Anaerobic Digestion of the Residue (Combination of Wastewater and Solid Waste) from a New Olive-Oil Manufacturing Process Based on an Olive Cold-Pressing System: Kinetic Approach and Process Performance

Round 1

Reviewer 1 Report

The article deals with a very interesting topic, and is very well-organised and written. A wide range of important aspects are carefully considered and discussed in terms of the efficiency of biomethanisation (methane yield coefficient, biodegradability of the substrates, kinetics, etc.). Consequently, I feel that this is suitable for publication in its present form in Processes.

Only a minor improvement should be considered in tables 1, 3 and 4, where the letters used to indicate significant differences might be confusing in its present form. Maybe the use of superscript would be better.

Author Response

We greatly appreciate the comments and suggestions and we thank you for the time and efforts devoted to our manuscript.

All the comments and suggestions have been incorporated into the text and we agree these have improved the content of the paper.

REVIEWER 1

 Comment 1: Only a minor improvement should be considered in tables 1, 3 and 4, where the letters used to indicate significant differences might be confusing in its present form. Maybe the use of superscript would be better.

RESPONSE: According to this suggestion, the letters used in Tables 1, 3, and 4 to indicate significant differences have been written as superscripts to avoid possible confusion.

Reviewer 2 Report

The novelty of the manuscript is not so much in the anaerobic digestion of alpeorujo, extensively studied by some of the signing authors, but in the comparison of it with different residues from the process of obtaining oil.

The novelty of the manuscript needs to be more clearly stated.

M&M section

The Abencor method has not been referenced or described.

In Table 1, the acronyms of the variables are missing. They have not been previously described.

In Table 1, the acronyms of the variables are missing. The last column of table 1 is not aligned with the other cells. The quality of the presentation has to be improved. They have not been previously described.

In table 2 it is necessary to unify the number of decimals to the significant figures. (see penultimate column, Hydroxitirosol line)

In the experimental procedure, the duration of the start-up and activation experiments of the anaerobic digesters has not been clearly described. What substrate are digesters started with? Is a white rector used? If they are activated three times with each substrate, review the sentence on page 12 line 429-433. There is a contradiction.

If endogenous metabolism is removed, show methane production results on target.

Page 5, line 190. Review the term energy. It should be heat energy

Línea 212-213. Un valor de desviación estándar de ser incluido.

Línea 233. Es necesario homogenizar sin decimales.

Tabla 3. Incluya e significado de a y b (idema otras tablas), defina Nd

Los valores de roducción de metano han de ser expresados en Standard temperatura and presure condicions, Reviar en NL el significado de N”

Líneas 388-394. Comprobar que la diferencia en la producción de metano se encuentra en la presencia de compuestos independizando los valores de producción de metano por la carga orgánica eliminada.

Results and Discussion

Line 212-213. A standard deviation value to be included.

Line 233. It is necessary to homogenize without decimals.

Table 3. Include the meaning of a and b (like other tables), define Nd

The methane production values ​​must be expressed in standard temperature and pressure conditions, review in NL the meaning of N”

Lines 388-394. Verify that the difference in methane production is found in the presence of compounds making the methane production values ​​independent from the organic load removed.

Lines 388-394. Verify that the difference in methane production is found in the presence of compounds making the methane production values ​​independent from the organic load removed.

Lines 444-448. Homogenize units to be able to compare.

Conclusions

The most relevant conclusions of the study should be clearly included. Highlighting whether the objectives and novelty have been met.

Author Response

We greatly appreciate the comments and suggestions and we thank you for the time and efforts devoted to our manuscript.

All the comments and suggestions have been incorporated into the text and we agree these have improved the content of the paper.

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Firstly, I appreciate the experimental work of the authors as it provides good coverage of chemical and biochemical analysis of the feedstock and final digestate. However, it will be more useful in the analysis and discussion of the results if the authors can provide data on fat and biogas composition.

Secondly, you provide the data on the soluble and total CODs, which is very useful to understand the dynamic of substrate conversion, but it seems that that is not used in most discussions and the kinetics study.

Thirdly, a simple Logistic model may be suitable for the feedstock containing a mixed portion of readily (or easily) degradable and slowly degradable substrates like in your cases.  I would suggest a two-step logistic (or Gompertz) model instead. These models can accommodate two (or more) groups of substrates having different levels of degradation difficulty.

Fourthly, I substituted the parameters given in Table 4 into the Logistic model and found that the model gave only correct B only when t > 10 d. And at t = 0, calculated B was very small (compared to ~100 CH4/kg VS). They are not consistent with the authors' claim.

Last but not least, in Lines 412-414:

"The shape of the curves of methane production – time observed in the present research indicates a rapid increase in methane generation after an initial lag period of 6 - 7 days, which revealed the occurrence of an adaptation stage prior to a rapid consumption of the most readily-available biodegradable components for the four mixture residues."

In your work, the lag period was very short (< 1 d because the inoculum was very active), judged by very fast CH4 generation during the first two days of digestion. The 7-9 lag before the second peak rate occurred was due to the hydrolysis of the slowly digestible substrate components.

Please, the authors should check the assumptions underlining the selection/use of the model and also correct any discrepancy in the parameter estimation.

Author Response

We greatly appreciate the comments and suggestions and we thank you for the time and efforts devoted to our manuscript.

All the comments and suggestions have been incorporated into the text and we agree these have improved the content of the paper.

Please see the attachment.

Author Response File: Author Response.pdf