Fe⁰-Supported Anaerobic Digestion for Organics and Nutrients Removal from Domestic Sewage

Round 1

Reviewer 1 Report

The authors have done great work. Nevertheless, some sections require revision including methodology and result/discussion as follows:

Abstract

1. Line 15 and 16: It is suggested to include one more line (to support) relate with first sentence of the abstract.
2. Line 17-22: COD, etc -Abbreviations should be defined at first mention in the main text
3. Line 22-24: the structure of the sentence needs to be touched up

Overall, it would be great to include information on type of bioreactor (system) and experimental conditions.

Introduction:

1.Line 34-43 : the structure of the sentence needs to be touched up. Refine the statement. Phosphorus is the limiting parameter and eutrophication will occur a certain P to Nitrogen ratio.

2. Line 41 and Line 80: Abbreviations should be defined at first mention in the main text
3. Line 85-86: the structure of the sentence needs to be touched up
4. Line 86 : please include the information on experimental condition

Materials and research method

1. Selection of SW and SI require justification. Lab grade Fe can be used to represent Fe instead.
2. Include information on sewage sludge to Fe ratio. In form of Line 69 : Pig manure sampling- It is suggested for the author to include the difference between these 3 samples. perhaps in terms of g COD or gVS per gVSS of sewage sludge.

3.it is recommended to include the method name (Hach method) for each of parameters.

4. author should include in methodology section on statistical analysis method used in this study.
5. presentation of table 2. Author need to justify the need experimental design, why SW and SI were not tested in same sample at same dosage.this will allow to have a fair comparison

Result and Discussion

1. Fig 4: this fig is on optimum dosage. It is suggested for the author to explain further on the concept of optimum dosage and why higher dosage remove less.

2. Please justify the selection of dosage, 10 g/L in Fig 5. Are they related to fig 4?
3. Sec 3.6: Discussion on pH should be included earlier as this the major parameters which determine the optimum dosage of Fe.
4. This is an anaerobic digestion (AD) system, at least biogas data/methane data must presented.
5. Equation on page 7,9, and 10: it is suggested to minimized, rearrange or move to introduction section.
6. is there any discussion related to elemental composition in Table 1?the elemental composition and Trace elements are among the important factor contributing to process performance in AD.

Conclusion

Can be improved and reflect on the optimum dosage of Fe and relate with process performance.

Author Response

The responses are summarized in the attached documents.

Thanks,

Dr. Noubactep

Author Response File: Author Response.pdf

Reviewer 2 Report

This paper deals a very interesting topic, anaerobic digestion treatment for domestic sewage. It focuses on Fe⁰ influence in anaerobic reactor performance, concerning organics and nutrients removal.

The research design is rigorous and appropriate.

However some result presentation should be revised and discussion detailed.

Concerning steel scrap what can explain so high Cu level? (even higher that Cr)

Concerning operating conditions, redox potential monitoring could have be informative, why was this parameter not considered?

The composition of inoculum and domestic sewage should be indicated, these data would be helpful for results discussion.

In part 2.3, limit of quantification should be indicated for all parameters.

The result presentation could be improved, in figures 3, 5 and 6, same marker should be used for same conditions.

It is not clear why all experimental condition tested are not reported in figure 3 (15g/L SI).

The table 3 caption should be completed, when or on which period were the residual pollutant concentrations measured. In table 3, 4a and 4b, nitrate and ammonium concentrations must be expressed in mg N-NO₃ and mg N-NH₄ (same molar mass 14g) and not as mg NO₃ (62 g/mol) and mg NH₄ (18 g/mol); the mass balance in the system requires to consider N. It should be checked if P concentration is given in P-PO₄mg/L or mg PO₄/L.

It is not clear why P removal performance decreased when Fe concentration raised from 10 to 30 g/L (maximum was obtained at 10g Fe/L, (line251 and repeated line 277). Could author discuss this result? Statistical analyse is only given for the comparison of system I and IV.

The P release in the initial 8 days (line 256-259) could not only be explained by pH evolution; it could be due to biomass release, as it is a well-known mechanism, described in wastewater treatment process. The P content in inoculum should be given in order to evaluate its contribution and validate or not this hypothesis.

The influence of temperature is noticed but these data are not provided. Could temperature monitoring be presented more precisely, what was the duration for temperature to reach 37°C  from 24°C? It did not last 8 days?

Solubility product associated to equation 6 should be given.

In the comparison between SI and SW systems, N removal results from the last 10 days should be at least noticed (line 354 to 357); here a significant difference was observed and no explanation was proposed.

The desirability indice should be defined. This part is not clear for me.

Globally an in-depth discussion is required (i) for results from systems IV, V and VI, (ii) for N results in system VII from 45 days to the end of the experiment. (iii) for pH increase in system VII from 45 days to the end of the experiment. P removal and iron concentration should have been affected by such pH condition?

As a perspective, the interest to continue such study in open reactor, to prevent the effect of substrate depletion could be mentioned?

Author Response

The responses are summarized in the attached documents.

Thanks,

Dr. Noubactep

Author Response File: Author Response.pdf

Reviewer 3 Report

This paper, entitled Fe0-supported Anaerobic Digestion for Organics and Nutrients Removal from Domestic Sewage, is a scholarly work and can increase knowledge on this topic of improvement of AD process especially by adding additives such as metals. This study is in the spotlight of current research work and is relevant to Water. The authors provide an interesting and original study. The manuscript is quite well written and well related to existing literature. The abstract and keywords are meaningful.

I have some specific and general comments:

• please provide more details about the sludge used as inoculum in subsection 2.1.1 in materials and methods section. What are TS, VS, pH, VFA content, ...? What about viscosity?
• please provide accuracy or standard deviation for Fe0 dosage values in Table 2.
• How were determined the Fe dosage concentrations? Is it from previous experiments of based on literature data?
• about the Fe reactivity (subsection 3.1 page 5), please provide costs analysis and discuss later in the manuscript about the potential applicability of such method at pilot scale or real scale. What could be the limitations vs gains? Please discuss about if is it realistic to propose such method for application at real scale.
• please provide error bars in Figure 2.
• please rename titles of subsection 3.3.1 to 3.3.3
• please provide error bars in Figure 3, Figure 4, Figure 5, Figure 6.
• please provide standard deviation in Table 3, Tables 4A and 4b.
• please discuss about the Fe addition during AD process, meaning by this comment, is it necessary or mandatory to add regurlarly Fe0 particles? What about the risk of settlement in the digester after a long period and what about the quality of digestate with such Fe0 particles? Is it possible to use digestate with such elements and to spread digestate on soils?

As it, this paper is not fully acceptable for publication and requires some amendments and additional information. I recommend the following decision: ACCEPT AFTER MINOR REVISION.

Author Response

The responses are summarized in the attached documents.

Thanks,

Dr. Noubactep

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Fig 3 and fig 5: it is suggested to include a specific  title for each of the graphs. Fig 3a,3b.3c...

Author Response

Implemented, thanks!

Reviewer 2 Report

The manuscript has been improved.

All changes are not coloured in blue (lines 126 to 134 ; 357 to 369; 411 to 416 ...). In the paragraph 2.3, the previous text is in blue and the new one in black.

Concerning the comment about nitrate, ammonium and phosphate concentrations, the change has been only done in table 4b (for N concentrations only), not in table 4a and in the text (line 512-516). The values have not been revised. Can author check these data?

Line 133, please replace "NH₄⁺-N of 53.1 mg NO₃^-N/L" by  "NH₄⁺-N of 53.1 mg NH₄⁺N/L"

On figure 3.b the markers for 10 and 30 g/L SI are not the same as for figures 3.a, c and d.

The presentation of Fe⁰ dosage should be revised (indicating only in the text and/or the caption the accuracy of 0.1mg/L Fe, in table 2).

Equations 5 and 6 are not balanced.

The presentation of standard deviations in table 3 still needs to be revised (four significant numbers for standard deviation are not appropriate).

In table 3, for a result, standard deviation is higher than Avrg data.

Line 332, "Figure 4 (d)" should be replaced by "Figure 3 (d)".

Discussion has been completed concerning nitrogen removal ; however, it would be interesting to also have a presentation of detailed results about ammonium and nitrate concentrations evolution, in order to impove the discussion and highlight the nitrate conversion to ammonium for instance.

Author Response

Implemented, see attached documents.

Thanks