarity.

 

Ensure uniform formatting, font size, and resolution across all figures.

Comments on the Quality of English Language

The manuscript's English language quality requires improvement to enhance clarity and readability. Frequent grammatical errors, awkward phrasing, and inconsistent terminology detract from the scientific rigor of the text. 

Author Response

The manuscript explores the incorporation of acoustic stimulation in anaerobic digestion systems to improve biogas production and wastewater treatment. While this is an intriguing concept, the article lacks the structural and scientific rigor necessary for publication. The introduction fails to provide a sufficient review of the state of the art, and the results lack clear evidence of the effectiveness of the proposed technology. Additionally, the discussion is overly focused on justifying the lack of improvement in biogas production without offering insights into potential antagonistic effects or strategies to enhance the technology.

This paper was intended as a description of the construction of an anaerobic digester coupled with post wastewater treatment and describes preliminary data collected from the system. It was not so much intended as a research paper but as a way to describe a complex system that can be referred to as more data becomes available.   

 

Specific Comments:

 

Introduction

Lines 28-29: The authors should provide quantitative data on greenhouse gas (GHG) emissions to avoid a superficial description of the problem. Why? There is tremendous amounts of data on GHG emissions and this is not a review paper? Precisely, this is not a review paper. The average person interested in this type of paper would likely be well versed on GHG emissions.

Lines 31-32: Rephrase and elaborate on why large-scale animal production poses a significant issue. How does it relate to GHG emissions and environmental degradation? Same as previous comments. We discuss a few of the issues with large scale agriculture. We feel an in depth discussion of this issue is distracting and unnecessary as the typical reader would already be cognizant of the issue.

Lines 36-37: Include exact figures for emissions from facultative lagoons to substantiate the discussion. I included a reference to one study.

Lines 37-40: Explain why the emissions from sulfur and nitrogen compounds are relevant. What are the broader environmental and health impacts? We discussed briefly the harmful impacts of hydrogen sulfide and added a few references to problems arises from ammonia either through emssions or its application to land.

Lines 75-80: The scientific contribution of the manuscript is unclear. The authors must articulate the novelty of the research and the knowledge gap it addresses. Additionally, the introduction of acoustic treatment is abrupt and unsupported by a state-of-the-art discussion. A thorough review of previous studies on sound-based technologies in anaerobic digestion is necessary to justify the approach. We added a brief discussion of our previous research on acoustic stimulation of anaerobic digestion and discussed possible mechanisms whereby this might aid in speeding the digestion of wastewater.

Lines 82-93: This section primarily describes the experimental system, which is not relevant to the introduction. Move this content to the methodology or justify its inclusion with a stronger connection to the study's objectives. We edited the Introduction to make it clearer that the objective was to describe the system.

Materials and Methods

Lines 171-173: The justification for selecting the specific musical pieces and sound frequencies used in the experiment is absent. The authors must clarify whether the choice of "Neptune, The Mystic" and other audio sources was arbitrary or based on prior evidence. Could the results differ with alternative soundtracks? We used this and other musical pieces in the past which was able to boost biogas production by 27 to over 100% that of control digesters except in cold weather when it increased biogas production by 74-fold, albeit that gas production in the control digester almost ceased entirely. We briefly mentioned some of our previous research.

Results and Discussion

Section 3.3 (Biogas Production): The results show no improvement in biogas production with acoustic stimulation. The discussion merely rationalizes this outcome without exploring potential antagonistic effects between the acoustic treatment and the anaerobic digestion process. The authors should provide a more comprehensive discussion on the implications of these results and suggest modifications to the experimental setup. We referenced two previous papers showing great improvements in biogas production using sound treatment as have numerous others who have shown the same results using ultrasonic treatment. This paper is about the design of a system with post digestion wastewater treatment and we are presenting preliminary data just as to show how such a system can be monitored as a means of evaluating a system’s efficacy for waste treatment.

Figures and Tables

The quality and readability of the figures are inconsistent. For example, Figure 3 is challenging to interpret, and the spectrogram lacks clarity. We added text to the Figure caption to improve the figure. A “waterfall” spectrogram with multiple frequencies such as this figure has is naturally messy looking but is a high resolution image.

 

Ensure uniform formatting, font size, and resolution across all figures. I increased the resolution of Figure 2 and modified the caption on Figure 3. Figure 1 was taken with a 16 MP camera and original copies of all figures were sent as in a zip file.

 

Comments on the Quality of English Language

The manuscript's English language quality requires improvement to enhance clarity and readability. Frequent grammatical errors, awkward phrasing, and inconsistent terminology detract from the scientific rigor of the text. Without specific examples I am unable to respond to this. I realize that everyone has a different style of writing that might not be appreciated by another.  

Reviewer 5 Report

Comments and Suggestions for Authors The authors set the stage to describe the construction of an anaerobic digester coupled with post-digestion low level aeration for agricultural wastewater treatment. The system employs sound to accelerate the speed of anaerobic digestion and as a substitute for mechanical mixing, reduce the concentration of solids in the digester effluent. The effluent then undergoes low level aeration to achieve partial nitrification to aid in the ammonia removal and is discharged to a small, constructed wetland to further reduce nitrogen and phosphorus concentration. The discharge from the wetland is send to a lagoon, serving as a reservoir for process water for the digester. It is a complex system which can improve the treatment of agricultural wastewater at low cost and in a efficient way. This work is interesting but there are some issues that have to be addressed.   1) In introduction, the authors should define the difference between this study with their previous published study (reference [21]).   2)  The sound treatment was of low intensity, as the authors mentioned in the text. What the authors would expect in terms of biogas production and quality, if the sound treatment was of higher intensity and/or last for longer time ?    

Author Response

The authors set the stage to describe the construction of an anaerobic digester coupled with post-digestion low level aeration for agricultural wastewater treatment. The system employs sound to accelerate the speed of anaerobic digestion and as a substitute for mechanical mixing, reduce the concentration of solids in the digester effluent. The effluent then undergoes low level aeration to achieve partial nitrification to aid in the ammonia removal and is discharged to a small, constructed wetland to further reduce nitrogen and phosphorus concentration. The discharge from the wetland is send to a lagoon, serving as a reservoir for process water for the digester. It is a complex system which can improve the treatment of agricultural wastewater at low cost and in a efficient way. This work is interesting but there are some issues that have to be addressed.   1) In introduction, the authors should define the difference between this study with their previous published study (reference [21]).   2)  The sound treatment was of low intensity, as the authors mentioned in the text. What the authors would expect in terms of biogas production and quality, if the sound treatment was of higher intensity and/or last for longer time ?    We edited the  paper to try and make it clearer that this is a description of the system along with early data representative of the kind of data that will be used in the future to monitor the digestion and wastewater treatment process. We also added a brief discussion of our previous research on acoustic stimulation of anaerobic digestion in Section 3.1. Thank you for your help with this paper.

Reviewer 6 Report

Comments and Suggestions for Authors

The title is attractive. Better to let the content reflect the title with detail work and data.

Comments for author File: Comments.pdf

Author Response

he title is attractive. Better to let the content reflect the title with detail work and data.  We edited the manuscript to make it clearer that the objective of this paper is more to describe the design of the system with just some preliminary data representative of that will be used to inform design changes in the system in the future. Thank you for your help.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

No Comments

Author Response

Thank you for your help on this manuscript.

Reviewer 4 Report

Comments and Suggestions for Authors

No further comments.

Author Response

Thank you for your help on this manuscript.

Reviewer 5 Report

Comments and Suggestions for Authors

-

Author Response

Thank you for your help on this manuscript.

Reviewer 6 Report

Comments and Suggestions for Authors

This is an experiment report style article but the experiment has not ben fully done becasue the sound's impact was not enough (as authors explained). It is encouraged to continue the experiment to test the sound's impact and compare with the references results.

The GHG emission part and condensate traps are not relative to the title of this article. This should be deleted.

The conclusions are too general. There should be the main points for readers to easily get. Because the sound's impacts are not yet reached, the conclusion may not have things to say.

Author Response

We thank you for your help in this process and have tried to address your concerns.

 

This is an experiment report style article but the experiment has not been fully done because the sound's impact was not enough (as authors explained). It is encouraged to continue the experiment to test the sound's impact and compare with the references results.

This paper reports the development and construction of a new anaerobic digester design for the Ag Engineering journal, with some preliminary data to show successful implementation of the design. The reviewer is correct that the experiment is continuing and the data preliminary; in fact, a number of experiments will be conducted on the various parts of the system and compared with results in the future. In this paper we wished to show some preliminary data since the full description of the system would be too large for a research paper. In this paper we included some preliminary data on the sound and especially discussed the aspect of recording the sound.

 

The GHG emission part and condensate traps are not relative to the title of this article. This should be deleted.

The condensate traps are part of the integrated system and the GHG measurements are part of the basic characterization of the design of the digester. We altered the title of the article to emphasize the preliminary nature of the paper and the condensate traps.

 

The conclusions are too general. There should be the main points for readers to easily get. Because the sound's impacts are not yet reached, the conclusion may not have things to say.

We expanded the conclusions section and reemphasized that this describes the initial design of the system with some preliminary data presented.