Fluctuation and Re-Establishment of Aerobic Granules Properties during the Long-Term Operation Period with Low-Strength and Low C/N Ratio Wastewater
, Qiangjun Yuan 1,3 and Fangyuan Chen 1,*Round 1
Reviewer 1 Report
This is an interesting work assessing the performance of stability of aerobic granules treating low strength wastewater with low COD/N ratio. The reactor operation period in this study is very long and therefore the results provided a good information for the long-term stability of granules.
The manuscript is clear, the experiments have been well conducted and the conclusions are clearly supported by the results. In my opinion, the topic is of undoubted interest and the conclusions are supported by the results. I think that a significant contribution in the field of study.
I suggest this paper should be accepted but after minor revisions that can be seen below.
Materials and methods
- The volume influent filling should be included
- After filling, a mixing with low aeration was carried out. However, no specific reference is made to airflow rate in this phase, only the airflow during the aeration phase was included. Additional information is required.
- The authors explain that a activate sludge from WWTP was seeded in the reactor. In my opinion is necessary include a Table with the main characteristics of this sludge.
- Due to low volume of experimental reactor. Does the take sample affect to the SRT or HRT?
Results
At the end of experimental period the MLSS increased to reached a maximum in 7g/L on day 270, after that biomass concentration decreased. I think that additional information is required to explain this behaviour
Conclusions
In my opinion, the lines 528-535 is a abstract not a conclusion.
Author Response
Response to Reviewer comments
Review 1#
- Materials and methods
(1) The volume influent filling should be included.
Response: the volume influent filling was not directly included in the part of Materials and methods in this paper, because it was indirectly described with the parameter of a volumetric exchange ratio of 50% (line 154), i.e., 1 L of influent filling volume. To avoid confusion, influent filling volume was added in line 152, i.e., “the influent filling volume was set as 1 L.”
(2) After filling, a mixing with low aeration was carried out. However, no specific reference is made to airflow rate in this phase, only the airflow during the aeration phase was included. Additional information is required.
Response: the airflow rate for the mixing with low aeration was neglected because the mixing was not for biodegradation of the pollutants in the system but only for better contact between the aerobic granules and newly-filled wastewater. It was very short, i.e., 1 min with an aeration rate of 1 L/min. This information has been supplemented.
(3)The authors explain that a activate sludge from WWTP was seeded in the reactor. In my opinion is necessary include a Table with the main characteristics of this sludge.
Response: Unfortunately, we did not analyze the main characteristics of the inoculated sludge. The main reason is that the inoculum does not affect granulation, which has been validated and reported by Liu, Y-Q., & Tay, J-H. (2015) in the paper pf ‘Fast formation of aerobic granules by combining strong hydraulic selection pressure with overstressed organic loading rate’. Water Research, 80, 256-266. Secondly, activated sludge from normally operated municipal wastewater treatment plants has more or less similar characteristics such as a size range of 100-200 um and SVI at around 100-200 mL/g. Therefore, no characterization was conducted for the vast majority of papers on an aerobic granular system with a seed of activated sludge.
(4) Due to low volume of experimental reactor. Does the take sample affect to the SRT or HRT?
Response: Sampling in this study was to evaluate either performance of the reactor or the characteristics of the aerobic granules. For performance assessment, most of the samples were taken from the effluent discharged from the reactor, which would not affect the liquid volume in the reactor and HRT. For the evaluation of the characteristics of granules, a totally 100 mL (0.1 L) of mixed liquid for SVI, MLSS and MLVSS testing was needed at 3-5 days intervals, which would barely affect the biomass in the reactor and SRT even taken the volume of the experimental reactor was 2 L. In addition, size and DGGE testing needed 5 and 30 mL sludge, respectively, which were tested for 4 and 2 times, respectively, during the whole operation period. Therefore, the samples taken in the experiment can be negligible regarding impacts on the operation, SRT and HRT of the system.
- Results
At the end of experimental period the MLSS increased to reached a maximum in 7g/L on day 270, after that biomass concentration decreased. I think that additional information is required to explain this behavior.
Response: in an aerobic granular reactor, biomass concentration was not controlled. After MLSS reached 7 g/L due to low SVI of the granular sludge, the decrease in biomass concentration was observed, which was quite similar to the decrease in biomass concentration on the operation day of around 130. This would be a sign that granule sludge might disintegrate again probably from DO restriction due to high biomass concentration. These sentences have been supplemented in the text.
- Conclusions
In my opinion, the lines 528-535 is a abstract not a conclusion.
Response: yes, we agree with the reviewer’s opinion and have deleted these lines from the conclusion part.
Reviewer 2 Report
The paper describes a laboratory experiment using synthetic wastewater with low carbon/nitrogen ratio mainly in aerobic treatment. Ordinary wastewater parameters and granules, as well as microbial species, have been followed during some 10 months. The paper is mainly well written, but I have some comments.
Some readers may read their first articles or they read the first English written papers. Help them so that you open all abbreviation as in line 17 COD and N and in line 70 SVI30!
Some of your sentences are too long as those starting in lines 60, 83, and 108. Please, cut too long sentences to shorter ones and think about if you must explain all details.
Lines 58-60 something is lacking.
Never start a sentence with But (Line 97)!
In the last paragraph of the Introduction should clearly explain the aims of the work starting as: We aimed to….
In Figs. 1, 3, 4, 5, and 7 increase the fonts since now reading this in a small screen or taking a paper copy without magnification is impossible to see the figure explanations. I had to use 125 % or 150 % magnification to be able to see those texts. In these figures, the figure font could also be bigger.
Is Fig 4 is A from low biomass concentration and B from high biomass concentration?
In Fig 2 A show that the x-axis is logarithmic! In Fig 2B volume percentage.
Table 1 is interesting but its columns for days are not clear. Instead of using two columns for days, you can have a row showing that the left one is for 85 days and the right one from 227 days. The other method is to present them as tables 1 A for days 85 and 1B for days 227 days, which would allow the normal portrait orientations and normal font size.
I warmly recommend that you will present the supplement figure as a normal Fig. 6 (and the present Figs. 6 and 7 as 7 and 8).
Increase the Figures 7 since the protozoa are there and they can be seen better with 150 or 200 % magnification. Is this microscopy figure taken without any staining? In this case, the 200 µm can be seen but not the days.
Correct all references and see from guidance for authors how they must be presented.
In some cases, space before[ should be before [
Author Response
Responses to Reviewer 2 comments
(1) Some readers may read their first articles or they read the first English written papers. Help them so that you open all abbreviation as in line 17 COD and N and in line 70 SVI30!
Response: thanks for the reviewer’s question. Sorry for the inconvenience we have made for the reading of these values. In correction, we have added the full names of these values for the first time use in the paper (Line 17 and Line 21).
(2) Some of your sentences are too long as those starting in lines 60, 83, and 108. Please, cut too long sentences to shorter ones and think about if you must explain all details.
Response: thanks for the reviewer’s suggestion, we have broken long sentences to shorter ones. Also, we tried to make them more concise. Please refer to the revised text.
(3) Lines 58-60 something is lacking.
Response: Lines 58-60 show the text like this “the granulation speed has been greatly increased. According to Liu and Tay (2015), granules formed within 24 hours by combining strong hydraulic selection pressure with overstressed organic loading rate [8]. Meanwhile, it was pointed out that the”. After a thorough examination, we did not find something is lacking. Maybe there was some display error on the version the reviewer read.
(4) Never start a sentence with But (Line 97)!
Response: thanks for the reviewer’s reminder. After careful examination, we found that “But” is inappropriate, so it has been changed to “Moreover,”
(5) In the last paragraph of the Introduction should clearly explain the aims of the work starting as: We aimed to….
Response: thanks for the reviewer’s question. We amended this part for the clarity (Line 135-138).
(6) In Figs. 1, 3, 4, 5, and 7 increase the fonts since now reading this in a small screen or taking a paper copy without magnification is impossible to see the figure explanations. I had to use 125 % or 150 % magnification to be able to see those texts. In these figures, the figure font could also be bigger.
Response: thanks for the reviewer’s comments and we are very sorry for the difficulties we have made for the reading of these figures. As suggested, we have enlarged the fonts in the mentioned figures to make them clear.
(7) Is Fig 4 is A from low biomass concentration and B from high biomass concentration?
Response: Fig 4 A is from lower biomass concentration with MLSS of 2.1 g/L, which presented stable cycle performance of the reactor when the aerobic granules initially formed before the disintegration; while Fig 4 B is from higher biomass concentration with MLSS of 4.0 g/L, which presented stable performance of the reactor when the aerobic granules regranulated after the disintegration. The main purpose to present Fig 4 is to show that even with different MLSS concentrations, nitrification was carried out complete. Even though this, it is hard to make a quantitative comparison between the performances of the aerobic granules during the two periods due to the different biomass concentrations. Therefore, the specific reaction rates of the aerobic granules were presented in Fig 5 for better understanding the deference of performances between the aerobic granules before and after the disintegration.
(8) In Fig 2 A show that the x-axis is logarithmic! In Fig 2B volume percentage.
Response: yes, it would be better to present Fig 2 A with a logarithmic x-axis. Fig 2B has a y-axis of volume percentage, because it presents the percentage of biomass with average size below 200 µm, which indicated the flocs compositions in the granular system at the time. This is a typical way to present size distribution in most published papers.
(9) Table 1 is interesting but its columns for days are not clear. Instead of using two columns for days, you can have a row showing that the left one is for 85 days and the right one from 227 days. The other method is to present them as tables 1 A for days 85 and 1B for days 227 days, which would allow the normal portrait orientations and normal font size.
Response: thanks for the reviewer’s suggestion. We have changed the table with a row showing that the left part is for day 85 and the right part is for day 227, which is clearer and easy to make a microbial comparison between the aerobic granules from the two days.
(10) I warmly recommend that you will present the supplement figure as a normal Fig. 6 (and the present Figs. 6 and 7 as 7 and 8).
Response: we put the DGGE band profile into supplement figure because all the bands have been explained explicitly in the table. Since there are so many figures and tables already in this paper, the profile was put in the supplement part to avoid redundancy.
(11) Increase the Figures 7 since the protozoa are there and they can be seen better with 150 or 200 % magnification. Is this microscopy figure taken without any staining? In this case, the 200 µm can be seen but not the days.
Response: thanks for the reviewer’s suggestion. The microscopic morphology of protozoa in figure 7 has a magnification of 100. Though it can be better to see protozoa with higher magnification of 150 or even 200, it is very important to see the connection between the aerobic granules and protozoa, which cannot show up all around at a larger magnification. Therefore, a smaller magnification was chosen in these pictures. Yes, this microscopy figure was taken without any staining, because the microscopy can take clear photos at this magnification without any ancillary measures. The days present on the pictures were added afterwards to show the day that the samples of the aerobic granules were taken.
(12) Correct all references and see from guidance for authors how they must be presented.
Response: thanks for the reviewer’s suggestion. We have checked all the references in the paper according to the guidance for authors and used Endnote to edit them to make sure they are correctly presented.
(13) In some cases, space before[ should be before [
Response: thanks for the reviewer’s careful examination. We have checked the whole paper to make sure it is normalized.
