Aeration to Improve Biogas Production by Recalcitrant Feedstock

Round 1

Reviewer 1 Report

This paper seems to me a report rather than a full article, 

Content are not sufficient for a full paper to be published,

Seems few repetition from the below link and need to be justify

https://lpelc.org/aeration-to-improve-biogas-production-by-recalcitrant-feedstock/ 

Author Response

We have expanded our Introduction and discussion of the results to address the reviewer’s concerns.

Some of the repetition from this link (https://lpelc.org/aeration-to-improve-biogas-production-by-recalcitrant-feedstock/) have been eliminated. This link contains a proceedings paper from a Waste to Worth Conference I attended in Raleigh, NC two years ago. Since we have modified the paper in response to the reviewer’s comments, hopefully some of this repetition has been eliminated.

Reviewer 2 Report

Comments and recommendations regarding manuscript Aeration to Improve Biogas Production by Recalcitrant Feedstock

General comments

This manuscript reports the results of an experimental study regarding the waste digestion enhanced by addition of low levels of aeration at small scales and without provision to retain the aeration in the digestate.

The topic of the study appears of interest for the readership of Environments, however in my view the study is lacking in several aspects.

In my opinion I think that the Introduction: Literature Review - It is inadequate. Authors should emphasize on works recently done (not older than 5 years).

Secondly, the main aim of the work is not clear, moreover Materials and Methods are weak. The “Digester descriptions” is insufficient. A simple and transparent drawing (Fig) would be very helpful considering that the authors emphasize in the manuscript: “The aim of this research was to construct prototypes of anaerobic digesters that would utilize a sub-surface manifold to retain aeration within the sludge layer of the digestate”. I would recommend the authors clearly explain and describe the digester.

The methodology is not clearly explained. The explanation and procedure are incomprehensible.  The section on materials and methods should answer the question HOW. Methodology not detailed enough. The text is very difficult to read.

The scientific/experimental approach is weak and findings and not well described (confusing). There are miss links between Introduction, Methodology and Results. In my opinion the authors have not discussed their results at all.

There are typos in the text, missing commas, etc.

I do believe there is valuable data to be published from this research, but as it stands the manuscript is weak and unrefined. Scientific quality is poor, there is potential for improvement, but this requires a lot of work from the authors. In conclusion at this stage I find the quality of this manuscript to be inadequate for publication in this Journal.

Author Response

We have expanded the Introduction and added more recent references. In addition, we expanded and clarified some of the concerns the reviewers had to make the rationale for this study clearer.

We modified the Materials and Methods section. In particular, we added a schematic of the digesters. We also edited the text of this section for clarity.

We expanded the Results and Discussion section. In addition to editing it foe clarity’s sake we expanded our discussion in regards to related research.

Reviewer 3 Report

Loughrin et al investigated that aeration impact on biogas production in anaerobic digestion. Authors reported that Aeration at 200- and 800-mL day-1 enhanced biogas production by 14 and 73% without damaging the system in small scale rector system. Based on the reported data it seems aeration has no or limited impact on biogas production. I have following major comments

1.    Reference is missing in line 50.

2.    The author needs to describe which facultative organisms was optimized after air introduction as described in front 53. Any reference?

3.    The introduction section is weak author need to summarize the recent state of the art on the problem statement. I would encourage the author to elaborate on recent studies.

4.    Figure 1:- Biogas production enhanced with the function of milliliters of aeration per day. Why the author select 0, 200, 800 and 2000? Why author jump from 800 to 2000 instead of regular interval of air injection? Any explanation? Aeration rate is randomly selected.

5.    Table 1, with 200 CH4 is 331 but with 800 methane is 314 unfortunately with control methane is 318 it seems aeration does not (likely) have impact on methane production. The result does not reflect the improvement as the author claimed 14 and 73% enhancement. 

6.    Biogas quality contains CH4, CO2, ammonia (NH3), oxygen (O2), carbon monoxide (CO), hydrocarbons, volatile organic compounds (VOC) as describing rent study on gas quality (Danish gas grid system—A study from the demand perspective ,ChemEngineering, 2018 – mdpi). Have you evaluated the impact of aeration on H2S or other components?

7.    Figure 2 legends were missing hard to follow.

8.    Result and discussion section is weak.  Authors need to compare and report previously researched result. Furthermore, no explanation about the microbial activity and O2 toxicity for methanogens.

Author Response

1.       References have been added to line 50.

2.       The amount of research into how microbial populations are affected by micro-aeration of wastewater is limited, however we added a reference from Fu et al. [9] that investigated this in depth.

3.       We have expanded the Introduction both to add more recent references and to clarify the reasons for this study.

4.       In regards to Figure 1 (now Figure 2), we chose 200 mL per day to add less than 2 mL of aeration per L of digestate per day to the digesters which seemed to be a reasonable choice in relation to previous researchers (see particularly Xu el al. [12]. We chose to increase this 10-fold to see if this would negatively affect biogas generation. The 800 mL per day was perhaps somewhat arbitrary. The lowest aeration rate however was based on an estimate based on the literature and the 800-mL per day was partially based on there being one aeration period for the 200 ml treatment and 10 aeration periods for the 2,000 mL treatment. The aeration periods for each digester had to be equally spaced throughout the day and not more than one digester could be aerated at a time since aeration to all digesters was controlled through one flowmeter. Reasonably, one could view the aeration rates as being arbitrary, but we did choose the lowest rate based on solid reasoning. Also, there is always going to be some guesswork involved in choosing aeration rates when using an unusual digester feedstock and a novel mechanism for introducing aeration to the digesters. If these aeration protocols had not yielded worthwhile results, we would have redone the experiments with different aeration rates.

5.       Table 1. These numbers to refer to methane concentration in the biogas, not biogas production as in Figure 1 (now Figure 2). We make a clear distinction in Figure 2 and Table 1 between gas quality and gas production. We also make this distinction clear in the text.

6.       We did not evaluate the impact of aeration on H2S or other components. However, we did add a discussion of possible affects of microaeration on H2S referring to previous research starting on line 226.

7.       We sis not change the legend on Figure 2 (now figure 3) but did modify the figure itself to make it easier to comprehend.

8.       We modified the Results and Discussion section considerably. In the future we plan 16s and 18s community analysis to determine shifts in prokaryotic communities and hope to identify any fungi that grew in response to the micro-aeration. The basis for oxygen toxicity to strict anaerobes is well known (lack of the enzyme catalase).  Any meaningful discussion of microbial activity would be dependent on community analyses.  

Round 2

Reviewer 1 Report

Thank you for your effort and changes on the basis of comments,

My advice is - if you could justify some results with the interpretation with some recent publication would be great,

Author Response

We added a couple of references to the possible role of fungi in enhancing both waste degradation and biogas production. In addition, we also re-checked the manuscript for grammar and clarity. 

Reviewer 2 Report

The authors significantly improved the manuscript

Author Response

Thank you for your help in improving the submission. 

Reviewer 3 Report

I would like to recommend to publish the manuscript.

Author Response

Thank you for your help in improving the submission. We rechecked the manuscript for clarity and grammar and added a couple of references to the possible role of fungi in anaerobic digestion.