Performance of Combined Olive Mills Wastewater Treatment System: Electrocoagulation-Assisted Adsorption as a Post Polishing Sustainable Process

    Comment 1: The EC experimental section lacks specifications regarding electrode material purity and surface pretreatment procedures (e.g., acid washing/polishing frequency), which may affect result reproducibility. We recommend supplementing details about electrode material specifications and pretreatment protocols.

Response 1: Thank you for this important comment. We have added more details in Section 2.4.1. Electrocoagulation Process on page 7, lines 256 to 274. While we did not specify the purity of the electrode material, we clarified that we used locally commercial iron plates. Additionally, we included information about the surface pretreatment procedures to enhance the redox reaction on the electrodes and to get reproducible results without any negative effect of sludge deposition on the electrodes.

Comment 2: The activation conditions for OS in adsorption experiments (e.g., pyrolysis temperature and duration) are not sufficiently described. Additional information is needed to verify the controllability of adsorbent performance.

Response 2: Thank you again for your comment. We appreciate your concern regarding the activation conditions for the biochar produced from OS in our adsorption experiments. We have included information about the physicochemical and surface characteristics of the biochar produced at pyrolysis different temperatures of 400, 500, and 600°C.

Comment 3: The significance testing of the RSM model (including ANOVA F-values and p-values) is incomplete. We request supplementation of regression model validation results (R², adjusted R², predicted R², and residual analysis) for all response variables.

Response 3: Thank you for your comment. We have added Table 7, which includes the P-values and F-values for each variable.

Comment 4: The study lacks model validation experiments (such as independent replicate experiments under optimized conditions). We recommend adding three sets of validation experimental data to demonstrate model reliability.

Response 4: Thank you for this important comment. We have revised Section 3.3.1 (Analysis of Variance) and added Figure 4, which includes three sets of experimental data for CODtotal, TKN, and TPh. Figure 4 illustrates the experimental versus predicted values.

Comment 5: The cost estimation only considers electricity consumption and electrode usage, omitting adsorbent regeneration/disposal costs, equipment depreciation, and sludge treatment expenses. We suggest including a comprehensive life cycle cost analysis to better evaluate practical application potential.

Response 5: Thank you for your important comment. We considered your comment in the revised version of the manuscript.

Comment 6: The synergistic mechanism between EC and adsorption (e.g., whether EC byproducts modify adsorbent surface properties) is not thoroughly discussed. We recommend conducting SEM/FTIR characterization to examine structural changes in OS before and after adsorption.

Response 6: Thank you for your comment. We have addressed your important recommendation by including SEM and FTIR characterization to analyze the structural changes in the OS before and after the adsorption process.

Comment 7: The comparison with similar studies (e.g., Al-Qodah et al.'s 2023 solar-assisted EC process) is insufficient. The discussion should better highlight this study's innovations and advantages.

Response 7: Thank you for your comment. We have revised the Results section to provide a more comprehensive discussion

Comment 8: The resolution of 3D response surface plots in Figures 2-7 is relatively low. We suggest using contour-overlaid surface plots to improve readability.

Response 8: Thank you for your comment. We have enhanced the resolution of the 3D response surface plots and incorporated contour-overlaid surface plots.

Comment 9: The units in Table 4's literature comparison data are inconsistent (e.g., COD units include both g/L and mg/L). Standardization of units is required.

Response 9: Thank you for your comment. We have standardized all units, converting COD, BOD₅, TPh, TS, and TSS to mg/L for consistency.

Comment 10: Some terminology is inconsistent (e.g., alternating use of "olive stone" and "olive pit"). We recommend uniformly using "OS" throughout the text.

Response 10: Thank you for your comment. We have standardized the terminology throughout the manuscript by adopting olive stone (OS) in place of olive pit (OP) to enhance consistency and clarity.

Comment 11: Variable definitions in Equations (5)-(6) are incomplete (e.g., F for Faraday's constant is not explained). A nomenclature table should be added for clarification.

Response 11: Thank you for your comment. We have revised the variable definitions for all equations in the research and included a nomenclature table for clarification. This ensured that all variables, including Faraday's constant (F), are clearly explained.

Comment 12: The conclusion section should provide more specific recommendations for future research.

Response 12: Thank you so much for this very important comment. Actually, we added the following recommendations to highlights some important gaps in this present research area.

Comment 1: No information on the type of material used for the electrodes (what type of steel for the Fe electrodes). Were the anodes and cathodes made of the same material? What does “Fe was used as electrode materials” mean?

Response 1: Thank you for this comment. We used locally commercial iron sheets for the electrodes in the electrocoagulation process. Both anodes and cathodes were made from the same material, ensuring consistency in performance. We clarified this in the revised manuscript.

Comment 2: No information about the course of the chemical reactions at the electrodes and in the solution. What types of ions are produced at the electrode, what types of reactions can take place near the electrodes. No description of the reaction of gas bubble formation at the cathode (what type of gas, under what conditions, etc.).

Response 2: Thank you for this important comment. As mentioned, we utilized Fe as the electrode material. We added the relevant chemical equations (2 to 4) in lines 256 to 260 on page 7, which described the formation of ferric hydroxide. This species acted as a coagulant for the pollutants present in the wastewater. Furthermore, we acknowledged the need to elaborate on the types of ions produced at the electrodes and the nature of the reactions occurring near them. At the anode, iron oxidized to produce ferric ions, while at the cathode; hydrogen gas was generated through the reduction of protons. The conditions for gas bubble formation, including current density and electrode surface characteristics, also played a crucial role.

Comment 3: Page 3 line 106 – “… and ease of sludge collection [32][33].” - the sludge after the electrocoagulation process is often difficult to remove. Please check the literature.

Response 3: Thank you for your comment. We have revised the text to state that "the advantages of EC include its flexibility, simplicity, lack of need for chemical additives, rapid startup time, ability to treat various pollutants, and reduced sludge generation [32][33]." This adjustment acknowledges the challenges of sludge removal while emphasizing that EC can lead to a reduced volume of sludge compared to some other treatment methods.

Comment 4: Page 4 line 153 - how long was the unpreserved sample stored?

Response 4: Thank you for this comment. We stored the unpreserved wastewater samples at 4°C for no longer than 24 hours to ensure the integrity of their characteristics prior to analysis.

Comment 5: chemical oxygen demand (COD); BOD5 - wrong unit - should be clearly labelled, e.g. g O2/L, similar to TN.

Response 5: Thank you for your comment. We have updated the table to reflect the correct units for BOD₅ and COD as g O₂/L, consistent with TN.

Comment 6: Page 5 2.2. Chemicals - sodium hydroxide (1 N NaOH) or sulfuric acid (1 N H₂SO₄), - unit of concentration not recommended. Notation of the concentration unit - hydrochloric acid (0.1 M HCl) - not recommended.

Response 6: Thank you for your comment. We have revised the text to remove the unit of concentration for sodium hydroxide, sulfuric acid, and hydrochloride acid.

Comment 7: Some of the chemicals used are from pre-made tests (e.g. HACH Lange DR 3900), this should be labelled or exactly what it is. E.g. Folin-Ciocalteu reagent;. Na2CO3 solution (page 6 line 224, 225, etc.)

Response 7: Thank you for your comment. We have revised the text to clearly label the Folin-Ciocalteu reagent as a commercially available reagent for phenolic analysis, and we have specified the sodium carbonate (Na₂CO₃) solution.

Comment 8: Page 5 2.3 Analytical measurement - unclear. What results were used for the analyses presented? Or perhaps the average? Which ones? Etc.

Response 8: Thank you for your comment. We have made revisions to clarify that the analyses for COD, BOD, TN, TPh were conducted in external laboratories, and that the average values from these measurements were used in our analyses. This adjustment enhances the clarity and reliability of our findings.

Comment 9: Page 9, line 285 – Why was pH 4 chosen?

Response 9: Thank you for your question. This pH level was selected because the zeta potential values of the biochar indicate that as pH increases beyond 5; the zeta potential becomes less negative. In contrast, at pH levels below 5, the zeta potential remains significantly negative, which enhances the stability and adsorption capacity of the biochar. Therefore, pH 4 was deemed optimal for maximizing adsorption efficiency while maintaining effective surface properties.

Comment 10: Page 9, line 306 – What quantities of samples were taken? Was the research material supplemented?

Response 10: Thank you for your comment. We prepared four tubes with 25 mL of OMW at initial concentrations of 0, 250, 500, and 750 mg/L. Each tube contained 0.1 g of biochar adsorbent. We have updated the manuscript to include these details for clarity.

Comment 11: Page 9, line 316 – “research and published studies” Name these publications.

Response 11: Thank you for your comment. We have revised the manuscript to include specific references for the "research and published studies" mentioned.

Comment 12: Figures 2-7 - no units for the parameters on the axes. Graph C is illegible.

Response 12: Thank you for your comment. We have included units for all parameters on the axes of Figures 4-9. Additionally, we have improved the legibility of Graph C.

Comment 13: Page 10 Figure 8 - no units for the given parameters.

Response 13: Thank you for your comment. We have made the necessary adjustments and included units for all parameters in all Figures.