Production of Volatile Fatty Acids in a Semi-Continuous Dark Fermentation of Kitchen Waste: Impact of Organic Loading Rate and Hydraulic Retention Time
Round 1
Reviewer 1 Report
- Page 11, line 412. The deleted word “is” should be removed.
- Page 12, line 458. The connection line between “ana-lysis” can be deleted.
Author Response
Response to Comments by Reviewer 1
We hope that the corrections made according to the comments of the Reviewer will accentuate the novelty issue and enhance quality of this paper.
Comments and Suggestions for Authors
- Page 11, line 412. The deleted word “is” should be removed.
Authors’ response:
Corrected. Page 13, line 484.
- Page 12, line 458. The connection line between “ana-lysis” can be deleted.
Authors’ response:
Corrected. Page 14, line 530.
Author Response File: 
Author Response.docx
Reviewer 2 Report
Paper is well written and interesting. Improvements are needed on results presentation.
- avoid using abbreviations in the abstract;
- insert a nomenclature reporting main symbols and abbreviations and their units of measure
- The English of the paper has poor quality, revise it through a mother tongue scientific editor
- line 82, mention the following method to optimize fermentation:
Liberti, F.; Pistolesi, V.; Mouftahi, M.; Hidouri, N.; Bartocci, P.; Massoli, S.; Zampilli, M.; Fantozzi, F. An Incubation System to Enhance Biogas and Methane Production: A Case Study of an Existing Biogas Plant in Umbria, Italy. Processes 2019, 7, 925. https://doi.org/10.3390/pr7120925
- check the enumeration of the paragraphs in section 2, they are numbered as 2.1. After the third paragraph insert another one named "analytical methods" in which you report the: type of analysis, the norm used, the instrument used (model, producer and origin). This information has to be reported also in a table.
- in the results section insert always the standard deviation when available
- after the results insert a discussion section where you compare the achievements of your study with the literature.
Author Response
Response to Comments by Reviewer 2
We hope that the corrections made according to the comments of this Reviewer will accentuate the novelty issue and enhance quality of this paper.
Paper is well written and interesting. Improvements are needed on results presentation.
Reviewer comment No 1.
- avoid using abbreviations in the abstract;
Author’s response:
All abbreviations have been removed from the abstract and replaced by the full names; Page 1, lines: 12-27.
Reviewer comment No 2.
- insert a nomenclature reporting main symbols and abbreviations and their units of measure
Author’s response:
A table with main symbols and abbreviations and their units of measure has been added on Page 2.
Reviewer comment No 3.
- The English of the paper has poor quality, revise it through a mother tongue scientific editor
Author’s response:
The article has been sent and revised by a scientific editor proficient in English. The corrections have been implemented into the whole text in the “track changes” mode.
Reviewer comment No 4.
- line 82, mention the following method to optimize fermentation:
- Liberti, F.; Pistolesi, V.; Mouftahi, M.; Hidouri, N.; Bartocci, P.; Massoli, S.; Zampilli, M.; Fantozzi, F. An Incubation System to Enhance Biogas and Methane Production: A Case Study of an Existing Biogas Plant in Umbria, Italy. Processes2019, 7, 925. https://doi.org/10.3390/pr7120925
Author’s response:
The above-mentioned article by Liberti et al. (2019) has been cited on page 3, line 90, as suggested by the Reviewer and additionally implemented into the section ‘References’ as a position number 18 (page 20, line 780).
Reviewer comment No 5.
- check the enumeration of the paragraphs in section 2, they are numbered as 2.1. After the third paragraph insert another one named "analytical methods" in which you report the: type of analysis, the norm used, the instrument used (model, producer and origin). This information has to be reported also in a table.
Author’s response:
The enumeration of the paragraphs in section 2 has been corrected.
After the last paragraph in Section 2 (Materials and Methods) there has been placed a table (Table 4) with listed type of analysis, the norm used and the instrument exploited (model, producer and origin); Page 6, line 241.
Reviewer comment No 6.
- in the results section insert always the standard deviation when available
Author’s response:
Due to Reviewer’s suggestion, the following explanation has been added to the text on page 5, line 207:
Each experiment on the fermentation process was performed in triplicate and the arithmetic average was taken for data interpretation. An error of the performed experiments was less than 10%.
Reviewer comment No 7.
- after the results insert a discussion section where you compare the achievements of your study with the literature.
Author’s response:
According to Energies Instructions for Authors (https://www.mdpi.com/journal/energies/instructions), theResults and Discussion sections may be combined. Our intention was to include the discussion of the results in direct connection with the obtained results, which, in our opinion, enables in-depth analysis of the obtained results.
Author Response File: Author Response.docx
Reviewer 3 Report
Dear Authors,
The results of the study are well presented, and the work is insightful and valuable to advance the state of knowledge. However, some issues should find your attention. In particular, materials and methods must be presented in more detail.
Major issues related to Materials and Methods:
- Section 2: The used kitchen waste is not adequately specified. Details must be amended: how much waste was collected, where (e.g. how many households), how was material transported and stored before analysis.
- Section 2: The used DS from the wastewater treatment plant is not adequately specified. What kind of technology is installed at that WWTP for the digestion (incl. type of digester, temperature, is there any co-digestion).
- Section 2: How was the pH during digestion experiments (to keep the pH at 7) monitored and kept constant? pH measured continuously? With all reactors equipped with sensor? How was NaOH dosage performed?
- Section 2: Does Table 1 report mean with standard deviation? That should be mentioned. Also: of how many repetitions?
- Section 2: No information is provided on how the microbial characteristics using the Illumina platform were determined. Reference to an earlier publication is made; that is O.K., but nevertheless the key information should be provided to the reader of this publication.
Arrangement of Contents
- In Section 3.1: Table 3 and most of the text related to this table is background knowledge from literature review. This is not appropriately placed here in the results section. It should be placed earlier, in the first section of the manuscript, as literature review.
Reference to Literature/ Acknowledgement of literature
- In Section 1, paragraph 2, first two sentences: these statements should be backed up by literature referencing.
- The literature used is generally fine, and the contents acknowledged. Nevertheless, there is a lot of more literature available from the last 5 years dealing with biological metabolites recovery from organic residues through fermentation. More relevant literature could be considered to strengthen the credibility of your work.
Other issues:
- Unit “ton(s)” should be replaced with “metric ton(s)” or “tonne(s)”, because simply “ton” is ambiguous (https://en.wikipedia.org/wiki/Ton).
- I strongly recommend not using a unit in the form “g/L/d”, this is mathematically not correct. Correct would be “g/(L*d)” or “g L-1 d-1”.
- Section 1, paragraph 1, second half of para, dealing with “Directive 75/442/EEC”: it should be mentioned that this is EU Directive …
- Table 3: What is “VSS”? What is “TVS”? Please amend.
Author Response
Response to Comments by Reviewer 3
We hope that the corrections made according to the comments of the Reviewer will accentuate the novelty issue and enhance quality of this paper.
The results of the study are well presented, and the work is insightful and valuable to advance the state of knowledge. However, some issues should find your attention. In particular, materials and methods must be presented in more detail.
Major issues related to Materials and Methods:
Reviewer comment No 1.
Section 2: The used kitchen waste is not adequately specified. Details must be amended: how much waste was collected, where (e.g. how many households), how was material transported and stored before analysis.
Reviewer comment No 2.
Section 2: The used DS from the wastewater treatment plant is not adequately specified. What kind of technology is installed at that WWTP for the digestion (incl. type of digester, temperature, is there any co-digestion).
Author’s response:
Details asked by the Reviewer regarding the KW specification and the WWTP have been given in the following revised Point 2.1 (no co-digestion is used in this WWTP):
The KW from local households (Lodz, Poland) was used as a substrate to study the production of VFAs. This organic substrate was composed of vegetable waste - approx. 44% w/w, fruit waste - approx. 30% w/w and other bio-waste (i.e., bread, coffee and tea grounds, cooked pasta) - approx. 25% w/w [30]. About 200 kg of fresh KW was collected from 70 households within 2-3 days and kept in the cold room at 4oC. These wastes were than shredded in a grinder to get a particle size of less than 3 mm. Thereafter, the thoroughly mixed substrate was stored at −20°C.
As the inoculum the DS from the Wastewater Treatment Plant in Lodz (Poland) was used in our research. Following the treatment technology in this plant, the thickened sludge is stabilized through methane fermentation under mesophilic conditions (35 ÷ 38°C), in four anaerobic digesters (10 000 m3, each) at the HRT of 20-26 days. Before initializing the DF process, the 4 L sample of DS brought from the treatment plant was heated at 70°C for 30 min in order to deactivate non-speculating forms of bacteria as it was performed in our previous study [31].
Reviewer comment No 3.
Section 2: How was the pH during digestion experiments (to keep the pH at 7) monitored and kept constant? pH measured continuously? With all reactors equipped with sensor? How was NaOH dosage performed?
Author’s response:
The following details regarding the maintenance of a constant pH 7 during the experiments has been added on page 5, line 182:
During the fermentation process the measurements of the pH were performed continuously using a pH electrode (InPro 3250/225/PT1000, Mettler Toledo, Switzerland). Based on the changing pH values during the fermentation process, NaOH was dosed automatically via a peristaltic pump (101 U/R, Watson Marlow, USA) connected to a pH regulator (KD7, Lumel, Poland) to keep a constant pH.
Reviewer comment No 4.
Section 2: Does Table 1 report mean with standard deviation? That should be mentioned. Also: of how many repetitions?
Author’s response:
Because additional table was added in row 121 the original Table 1 the Reviewer questions are now pertinent to Table 2. Hence, the following text has been added (Page 4, line 166):
The characteristics of the KW and DS are presented in Table 2 where the first number in each column signifies an arithmetic mean from 3 independent measurements whereas the second number quantifies the standard deviation.
Reviewer comment No 5.
Section 2: No information is provided on how the microbial characteristics using the Illumina platform were determined. Reference to an earlier publication is made; that is O.K., but nevertheless the key information should be provided to the reader of this publication.
Author’s response:
The following text referred to microbial characteristic has been added on page 6, line 229:
The total genomic DNA was extracted from the liquid fraction of a fermentation mixture. Amplification of conserved bacterial 16SrRNA gene with the fragment covering V3 and V4 regions was done in triplicate. Obtained amplicons were purified, pooled in equimolar ratio and indexed according to the Nextera indexing strategy by PCR (Illumina). Sample indexing allowed pooling of amplicons for sequencing run and further extraction of the sample sequence reads from large batch of sequencing results data. Sequences were grouped based on their taxonomic classification and highly similar sequences were clustered into operational taxonomic units.
Arrangement of Contents
Reviewer comment No 6.
In Section 3.1: Table 3 and most of the text related to this table is background knowledge from literature review. This is not appropriately placed here in the results section. It should be placed earlier, in the first section of the manuscript, as literature review.
Author’s response:
Thank you for your suggestion; we have placed a short literature review on Page 3, line 102, as follows:
"The values of different OLRs and HRTs used in different DF processes are presented in Table 1. However, a wide range of examined OLRs and HRTs precludes quantitative comparison with literature data but only makes it possible to analyze the trends in changing values. Lim et al. [20] studied the effect of the OLR on acidic fermentation of the FW and DS in a semi-continuous system at the HRT of 8 d and pH 5.5. They have observed that VFAs synthesis increased at higher OLR values. Gou et al. [22] searched for the possible relationship between the OLR and temperature by co-fermenting activated sludge and FW in a semi-continuous system. At higher temperature and at higher OLR values these researchers achieved the highest VFAs production [22]. Scoma et al. [23] have observed that shorter HRTs turned into a lower accumulation of VFAs when fermenting in continuous mode dephenolized olive mill wastewater with the acidogenic inoculum. The positive impact of increasing HRT on the DF process was also reported by Lim et al. [20], who studied the effect of different HRT values on DF using FW as a substrate and DS as an inoculum."
Additionally, due to Reviewer’s suggestion, we have also placed the former Table 3 in the first section of the manuscript (Page 3, line 121).
Reference to Literature/ Acknowledgement of literature
Reviewer comment No 7.
In Section 1, paragraph 2, first two sentences: these statements should be backed up by literature referencing.
Author’s response:
Due to Reviewer’s suggestion, the first sentence in Section 1, paragraph 2 (Page 2, line 63) has been backed up by the Reference 7:
- Zhou, M.; Yan, B.; Wong, J.W.C.; Zhang, Y. Enhanced volatile fatty acids production from anaerobic fermentation of food waste: A mini-review focusing on acidogenic metabolic pathways. Bioresour. Technol. 2018, 248, 68–78, doi:10.1016/j.biortech.2017.06.121.
The second sentence in Section 1, paragraph 2 (Page 2, line 66) has been backed up by the reference 5:
- Lee, W.S.; Chua, A.S.M.; Yeoh, H.K.; Ngoh, G.C. A review of the production and applications of waste-derived volatile fatty acids. Chem. Eng. J. 2014, 235, 83–99, doi:10.1016/j.cej.2013.09.002.
Reviewer comment No 8.
The literature used is generally fine, and the contents acknowledged. Nevertheless, there is a lot of more literature available from the last 5 years dealing with biological metabolites recovery from organic residues through fermentation. More relevant literature could be considered to strengthen the credibility of your work.
Author’s response:
We do recognize your concern and basically agree with your comment. However, due to limited number of pages, we had to select the most relevant references within the assigned number of 60 citations.
Other issues:
Reviewer comment No 9.
Unit “ton(s)” should be replaced with “metric ton(s)” or “tonne(s)”, because simply “ton” is ambiguous (https://en.wikipedia.org/wiki/Ton).
Author’s response:
The suggested amendments have been done in the article: Page 1, lines: 36, 37, 42, 43.
Reviewer comment No 10.
I strongly recommend not using a unit in the form “g/L/d”, this is mathematically not correct. Correct would be “g/(L*d)” or “g L-1 d-1”.
Author’s response:
As suggested by the Reviewer, the dimension “g/L/d” has been replaced by “g/(L*d)” in the whole article.
Reviewer comment No 11.
Section 1, paragraph 1, second half of para, dealing with “Directive 75/442/EEC”: it should be mentioned that this is EU Directive …
Author’s response:
Due to Reviewer’s comment, this information has been added to the article in Section 1, paragraph 1 (Page 2, line 51).
Reviewer comment No 12.
Table 3: What is “VSS”? What is “TVS”? Please amend.
Author’s response:
This information has been added to the newly added list with Nomenclature (Section 1) on Page 2.
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
accept
Reviewer 3 Report
Dear Authors, the work is now comprehensively presented. It is insightful and interesting to read.
