Study of Chlorella sorokiniana Cultivation in an Airlift Tubular Photobioreactor Using Anaerobic Digestate Substrate

Round 1

Reviewer 1 Report

In the present manuscript the authors are presenting the results of the evaluation of the treatment capacity of a high-nitrogen anaerobic digestion effluent as a nutrient source for a microalgal strain (C. sorokiniana) in a horizontal tubular photobioreactor. Their first objective is to assess how the concentration and composition of the digestate influence microalgae growth, while secondly, they identify the preferred nitrogen forms assimilated by the microalgae during long-term, continuous operation.

The manuscript is very well-written and easy to follow. The authors fully support and validate their claims, while they help demonstrate how important a process scale-up is for fully understanding its kinetics and all of the parameters that have an essential role in it.  The figures are well presented and explained, while the supplementary material is very helpful in order for the reader to completely understand this study.

Below, there is a list of specific issues that should be addressed by the authors, before the manuscript can be published.

1. In my opinion, a list of abbreviation on the beginning of manuscript could be helpful.

2. It is not quite clear to me how the regulation of the temperature in the photobioreactor is achieved. The authors should add the appropriate information in the Materials and Methods section, where they already describe the PBR set-up.

3. At Table 3, in the end of 3.2 section after line 530, a border line between the last two nutrient media (ADE_1 and ADE_2) is missing.

4. Table 4’s legend (line 553) should be corrected from Table 3 to Table 4.

Author Response

We would like to thank the reviewer for comments; corresponding issues have been addressed according to the attached file.

Author Response File: Author Response.pdf

Reviewer 2 Report

This research aims to study the effectiveness of the growth of Chlorella sorokiniana alga in a horizontal air-lift tubular photobioreactor (ATPBR) on anaerobic digestate waste at different concentrations, while sometimes supplementing the waste by adding an external source of phosphate to maintain the optimal ratio between the elements nitrogen and phosphorus for the optimum growth microalgae. Therefore, the title of the research must also be changed to become more expressive of the content, by replacing the word microalgae directly with the name alga Chlorella sorokiniana.

A variety of modern scientific research was used, reaching 47 references related to the field of study without excessive self-citations.

12 figures and 3 tables were used to present and discuss the results, and 12 mathematical equations were used to calculate the productivity and removal of nitrogen and phosphate sources in the various experiments.

Despite the effort expended in this study, it lacked any statistical analysis of the results to determine the qualitatively significant effect on C. sorokiniana growth and water treatment. In addition, most of the figures contain errors in writing the chemical symbols on the x- and y-axes, and sometimes the number zero is used instead of the letter “O” as a symbol for oxygen.

It was also mentioned that the measurements were made with replica samples. Still, all the figures and the text for the results and discussion part did not mention these calculations, whether standard deviation or standard error.

Check the attached pdf file to clarify all of the edits mentioned above. The work is excellent, but it still needs some revision and additions to be reconsidered for publication.

Comments for author File: Comments.pdf

Author Response

We would like to thank the reviewer for comments; corresponding issues have been addressed according to the attached file.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Thank you for the correction
The manuscript is accepted now in the present form for publication