Behavior of UV Filters, UV Blockers and Pharmaceuticals in High Rate Algal Ponds Treating Urban Wastewater

Round 1

Reviewer 1 Report

The manuscript contains the information on the behaviour of UV filters, UV blockers and pharmaceuticals in high rate algal ponds treating urban wastewater. It is an interesting content, but arranged structure needs to be further improved. Therefore, it needs revision before it is published in this journal. The following issues should be carefully addressed.

1. English language improvement is highly recommended.
2. Authors should further explain the novelty of the study in Introduction.
3. More details about the experimental conditions should be provided.
4. Please explain why high rate algal pond was selected in this work.
5. Give more detail analysis and discussion in the section of “Discussion”, and it should be amended by including a brief analysis respect to the use of the reported results to other scale;
6. Conclusions should be rearranged.

Author Response

Reviewer #2:

 

The manuscript contains the information on the behaviour of UV filters, UV blockers and pharmaceuticals in high rate algal ponds treating urban wastewater. It is an interesting content, but arranged structure needs to be further improved. Therefore, it needs revision before it is published in this journal. The following issues should be carefully addressed.

 

1. English language improvement is highly recommended.

 

Following your suggestion, English language has been reviewed by a native-speaker colleague. Changes were marked on text.

 

2. Authors should further explain the novelty of the study in Introduction.

 

We appreciate the reviewer comment, and the final paragraph in the intro section has been updated accordingly. Nevertheless, all through the introduction it has been made clear the novelty of the use of these green, low-cost treatments as feasible alternatives to conventional, intensive wastewater treatment plants. A sentence was inserted at the end of the introduction, clarifying the novelty of the study and the gaps in the literature justifying the evaluation of the evaluated compounds.

 

3. More details about the experimental conditions should be provided.

 

The text has been changed and highlighted in the text.

 

4. Please explain why high rate algal pond was selected in this work.

 

As explained in the introduction of the manuscript, from lines 42 to 69, high rate algae ponds are alternative, green and low-cost systems which have demonstrated to be highly  efficient for sewage treatment. These are nature-based systems very easy to operate, with very reduced operation and maintenance costs, which don´t require any chemical input. These are all great advantages compared to conventional, activated sludge based treatment plants. The selection of natural systems that are able to treat sewage and still remove other pollutants (such as pharmaceuticals, UV filters and UV blockers) at low cost, are important to expand the possibility of simple and sustainable operation technologies with the delivery of an effluent from high quality.

 

5. Give more detail analysis and discussion in the section of “Discussion”, and it should be amended by including a brief analysis respect to the use of the reported results to other scale.

 

We appreciate this comment. The mechanisms that act in the ponds are the same regardless of the scale. Furthermore, studies with HRAPs at real-demonstrative scale are basically inexistent, with the exception of the study by Villar-Navarro et al., 2018, who worked with 9.6 m3 ponds, and it has been referred to in the text. Most of previous studies on HRAPs/microalgae removal capacity are based on lab-scale experiments or similar-smaller ponds to those in the present study. Last of all, the target compounds evaluated in this study have barely been addressed in similar, previous studies, which enhances the novelty of this work, but makes it more difficult to find previous articles to use as reference.  

 

6. Conclusions should be rearranged.

We have updated the conclusion section accordingly

Author Response File: Author Response.docx

Reviewer 2 Report

The paper is well written and complete and it is of interest for the journal’s readers, even if the topic is not completely new. Introduction and discussion of the results are adequate and easily understandable. Sufficient and clear details have been provided for the description of the experimentation.

Please highlight the differences with a very recent study published by the same authors in the Journal of Environmental Management - https://doi.org/10.1016/j.jenvman.2020.111081

The paper can be published after minor revisions:

Line 61 – not clear to which “their” is referring.

Line 67 – put a comma after (N and P)

Line 120 – it can be of interest repeating the study in a different season…

Line 138 – delete one of the “were purchased from”

Line 200 – if an uncertainty factor was used, that should be highlighted in Table 2

Line 205 – detail the considered trophic levels

Line 238-240 – the two sentences must be rephrased for an improved clarity (especially because they are the first two sentences of the section)

Line 338 – the sentence must be rephrased

Lines 342 – 343 – I think that “its” must be replaced by “their”

Line 361 – on average

Line 376 – derived from

Line 390 – as it can be observed

Line 464 – tray to rephrase the sentence thus avoiding the parenthesis (primarily treated)

Line 476 – pay attention to “either”

Author Response

Reviewer #2:

The paper is well written and complete and it is of interest for the journal’s readers, even if the topic is not completely new. Introduction and discussion of the results are adequate and easily understandable. Sufficient and clear details have been provided for the description of the experimentation.

1. Please highlight the differences with a very recent study published by the same authors in the Journal of Environmental Management https://doi.org/10.1016/j.jenvman.2020.111081

 

The main differences were that the study published in Journal of Environmental Management was carried out in a hybrid photobioreactor: This is, a closed photobioreactor which counted on two open containers at the end of the horizontal tubes to aid with the excessive accumulation of DO within the reactor. The feeding water was also different, as it was a mix of agricultural runoff water with secondary effluent from a WWTP nearly. The hybrid PBRs were carrying out tertiary treatment, and the potential reuse of the resulting effluent was also evaluated. On the contrary, in the present study a high rate algal pond (open system) was evaluated treating urban sewage. It is true, however, that the methodology and the analytes applied are the same, although the compounds detected in each feed water, as well as the concentrations, were different.

 

2. The paper can be published after minor revisions:

 

Thank you for all indication. The changes were all made and marked in the text.

 

• Line 61 – not clear to which “their” is referring.
• Line 67 – put a comma after (N and P)
• Line 120 – it can be of interest repeating the study in a different season…

We don´t fully agree with the reviewer on this comment, as the study was developed under the best conditions for microalgae to treat wastewater (maximum biomass productivity). Previous studies considering seasonality have already demonstrated that efficiency decreases in those seasons with lower irradiation rates.

• Line 138 – delete one of the “were purchased from”
• Line 200 – if an uncertainty factor was used, that should be highlighted in Table 2
  3. This information has been provided in section 3.4.
• Line 205 – detail the considered trophic levels
  3. This is explained in detailed in section 3.4
• Line 238-240 – the two sentences must be rephrased for an improved clarity (especially because they are the first two sentences of the section)
• Line 338 – the sentence must be rephrased
• Lines 342 – 343 – I think that “its” must be replaced by “their”
• Line 361 – on average
• Line 376 – derived from
• Line 390 – as it can be observed
• Line 464 – tray to rephrase the sentence thus avoiding the parenthesis (primarily treated)
• Line 476 – pay attention to “either”
•  

All these points have been addressed in the revised version.

Author Response File: Author Response.docx

Reviewer 3 Report

• Does the removal efficiency differ between seasons (winter versus summer)?
• What are the Abiotic factors that might affect the removal process?
• How does the fluctuation in the physicochemical characterization (temperature and nutrients) of the influent affect the treatment process?
• Add a table showing the chemical structure of each compound removed

 

Author Response

Reviewer #1:

 

 

1. Does the removal efficiency differ between seasons (winter versus summer)?

 

Thank you for your question. Seasonality was not considered in this study, and only one sampling campaign was carried out during summer. However, it is well known that the seasonal factor influences the final quality of the treated effluent: higher sun irradiance leads to higher microalgae biomass and, indirectly, to higher depuration in terms of organic matter and nutrients removal. Higher biomass productivity allows also to work with shorter HRTs. This is the main reason why the sampling campaign was carried out during the month of July.

 

2. What are the abiotic factors that might affect the removal process?

 

Photodegradation is an important abiotic factor that influences the degradation of organic micropollutants, especially those highly photosensitive (i.e. diclofenac); this is discussed in the text (lines 443-446 of the manuscript). Other important abiotic parameters to consider are pH and dissolved oxygen, which can influence the ionic stability and oxidation of some micropollutants. In addition, and as explained in the previous comment, abiotic factors also influence the growth of microalgae in the system (i.e: solar radiation and temperature), which indirectly influences their removal. It is important to highlight that all removal routes (biotic and abiotic) were discussed in the manuscript according to the chemical characteristics of the compounds.

 

 

3. How does the fluctuation in the physicochemical characterization (temperature and nutrients) of the influent affect the treatment process?

 

The concentration of nutrient input, mainly carbon, phosphorus and nitrogen, affects the growth of microalgae in the system. The limitation or fluctuation in the concentration of any of these compounds, coupled for example with the deficiency or excess of solar radiation can cause low productivity of microalgal biomass. As the system uses microalgae to promote sewage treatment, low biomass productivity can decrease the final quality of treated sewage. In the present study, however, sampling was carried out under the most optimum conditions regarding physical and chemical conditions and the removal efficiencies, as shown in Table 1, are excellent.

 

4. Add a table showing the chemical structure of each compound removed

 

Table S1 in the Supplementary Materials  has been updated to meet this request.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The manuscript's quality has been substantially improved. I recommend its acceptance for publication in its present form.