Evaluating Pilot-Scale Floating Wetland for Municipal Wastewater Treatment Using Canna indica and Phragmites australis as Plant Species

Round 1

Reviewer 1 Report

Overall the manuscript present interesting subject. However, the authors should emphasis the novelty of the study. The manuscript fells to convince that FTW and plants in CW, have advantages over other CW treatments.

Introduction
Page 2 line 44: Why? Could you cite a study that showed that FTW is more economical?
Page 2 lines 55 – 56: How it more advantage over sub-surface CW and vertical flow CW?

Martials and Methods

Page 2 line 81: What is L*W*D? D is diameter? The cells are round? Please, clarify.
Page 3 figure 1: Improve the quality of the figure. Add dimensions (length, wide, height)
Page 3 table 1: Do not show calculation, show the final value
Page 4 line 102: Total? Or for each cell? How does the flow was measured? How was the flow regulated between the cells, and ensured equal flowrate for each cell
Page 4 line 103: Where was the inlet? at the bottom of the cell or on the water surface?
Page 4 line 106: What it includes? Domestic wastewater, runoff, industrial wastewater?
Page 4 line 111: How it possible, if the tank was filled every 3 days, and the flow rate 400 L/day = 1200L/3days. The tank can store for only 2.5 days.
Furthermore, as the tank gets empty, the water level is decreased, and probably the flow rate decreases.
Page 4 line 113: During the 3 days, there were changes in the quality of the wastewater?
Page 4 line 118: All parameters were measured with one probe/electrode? Probably a probe/electrode measures EC and the instrument convert EC to TDS. How it's done? Using which equation?
Page 4 line 121: Total ammonia? Or just NH3? In the manuscript results of total ammonia was not reported.
Page 4 line 122: Please specify the APHA method number
Page 4 equation 1: This is hardly a mass-balance. Mass balance has units of kg/day
Page 4 line 132: Please, provide reference
Page 4 line 134: Units?
You can not calculate the K_T based on single HRT value. You have to conduct experiments where HRT were changed, in order to calculate K_T
Results and discussion

Page 5 line 155: The BOD and COD are relatively low, it is not typical domestic wastewater (low strength wastewater). Was the wastewater were mixed with rain or runoff? Please, give more information on the wastewater – urban area, population etc.
Page 5 line 159: See text book such as - Wastewater engineering: treatment and reuse / Metcalf & Eddy, Inc.
Page 6 table 3: How many samples were measured? For each treatment? There is no TDS removal. Nitrate and phosphorous removal is insignificant
Page 6 line 172: p value of what? What is the Null hypothesis - H0? Redaction of 0.3 in pH is insignificant. From the box-plots it seems that it's not statistically significant

Page 6 line 176: From the water surface? From the plants? Plants uptakes CO₂ during the day
Page 6 line 180: If you didn't conduct mass balance on the carbonates, you can't say that
Page 7 line 193: Diffusion of oxygen is very slow
Page 7 line 214: Temperature and EC effects the viscosity. Not the other way around. How it's relevant?
Page 8 line 223: Changes of TDS are negligible. It's not decay, there is not a biological or chemical process involved.
Page 8 line 226: How did the TDS was removed? Uptake by plants?
Page 8 line 235: BOD is organic matter; Microorganism consumes the organic matter
Page 8 lines 243 - 246: Is there any statistical differences between the treatments (C1,C2,C3 and C4)?
Page 8 line 249: Probably there are microorganisms suspended in the water (not attached), and/or settled in the cell.
Page 8 line 252: Why? C4 BOD removal is not different from the other treatments
Page 8 - 9 lines 252 - 254: All are not significantly different
Page 9 lines 267 – 268 : Why?
Page 9 line 283 : It is very low redaction. 61% from 2.5 mg/L is negligible.
Page 9 lines 289 – 291: How can you report on nitrogen and nitrate removal without measuring total ammonia nitrogen, and total nitrogen?
Page 10 Figure 5a: Please, change the scale of y axis to be between 0 - 6 mg/L
Page 10 table 4: What is the number off samples? What is the standard error/deviation?
Conclusion

General: The conclusions chapter is badly written; Avoid writing in bullets; Write an opening paragraph, giving overview on the study. Summary and conclusion chapter should standalone.
Page 10 line 320: For which use? Does the effluent meets the reuse or discharge standards?
Page 11 line 324: The effluent DO concentration was below 2.5 mg/L. Hardly it could be an improvement
Page 11 line 327: Why?
Page 11 line 333: Maybe, it is statistically significant, however differences of 5% is negligible
Page 11 line 342: Raw wastewater contains ammonia, less nitrate.

Comments for author File: Comments.pdf

No comments

Author Response

• Overall the manuscript present interesting subject. However, the authors should emphasis the novelty of the study. The manuscript fells to convince that FTW and plants in CW, have advantages over other CW treatments.

 

Response: Thanks for the detailed review of the manuscript and providing comments/suggestions for improving the manuscript. We revised the manuscript substantially to address the concerns raised by the reviewer (Lines 44-63).

 

• Introduction

 

• Page 2 line 44: Why? Could you cite a study that showed that FTW is more economical?

 

Response: Thanks for the suggestion. The appropriate citations have been added in Line 44 (Abed et al. 2017; Colares et al. 2020; San Miguel et al. 2023).

 

• Page 2 lines 55 – 56: How it more advantage over sub-surface CW and vertical flow CW?
  
  

Response: The advantages of FTW over sub-surface CW (horizontal and vertical flow) is added in Line 56-64, as given below.

 

“In FTWs, plants grow hydroponically on a floating mat (buoyant raft), the hanging root network remains in contact with the water column receiving nutrients under the floating mat whereas, the leaves and shoots of the plant grow above the mat (Colares et al. 2021; Saravanan et al. 2021). Unlike CWs, these FTWs are suitable for the fluctuating water levels in the water column as these buoyant mats are designed to move up and down, like in a hydroponic system (Sharma et al. 2021). In FTWs, the development of a larger root surface area supports the development of the biofilm responsible for biochemical processes (i.e., microbial process), and physical processes (i.e., filtration). The dense root network also enhances the phyto-uptake of the nutrients (Oliveira et al. 2021a; Pishgar et al. 2021). The hanging root system of FTW has a higher uptake of nutrients from the water column compared to the rooted vegetation of other intensified CWs (Park et al. 2019). Unlike CWs, the use of solid filter media (gravel, sand, etc.) is not required in FTW, and consequently, they are less susceptible to clogging and can easily be de-sludged, if required (Park et al. 2019; Oliveira et al. 2021).”

 

• Materials and Methods

 

• Page 2 line 81: What is L*W*D? D is diameter? The cells are round? Please, clarify.
  
  

Response: Thanks for pointing out this. The treatment cells are rectangular as indicated in figure-1 and Line 87-88. L*W*D is the Length, Width, and Depth of the treatment cells, and the same has been elaborated in Lines 89-90.

 

• Page 3 figure 1: Improve the quality of the figure. Add dimensions (length, wide, height).

 

Response: Thank you for the suggestion. The dimensions of the treatment cell are given separately in Figure 1.

 

 

• Page 3 table 1: Do not show calculation, show the final value

 

Response:  Thank you for the suggestion. The corrections were made in Table 1.

 

• Page 4 line 102: Total? Or for each cell? How does the flow was measured? How was the flow regulated between the cells, and ensured equal flowrate for each cell?

 

Response: Thanks for the comment to bring in more clarity for the readers. We had re-written the sentences to bring in more clarity (Lines 121-124)-

“The experiment setup was operated at a flow rate of 403 L/day (280 ml/min) in continuous mode for each treatment cell and the inlet was kept above the water column of the treatment cell. The flow rate to each cell was manually measured every four hours and regulated to maintain the desired flow rate.”

 

• Page 4 line 103: Where was the inlet? at the bottom of the cell or on the water surface?

 

Response: The inlet was above the water column of treatment cells (Line 122).

 

• Page 4 line 106: What it includes? Domestic wastewater, runoff, industrial wastewater?

 

Response: It mainly includes domestic wastewater from the residential colony (Line 126).

 

 

• Page 4 line 111: How it possible, if the tank was filled every 3 days, and the flow rate 400 L/day = 1200L/3days. The tank can store for only 2.5 days.
  Furthermore, as the tank gets empty, the water level is decreased, and probably the flow rate decreases.

 

Response: Thanks for pointing out this typo mistake. The size of the tank was 2000 L and the tank was refilled every alternate day. We had corrected it in Lines 128 & 132. Further, to offset the change in the flow rate due to a decrease in the water level of the tank, the flow rate was measured every 4 hours of operation and the flow was adjusted accordingly (Line 121-124).

 

• Page 4 line 113: During the 3 days, there were changes in the quality of the wastewater?

 

Response: As indicated in response 9, the tank was refilled every alternate day, and therefore, a little change in the wastewater quality was expected. However, we measured the water quality of both the inlet and outlet of FTW cells to offset this change.

 

• Page 4 line 118: All parameters were measured with one probe/electrode? Probably a probe/electrode measures EC and the instrument convert EC to TDS. How it's done? Using which equation?

 

Response: pH and EC were measured using two separate electrodes. However, TDS and EC were measured using the same electrode. The TDS is calculated by the instrument from the EC values using the proportionality constant (Ke)

TDS (mg/L) = Ke * EC (μS/cm)

Where Ke is taken as 0.55.

 

• Page 4 line 121: Total ammonia ? Or just NH3? In the manuscript results of total ammonia was not reported.

 

Response: In this study, we only measured nitrate and phosphate for the nutrient information, and therefore, the sentence has been revised.

 

• Page 4 line 122: Please specify the APHA method number

 

Response: The APHA Method numbers have been added in lines 142-143.

 

“Chemical oxygen demand (COD), dissolved oxygen (DO), nitrate (NO₃), and phosphate (PO₄^3-) were measured using APHA 5220-B, APHA 4500-O C, APHA 4500-NO₃ B, and APHA 4500-P C methods, respectively (APHA, 2017).”

 

• Page 4 equation 1: This is hardly a mass-balance. Mass balance has units of kg/day

 

Response: Thanks for pointing out the mistake. The author means the change in parameter concentration and removal efficiency, and accordingly, the sentence is corrected (Line 144-146).

 

• Page 4 line 132: Please, provide reference

 

Response: As suggested, appropriate citations have been provided (Line 153).

 

• Page 4 line 134: Units?

 

Response: As suggested, the units have been provided in the revised manuscript (Lines 154-155).

 

• You cannot calculate the K_Tbased on single HRT value. You have to conduct experiments where HRT were changed, in order to calculate K

 

Response: We used the first-order plug flow equation to estimate the K_T. Since we operated the system at one HRT, the same was used to estimate the K_T. We adopted a similar procedure as followed in other studies such as Sooknah and Wilkie 2004; Jamwal et al. 2021.

 

• Results and discussion

 

• Page 5 line 155: The BOD and COD are relatively low; it is not typical domestic wastewater (low strength wastewater). Was the wastewater were mixed with rain or runoff? Please, give more information on the wastewater – urban area, population etc. Page 5 line 159: See text book such as - Wastewater engineering: treatment and reuse / Metcalf & Eddy, Inc.

 

Response 18: The BOD and COD in the domestic wastewater of India vary from 110-400 mg/l and 250-1000 mg/l respectively depending upon the municipal water supply/usage and the characteristics of the domestic wastewater used in this study were observed in this range. However, to bring more clarity on the factors responsible for variation in the quality, modifications have been made in the revised manuscript (Lines 210-212).

 

“The urban drain from where the wastewater was collected receives the wastewater largely from households (>1000 people) and nearby small commercial establishments therefore, variation in the physicochemical parameters was observed. The drain receives the runoff from the surrounding area which may affect the organic load in the drain.”

 

• Page 6 table 3: How many samples were measured? For each treatment? There is no TDS removal. Nitrate and phosphorous removal is insignificant

 

Response: Water quality was monitored every 3 days at the inlet and outlet of each treatment cell (C1, C2, C3, and C4) for a period of two months (March 2021 to April 2021). A total of 18 samples were taken from each treatment cell. Details are given in Lines 134-135 and Line 237.

 

As suggested, Table 3 has been revised and only the significant reduction/increase in the parameters has been shown in the revised manuscript.

 

The reviewer has rightly indicated that the nitrate and phosphate removal in some cells was insignificant, however, nitrate removal was relatively high in the treatment cell with 100% coverage of Canna indica and phosphate removal was relatively high in 50% coverage of Phragmites australis and control with algal growth.

 

• Page 6 line 172: p value of what? What is the Null hypothesis - H0? Redaction of 0.3 in pH is insignificant. From the box-plots it seems that it's not statistically significant

 

Response: Thanks for the suggestion. P value is for the t-test. As suggested we have incorporated the Null hypothesis in the revised manuscript (Lines 196-200).

 

“In this study, the null hypothesis was that the floating mats with different plant species and coverage may not contribute to the pollutant removal from the treatment cells. For the same, pollutant reduction from each treatment cell was compared with the influent, and the t-test was performed. The null hypothesis is rejected for the p values less than 0.05.”

 

Regarding the change in pH values during treatment, although it seems that the change in average values is insignificant, however, t-test on experimental values for 2 months of observation indicates that the change in pH values for C1, C2, and C3 was significant whereas insignificant for C4. Accordingly, changes have been made in the revised manuscript to bring in more clarity (Line 243-244).

 

“The change in pH values for C1, C2, and C3 were observed to be statistically significant and insignificant for C4.”  

 

• Page 6 line 176: From the water surface? From the plants? Plants uptakes CO₂during the day

 

Response: Thanks for the suggestion. We revised the lines to bring more clarity (Lines 244-252).

 

“Reduced pH in the treatment cells than the control can be attributed to the presence of FTW mats with 100% (C1- Canna indica, and C2- Phragmites australis) and 50% coverage (C3- Phragmites australis). The FTW mats prohibit the penetration of sunlight into the water column and in turn the algal growth which consumes the CO₂ in the water and results in an increase in pH (Liu et al. 2016). Furthermore, the pH reduction in the cells with FTW could be due to the wetland plants which excrete organic acids in their rhizosphere through their roots which later break into a proton and organic bases (Neumann and Römheld 1999).”  

 

• Page 6 line 180: If you didn't conduct mass balance on the carbonates, you can't say that

 

Response: The sentence has been removed in the revised manuscript (Line 253).

 

• Page 7 line 193: Diffusion of oxygen is very slow

 

Response: We agree with the reviewer that the diffusion of oxygen is very slow but it can result in DO levels in the inland water bodies as high as 6-8 mg/l depending on the water temperature and atmospheric pressure, and plays a significant role in the biochemical processes.

 

Several researchers have indicated that the vegetation coverage or any other physical barrier like FTW beds limits the diffusion of atmospheric oxygen into the water column and in turn the Do levels. Accordingly, the statement has been revised for more clarity to readers (Lines 266-268).

 

“In the treatment cells, the FTW bed limits the diffusion of atmospheric oxygen (O₂) to the water column and thus acts as the physical barrier (Colares et al. 2020, Samal et al. 2021).”

 

• Page 7 line 214: Temperature and EC effects the viscosity. Not the other way around. How it's relevant?

 

Response: The sentence is removed in the revised manuscript.

 

• Page 8 line 223: Changes of TDS are negligible. It's not decay, there is not a biological or chemical process involved.

 

Response: Thanks for the suggestion. We removed the decay rate information from Table 4 and corrections were made between Lines 315-316.

 

• Page 8 line 226: How did the TDS was removed? Uptake by plants?

 

Response: The slight reduction of TDS can be due to physical adsorption, precipitation, and microbial degradation in the treatment cells (Line 313-314).

 

• Page 8 line 235: BOD is organic matter; Microorganism consumes the organic matter

 

Response: Thanks. Suitable changes have been made.

 

• Page 8 lines 243 - 246: Is there any statistical differences between the treatments (C1, C2, C3 and C4)?

 

Response: Statistically slight difference (1% to 3%) was observed at different coverage of plant species in the treatment cells in this study. The removal was rather high in C3 with 50% coverage of Phragmites australis (53%) and C2 with 100% coverage of Phragmites australis (52%). Further long-term studies are recommended to understand the influence of species coverage on pollutant removal, without algal growth.

 

• Page 8 line 249: Probably there are microorganisms suspended in the water (not attached), and/or settled in the cell.

 

Response: The probability of microorganisms in the suspension or settled at the bottom of the FTW cells is negligible as the system is in continuous flow and there is no arrangement for arresting them and recycling them back into the cells. Further, the presence of microbes in the suspension and carryover will significantly increase the TSS as well as BOD/COD of the treated water. Some sloughing of microbes from the FTW beds and hanging roots is expected. Moreover, the system was designed in such a way as to minimize the sludge accumulation at the bottom, and the same was observed after completion of experiment.

 

• Page 8 line 252: Why? C4 BOD removal is not different from the other treatments

 

Response: Although the BOD removal in control (C4) was relatively lower than the other treatment cells (C1, C2, and C3), the removal was comparable. This may be due to the presence of significant concentrations of algae with whom heterotrophic bacteria may be associated and the symbiotic effect of both has resulted in the reduction of BOD/COD in the control (Lines 371-374).

 

“The significant removal of BOD and COD in the control cell, C4, may be due to the presence of algae providing niche, Phycosphere, for heterotrophic bacterial growth (Ramanan et al. 2016), and the symbiotic effect of both may have resulted in the oxidation of organics.”

 

• Page 8 - 9 lines 252 - 254: All are not significantly different

 

Response: Yes, agreed. We found a significant reduction of COD only in the treatment cell C3 with 50% coverage of Phragmites australis, indicating the influence of plant coverage as is the case for BOD reduction.

 

• Page 9 lines 267 – 268 : Why?

 

Response: At higher depth, dead zones devoid of root system and microbes exist limiting the efficacy of the system (Lines 385-386).

 

• Page 9 line 283: It is very low redaction. 61% from 2.5 mg/L is negligible.

 

Response: We addressed this against Comment 19.

 

• Page 9 lines 289 – 291: How can you report on nitrogen and nitrate removal without measuring total ammonia nitrogen, and total nitrogen?

 

Response: Thanks for the comment. Ammonia nitrogen and total nitrogen measurements would have definitely provided insight into the nitrogen processes taking place in the cells and would have provided insightful information taking place. However, here we are discussing only nitrate removal and not total nitrogen.  

 

• Page 10 Figure 5a: Please, change the scale of y axis to be between 0 - 6 mg/L

 

Response: Thanks for the suggestion. Figure 5a is modified in the manuscript.

 

 

• Page 10 table 4: What is the number off samples? What is the standard error/deviation?

 

Response: The number of samples was 18. The standard deviation is added in the Table 4

 

• Conclusion

 

• General: The conclusions chapter is badly written; Avoid writing in bullets; Write an opening paragraph, giving overview on the study. Summary and conclusion chapter should standalone.
  Page 10 line 320: For which use? Does the effluent meets the reuse or discharge standards?
  Page 11 line 324: The effluent DO concentration was below 2.5 mg/L. Hardly it could be an improvement.

Page 11 line 327: Why?

Page 11 line 333: Maybe, it is statistically significant, however differences of 5% is negligible.
Page 11 line 342: Raw wastewater contains ammonia, less nitrate.

 

Response: Thank you for the suggestion. We rewrote the conclusion section 4 and addressed the given comments accordingly.

Author Response File: Author Response.docx

Reviewer 2 Report

As the purpose of the investigation  was  the evaluation of the system, the work is interesting. But It is not very clear the effectiveness of the treatment applied with the macrophytes in relation to the control. 

Line 86-  It is important to note that Phragmites in cold countries goes into dormancy in the colder months, jeopardizing the effectiveness of treatment.

Line 400- Reference written in capital word. Please, correct.

Author Response

1. As the purpose of the investigation was the evaluation of the system, the work is interesting. But It is not very clear the effectiveness of the treatment applied with the macrophytes in relation to the control. 

 

Response: The BOD removal of the control was relatively lower than the treatment cell (C1, C2, and C3). In this study, we found BOD removal high in treatment cell C3 with 50 % coverage of Phragmites australis (53%), than the other treatment cell and the control, which indicates the coverage of plants in the treatment basin also influences the pollutant removal. In C3, the patches of algal growth were also observed (but were not quantified) which could have resulted in the photosynthetic O₂ generation and could have enhanced the microbial degradation of organic matter. Similarly, C4 (control) has sufficient space for atmospheric O₂ diffusion and photosynthesis for algal growth; although no plants are present, the growth of algae could have influenced the organic matter degradation in C4. Moreover, nutrient such as nitrate removal was also observed to be higher in the treatment cell C1 than in the control. Further long-term studies are recommended to understand the influence of species coverage on pollutant removal, without algal growth.

 

2. Line 86-  It is important to note that Phragmitesin cold countries goes into dormancy in the colder months, jeopardizing the effectiveness of treatment.

 

Response: Yes, we agree that the temperature affects the performance of the plant species in the FTW. However, for tropical countries, like India, Phragmites australis is a perennial species and no significant impact on treatment performance is observed during winter months.

 

3. Line 400- Reference written in capital word. Please, correct.

 

Response: Thanks. The reference is corrected in the revised manuscript. Moreover, all the references have been checked and corrected.

Author Response File: Author Response.docx

Reviewer 3 Report

The Research work presented by Yadav et al entitled "Evaluating Pilot-Scale Floating Wetland for Municipal 2 Wastewater Treatment using Canna indica and Phragmites australis as Plant Species" is quite interesting, scientifically sound, and updated. More such types of research work are required.  I recommend acceptance of the manuscript. My only suggestion to the author is; to rewrite the conclusion section in the paragraph.

Author Response

1. The Research work presented by Yadav et al entitled "Evaluating Pilot-Scale Floating Wetland for Municipal 2 Wastewater Treatment using Canna indica and Phragmites australis as Plant Species" is quite interesting, scientifically sound, and updated. More such types of research work are required.  I recommend acceptance of the manuscript. My only suggestion to the author is; to rewrite the conclusion section in the paragraph.

Response 1: Thank you for your suggestion. We have significantly improved the conclusion section in the revised manuscript.

 

Author Response File: Author Response.docx