Comprehensive Review of Hydrothermal Pretreatment Parameters Affecting Fermentation and Anaerobic Digestion of Municipal Sludge

Round 1

Reviewer 1 Report

please find my comments in the attached file

Comments for author File: Comments.pdf

Author Response

Respond to reviewers:

Reviewer 1:

1. The topic of this manuscript is highly relevant, because there is a lot of research on HTP as pretreatment prior to anaerobic digestion. A summary of the current status is therefore of great interest to many scientists. The introduction is too extensive. It takes 78 (!) lines before the actual objective of this review is defined. Authors may presume that most of the readers are familiar with the background of HPT and AD processes and their impact to a sustainable economy. I recommend to shorten the introduction massively.

Author’s response: Authors appreciate this comment. The introduction has been shortened.

 

2. It was confusing to me that results of HTP pretreatment of sewage sludge, biowaste and municipal solid waste (MSW) are mixed. For my understanding, it makes a huge difference whether e.g. MSW, food waste or sewage sludge are matter of investigation?

Author’s response: Authors acknowledge this insightful comment. Yes, hydrothermal pre-treatment has different impacts depending on the type of waste, therefore considering the parameters affecting HTP only sludge in WAS, TWAS, and mixed sludge has been considered while only for combined pre-treatment all waste type has been studied.

 

3. The section about full-scale HTP technologies is of great interest too, but the names of the companies are highlighted very often, so that the neutral analysis seems at least questionable.

Author’s response: Thank you for the great comment. The bias has been removed by changing the language and removing the repeated company names.

Cambi Inc. has developed a thermal hydrolysis process based on the type of sludge being processed. This company reported the HTP condition at 165^ºC for a 30-minute treatment depending on the characteristics of the processed sludge. A significant improvement in biogas production by about 150% is reported. The solubilization of solids is about 30%, as per the company's website. The thermal hydrolysis process (THP) patented is a pretreatment of sludge combined with anaerobic digestion. Here, THP works by dissolving and disintegrating sludge using pressure and temperature. To increase the efficiency of THP, the sludge needs to be dewatered at around 16% solid concentration before passing to the equalization tank. The equalization tank is responsible for accommodating large flow rate variations, which is essential for thermal pretreatment to function at a constant flow rate. The pre-heated sludge was then introduced to the reactor and heated again with the higher temperature reaching 165 ^ºC and 8 to 9 bar pressure. After 30 minutes of retention time, the treated sludge is pumped into the flash tank with a solid concentration of approximately 12% to 15%. Exelys, a subsidiary of Veolia group, has reported solubilization of solid by 30% at 165 ºC for 30 minutes, increasing biogas production by 150%. When HTP is associated with conventional digestion, it also delivers around a 40% reduction of sludge which could be readily available for disposal, and a 30% more biogas production. The process works in batch mode, combining thermal hydrolysis and anaerobic digestion. The dehydrated sludge goes through a batch thermal hydrolysis phase during which steam is injected in reactors operating under specific pressure (9 bar) and temperature (165 ^ºC) conditions for approximately 30 minutes. The process handles all kinds of organic, industrial, or municipal sludge and can also handle grease. The advantages of using such thermal hydrolysis pretreatment process are reduced sludge volume, improved sludge quality, and increased biogas production.

 

4. A major weakness of this manuscript is that the cited articles are not critically reviewed. The authors merely gathered numbers and data which are summarized and presented in a structured way. This is generally okay, but inconsistencies in results, differences in methodology or contradictory results are not discussed which is a pity. Such kind of review would be even more interesting. To give one example: at P7/L251, an increase in biogas production of 67.8 % is cited. Such an increase can hardly be explained plausibly and the reference values, methodology or causes for this finding should be critically questioned. Nevertheless I appreciate the work done and recommend the manuscript for publication after minor revisions.

Author’s response: The authors appreciate the reviewer’s comment and the inconsistencies in results, differences in methodology or contradictory results are discussed in couple of places.

 

On the other hand, as shown in Table 1, there some inconsistence and contradictory result for the degree for solubilization and the variation is significant. For example, Xue et al. [38] reported a degree of solubilization of 85% which is very high compared to most of the reported in the literature (20%-50%), this was due to the high particulate SOD (pCOD) of about 158 g/L and the extended time for the pretreatment of 72 hours which is not common for the HTP. Furthermore, Zang et al. [42] reported low degree of solubilization of only 9.2% which was due to the low temperature applied of 55 ^ºC. So, the contradictions in the results in the literature are mainly due to the different in pretreatment conditions and/or the nature of the feedstock (source, solids content, pCOD content, sludge age, etc.).

 

Qiao et al. [34] also researched the evaluation of biogas production from different biomass wastes with or without hydrothermal pretreatment. After hydrothermal pretreatment at typical conditions (170 ^ºC at 60 minutes), the biogas production of municipal sewage sludge increased 67.8 % as compared with the untreated sample. This high number might be due to the nature of the sludge source as the sludge in this study was collected form a WWTP that uses a biological A/O process and membrane bioreactor which different from a typical sludge collected from an activated sludge process in term of solids content and the nature. Furthermore, the authors diluted by the sludge by adding tap water before applying the thermal pretreatment.

 

Remarks in particular

5. figure 1: axis label is “Axis Label” → please correct

Author’s response: Thanks for this comment. The axis label has been corrected.

 

6. throughout the manuscript: A space must be placed between the number and the unit, also between the number and “%”

Author’s Response: Thank you very much for the comment. The spaces have been added.

 

7. table 1: please explain abbreviations: “ND” probably stands for not determined, but how can the retention time in a batch-process be “0 min” ? I suppose the authors want to express that this parameter was not mentioned in the respective publication?

Author’s response: Authors acknowledge this comment. The table has been updated by expanding ND abbreviation and changing the 0 retention time to ND since yes, the RT was not determined by the publications.

Substrate characteristics (for the raw sample only)						HTP Condition		System Configuration		Significant Results		Reference
Substrate	TCOD	TS	VSS	pH	Solid Content	Temperature	Retention Time	Reactor mode	Temperature	Solubilization	VFAs Production
	g/L	g/L	g/L	-	%	ºC	min	B/S	M/T	%	g VFAs/L
TWAS	88.8	ND*	ND	7.6	ND	170 -320	30	Batch	Mesophilic	ND	0.58	[34]
TWAS (Lab scale)	108	99.8	75.7	ND	9.98	170	5- 30	Batch	Mesophilic	48.1	0.52	[20]
TWAS (Pilot scale)	90.17	76.8	54		7.68	170	5- 30	Batch	Mesophilic	ND	0.37
TWAS	49.6	34	22.7	6.3	3.4	150-240	5- 30	Batch	Mesophilic	49.0	2.52	[13]
TWAS	62	44.19	33.38	ND	4.4193	150-270	ND	Batch	Mesophilic	46	3.31	[17]
WAS	55.3	ND	ND	ND	ND	70-90	15-60	Batch	Mesophilic	17.8	2.74	[25]
WAS	166	167	ND	ND	16.7	60-180	15-180	Batch	Mesophilic	3.7	0.03	[22]
Urban WAS	ND	12.91	8.58	7.15	1.29	60-120	ND	Batch	Mesophilic	ND	0.23	[23]
Industrial	ND	13.44	7.92	8.07	1.34						0.28
TWAS	ND	40.59	31.86	6	4.05	120-200	60	Batch	Mesophilic	ND	2.94	[21]
Raw Sludge	68.68	60.15	45.26	6.41	15.67	155-175	30	Semi-continuous	Mesophilic	15.5	5.15	[26]
									Thermophilic	9.2	5.90
WAS	134	ND	ND	6.4		130-180	ND	Semi-continuous	Mesophilic	22.3	4.54	[30]
*ND: Not Determined

 

 

 

8. figure 4: is Wikipedia a reliable source?

Author’s response: Thanks for the insightful comment. The unreliable source has been removed.

 

Figure 4. Largest Full Scale Thermal Hydrolysis Plants as of 2021 (Source: Adoptd from company’s websites)

 

 

9. References: some references are not traceable: 4 / 7 / 17 / 31

Author’s response: We appreciate the comment. The references have been corrected.

6. Mancini, G. (2019). Different approaches to enhance the biogas production from the anaerobic digestion of lignocellulosic materials To cite this version : HAL Id : tel-02374280. Different approaches to enhance the biogas production from the anaerobic digestion of lignocellulosic materials - TEL - Thèses en ligne (archives-ouvertes.fr)
7. Abdulazeez, M. (2019). Scholarship @ Western Co-digestion of Food Waste and Thickened Waste Activated Sludge: Microbial Communities and Inhibition Control using Biochar. Available from ProQuest Dissertations & Theses A&I; ProQuest Dissertations & Theses Global. (2714864402). Retrieved from http://ezproxy.lib.ryerson.ca/login?url=https://www.proquest.com/dissertations-theses/co-digestion-food-waste-thickened-activated/docview/2714864402/se-2
8. Kim D., Lee K., and Park K. (2015). Enhancement of biogas production from anaerobic digestion of waste activated sludge by hydrothermal pre-treatment. nternational Biodeterioration and Biodegradation (2015) 101 42-46. https://doi.org/10.1016/j.ibiod.2015.03.025
9. Tran, K. C. (2017). Anaerobic digestion of microalgal biomass: effects of solid concentration and pre-treatment. University of Southampton, 182. https://eprints.soton.ac.uk/415791/ %0Ahttps://eprints.soton.ac.uk/415791/1/Final_e_thesis_for_e_prints_Khanh_Tran_24542342.pdf

Author Response File: Author Response.docx

Reviewer 2 Report

General comments:
 Manuscript ID processes-2015434. Titled "Review of Hydrothermal Pre-treatment of Thickened Waste Activated Sludge for Fermentation and Anaerobic Digestion Processes." for possible publication in processes.

This paper’s topic is interesting but authors have to proofread the manuscript and improve the English and remove grammar mistakes. The referencing inside the manuscript needs to be corrected in many places and authors must add new references. The abstract should be improved by adding current research gap on this topic also add quantitative data instead of irrelevant discussion. The introduction section also needs improvements by adding new work already published globally and in Canada. Moreover conclusion parts should be revised by adding possible suggestions for future researchers.

Specific comments:

1.      Title is not representing the research background. Title should be modified/revised.

2.      Abstract should be revised by adding some words about research gap.

3.      Also add quantitative data in abstract.

4.      Line 24; Write correct spelling of Biogas

5.      Introduction; please add global data about production of waste activated sludge and also add previous work done for processing and handling of sludge in Canada and different countries of world.

6.      Also add research gap in introduction section.

7.      Authors must add new section about all methods used in world for handling and processing of Waste sludge.

8.      Also add section about cost analysis of different methods adopted for processing of Sludge.

9.      Please include following details in Conclusion section.

·         Write quantitative data by comparing different research work already done with solid justification.

·         Add some words about novelty of your review work.

·         Give your suggestions for future research work about this topic.

Comments for author File: Comments.docx

Author Response

Respond to reviewers:

Reviewers 2:

1. This paper’s topic is interesting but the authors have to proofread the manuscript and improve the English and remove grammar mistakes.

Author’s response: Authors appreciate this feedback. Manuscript has been thoroughly reviewed for grammar check and English has been improved.

 

2. The referencing inside the manuscript needs to be corrected in many places and authors must add new references.

Author’s response: The references in the manuscript have been reviewed and corrected and new references have been added.

List of added references:

6. Ehalt MacEdo H., Lehner B., Nicell J., Grill G., Li J., Limtong A., Shakya R. (2022). Distribution and characteristics of wastewater treatment plants within the global river network. Earth Syst. Sci. Data, 14, 559–577.
7. Qadir M., Drechsel P., Jiménez Cisneros B., Kim Y., Pramanik A., Mehta P., Olaniyan O. (2020). Global and regional potential of wastewater as a water, nutrient and energy source. Nat. Resour. Forum, 44, 40–51.
8. Zhang Q., Hu J., Lee D.J., Chang Y., Lee Y.J. (2017). Sludge treatment: Current research trends. Bioresour. Technol., 243, 1159–1172.
9. Environment and Climate Change Canada. Canadian Environmental Sustainability Indicators: Greenhouse Gas Emissions; Environment and Climate Change Canada: Winnipeg, MB, Canada, 2020.
10. EPA Basic Information about Biosolids. Available online: https://www.epa.gov/biosolids/basic-information-about-biosolids (Accessed on Nov 13, 2022).
11. Shanmugam K., Gadhamshetty V., Tysklind M., Bhattacharyya D., Upadhyayula V.K.K. (2022). A sustainable performance assessment framework for circular management of municipal wastewater treatment plants. J. Clean. Prod., 339, 130657.
12. Norouzi O., Dutta A. (2022). The Current Status and Future Potential of Biogas Production from Canada’s Organic Fraction Municipal Solid Waste. Energies 2022, 15, 475. https://doi.org/10.3390/en15020475
13. Papastergiadis E., Sklari S., Zouboulis A., Chasiotis A., Samaras P. (2015). The use of steelmaking slag for sewage sludge stabilization. Desalin. Water Treat., 55, 1697–1702.
14. Kosowski P., Szostek M., Pieniazek R., Antos P., Skrobacz K., Piechowiak T., Zaczek A., Józefczyk R., Balawejder M. (2020). New approach for sewage sludge stabilization with ozone. Sustainability, 12, 886.
15. Kozak J., Wlodarczyk-Makula M., Popenda A. (2021). Impact of Aerobic Stabilization of Sewage Sludge on PAHs Concentration in Reject Waters. J. Ecol. Eng., 22, 27–35.
16. Hanum F., Yuan L.C., Kamahara H., Aziz H.A., Atsuta Y., Yamada T., Daimon H. (2019). Treatment of sewage sludge using anaerobic digestion in Malaysia: Current state and challenges. Front. Energy Res., 7, 19.
17. Water Environment Federation. Design of Municipal Wastewater Treatment Plants; Water Environment Federation: Alexandria, VA, USA, 2009; Volume 30.
18. Acharya B., Sule I., Dutta, A. (2012). A review on advances of torrefaction technologies for biomass processing. Biomass Convers. Biorefinery, 2, 349–369.
19. Libra J.A., Ro K.S., Kammann C., Funke A., Berge N.D., Neubauer Y., et al. (2011). Hydrothermal carbonization of biomass residuals: A comparative review of the chemistry, processes and applications of wet and dry pyrolysis. Biofuels, 2, 71–106.
20. Hu M., Ye Z., Zhang H., Chen B., Pan Z., Wang J. (2021). Thermochemical conversion of sewage sludge for energy and resource recovery: Technical challenges and prospects. Environ. Pollut. Bioavailab., 33, 145–163.
21. Chen Y., Yi L., Li S., Yin J., Jin H. (2020). Catalytic gasification of sewage sludge in near and supercritical water with different catalysts. Chem. Eng. J., 388, 124292.

 

55. Wei Y., Gao J., Shi Z., Li X., Ma W., Yuan H. (2022). Effect of hydrothermal pretreatment on two-stage anaerobic digestion of food waste and Enteromorpha: Digestion performance, bioenergy efficiency, and microbial community dynamics. Fuel 318 (2022) 123639. https://doi.org/10.1016/j.fuel.2022.123639
56. Cheng J., Yue L., Hua J., Dong H., Zhou J., Li Y-Y. (2020). Hydrothermal alkali pretreatment contributes to fermentative methane production of a typical lipid from food waste through co-production of hydrogen with methane. Bioresour Technol 2020;306: 123164. https://doi.org/10.1016/j.biortech.2020.123164.

 

3. The abstract should be improved by adding current research gap on this topic also add quantitative data instead of irrelevant discussion.

Author’s response: Authors appreciate this insightful comment. The abstract has been revised accordingly.

Abstract: Municipal solid waste treatment and disposal have become one of the major concerns in waste management due to the excessive production of waste and higher-level of pollution. To address these challenges and protect the environment in sustainable ways, the hydrothermal pretreatment (HTP) technique coupled with anaerobic digestion (AD) becomes a preferred alternative technology that can be used for municipal solid waste stabilization and production of renewable energy. However, the impact of HTP parameters such as temperature, retention time, pH, and solid content on the fermentation of TWAS is yet to be well studied and analyzed. Hence this study was conducted to review the effect of hydrothermal pretreatment of thickened waste-activated sludge (TWAS) on fermentation and anaerobic digestion process. Many studies reported fermentation of TWAS at pretreatment temperature ranges from 160 ^ºC to 180 ^ºC resulted in a 50 % increase in volatile fatty acids (VFAs) yield compared to no pretreatment. Whereas, for the AD process, HTP in the range of 175 ^ºC to 200 ^ºC with a 30-60 minute retention time was considered as the optimal condition for higher biogas production with 30 % increase in biodegradability, and higher than 55 % of the increase in biogas production. Even though there is a direct relationship between increased HTP temperature and the hydrolysis of TWAS, a pretreatment temperature range beyond 200 ^ºC alters the biogas production. Solid content (SC) of sludge plays a crucial role in HTP, where in practice up to 16% SC has been utilized for HTP. Further, combined alkaline-HTP enhances the process performance.

 

4. The introduction section also needs improvements by adding new work already published globally and in Canada.

Author’s response: We appreciate the reviewer’s comment, and one section has been added to the introduction for the global data about production of waste activated sludge and sludge processing and handling. New references were also added to the results section.

The volume of worldwide municipal wastewater of 360–380 km³/year was estimated to be produced in 2020 and is predicted to increase by 24% by 2030 and by 51% by 2050 [6, 7]. In 2017, the annual global municipal sewage sludge production rate was estimated to be 45 million tons as dry solid (DS) [8]. The municipal sewage sludge produced from UAS, and Canada are 14 and 1.2 million tons, respectively [9, 10]. According to Shanmugam et al. [11], global sewage sludge production is predicted to reach 127.5 million tons as DS by 2030. The most used thickening processes of municipal sewage include gravity thickening, dissolved air flotation, and rotary drum thickening, and centrifuge. The type of thickening selected is usually determined by the size of WWTP, its physical constraints, and the planned further downstream operations. Stabilization is usually achieved by chemical, biological, and thermochemical treatments [12]. The typical chemical stabilization is alkaline stabilization by adding lime, or lime and waste solid materials, to the thickened liquid sludge, to raise the pH to greater than 12 for several days [13]. Recently, ozonation and other advanced oxidation processes such as Peroxone, have been used for the chemical stabilization prior to the biological stabilization [14]. Biological stabilization can be either aerobic stabilization [15] or anaerobic stabilization [16]. According to the WEF Manual of Practice [17], AD is generally applied for WWTPs, treating wastewater incoming flows greater than about twenty million liters per day whereas AS is typically applied in smaller WWTPs. On the other hand, there are a range of thermal stabilization technologies such as drying, torrefaction, hydrothermal carbonization, hydrothermal liquefaction, pyrolysis, gasification, and incineration. Drying is an evaporation process by boiling the sludge. Torrefaction is a thermochemical process performed at 200–300 ºC, at atmospheric pressure at a relatively low residence time [18]. Hydrothermal carbonization is performed at 180–250 ºC and the residence time ranges from 0.25 to 2 h [19]. Hydrothermal treatment is performed at 300–360 ºC under pressurized water and the residence time ranges from 0.25 to1 h [20]. Pyrolysis is applying the heat in a gaseous or liquid environment without oxygen [19]. Gasification is a process that converts carbonaceous organic or fossil-based materials at high temperatures (>700 ºC) without combustion, while incineration must be performed above 850 ºC [21].

 

On the other hand, Wei et al. [55] studied the effects of HTP on the performance of two-stage anaerobic digestion of food waste.  They reported that the highest biomethane yield 591 mL/g VS was obtained at HTP temperature of 140 ^ºC, which was about 12% higher than that obtained from the untreated sample. In another study, the biomethane yield of 878 mL/gVS was achieved from the pretreated food waste with HTP at 220 ^ºC compared to 637 for the untreated [56].

 

 

5. Moreover conclusion parts should be revised by adding possible suggestions for future researchers.

Author’s response: The very insightful comment is appreciated. Recommendations have been added to the conclusion.

Conclusion

Article review made by this research reviewed the parameters affecting the hydrothermal pretreatment prior to fermentation and AD.  The impact of individual parameters such as temperature, RT, pH, SC and combined HTP and other pretreatments were analysed. According to this study, HTP improves the solubilization and biodegradability of the TWAS and enhances VFA and biomethane production.

The following major conclusions are drawn:

• HTP condition operated both in batch, and the semi-continuous reactor has increased dissolution of organic matter and suspended solids removal efficiency.
• Most of the reviewed articles on Fermentation of TWAS revealed that higher VFAs yield was observed at pretreatment temperature ranges from 160 ^ºC to 180 ^º Waste activated sludge treated with the hydrothermal pretreatment technique resulted in a 35 to 50 % increase in VFAs yield compared with the raw sample.
• Temperature, retention time, and solid content are considered the most important parameters affecting the hydrothermal pretreatment of TWAS, while the temperature is the dominant factor.
• HTP in the range of 175 ^ºC to 200 ^ºC with a 60-minute retention time was the optimal condition for increased biogas production. At the optimum condition, it was generally observed a 30 % increase in biodegradability of waste-activated sludge, which ended up in higher biogas production.
• Most of the studies reported that HTP increases the hydrolysis of WAS up to a specific temperature range. The temperature range beyond 200 ^ºC showed a significant reduction in VFAs and biogas production. In addition, a lower temperature cannot efficiently decompose the complex organics in the AD process unless combined with other pretreatment techniques.

To further understand the impact of thermal hydrolysis on fermentation and AD, further research is required to intensify these processes. There is a great potential of reducing the HRT of the AD with HTP integration therefore studying HRTs below 20 days up to 8 days would be valuable. Another gap in the literature to be filled is the different configurations of the HTP, fermentation and AD varying the HTP parameters and HRT of the fermentation and AD. 

 

Specific comments:

6. Title is not representing the research background. Title should be modified/revised.

Author’s response: Thank you for the great comment title has been revised to the following.

Comprehensive Review of Hydrothermal Pretreatment parameters Affecting Fermentation and Anaerobic Digestion of Municipal Sludge

 

7. Abstract should be revised by adding some words about research gap. Also add quantitative data in abstract.

Author’s response: Authors appreciate this insightful comment. The abstract has been revised accordingly.

Abstract: Municipal solid waste treatment and disposal have become one of the major concerns in waste management due to the excessive production of waste and higher-level of pollution. To address these challenges and protect the environment in sustainable ways, the hydrothermal pretreatment (HTP) technique coupled with anaerobic digestion (AD) becomes a preferred alternative technology that can be used for municipal solid waste stabilization and production of renewable energy. However, the impact of HTP parameters such as temperature, retention time, pH, and solid content on the fermentation of TWAS is yet to be well studied and analyzed. Hence this study was conducted to review the effect of hydrothermal pretreatment of thickened waste-activated sludge (TWAS) on fermentation and anaerobic digestion process. Many studies reported fermentation of TWAS at pretreatment temperature ranges from 160 ^ºC to 180 ^ºC resulted in a 50 % increase in volatile fatty acids (VFAs) yield compared to no pretreatment. Whereas, for the AD process, HTP in the range of 175 ^ºC to 200 ^ºC with a 30-60 minute retention time was considered as the optimal condition for higher biogas production with 30 % increase in biodegradability, and higher than 55 % of the increase in biogas production. Even though there is a direct relationship between increased HTP temperature and the hydrolysis of TWAS, a pretreatment temperature range beyond 200 ^ºC alters the biogas production. Solid content (SC) of sludge plays a crucial role in HTP, where in practice up to 16% SC has been utilized for HTP. Further, combined alkaline-HTP enhances the process performance.

 

8. Line 24; Write correct spelling of Biogas

Author’s response: Thank you for noting this. Biogas word has been corrected.

Keywords: Hydrothermal pretreatment, thickened waste activated sludge, Fermentation, Anaerobic Digestion, Volatile Fatty Acids, Biogas

 

 

9. Introduction; please add global data about production of waste activated sludge and also add previous work done for processing and handling of sludge in Canada and different countries of world. Also add research gap in introduction section.Authors must add new section about all methods used in world for handling and processing of Waste sludge. Also add section about cost analysis of different methods adopted for processing of Sludge.

Author’s response: We appreciate the reviewer’s comment, and one section has been added to the introduction for the global data about production of waste activated sludge and also add previous work done for processing and handling of sludge.

The volume of worldwide municipal wastewater of 360–380 km³/year was estimated to be produced in 2020 and is predicted to increase by 24% by 2030 and by 51% by 2050 [6, 7]. In 2017, the annual global municipal sewage sludge production rate was estimated to be 45 million tons as dry solid (DS) [8]. The municipal sewage sludge produced from UAS, and Canada are 14 and 1.2 million tons, respectively [9, 10]. According to Shanmugam et al. [11], global sewage sludge production is predicted to reach 127.5 million tons as DS by 2030. The most used thickening processes of municipal sewage include gravity thickening, dissolved air flotation, and rotary drum thickening, and centrifuge. The type of thickening selected is usually determined by the size of WWTP, its physical constraints, and the planned further downstream operations. Stabilization is usually achieved by chemical, biological, and thermochemical treatments [12]. The typical chemical stabilization is alkaline stabilization by adding lime, or lime and waste solid materials, to the thickened liquid sludge, to raise the pH to greater than 12 for several days [13]. Recently, ozonation and other advanced oxidation processes such as Peroxone, have been used for the chemical stabilization prior to the biological stabilization [14]. Biological stabilization can be either aerobic stabilization [15] or anaerobic stabilization [16]. According to the WEF Manual of Practice [17], AD is generally applied for WWTPs, treating wastewater incoming flows greater than about twenty million liters per day whereas AS is typically applied in smaller WWTPs. On the other hand, there are a range of thermal stabilization technologies such as drying, torrefaction, hydrothermal carbonization, hydrothermal liquefaction, pyrolysis, gasification, and incineration. Drying is an evaporation process by boiling the sludge. Torrefaction is a thermochemical process performed at 200–300 ºC, at atmospheric pressure at a relatively low residence time [18]. Hydrothermal carbonization is performed at 180–250 ºC and the residence time ranges from 0.25 to 2 h [19]. Hydrothermal treatment is performed at 300–360 ºC under pressurized water and the residence time ranges from 0.25 to1 h [20]. Pyrolysis is applying the heat in a gaseous or liquid environment without oxygen [19]. Gasification is a process that converts carbonaceous organic or fossil-based materials at high temperatures (>700 ºC) without combustion, while incineration must be performed above 850 ºC [21].

Please include following details in Conclusion section.

• Write quantitative data by comparing different research work already done with solid justification.
• Add some words about novelty of your review work.
• Give your suggestions for future research work about this topic.

Author’s response:

Article review made by this research reviewed the parameters affecting the hydrothermal pretreatment prior to fermentation and AD.  The impact of individual parameters such as temperature, RT, pH, SC and combined HTP and other pretreatments were analysed. According to this study, HTP improves the solubilization and biodegradability of the TWAS and enhances VFA and biomethane production.

The following major conclusions are drawn:

• HTP condition operated both in batch, and the semi-continuous reactor has increased dissolution of organic matter and suspended solids removal efficiency.
• Most of the reviewed articles on Fermentation of TWAS revealed that higher VFAs yield was observed at pretreatment temperature ranges from 160 ^ºC to 180 ^º Waste activated sludge treated with the hydrothermal pretreatment technique resulted in a 35 to 50 % increase in VFAs yield compared with the raw sample.
• Temperature, retention time, and solid content are considered the most important parameters affecting the hydrothermal pretreatment of TWAS, while the temperature is the dominant factor.
• HTP in the range of 175 ^ºC to 200 ^ºC with a 60-minute retention time was the optimal condition for increased biogas production. At the optimum condition, it was generally observed a 30 % increase in biodegradability of waste-activated sludge, which ended up in higher biogas production.
• Most of the studies reported that HTP increases the hydrolysis of WAS up to a specific temperature range. The temperature range beyond 200 ^ºC showed a significant reduction in VFAs and biogas production. In addition, a lower temperature cannot efficiently decompose the complex organics in the AD process unless combined with other pretreatment techniques.

To further understand the impact of thermal hydrolysis on fermentation and AD, further research is required to intensify these processes. There is a great potential of reducing the HRT of the AD with HTP integration therefore studying HRTs below 20 days up to 8 days would be valuable. Another gap in the literature to be filled is the different configurations of the HTP, fermentation and AD varying the HTP parameters and HRT of the fermentation and AD. 

Author Response File: Author Response.docx