Effects of Lighter Dose of Oxytetracycline on the Accumulation and Degradation of Volatile Fatty Acids in the Process of Thermophilic Anaerobic Digestion of Swine Manure
Round 1
Reviewer 1 Report
Dear authors,
the manuscript entitled “Effects of lighter dose of Oxytetracycline on the accumulation and degradation of volatile fatty acids in the process of thermophilic anaerobic digestion of swine manure” is focused on negative effects on methane production due to the presence of OTC in swine manure.
Some clarifications are required to confirm the quality of the work.
Line 18: how much does methane production decrease?
Line 30 and line 79: Based on the reported OTC concentration range for pig slurry, on what basis was it determined that 0-1 mg / L was an adequate range to indicate low concentration?
Lines 49-50: which antibiotics? reference?
Lines 71-74: what exactly is meant by activated sludge and rinses? Is adding a certain amount of OTC to the slurry comparable to the amount of OTC found in the slurry, after being digested (even if partially) by the animal? Conversely in figure 1 the pig eats OTC.
Table 1: are the TS values for Inoculum and swine manure correct?
Line 100: it would have been much more useful to analyze ammonia nitrogen rather than TN. Why wasn't it done? The authors are aware of this because they report it in the discussion of the results.
Line 121: I did not understand why a model is needed to quantify the yield if methane is measured frequently.
Line 158: Isn't R1 without antibiotics?
Line 160-166: I cannot understand this passage well because it is not seen in figure 2.
Figure 2: it is necessary to distinguish figure 2 to 2b
Line 217: probably also due to the volatilization of CO2. if there had been a trend in ammonia nitrogen the discussion would have been clearer.
Figure 3a-d: the y-axes scales must be equal
Line 331: Does OTC Promotes the rupture of cells? to what extent is the rupture of these cells more relevant than other substances in the slurry, in terms of degradable substrate to produce vfa?
Figure 4b:
Figure 6 and lines 387-392: I did not understand if the decrease in methanogenesis is due to the increase in acidification or to the action of otc on methanogens?
Based on the results obtained, it is important to clarify the OTC concentration range chosen for the tests. OTC has limited effects on methane reduction (how much is it reduced?). Could the range 0-1 mg / l be considered too limited?
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript concerns an important issue which is the study of the effect of low-concentration oxytetracycline (OTC) on the dynamic changes of volatile fatty acids (VFAs) in anaerobic fermentation at a higher temperature (55°C) and the underlying mechanisms.
Oxytetracycline is one of the most widely used antibiotic in swine breeding. The presence of residues of this antibiotic in swine manure may contribute to the contamination of the environment with oxytetracycline and its metabolites, leading to antibiotic resistance in bacteria. An effective method of eliminating veterinary drugs from swine manure is anaerobic digestion. This means that the research presented in the manuscript is topical.
The aim of the research is clearly defined. The introduction provides sufficient background on the topic. The results are presented in a clear and comprehensible way. However, there are few points which need to be clarified or improved before publication.
Some experimental details in the section Analytical methods are missing:
- There is no information about the used column, temperature program, used carrier gas and temperature of injector in the case of VFAs analysis.
- There is no information about the used column in the case of methane analysis.
- There is no information which mode in LC-MS/MS analysis was used for quantitative determination. If MRM transition was used for quantitative determination of OTC m/z of precursors and products (in MRM transition) should be added. How were the changes in efficiency of ionization controlled? In my opinion, it is important because during the long series of measurements of complex samples, the detector response may gradually change due to the contamination of ESI source. It is advantageous to use an internal standard in analyses for example 13C labelled standard of OTC. How many times was the injection of analyzed sample repeated?
Please provide the missing information.
Overall, the manuscript presents interesting research which can provide a theoretical basis and reference for the high-temperature anaerobic digestion of swine manure containing OTC.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Dear authors,
your comments and corrections are satisfactory, but I did not find in the manuscript the revisions you proposed for points 13 and 14.
Author Response
Dear authors,
your comments and corrections are satisfactory, but I did not find in the manuscript the revisions you proposed for points 13 and 14.
We are very grateful for your professional review.
Response: We apologized for not giving a clear expression of this sentence in the article. For the point 13, the statement was added in the revised manuscript (Line 344-357). For the point 14, the information have be added in our revised manuscript (Line 432-434) as follow “Finally, the methanogen was suppressed to a certain extent by OTC, which reduced methane production and caused the temporary accumulation of VFAs.”
Author Response File: Author Response.pdf
