Review Reports

Round 1

Reviewer 1 Report

The authors investigated the methane potential from available by-products during the primary treatment process stage of different municipal wastewater treatment plants. The aim of the study was to justify that these by-products could be used for energy production by anaerobic digestion to reduce the carbon footprint of WWTPs. I believe this is a well thought and well written scientific manuscript that promotes the AD scientific knowledge.

Minor comments that could further improve the manuscript:

1. Title: “Estimation of Energy Recovery Potential from Primary Residues of a Municipal Wastewater Treatment Plant” should be “Estimation … Residues of Municipal Wastewater Treatment Plants”. You have checked 4 WWTPs
2. Lines 253 to 256 authors mention low BMP from floatings with high fat content and in lines 280 to 284 describe high methane production from fats based on bibliography. Why floatings from grit chambers had the lowest BMP even though as mentioned they had high content of fats? Please further analyse this issue (bibliography etc.)
3. What was the purpose to measure the methane potential of samples after 30 days of anaerobic treatment, when in typical industrial biogas plants, the hydraulic retention time is not often more than 30 days?
4. In the paper title it is mentioned the energy recovery potential from the residues (from primary treatment stage) of WWTPs. Could the authors provide any data about the volume/time of these by-products in the WWTPs? This could help to have a clearer estimation of the energy recovery potential from the WWTPs.
5. Line 127 “Samples were stored at -4°C in a constant…”. Is this temperature correct?
6. In Figure 1 TS, VS and FATS content are illustrated for the different samples. Even though differences on these characteristics are expected due to the types of waste that end up at WWTPs, was the samples obtained at the same period? In example the same week or month of the year?
7. In general, please reduce sentences that are very long. You have sentences with more than 4 lines.
8. Line 25: “…doubling times less than 1 day.”
9. Line 156: “…each gas sample. The obtained…”
10. Line 228: “…to one of the treatment plants…”

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

My comments and suggestions are fully described in the attached file.

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

Improvement of energy balances of wastewater treatment plants is one of the key issues of reserach and development activities in the field of wastewater treatment and bioenergy production. Manuscript sustainability-1264676 focuses on the determination of biomethane potential of screening, floating stream from different steps of wastewater treatment technology. Introduction section summarizes the relevance of the studíy and research motivations. Materials and methods are described clearly. Although the mnauscript contains interesting results but, in my opinion, the presenting of results of BMP tests cannot be considered as a ‘novel’ study. Caharacterization of different raw materials (fat, TS, VS, P and N content) can provide sueful information, but, these materials cannot be digested alone. In my opinion, the main question is that how affect the feed of this material into the ’normal’ sludge AD line the anaerobic digestion kinetics, methane content and the stability of the digesters and how is changed the digestate composition. Another important aspect is the energy balances and the economy of using these streams in AD. Authors have not presented these data.

Author Response

Please see the attachment

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The authors have done a proper job revising the article. I think it could be published in its current form. 

Reviewer 3 Report

Manuscript sustainability-1264676 has an interesting topic that has practical and technological relevance, as well. Authors have revised the manuscript thoroughly according to reviewers’ comments and suggestions. In the revised manuscript the Introduction section has been amended, and the relevance of the study has been more highlighted. Materials and methods section has more detailed information. The experimental data and results have been discussed in more details, with relevant and new references. After the revision the overall scientific quality of manuscript has improved significantly. In my opinion, the manuscript contains significant and interesting findings for the readers. I accept all answers and modification made by the authors and recommend manuscript sustainability-1264676 for publishing.