Techno-Economic Assessment of PEM Electrolysis for O2 Supply in Activated Sludge Systems—A Simulation Study Based on the BSM2 Wastewater Treatment Plant
Round 1
Reviewer 1 Report
This is an interesting work, and I recommend minor revision.
1. The abstract was too long, and it should be rewritten.
2. Alkaline water electrolysis was more flexible and low-cost for production of O2. However, the author chose PEM electrolysis. The advantages should be claimed.
3. O2 was mainly produced by the air cryogenic technology. Please compare the price of O2 by PEM electrolysis with that by air cryogenic technology.
Author Response
We are very grateful to the reviewer for his comments, our response can be found below:
- The abstract was too long, and it should be rewritten.
We have made a new, shorter version of the abstract.
- Alkaline water electrolysis was more flexible and low-cost for production of O2. However, the author chose PEM electrolysis. The advantages should be claimed.
Although alkaline electrolysers are more price-competitive, PEM electrolysis was chosen because of its shorter response time. PEM electrolysis is therefore better qualified for dynamic operation in response to volatile renewable energy supply, which is considered together with conventional energy in the TEA. An explanation for the selection of PEM electrolysis was added to Section 2.1 (lines 103-106).
- O2 was mainly produced by the air cryogenic technology. Please compare the price of O2 by PEM electrolysis with that by air cryogenic technology.
The references for the cost inventory in Section 2.4.1. are for conventional O2 production processes like cryogenic air separation units, the selected value of 100 €∙t-1 for the reference year in 2020 represents then typical market values for such conventional technologies.
Reviewer 2 Report
This study provides a quantitative analysis of the benefit of integrating of green-energy powered electrolysis cell within wastewater treatment plants. Thus, it makes an important contribution towards the question of how these infrastructures can benefit directly from renewable energy sources. In this way, these presented findings might impact a wide range of readers and further studies. The study is well-defined and structured. I think that most of the simplifications and methods are valid for the defined research questions and objectives of the study. There are, however, some questions open and demands of improvement that are need for a final publication. These points are listed below.
a. One fundamental parameter from this study is missing, namely the size of the WWTP. This is key to understand the analysis, e.g., how can we extrapolate the number of electrolysers or costs from this study to other plants. I would also strongly suggest including some comments regarding such normalized parameters (regarding WWTP loading or population equivalent) into the final discussion. This way the reader can relate to the results much better.
b. A brief description of the implemented model is also strongly recommended. This might be very helpfull for readers which are not familiarized with the ASM models.
c. More details regarding the O2-supply with pure O2 are needed. This very important to understand the results and findings from the analysis. Thus a more complete description might help to clarify several issues e.g.: which was the selected set O2 concentration? The saturation concentration is affected by the 100% O2 gas stream? Is the use a typical AS alpha factor here supported by literature? Or is a conservative estimate?
d. Why was a baseline scenario representing current WWPTs (i.e., without electrolysis cell) not included in this analysis?
e. Some abbreviations are not explained along the text, which difficult the reading. Please include also the full parameters name within Table and Figures Labels. This helps the reading very much as many tables are listed with different cost parameters which are many along the manuscript and is very difficult to keep with them all.
f. Some temperature effects were neglected as already explained in the text. Nonetheless, it is not very clear which effects were considered (Lines 139 and 425 for example). Hence, please provide which was temperature variation and which processes were temperature dependent. Such temperature variations along the year and other simulation results could perhaps be included as support material.
g. A final remark here is the classification of higher H2 prices within the optimistic scenario. Altough it is clear that this is optmistic for a cost perspective, a small explanation here can perhaps be considered.
Author Response
We are very grateful to the reviewer for his comments, our response can be found below:
- One fundamental parameter from this study is missing, namely the size of the WWTP. This is key to understand the analysis, e.g., how can we extrapolate the number of electrolysers or costs from this study to other plants. I would also strongly suggest including some comments regarding such normalized parameters (regarding WWTP loading or population equivalent) into the final discussion. This way the reader can relate to the results much better.
The capacity of the BSM2 WWTP has been specified in Section 2.2. (line 128), further discussion on the size of the plant, how it compares to real municipal WWTPs and an example of an extrapolation for estimating the capacity of a PEM for a smaller WWTP have been added to Section 3.3 (lines 575-585).
- A brief description of the implemented model is also strongly recommended. This might be very helpfull for readers which are not familiarized with the ASM models.
A more detailed description of the ASM1 model has been added to Section 2.2. (lines 129-135).
- More details regarding the O2-supply with pure O2 are needed. This very important to understand the results and findings from the analysis. Thus a more complete description might help to clarify several issues e.g.: which was the selected set O2 concentration? The saturation concentration is affected by the 100% O2 gas stream? Is the use a typical AS alpha factor here supported by literature? Or is a conservative estimate?
The set-point for O2 concentration in the activated sludge system, according to the default control strategy for the BSM2 plant used in this study, is 2 mg L-1 for the second nitrification tank. A reference to this value has been added to Section 2.2. (lines 148-149).
The O2 saturation concentration for the pure O2 gas stream is around five times higher than the one for air. Given that the O2 saturation concentration at 15°C is defined as 8 mg L-1 for the BSM2, a simple estimation of the corresponding saturation concentration for pure O2 can be done dividing the value for air by 0.21 (relative content of O2 in air), resulting in around 38 mg L-1. This change has been considered in the model as a modification of the original BSM2. Eq. 1 shows that, for the same dissolved oxygen concentration and kLa value, higher saturation concentration for pure O2 leads to higher oxygen transfer rates (OTR). For this reason, the kLa values calculated by the PI controller are lower for the case of pure O2 than for the use of air, i.e. less gas flowrate is required. The model then balances increased differences between dissolved and saturation O2 concentrations with lower kLa values, resulting in practically the same OTR used as basis for the dimensioning of the electrolyser plant. Further clarification regarding the modification of the BSM2 for the simulation of pure O2 supply has been added to Section 2.2. (lines 156-161), while a discussion regarding the effects on the simulations results has been added to Section 3.1. (lines 445-454).
An alpha factor around 0.6 have been reported in literature for pure O2 activated sludge plants. A corresponding reference has been added to Section 2.2. (lines 154-155).
- Why was a baseline scenario representing current WWPTs (i.e., without electrolysis cell) not included in this analysis?
To the authors best knowledge this is the first simulation-based study of the economics of electrolysis O2 supply to biological treatment in a WWTP. For simplicity, the scope of the TEA in this first study was restricted to the installation and operation of an electrolyser and its associated components for O2 supply to biological treatment steps. Including a baseline scenario would mean expanding the scope of the TEA to include the costs related to the WWTP. However, further studies should consider the costs of operating the WWTP either with air or pure O2, the investments necessary for adapting the WWTP to the use of pure O2 and the effects of pure O2 in the processes involved in activated sludge systems.
- Some abbreviations are not explained along the text, which difficult the reading. Please include also the full parameters name within Table and Figures Labels. This helps the reading very much as many tables are listed with different cost parameters which are many along the manuscript and is very difficult to keep with them all.
Brief descriptions of the abbreviations not explained in the first version have been added to Section 2.4.1. (lines 307-310, 315-316, 336-337, 341-342), as well as the full parameter names in Table and Figure labels.
- Some temperature effects were neglected as already explained in the text. Nonetheless, it is not very clear which effects were considered (Lines 139 and 425 for example). Hence, please provide which was temperature variation and which processes were temperature dependent. Such temperature variations along the year and other simulation results could perhaps be included as support material.
Further explanation regarding the considered effects of temperature has been added to Section 2.2. (lines 137-142).
- A final remark here is the classification of higher H2 prices within the optimistic scenario. Altough it is clear that this is optmistic for a cost perspective, a small explanation here can perhaps be considered.
An explanation regarding the classification of H2 prices has been added to Section 2.4.2. (lines 435-439).
Reviewer 3 Report
The authors developed a model to simulate oxygen supply for a municipal waste water treatment plant, which includes an activated sludge system and polymer electrolyte membrane electrolyzer plant. Two scenarios were considered and studied. The model can effectively simulate oxygen supply for an operation period of one year. The PEME plants in both scenarios were able to cover the oxygen demand for more than 99% of the simulated time. This work is very valuable for the BSM2 waste water treatment plant and has enough content. The manuscript is well-written and fits for the journal. There are just a few issues that should be addressed before the acceptance. Please see comments below.
1. Abstract and Conclusion sections are too long, and they should be simplified.
2. Introduction, the authors should state what is the main purpose and general work of this study in the last paragraph.
3. All the equations are coming from other literatures, and just wondering what is the novelty or contribution of this work, which should be highlighted.
4. Figure 3. It should be introduced and discussed more in details. What does the symbol and line mean? Not just a brief description in Section 3.1, which should be expanded.
5. Has the model been validated and how?
Author Response
We are very grateful to the reviewer for his comments, our response can be found below:
- Abstract and Conclusion sections are too long, and they should be simplified.
We have made a new, shorter version of the abstract and conclusions.
- Introduction, the authors should state what is the main purpose and general work of this study in the last paragraph.
We have added a more detailed description of the main objective of the study to Section 1. (lines 92-98).
- All the equations are coming from other literatures, and just wondering what is the novelty or contribution of this work, which should be highlighted.
The main novelty of the work is the combination of activated sludge and electrolyser models for the simulation and economic analysis of O2 supply. Although the mathematical models for both the WWTP and the PEM electrolyser come from references, as well as the cost estimation methodology for the TEA, the authors could not find another scientific publication which made a dimensioning and TEA for an O2 supply process to an activated sludge system. The main contribution is then to describe a methodology for the analysis of such projects on the basis of the BSM2 model, which is a widely recognized benchmark for the simulation of municipal wastewater treatment. Further discussion on this regard has been added to Section 3.4. (lines 551-553).
- Figure 3. It should be introduced and discussed more in details. What does the symbol and line mean? Not just a brief description in Section 3.1, which should be expanded.
Further explanation on Figure 2 (Figure 3 in the first version) has been added to the label. (lines 463-465)
- Has the model been validated and how?
The model has not been validated, further experimental investigations in laboratory scale should address the effects of electrolysis O2 supply on the biochemical processes in activated sludge systems. A comment on this has been added to Section 3.4. (lines 567-569).
Round 2
Reviewer 2 Report
The authors provide a careful response to all comments, and the manuscript improved significantly after this revision. I would like to add just one more final revision suggestion regarding the revised abstract:
-In the abstract, figures regarding the dimensions of the electrolyser are provided within the main findings of the study – which are certainly very relevant. However, as in the first manuscript version, those figures lose much of their meaning without providing some indication of the WWTP size. Thus, please provide an indication also here the WWTP size.
Author Response
We thank the reviewer for his/her further commentaries in the second round. We have added specifications on the BSM2 WWTP size in the abstract as in the main text of the publication.
