Effect of Effluent Recirculation on the Performance of an Anaerobic Baffled Reactor in Municipal Wastewater Treatment: A Modeling Approach
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsTitle: Should state “performance of an anaerobic baffled reactor”
GENERAL COMMENTS:
This work is really clever and definitely needed. Decentralized systems are utilized in many areas around the world even in “developed” countries such as the U.S. and Canada because the distances between houses/farms can be too far to put in piping for a decentralized wastewater treatment system. The ABR is currently considered one of the most advanced on-site wastewater treatment systems available. In many places in the U.S. where centralized wastewater treatment is not feasible, regulatory agencies are requiring the installation of these systems in order to protect waterways from pollution. Modeling the performance of a system like this is valuable to current designers as well as operators of these systems in order to tune parameters to ensure optimal performance. I enjoyed the way you modeled the operation of an ABR in the GPS-X system.
The introduction is too long. You want to just include the information necessary for a reader to understand the importance of what you are studying. You could explain the importance of studying ABR performance in one paragraph by just briefly describing the need for effective, low-cost decentralized wastewater systems. You should provide more details on why there is a need for research on ABRs and why there is a need for high-quality models of ABR systems. You want to specifically explain your choice of GPS-X and ABRs and explain the value of your proposed work in advancing the science of this area.
The methods section does not contain enough details for a reader to replicate your work. There are details missing relevant to the composition of the materials used and the operation of the reactor. Most importantly, there is insufficient explanation of the implementation of recirculation and the relationship to HRT especially as a large part of your introduction was devoted to explaining the importance of this. Presumably recirculation is related to longer HRT but there is nothing relating your effluent recirculation rates to HRT. Also, it was not clear at first if everything before Section 2.6.6. is conducted without recirculation given your emphasis on recirculation in the Introduction. It would be helpful to describe that you are modeling the system first without circulation and ensuring the model is accurate before introducing recirculation.
It would be helpful if the Results and Discussion section included a discussion of the relationship between influent COD, HRT, and removal efficiency. The studies that are referenced all use different types of wastewater with different COD compositions so it would be helpful to have a common reference value to compare the present study with these other studies. This would help the reader know if the results obtained here are comparable to the results obtained in other studies. This is very important for ensuring that the modeling performed here is representative for ABR systems in general.
There is insufficient discussion in the Methods and the Results and Discussion sections about the use of sensitivity analysis and why parameters were adjusted as they were. Presumably there was some sort of statistical evaluation that suggested which parameters were responsible for the majority of the operational variance and these were the parameters targeted for adjustment. However, some of the adjustments are quite large from model defaults and it is unclear where these suggested changes originated from.
SPECIFIC COMMENTS
Line 43: should read “severe threats to ecosystems”; remove “the”
Paragraph from lines 52-68: This should be shortened to 2-3 sentences regarding the disadvantages or impracticality of centralized wastewater treatment in developing countries. Readers who are interested in the biology and chemistry of ABRs should already be aware of the energy costs and infrastructure needs of centralized vs. decentralized systems. The information in this paragraph makes the introduction section longer without contributing necessary information to understand your research.
Paragraph from lines 69-77: An important consideration of sludge blanket reactors is the potential to segregate the hydraulic retention time from the solids retention time. This was not mentioned here but is relevant towards understanding the impact of effluent recirculation.
Line 70: extra space before “simple”
Line 73 starts with “Although” but there is no clarifying clause, making this a partial sentence.
Line 76: should read “Despite these advantages”; should read “ABR still has the drawback”
Line 88: more consideration should be given here to explaining how the specific characteristics of the wastewater impact the effects of effluent recirculation (e.g., pH, TSS, COD, etc.)
Line 101: should be “artificial neural networks”
Line 102: You state that predictions should be generated using various modeling computer applications but you do not explain why. Presumably, you are stating that instead of machine learning techniques, alternative modeling applications should be used. But the reader may not know what these modeling applications are and what they do and why they are better than machine learning for this application.
Line 135: I would rewrite this statement to say “The novelty of this study is in its unique”.
Paragraph from lines 124-145: You repeat the study characteristics twice in the paragraph (namely, that you are comparing the ABR performance at four distinct HRTs). This is repetitive and unnecessary. You do not need to provide these details in the introduction. Instead, you can provide a general overview of the parameters you assessed and relate this to the importance of creating an accurate model of ABR performance.
Line 149: should read “having a total working volume”
Line 151: should read “having a diameter of 25 mm”
Line 153: extra spacing between “of” and “the”
Line 153: should read “The inlet and outlet of the ABR”
Line 154: extra spacing before 305
Line 157: within is one word, not “with in”
Line 159: should be “corresponding”, not “corresponded”
Line 159: should be “A schematic”
Line 160: should be Figure 1, not Figure-1
Line 163, Section 2.2. You need to provide a recipe for your synthetic wastewater solution or a reference to a previous publication that contains the recipe.
Line 169: should be “proportions”
Line 170” should be “a wetland”. You should provide characteristics of the wetland to give the reader more information regarding the type of inoculum. If this is a peat wetland, the pH is going to be quite acidic and the types of methanogens present are quite different. Provide information regarding the pH, DOC, and relevant seasonal changes.
It would be a good idea here to explain your choice of inoculum. In many parts of the world, an ABR is one possible version of a decentralized wastewater treatment system also referred to as a septic system. For these types of on-site, decentralized systems, the inoculum comes from the influent wastewater which is typically human waste. As you are trying to replicate this in the lab, you should explain that your choice of inoculum is to reflect what would occur with the installation of a new ABR system. In other words, you are not using a mature inoculum from another ABR system (or other type of waste treatment system) to start your ABR reactor because this does not reflect the reality of these systems. It would help the reader understand your choices better as most wastewater treatment papers use inocula from other wastewater treatment systems.
Paragraph from lines 182-190: Describe your start-up period in more detail. Presumably you started with a low flow rate to allow establishment of the microbial community without wash-out and then increasingly ramped up flow rate? Did you also increase your feeding rate during this time?
Line 219: should read “the model was comprised of”
Line 246: should be “affect” not “effect”
Table 2: Why are your influence COD rates fluctuating so greatly with a synthetic wastewater? Is this with or without recirculation?
Lines 341-342. You mentioned that higher influent COD could contribute to higher removal efficiencies which suggests a first order rate equation. This could be somewhat modeled by comparing removal efficiencies in different studies given different COD concentrations and the same HRT. At the least, you should normalize the removal efficiencies provided by other studies by comparing COD to HRT in order to compare these.
Line 355: extra spacing between “regarding” and “TSS”
Paragraph from lines 346-360: Your results and the results of other studies seem to suggest there is an upper limit for removal efficiency from ABRs. Is this due to a sludge washout or some other aspect of operation? In your introduction, you stated that ABRs may be considered inappropriate for wastewater treatment due to their inability to meet effluent guidelines. It would be good here to provide some discussion on the ability of an ABR to meet greater removal efficiencies.
Line 368: should be “affecting”, not “effecting”
Line 372: extra spacing between “stochiometric” and “parameter”
Line 374: should read “within the first compartment”
Paragraph from lines 370-383: Why did you make such a large change in uhcon in C1 to 13.5 from 3.2? Is this supported from kinetic modeling of heterotrophic growth from 20C to 35C?
Line 397: extra spacing between “of” and “3.0”
Line 438: The title for this section does not make sense. What is the use of “wise” in this context? Do you mean “Model compartment level COD reduction”? For instance, modeling COD removal in each compartment?
Line 439: I don’t think you mean “wise”. Perhaps you were thinking “compartment-wide”? But this would not be the best way to state this.
Figure 8: The use of “variations” in this context is confusing. I read variation as the variance in COD removal values but if this is the case then you cannot have a negative variance. You can have a reduction in variance but it would never become negative. I have no understanding of what you mean by a negative variation in this context. Also, you need error bars so we can see if the changes in removal efficiency are significant.
Line 482: Paragraph is not indented
Line 483: Use 0 or “zero” instead of “nil”
Paragraph for lines 496-507: This paragraph is a bit confusing because your baseline operation is with a 24 h COD. So there is no “increase” in removal efficiency at an HRT of 24 hr because this is your baseline. You can say that there was greater removal efficiency at an HRT of 24 hr (comparing this value to the values for other HRTs) but you cannot state there is an increase as you are starting with an HRT of 24 hr.
Line 519: extra spacing between “reactor” and “beyond”
Figure 9: Again, the use of variation here is confusing. Also, you changed the variable naming scheme to “Vari” from “Var” which is an added layer of confusion for the reader. You also need error bars.
Line 549: The paragraph is not indented.
Line 571: Should be Conclusions
Line 579: Should read “along with an increase in RR”
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThis study addresses a critical gap in understanding the role of effluent recirculation in ABR performance under steady-state mesophilic conditions. The integration of GPS-X modeling with experimental validation is a notable strength, providing a robust framework for predictive analysis. This research has found that such as the ineffectiveness of recirculation at longer HRTs and performance decline at 8 h—offer practical insights for optimizing decentralized wastewater treatment systems. It has certain research significance. However, there are some deficiencies in the presentation and content of the manuscript, which need to be revised more substantially.
- The abstract section requires re-writing.. It is necessary to highlight the key findings, especially the differential impact of effluent recirculation across HRTs. Specify numerical ranges for COD/TSS removal efficiencies under recirculation to enhance clarity.
- Suggest expanding the literature review to better contextualize the novelty of this study. For instance, explicitly compare prior findings on ABR performance with/without recirculation and emphasize the gap in steady-state mesophilic conditions. Highlight why GPS-X modeling is uniquely suited for this investigation.
- Itshould elaborate on the sensitivity analysis methodology in chapter 2.6.4. Specify which parameters were prioritized, how adjustments were validated, and the rationale behind modifying parameters.
- It should add error bars in Figures 5, 6, 8, and 9 to indicate the variability of experimental data.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
