Changes made to the manuscript ‘Development of Intelligent Anaerobic Wastewater Treatment System: Challenges and Opportunities’ (WCSE-D-15-00001), based on the
Comment by Editor
Comment # / Page (Revised) / Comments / Changes1 / Page 1 and 2 / Please include a statement or two on how your research can contribute to the 'water conservation' practices. / A statement on how the research contributes to water conservation practices has been added in the introduction as highlighted in the manuscript.
Comments by Reviewer #1
Comment # / Page (Revised) / Comments / Changes1 / - / The title is too general; It does not reflect the content of the manuscript. / The title has been revised to ‘Optimization of Wastewater Anaerobic Digestion using Mechanistic and Meta-heuristics Methods: Current Limitations and Future Opportunities’ to reflect the content of the manuscript
2 / Page 1 (Line 26 – 29) / Abstract:
The authors should conclude the best control system for anaerobic digestion which can be based on several criteria such as cost effectiveness, overall performance, R2 etc. / We thank the reviewer for the comments and we have revised the abstract to include the conclusion of this review.
3 / Page 3 (Line 39) / Section 2:
Page 3 Line 39: state the complete name for POME / The complete name for POME has been provided in Page 3 line 29.
4 / Page 5 (Line 4-8) / Section 3:
Dilution rate should be discussed as it appears many times in Table 5. / Dilution rate can be regarded as the organic loading rate. A more thorough discussion on dilution rate has been added to the manuscript in Page 5 (Line 4-8)
5 / Page 4 (Line 38; Page 5 Line 2) / Section 3.2:
Page 4, Line 37, Typo error: mAcidoMethanogenesis
Line 42-44, reference? / Thank you for highlighting this. The correction has been made with a reference added to the sentence in Line 42-44 (now Page 5, Line 2).
6 / Page 5 (Line 18 – 23) / Section 3.3:
The authors should state the recommended OLR range. / Thank you for the comment. However, it is not possible to give a recommended OLR range for anaerobic digestion because it is very much dependent on the content in the wastewater and how it affects the growth of microbial population in the digester.
In order to clarify this, we have added brief sentences into the manuscript to avoid having readers being misled.
7 / - / Section 4.1:
Page 5, Line 24-28: Please define all the parameters and state their units: k1,k2,…, Ks1… (Please check for other equations as well) / In order to improve the presentation of the equations and also definitions of the parameters involved, we have moved them to a table (Table 2).
8 / Page 7 (Line 17) / Page 7, Line 14: stoichiometric coefficients, Vi,j / Coefficient was defined and highlighted in the manuscript.
9 / - / Page 16, line 33-38: The sentence is repeated. / The repeated sentence was deleted from the manuscript.
10 / - / The authors should compare the Performance of various anaerobic reactors for industrial wastewater treatment / A comparison on the performance of various anaerobic reactors for industrial wastewater treatment has already been made under Section 2, Table 1 of the manuscript.
11 / Page 37 - 38 / Section 6:
The authors should add a comparison table of the three types of the controller including the advantages and disadvantages of each system. / The comparison of different controllers were discussed in respective sub-sections in the existing manuscript. We have decided not to create additional tables in view of this as it will take up space.
12 / - / The authors should include more discussions/results in each system with the support of appropriate references. / We thank the reviewer for the comments. We have added more discussion with the support of appropriate reference throughout different parts of the manuscript where the additional references as listed at the end of this document.
13 / Page 17 (Line 35-37) / Section 7: For each reference, the controller's performance can be summarized in Table 5. Table 5 should be discussed appropriately in Section 7 and the authors should identify the best controller system and state the area of improvement. / Table 5 (now Table 6) were discussed in the individual sub-sections of section 6 but it was not referred anywhere in the manuscript. We have noted this problem and referred to the table in Section 6 (Page 15 Line 5-7).
It is not possible to identify a best controller system to fit all anaerobic digestion system. We have discussed about the current gaps and possible suggestions for improvements in Section 7. We propose that in-depth studies have to be performed to determine the best controller for a system where further optimization can be done to achieve best performance (Page 17, Line 35-37)
14 / Page 18 (Line 20 – 25) / Conclusion:
Please state clearly the area of improvement/future research area of the current control strategies for anaerobic digestion / We thank the reviewer for the comments and we have revised the conclusion to reflect the reviewer’s suggestions.
15 / - / Table 4: The diagrams are not clear. / Clearer diagrams are now attached in the revised manuscript.
16 / - / Table 5: Typo error for 'COD' / Typos for COD were corrected accordingly and highlighted in Table 5 (now Table 6).
Comments by Reviewer #2
Comment # / Page (Revised) / Comments / Changes1 / Page 3 (Line 1) / Page 2, lines 48-49: The stage acetogenesis was not mentioned in the sequence that involves the production of biogas from anaerobic digestion. / Acetogenesis was not explicitly mentioned because some literatures grouped acetogenesis under acidogenesis process. Nevertheless, we have added the acetogenesis stage in the manuscript.
2 / - / Table 1: Authors should include more updated/recent references related to the performance of novel anaerobic reactor configurations for the treatment of different kinds of industrial wastewater. / Table 1 has been updated to include more recent references on the performance of different reactors used for industrial wastewater treatment.
3 / - / All abbreviated terms should be spelled out at their first appearance (For example, POME, PID, TIC, UASB-HCPB). / All abbreviated terms are spelled out accordingly. Changes made have been clearly highlighted in the manuscript.
4 / Page 5 (Line 25-31) / Anaerobic digestion is considered a very complex process that can be influenced by many factors including temperature, pH, VFA concentration, organic loading rate, and hydraulic retention time, mentioned in the manuscript. The presence of toxic inhibitory compounds (e.g., herbicides, ammonia, antibiotics, heavy metals, high sodium concentration, even oxygen) can adversely affect the anaerobic process. Carbon and nutrients availability is another parameter that should also be taken into consideration. On the other hand, hydrogen is an important intermediate in the methane production, and it has been shown that a stable anaerobic digester has a very low dissolved hydrogen concentration and converts most organic substrates directly to acetic acid. Authors should rewrite the respective section in a more realistic way based on more literature evidences. / We agree with the reviewer that there are many other parameters that will play a role to influence the efficiency of an anaerobic digester. However, only the major parameters that are influential to anaerobic digestion were discussed in this manuscript as adding more sections related to other parameters influencing anaerobic digestion will change the focus of this paper entirely. Therefore, we have added an extra section to provide readers with some references related to the influence of other compounds (i.e.: herbicides, hydrogen concentration etc.) so that they can refer to for further information if they would like to know more about those compounds.
5 / Page 15 (Line 31 – 41); Page 16 (Line 1 – 10) / The destabilization of anaerobic digestion is considered related to the unbalanced microbial consortia involved. Many experimental and process parameters may be responsible for the failure of biogas production and wastewater treatment. Under this circumstance, how can the mechanistic and meta-heuristic models be applied to the system to better control the experimental/process parameters? Authors should mention to further implement these models in this aspect in order to emphasize the challenges and opportunities. / The following has been incorporated into the write-up to emphasize the challenges and opportunities
Mechanistic Models
1. Mass Balance
The monitoring of anaerobic digestion is admittedly difficult and complex, as it is a multivariate process. This is further complicated with the limited reliable on-line sensors for the measurement of all required parameters and substances involved [126]. The mass balance model, however, is mainly interested in monitoring one or all of the following (1) COD, (2) VFA and the (3) reduction of model complexity. This is where this model is advantageous, by limiting the number of parameters required and assumptions made. Moreover, it ensures reliable and extended time periods of useful operations.
2. ADM 1
ADM-1 is undeniably the most robust model that has been applied to develop control schemes for anaerobic digestion. However, for its implementation, it is crucial to have comprehensive characterization of the substrate. This can be difficult for anaerobic digestion with significant variation in the feed characteristics throughout the year, especially in wastewater treatment processes. For example, the olive mill wastewater (OMW) characteristics would vary greatly between mills [89, 94] and in the case of POME, it would vary according to season and type of fruits processed in the mill [94]. This could reduce the robustness of the control systems developed for such systems and hence future development should be focused on modification of the ADM-1 to cater for wastewaters with varying characteristics.
Meta-heuristics
The methods reviewed in the meta-heuristic models are likened to a black box. Where these models seek to form input/ output relationship through non-linear and fuzzy set methods. This is advantageous as it is capable of possibly simplifying the number of parameters used in the mechanistic approach to AD. The exception to this would be the PSO method, which only seeks to maximize/minimize based on the way we configure the search and the function provided.
In the general meta-heuristic models, designed models are not constrained by the number and type of parameters. However, it should be noted that the choice of parameters will influence the final performance of the model. As such, the meta-heuristic approach is likened to a trial and error method. Therefore, it was proposed that we leverage on the strengths of individual methods and form a directed approach in the hybrid meta-heuristic models.
These arguments and points are argued in the individual section write-ups.
6 / Page 2 (Line 21 – 30) / Reviews related to the utilization of mathematical models in anaerobic digestion have already been published elsewhere, showing the anaerobic process modeling is a mature and well-established field (some references shown below).
Batstone DJ, Puyol D, Flores-Alsina X, Rodriguez J (2015) Mathematical modeling of anaerobic digestion processes: applications and future needs. Rev Environ Sci Technol. In press.
Kythreotou N, Florides G, Tassou SA (2014) A review of simple to scientific models for anaerobic digestion. Renew Energ 71:701-714.
Donoso-Bravo A, Mailier J, Martin C, Rodriguez J, Aceves-Lara CA, Wouwer AV (2011) Model selection, identification and validation in anaerobic digestion: A review. Water Res 45:5347-5364.
Then, what would be the contribution of this review, which will justify its publication? / The differences between the review papers and this review paper are discussed and comparisons were made under ‘Introduction’. Differences were compared in terms of focus of control implemented on the system and target of end objectives.
"Despite extensive studies in modelling and optimizing anaerobic digester (Batstone, et al, in press; Kythreotou et al, 2011), many of the recent developments in the process control of the anaerobic digestion technology were not reviewed in recent years. These studies also did not discuss the use of several meta-heuristics approaches in modelling and controlling the digester. Meta-heuristic started gaining traction in recent years and has been used in various application too. Hence, it is important to conduct a comprehensive review in this area to provide proper insight to decision makers for effective handling of wastewater processes to reduce operational costs. Therefore, this review serves to identify the advances in anaerobic digestion of wastewater, modelling and process control of these systems from year 2000 onwards. "
7 / - / Authors proposed the implementation of control process to improve the wastewater treatment as well as the biogas production. Considering the nonlinear nature of the process (e.g., presence of inhibitors, organic matter and nutrients loading, microbial kinetics) and the different anaerobic reactors design, how can the advanced models (e.g., meta-heuristic models) successfully be applied? / The possible ways of successfully implementing the advanced models to improve wastewater treatment and biogas production were listed in the sections below:
· Mass balance model for process control is summarized and is as stated under the final paragraph of section 4.1.
· Possible ways to improve the application of ADM1 for wastewater treatment can be found in the final paragraph of section 4.2
· Similarly discussion on how meta-heuristic models can be successfully applied can be found in the final paragraph in each section of the write up (5.1, 5.2 and 5.3).
Therefore, no significant changes were made with respect to this comment.
8 / - / Page 16, lines 32-38: Authors provided the info redundant. / The sentences were revised and changed accordingly to remove redundant information.
9 / - / Figures 1-4 were not mentioned/explained/cited in the main text. / Figures 1 to 4 are now mentioned and cited in the main text respectively. These has been highlighted in the manuscript.
10 / - / Authors may consider adding long tables separately as Supplementary Materials. / Thank you for the suggestion. We have rearranged the information in the tables and they should be easier for reference to the readers.
11 / - / References were not listed following the journal instructions. The standard abbreviation of the journal might be provided. / References were edited according to the format required
12 / - / References 39 and 40 are repeated. / The extra references were removed from the manuscript
Second Revision