Digestate of Fecal Sludge Enhances the Tetracycline Removal in Soil Microbial Fuel Cells
Round 1
Reviewer 1 Report
In this work, ferrate-pretreated fecal sludge digestate was added to soil microbial fuel cells for tetracycline (Tc) degradation, and its effect was evaluated based on microbiome response and SMFC performance without the addition of any other chemicals decreasing the Tc adsorption capacity in soil.
This is a timely, thorough and well-executed study proposing an interesting alternative of simultaneous disposal of fecal sludge digestate and bioremediation of antibiotics-contaminated-soil.
The experimental details are accurately outlined and the results are comprehensively described accompanied with well prepared figures. From the technical point of view I have no concerns.
However, there is a distinct gradient of (improving) English language level noticeable reading from the introductory section towards the final result part. Prior publication, I would strongly recommend that particular some parts of the text in the abstract & introduction are getting adjusted as well as grammar errors removed throughout the whole document for better readability and/or clarification.
To give some suggestions, in the abstract the authors wrote:
"The TC removal efficiency of the SMFC with fecal sludge digestate (SMFC-FSD) was 64.5% after 25 d, which was 25.2% and 21.4% in the SMFC without fecal sludge digestate (SMFC) and the OC-SMFC, respectively."
Abbreviations not introduced so far to the reader should be avoided here and the outcome might be quite easier to understand writing something like:
After 25 days, the SMFC with fecal sludge digestate showed TC removal efficiency of 64.5%, compared to values of 25.2% and 21.4% observed for a SMFC- and an open-circuit SMFC-reactor operating without the addition of fecal sludge digestate, respectively.
or instead of the previous line in the abstract:
"The addition of FS digestate was beneficial to the electricity generation by SMFC, and thus to facilitate removal efficiency of TC in the SMFC. "
The addition of FS digestate was beneficial to the electricity generation by SMFC, and thus enhancing the removal efficiency of TC in the SMFC.
Just to give few suggestions for the introduction section:
Instead of
“Additionally, the soil borne antibiotics might contaminate groundwater via rainfall and irrigation, thus the groundwater pollution caused by antibiotics needs to be further explored”.
one could write:
Furthermore, soil-borne antibiotics could/might contaminate groundwater via rainfall and irrigation, thus there is a need for further research on antibiotic-related groundwater pollution.
or for the line
“Hence, it is an urgent practical problem need to understand the degradation mechanism of TC in soil and seek cost-effective methods to remediate TC-contaminated soil. “
one could state:
Hence, understanding the degradation mechanism of TC in soil and finding cost-effective remediation methods for TC contamination is an urgent practical issue.
These days there are plenty of online grammar tools and text checkers available (free of charge) that can conveniently assist in this context.
Wish the authors best of luck with the corrections! :)
Author Response
Reviewer #1:
In this work, ferrate-pretreated fecal sludge digestate was added to soil microbial fuel cells for tetracycline (Tc) degradation, and its effect was evaluated based on microbiome response and SMFC performance without the addition of any other chemicals decreasing the Tc adsorption capacity in soil.
This is a timely, thorough and well-executed study proposing an interesting alternative of simultaneous disposal of fecal sludge digestate and bioremediation of antibiotics-contaminated-soil.
The experimental details are accurately outlined and the results are comprehensively described accompanied with well prepared figures. From the technical point of view I have no concerns.
However, there is a distinct gradient of (improving) English language level noticeable reading from the introductory section towards the final result part. Prior publication, I would strongly recommend that particular some parts of the text in the abstract & introduction are getting adjusted as well as grammar errors removed throughout the whole document for better readability and/or clarification.
Reply: Thanks for reviewer’s positive comments.
- "The TC removal efficiency of the SMFC with fecal sludge digestate (SMFC-FSD) was 64.5% after 25 d, which was 25.2% and 21.4% in the SMFC without fecal sludge digestate (SMFC) and the OC-SMFC, respectively."
Abbreviations not introduced so far to the reader should be avoided here and the outcome might be quite easier to understand writing something like:
After 25 days, the SMFC with fecal sludge digestate showed TC removal efficiency of 64.5%, compared to values of 25.2% and 21.4% observed for a SMFC- and an open-circuit SMFC-reactor operating without the addition of fecal sludge digestate, respectively.
or instead of the previous line in the abstract:
"The addition of FS digestate was beneficial to the electricity generation by SMFC, and thus to facilitate removal efficiency of TC in the SMFC. "
The addition of FS digestate was beneficial to the electricity generation by SMFC, and thus enhancing the removal efficiency of TC in the SMFC.
Reply: The abbreviations in the abstract have been changed to the full explanations.
Line 14-16, page 1: The addition of FS digestate was beneficial to the electricity generation by SMFC, and thus enhancing removal efficiency of TC in the SMFC.
Line 16-24, page 1: After 25 days, the SMFC with fecal sludge digestate showed TC removal efficiency of 64.5%, compared to values of 25.2% and 21.4% observed for a SMFC- and an open-circuit SMFC-reactor operating without the addition of fecal sludge digestate, respectively. In addition, the addition of FS digestate was in favor of electricity generation by SMFCs, and the average current density and the maximum power density of the SMFC with fecal sludge digestate were 0.054 A/m3 and 8.85 W/m3, respectively. The enrichment of Desulfuromonas and Pseudomonas in the electrode biofilms might account for the high TC removal efficiency and electricity generation. The SMFC with fecal sludge digestate provides a new approach of simultaneous disposal of fecal sludge digestate and bioremediation of antibiotics-contaminated-soil.
- Instead of
“Additionally, the soil borne antibiotics might contaminate groundwater via rainfall and irrigation, thus the groundwater pollution caused by antibiotics needs to be further explored”.
one could write:
Furthermore, soil-borne antibiotics could/might contaminate groundwater via rainfall and irrigation, thus there is a need for further research on antibiotic-related groundwater pollution.
or for the line
“Hence, it is an urgent practical problem need to understand the degradation mechanism of TC in soil and seek cost-effective methods to remediate TC-contaminated soil. “
one could state:
Hence, understanding the degradation mechanism of TC in soil and finding cost-effective remediation methods for TC contamination is an urgent practical issue.
Reply: These parts have been changed to a distinct expression as the reviewer mentioned. And other parts have also been clarified.
Line 37-39, page 1: Furthermore, soil-borne antibiotics might contaminate groundwater via rainfall and irrigation, thus there is a need for further research on antibiotic-related groundwater pollution.
Line 39-41, page 1: Hence, understanding the degradation mechanism of TC in soil and finding cost-effective remediation methods for TC contamination is an urgent practical issue.
Author Response File: 
Author Response.docx
Reviewer 2 Report
This paper, entitled Digestate of fecal sludge enhances the tetracycline removal in soil microbial fuel cell, is a scholarly work and can increase knowledge on this domain. The content is relevant to Water, the authors provide an interesting and original study. The abstract and keywords are meaningful, the manuscript is quite well written and well related to existing literature.
I have some specific and general comments:
- Lines 80-81, authors wrote that "The basic parameters of FS was de- 80
scribed in our previous research [14]". I understand that some data were given in previous study, but please provide some data in this study, especially in Materials and Methods section about Fecal sludge for a better understanding of the content.
- If I understand right, AD process was carried out at small scale in 250 mL bottles. By this way, the produced digestate is in small amount. Is there's any other way for digestate production? What is the I/S ratio of these assays? Please provide more date and information about active sludge working as inoculum (origin, characteristics,...). Please provide a scheme of the experimental setup. From my point of view, this point about the AD process for digestate production is a limitant point of this study, due to the fact that highest amount of digestate are required for such assays.
- How many samples were treated? How many replication were carried out? Is there any control with other method in order to compare the efficiency of this method?
- Please provide a detailed legend for the Figure 1. Same comment for Figure 2. Please check the legend of Figure 2a (degrdation instead of degradation) and Figure 2b. sCOD degrdation rate instead of degradation rate.
- Is there any experiments scheduled at highest scale? At pilot scale? What could be the applicability of such method in real conditions?
- Please discuss about costs analysis. Please discuss about advantages and limitations of such method, gains vs limits.
- What is the explanation of this better efficiency of tetracycline removal with pretreated fecal sludge? Please discuss about this conclusion and propose some hypothesis of the involved mechanism? Is it due to a specific enzymatic activity (due to specific microorganisms, individualy or in mixture, synergism behavior,...). As it, this point is only descriptive and there's no temptative of explanation. Please try to propose a scenario based on our experiments and on existing literature.
As it, this paper is not fully acceptable for publication and requires at least a major revision. Some points need to be completed and detailed in depth. Some amendments and modification are requested according to the comments listed previously.
Author Response
Response to Reviewers’ Comments
Reviewer #2:
Manuscript: This paper, entitled Digestate of fecal sludge enhances the tetracycline removal in soil microbial fuel cell, is a scholarly work and can increase knowledge on this domain. The content is relevant to Water, the authors provide an interesting and original study. The abstract and keywords are meaningful, the manuscript is quite well written and well related to existing literature.
Reply: Thank you very much for valuable comments and suggestions for improving the quality of this paper.
- Lines 80-81, authors wrote that "The basic parameters of FS was described in our previous research [14]". I understand that some data were given in previous study, but please provide some data in this study, especially in Materials and Methods section about Fecal sludge for a better understanding of the content.
Reply: We now rewrote some sentences and added more information regarding the basic information of the original fecal sludge in order to further improve the understanding of the content. The moisture content of the FS was 70% (v/v), the pH of the FS was 6.3 and C/N ratio was 8.8. The total chemical oxygen demand of the FS was 47665 mg/L. The volatile solid was 36.5 g/L and the total organic carbon was 28742 mg/L.
Line 81-84, page 2: The moisture content of the FS was 70% (v/v), the pH of the FS was 6.3 and C/N ratio was 8.8. The soluble chemical oxygen demand (SCOD) of the FS was 7316 mg/L. The total chemical oxygen demand of the FS was 47665 mg/L. The volatile solid was 36.5 g/L and the total organic carbon was 28742 mg/L.
- If I understand right, AD process was carried out at small scale in 250 mL bottles. By this way, the produced digestate is in small amount. Is there's any other way for digestate production? What is the I/S ratio of these assays? Please provide more date and information about active sludge working as inoculum (origin, characteristics,...). Please provide a scheme of the experimental setup. From my point of view, this point about the AD process for digestate production is a limitant point of this study, due to the fact that highest amount of digestate are required for such assays.
Reply: Besides the 250 mL bottles, the digestate was also produced by fermenter with a total volume of 2 L. As a result, the digestate was adequate for the following experiment. The active sludge was obtained from Wenchang sewage treatment plant (Harbin, Heilongjiang, China) as inoculum with a 1:10 (v/v) I/S ratio. The total suspended solid was 14.6 g/L, the pH of the sludge was 6.6. The SCOD of the sludge was 280 mg/L. The sludge was added into the anaerobic digestion reactors as inoculum after mixing well. The scheme of the experiment was added in the supporting information as Fig. S1. As you mentioned, the amount of digestate was needed for the next experiment. So we use a large fermenter (5 L) for the production of FS digestate.
Line 86-87, page 2: The digestate was made in a 2 L fermenter by anaerobic digestion (AD) process of PF-pretreated FS for 10 days.
Line 87-91, page 2: The active sludge was obtained from Wenchang sewage treatment plant (Harbin, Heilongjiang, China) as inoculum with a 1:10 (v/v) I/S ratio. The total suspended solid was 14.6 g/L, the pH of the sludge was 6.6. The SCOD of the sludge was 280 mg/L. The sludge was added into the anaerobic digestion reactors as inoculum after mixing well.
- How many samples were treated? How many replication were carried out? Is there any control with other method in order to compare the efficiency of this method?
Reply: There are 3 reactors in this experiment. The first reactor is the SMFC added with potassium ferrate-pretreated FS digestate, the second reactor is the SMFC without FS digestate and the last reactor is the open circuit reactor without FS digestate. The samples in this experiment were collected with triplicate times at once. The SMFC reactors with normal FS digestate and the open circuit reactor are the control in order to measure the TC removal efficiency in SMFC with PF-pretreated digestate.
Line 107-110, page 3: The concentration of SCOD and TC during the leaching process was monitored every 4 hours with sampling the leachate at the outlet of the reactor. Each sample was collected with triplicate times at once. The error bar was performed with standard deviation.
Line 124-125, page 3: The reactor ran for 25 days, and samples were taken from the water tank and soil every 5 days. The samples were collected with triplicate times at once. The error bar was performed with standard deviation.
Line 129-131, page 3: There were three reactors set in this study, including SMFC without FS digestate addition (SMFC), SMFC with FS digestate (SMFC-FSD), and open-circuit reactor without FS digestate addition (OC-SMFC). All reactors were operated for 42 days at 25 ± 1℃.
- 4. Please provide a detailed legend for the Figure 1. Same comment for Figure 2. Please check the legend of Figure 2a (degrdation instead of degradation) and Figure 2b. sCOD degrdation rate instead of degradation rate.
Reply: Thanks for the comment. We have rewritten the figure legend of Figure 1 and Figure 2.
Figure 1. The concentration changes of soluble chemical oxygen demand (SCOD) and tetracycline concentration at the sampling point of the water tank during leaching process of SMFC-FSD were compared with those of SMFC (a). The final concentration of SCOD and tetracycline in the total leaching solution of the two reactors after the leaching process (b).
Figure 2. Degradation performance (a) of SCOD and TC in the drenching solution of SMFC, SMFC-FSD and OC-SMFC with a 5-day interval. The degradation rate (b) of SCOD and TC in different SMFC reactors: 5. 10, 15, 20 and 25 represent the average degradation rate in the 0-5th day, 5-10th day, 10-15th day, 15-20th day and 20-25th day respectively.
Figure 1. The concentration changes of soluble chemical oxygen demand (SCOD) and tetracycline concentration at the sampling point of the water tank during leaching process of SMFC-FSD were compared with those of SMFC (a). The final concentration of SCOD and tetracycline in the total leaching solution of the two reactors after the leaching process (b).
Figure 2. Degradation performance (a) of SCOD and TC in the drenching solution of SMFC, SMFC-FSD and OC-SMFC with a 5-day interval. The degradation rate (b) of SCOD and TC in different SMFC reactors: 5. 10, 15, 20 and 25 represent the average degradation rate in the 0-5th day, 5-10th day, 10-15th day, 15-20th day and 20-25th day respectively.
- Is there any experiments scheduled at highest scale? At pilot scale? What could be the applicability of such method in real conditions?
Reply: This experiment is mainly focused on the tetracycline degradation and microbial diversity of electrode on a laboratory scale. Some phenomena could hardly be determined at pilot scale. As a result, this research did not do any pilot scale for now. For the further study, the actual effect of tetracycline degradation on SMFC placing in the tetracycline-contaminated soil could be determined based on this research. The pretreated septic tank supernatant could be the digestate which brought the soluble organic matters and tetracycline to the electrodes.
Line 324-326, page 11: For the further study, the actual effect of tetracycline degradation on SMFC placing in the tetracycline-contaminated soil could be determined based on this research. The pretreated septic tank supernatant could be the digestate which brought the soluble organic matters and tetracycline to the electrodes.
- Please discuss about costs analysis. Please discuss about advantages and limitations of such method, gains vs limits.
Reply: The economic analysis was taken out in the 3.6 part. The whole system mainly contained electrode system, automatic control system and water tank. The anode of the SMFC is the carbon fiber which costs 0.14 USD/cm2, and the cathode is the active carbon which costs 0.37 USD/cm2. The water tank is made up of polymethyl methacrylate which the costs 0.04 USD/mL/reactor (include processing fee). The biological TC treatment method with anaerobic digestion by using active sludge usually took 0.03 USD/mL. Although the total cost of MFC-FSD is higher than that of the anaerobic digestion, the TC degradation rate is higher than the anaerobic digestion.
Line 325-331, page 11: The whole system mainly contained electrode system, automatic control system and water tank. The anode of the SMFC is the carbon fiber which costs 0.14 USD/cm2, and the cathode is the active carbon which costs 0.37 USD/cm2. The water tank is made up of polymethyl methacrylate which the costs 0.04 USD/mL/reactor (include processing fee). The biological TC treatment method with anaerobic digestion by using active sludge usually took 0.03 USD/mL [47]. Although the total cost of MFC-FSD is higher than that of the anaerobic digestion, the TC degradation rate is higher than the anaerobic digestion.
- What is the explanation of this better efficiency of tetracycline removal with pretreated fecal sludge? Please discuss about this conclusion and propose some hypothesis of the involved mechanism? Is it due to a specific enzymatic activity (due to specific microorganisms, individualy or in mixture, synergism behavior,...). As it, this point is only descriptive and there's no temptative of explanation. Please try to propose a scenario based on our experiments and on existing literature.
Reply: The high concentration of soluble organics in the pretreated-FS digestate provide sufficient volatile fatty acids for the growth of electroactive bacteria. The electron transfer generated by electroactive bacteria including Desulfuromonas and Geobacter could enhance the extracellular electron transport of other microorganisms. Moreover, the oxygen as an electron acceptor on the cathode enhance the growth of Pseudomonas, Pseudoxanthomonas and SWB02. As a result, the biofilm on the cathode has the ability to degrade the TC.
Line 311-319, page 11: The high concentration of soluble organics in the pretreated-FS digestate provide sufficient volatile fatty acids for the growth of electroactive bacteria. The electron transfer generated by electroactive bacteria including Desulfuromonas and Geobacter could enhance the extracellular electron transport of other microorganisms [45]. Moreover, the oxygen as an electron acceptor on the cathode enhance the growth of Pseudomonas, Pseudoxanthomonas and SWB02 [46]. As a result, the biofilm on the cath-ode has the ability to degrade the TC. It is worth noting that the lower relative abundance of Pseudomonas in the SMFC-C might partly explain why the SMFC-FSD reactor had better TC removal and power generation performance than SMFC reactor.
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
The authors provide detailed answers and a revised version of their manuscript, taking into account all the comments and requests of amendments made in the previous review. All the answers are well justified and detailed. The quality and the content of the document are now improved. I agree with all the comments and the amendments made in the manuscript. As it, the manuscript is now fully acceptable for publication and I recommend the following decision: ACCEPT IN PRESENT FORM.
