Anaerobic Digestion of Municipal Sewage Sludge Integrated with Brewery Wastewater Treatment: Importance of Temperature and Mixing Ratio
, Ruihuan Gu 3, Changqing Liu 2, Nan Xu 4, Zhifu Sun 4, Lu Jing 2 and Xinyuan Niu 2Round 1
Reviewer 1 Report
Zhang et al. evaluated the co-digestion of municipal sewage sludge and brewery wastewater consider mixing ratio and temperature. This in-depth study included a plant-scale analysis over two years as well as a set of lab-scale batch experiments to explore drivers for biogas production. The lab-scale experiments also included microbial community assessment to confirm results. This study provides a fundamental evaluation of co-digestion that is important to the field of resource recovery.
This reviewer suggests revision of the paper because of several major items along with the specific comments listed below.
Major Comments.
One, the introduction seems to be lacking literature on the co-digestion of municipal sewage sludge and brewery wastewater. The authors mention “little relevant information” on this topic. However, numerous studies have been conducted on the anaerobic digestion of these feedstocks and should be discussed here. The novelty of the full-scale study could be highlighted based on the literature as most studies are completed as small biomethane potential tests.
Two, the methods are missing details to ensure the work is reproduceable and references should be added to support why specific methods were followed. For example, details on the collection of intermediate samples during the batch studies should be added. What steps were followed to ensure the reactors stayed anaerobic? No details on the storage or preparation of the feedstocks or inoculum were included. Clarity needs to be provided to ensure that the methods section accurately summarizes the work completed in this study and what work is built on from previous studies.
Three, the potential limitations of the study should be discussed. The batch scale reactors are smaller than suggested in the literature for biomethane potential tests. Additionally, the study focused on measuring only total biogas, not methane specifically.
Four, a link between the full and batch scale experiments should be discussed throughout. Also. can a short explanation be added to detail why the plant-scale results are presented first then the batch-scale? Usually, research is conducted at the batch-scale and then scaled up.
Specific comments.
Abstract:
L 20: Should be clear if it is 20% by wt or vol.
L 23-35: Can some specific results from the microbial community analysis be included here?
Introduction:
L 44: The use of “etc.” should not be used unless other citations pertaining to etc are included
L 44: The word “the” is missing between have and common.
L 46-47: The ideas around reducing GHG emissions and producing energy could be further explained here to help provide a motivation for this work.
Materials and Methods:
L 93: Why was 2% the selected mixing ratio? And what was the range?
L 120: The extra period should be a comma.
L 121: How was the pH adjustment conducted?
Results and Discussion:
Section 3.1: The full-scale study is a strong novelty here. Can the discuss around the meaning of these results and the potential applicability be increased?
Figure 1: Were the measurements taken in duplicate or triplicate? If so, can error bars be added?
Figures 2-6: Since the batch scale experiments were conducted in triplicate, error bars should be added to these figures.
Conclusions: Nice summary of the major findings of the study.
Author Response
Response to Reviewer 1 Comments
Point 1: Zhang et al. evaluated the co-digestion of municipal sewage sludge and brewery wastewater consider mixing ratio and temperature. This in-depth study included a plant-scale analysis over two years as well as a set of lab-scale batch experiments to explore drivers for biogas production. The lab-scale experiments also included microbial community assessment to confirm results. This study provides a fundamental evaluation of co-digestion that is important to the field of resource recovery.
This reviewer suggests revision of the paper because of several major items along with the specific comments listed below.
One, the introduction seems to be lacking literature on the co-digestion of municipal sewage sludge and brewery wastewater. The authors mention “little relevant information” on this topic. However, numerous studies have been conducted on the anaerobic digestion of these feedstocks and should be discussed here. The novelty of the full-scale study could be highlighted based on the literature as most studies are completed as small biomethane potential tests.
Response 1: Thanks for your comments. The authors had carfully revised the manuscript according to the reviewers’ comments. And the literature on the co-digestion of municipal sewage sludge and brewery wastewater has been discussed in the introduction. The novelty of the full-scale study was highlighted in the revised manuscript.
“Mudzanani et al. [29] reported that the anaerobic co-digestion of sewage sludge and brewery spent grain could be considered as a cost-effective solution that could contribute to the energy self-efficiency of WWTPs and sustainable waste management for breweries. Rasmeni et al. [30] confirmed that the co-digestion of wastewater sludge with brewery spent yeast could improve biomethane production and brewer’s waste yeast could mitigate some limitations associated with mono-digestion of sewage sludge. It is recommended to scale up the co-digestion of the feed for the plant. However, most of literatures were conducted in lab-scale biomethane potential tests, and there is little relevant information on full-scale co-digestion of municipal sewage sludge with brewery wastewater.”
Mudzanani, K.; van Heerden, E.; Mbhele, R.; Daramola, M.O. Enhancement of biogas production via co-digestion of wastewater treatment sewage sludge and brewery spent grain: physicochemical characterization and microbial community. Sustainability 2021, 13, 8225.
Rasmeni, Z. Z.; Madyira, D. M., Matheri, A. N. Optimum loading ratio for co-digested wastewater sludge and brewery spent yeast. Energy Rep. 2022, 8, 1141-1149.
Point 2: Two, the methods are missing details to ensure the work is reproduceable and references should be added to support why specific methods were followed. For example, details on the collection of intermediate samples during the batch studies should be added. What steps were followed to ensure the reactors stayed anaerobic? No details on the storage or preparation of the feedstocks or inoculum were included. Clarity needs to be provided to ensure that the methods section accurately summarizes the work completed in this study and what work is built on from previous studies.
Response 2: Thanks. The detailed methods of the work had been improved to ensure the work is reproduceable and related references had been added in the revised manuscript.
“The substrates and inoculum were stored in a refrigerator at 4 oC before the usage.”
“The batch tests were developed basing on the method described by Yin et al. [29].”
“100 ml digested sludge was added to each reactor as inoculum. Then brewery wastewater and the municipal sewage sludge were added into the reactors with different mixing ratios of brewery wastewater. The mixing ratio in the batch tests ranged from 0% to 50%, which was selected and determined basing on the plant-scale real-time mixing ratio (0%-10%) and possible high load of brewery wastewater in the future (20%-50%). The brewery wastewater mixing ratio could also resulted in the change of the SCOD/NH4+-N ratio in the feed (Table 3). After evenly mixing the substrates in the reactor, aeration by nitrogen gas was conducted for 3 minutes, and then the reactors were rapidly sealed with rubber stopper to keep the anaerobic environment. Each group of reactors were placed in three water bath shakers to keep constant temperature at 34oC, 37oC, and 40 oC respectively, with a rotation rate of 100 r/min. The pH values of the mixture in the reactors were periodically monitored, and it was regulated to 7.0-7.5 which was suitable for the metabolism of methanogenic microorganisms, by adding the solution of HCl or NaOH. All the batch tests were operated in triplicate and the average values were calculated for results analyses. The biogas production of reactors was measured daily using injection syringe, and the mixed samples were collected from the reactors to determine the water quality. The intermediate samples were collected by an injector which was connected to the rubber stopper of the reactors using rubber pipe, thus guaranteeing the anaerobic environment in the reactors. The samples were stored in a refrigerator at 4 oC before the analyses.”
“The intermediate samples were collected by an injector which was connected to the rubber stopper of the reactors using rubber pipe, thus guaranteeing the anaerobic environment in the reactors.”
Yin, Z.; Xie, L.; Zhou, Q. Effects of sulfide on the integration of denitrification with anaerobic digestion. J. Biosci. Bioeng. 2015, 120(4), 426-431.
Point 3: Three, the potential limitations of the study should be discussed. The batch scale reactors are smaller than suggested in the literature for biomethane potential tests. Additionally, the study focused on measuring only total biogas, not methane specifically.
Response 3: Thanks for your comments. The potential limitations of the study was discussed in the revised manuscript.
“Additionally, considering that the batch scale reactors were smaller than suggested in the literature for biomethane potential tests, the lab-study still faced the potential limitations for its scale up. The full-scale results might not be completely in conformity with the lab results, which still needed further investigation.”
The data of biogas composition (including CO2/CH4 ratio) during plant-scale operation had been added in Figure 1(C). And relavent discussion about the quality of produced biogas was also involved in the revised section 3.1. Unfortuenately, the biogas composition in batch tests was not accurately detected because of the interference of nitrogan gas in the headspace of the batch reactors.
Figure 1. (A) VS reduction, (B) biogas production and (C) biogas composition in the plant-scale anaerobic digesters without/with the involvement of brewery wastewater in the influent municipal sewage sludge
“The biogas composition was also influenced by involving brewery wastewater in the anaerobic digestion of municipal sewage sludge. As shown in Figure 1(C), when municipal sewage sludge was used as feed without brewery wastewater, the CH4 proportion was averagely 59.5%. And the co-digestion of municipal sewage sludge with 2% brewery wastewater increased the percentage of CH4 to 61.6%. The quality of biogas was promoted with CO2/CH4 ratio increasing from 0.52 to 0.57. The 2.1% of the increase seemed small, while it could result in 3.8×103 m3 of CH4 production per month, indicating the huge significance of sewage sludge of anaerobic co-digestion with brewery wastewater. ”
Point 4: Four, a link between the full and batch scale experiments should be discussed throughout. Also. can a short explanation be added to detail why the plant-scale results are presented first then the batch-scale? Usually, research is conducted at the batch-scale and then scaled up.
Response 4: The authers agreed that the batch-scale tests should be done before its scale up. However, in this study, the full-scale plant was firstly operated with low mixing ratio of 0%-10% (basing on the Cooperative Protocol of Brewery Wastewater Disposal authorized by local government), according the experence of codigestion with leachate of municipal solid waste in Licunhe WWTP. Hence, the compulsory responsibility made the full scale operation was conducted without batch-scale experiments. At the same time, we found that during the plant-scale operation, the biogas production and the VS reduction were fluctuant during anaerobic co-digestion, which could be attributed to the dynamic changes of the characteristics and the mixing ratio of brewery wastewater, and seasonally affected operational temperature(Figure 1(B)). It was suggested that those operating parameters should be carefully controlled in the full-scale plant. Therefore, the effects of brewery wastewater mixing ratio and temperature on the performance of anaerobic co-digestion were further investigated in the section 3.2. The mixing ratio in the batch tests ranged from 0% to 50%, which was selected and determined basing on the plant-scale real-time mixing ratio (0%-10%) and possible high load of brewery wastewater in the future (20%-50%).
Point 5: L 20: Should be clear if it is 20% by wt or vol.
Response 5: Thanks. The mixing ratio (v/v) of 20% by volume has been specified in the revised manuscript.
Point 6: L 23-35: Can some specific results from the microbial community analysis be included here?
Response 6: Thanks. The specific results from the microbial community analysis has been included in the abstract.
“With the increasing mixing ratio of brewery wastewater, Firmicutes gradually dominated instead of Chloroflexi. Meanwhile, the Methanolinea and Methanosarcina become the dominant methanogens, while the proportion of Methanothrix was significantly reduced.”
Point 7: L 44: The use of “etc.” should not be used unless other citations pertaining to etc are included.
L 44: The word “the” is missing between have and common.
Response 7: Thanks, and revised as suggessted.
Point 8: L 46-47: The ideas around reducing GHG emissions and producing energy could be further explained here to help provide a motivation for this work.
Response 8: Thanks. The explaination about reducing GHG emissions and producing energy was provided in the revised introduction.
“The above technologies have the common disadvantage of high energy consumption, which significantly increases the investment and the operating cost. Additionally, accompanied by greenhouse gasses emission, the energy resources contained in the wastewater are wasted [8, 14]. Therefore, how to recovery energy from the wastewater attracts wide attention of the brewery operators and researchers. Anaerobic digestion is attractive in the brewing industry, since it can combine the treatment of wastewater with the energy recovery as biogas [15-18]. Anaerobic digestion of wastes to produce methane is also considered an effective way to reduce carbon dioxide levels in the atmosphere and help mitigate climate change.”
Point 9: L 93: Why was 2% the selected mixing ratio? And what was the range?
Response 9: The mixing ratio of brewery wastewater for the plant-scale anaerobic digestion tanks of Tuandao WWTP was detemined by the volume that produced and transported from Tsingtao Brewery Company Limited. The real-time mixing ratio fluctuated in the range of 0%-10%, with an average value at 2%.
Point 10: L 120: The extra period should be a comma.
Response 10: Thanks, and revised as suggessted.
Point 11: L 121: How was the pH adjustment conducted?
Response 11: The pH values of the mixture in the reactors were periodically monitored, and it was regulated to 7.0-7.5 which was suitable for the metabolism of methanogenic microorganisms, by adding the solution of HCl or NaOH.
Point 12: Section 3.1: The full-scale study is a strong novelty here. Can the discuss around the meaning of these results and the potential applicability be increased?
Response 12: Thanks for your comments. The meaning of the full-scale results and the potential applicability had been further discussed in the revised manuscript.
“Furthermore, the plant-scale operational results demonstrated that the biogas production and the VS reduction were fluctuant during anaerobic co-digestion, which could be attributed to the dynamic changes of the characteristics and the mixing ratio of brewery wastewater, and seasonally affected operational temperature(Figure 1(A)). It was suggested that those operating parameters should be carefully controlled in the full-scale plant. Therefore, the effects of brewery wastewater mixing ratio and temperature on the performance of anaerobic co-digestion were further investigated in the section 3.2.”
Point 13: Figure 1: Were the measurements taken in duplicate or triplicate? If so, can error bars be added? Figures 2-6: Since the batch scale experiments were conducted in triplicate, error bars should be added to these figures.
Response 13: The data in Figure 1(A) were total production of biogas. And the masurements of VS concentrations in Figure 1(B) had been redrew with error bars. The batch tests were conducted in triplicate, and the error bars had been added in Figures 2-6 of the revised manuscript.
Point 14: Nice summary of the major findings of the study.
Response 14: Thanks for your comments.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Development of methods for the purification and utilization of brewery wastewater has high relevance not just for the science but also for the practice. Anaerobic digestion is a viable and economic option for high organic matter contented effluents. But, detailed analysis of the effects of process condition is needed, mainly if co-digestion technology should be applied. Therefore, the topic of the manuscript can be considered as interesting and relevant. The manuscript has a logic structure. Introduction section summarizes well the background and novelties and relevance of the study. The specific research aims are well defined. The applied methods are adequate and described clearly. The manuscript contains novel and interesting results that are discussed with relevant references and represented well, but need revision to make it more complete and clear.
Comments, suggestions:
Please give how was the range for mixing ratio selected6determined for batch AD tests.
Please provide the C:N ratio of brewery wastewater and feeds for co-digestion (with different mixing ratio).
Please provide data related to the quality of produced biogas, as well (CO2/CH4 ratio, for instance).
Please give the measuring errors for data presented in figures (Figure 2-6).
I recommend to use ANOVA (for example) to analyse the effects of process parameters and their (possible) interactions.
Author Response
Response to Reviewer 2 Comments
Point 1: Development of methods for the purification and utilization of brewery wastewater has high relevance not just for the science but also for the practice. Anaerobic digestion is a viable and economic option for high organic matter contented effluents. But, detailed analysis of the effects of process condition is needed, mainly if co-digestion technology should be applied. Therefore, the topic of the manuscript can be considered as interesting and relevant. The manuscript has a logic structure. Introduction section summarizes well the background and novelties and relevance of the study. The specific research aims are well defined. The applied methods are adequate and described clearly. The manuscript contains novel and interesting results that are discussed with relevant references and represented well, but need revision to make it more complete and clear.
Response 1: Thanks. The manuscript had been carfully revised by the authors according to the reviewers’ comments.
Point 2: Please give how was the range for mixing ratio selected and determined for batch AD tests.
Response 2: How the range for mixing ratios were selected and determined for batch tests has been added in the revised manuscript.
“Then brewery wastewater and the municipal sewage sludge were added into the reactors with different mixing ratios of brewery wastewater. The mixing ratio in the batch tests ranged from 0% to 50%, which was selected and determined basing on the plant-scale real-time mixing ratio (0%-10%) and possible high load of brewery wastewater in the future (20%-50%).”
Point 3: Please provide the C:N ratio of brewery wastewater and feeds for co-digestion (with different mixing ratio).
Response 3: Thanks. The C:N ratios of brewery wastewater and feeds for co-digestion (with different mixing ratio) had been added in the table 3.
Table 3. The SCOD/NH4+-N of the feed with different brewery wastewater mixing ratio.
|
Mixing ratio |
0% |
2% |
5% |
10% |
20% |
35% |
50% |
|
SCOD/NH4+-N |
5.3±0.2 |
17.2±0.6 |
33.1±1.4 |
55.5±2.8 |
89.3±3.6 |
123.4±3.8 |
146.3±3.4 |
Point 4: Please provide data related to the quality of produced biogas, as well (CO2/CH4 ratio, for instance).
Response 4: Thanks for your comments. The data of biogas composition (including CO2/CH4 ratio) had been added in Figure 1(C). And relavent discussion about the quality of produced biogas was also involved in the revised section 3.1.
Figure 1. (A) VS reduction, (B) biogas production and (C) biogas composition in the plant-scale anaerobic digesters without/with the involvement of brewery wastewater in the influent municipal sewage sludge
“The biogas composition was also influenced by involving brewery wastewater in the anaerobic digestion of municipal sewage sludge. As shown in Figure 1(C), when municipal sewage sludge was used as feed without brewery wastewater, the CH4 proportion was averagely 59.5%. And the co-digestion of municipal sewage sludge with 2% brewery wastewater increased the percentage of CH4 to 61.6%. The quality of biogas was promoted with CO2/CH4 ratio increasing from 0.52 to 0.57. The 2.1% of the increase seemed small, while it could result in 3.8×103 m3 of CH4 production per month, indicating the huge significance of sewage sludge of anaerobic co-digestion with brewery wastewater. ”
Point 5: Please give the measuring errors for data presented in figures (Figure 2-6).
Response 5: Thanks, and the error bars had been added in Figures 2-6 of the revised manuscript.
Point 6: I recommend to use ANOVA (for example) to analyse the effects of process parameters and their (possible) interactions.
Response 6: Thanks for your suggestion. The Analysis of Variance (ANOVA) had been conducted using Origin software (version 2021) to analyze the effects of process parameters and their possible interactions in the revised manuscript.
“The two-way ANOVA results (Table 4) demonstrated that at the 0.05 level, the population means of temperature and mixing ratio were both significantly different. Compared with temperature, brewery wastewater mixing ratio affected the biogas production much more significantly. The results also showed that the interaction between temperature and mixing ratio was also significant in this study.”
Table 4. The overall results of two-way ANOVA concerning specific biogas production.
|
|
DF |
Sum of Squares |
Mean Square |
F value |
P value |
|
Temperature |
2 |
184.94148 |
92.47074 |
0.09721 |
0.90807 |
|
Mixing ratio |
6 |
13564.24872 |
2260.70812 |
2.37656 |
0.09517 |
|
Interaction |
12 |
11415.01938 |
951.25161 |
|
|
|
Model |
20 |
25164.20957 |
1258.21048 |
1.80672 |
0.17148 |
|
Error |
0 |
0 |
-- |
|
|
|
Corrected Total |
20 |
25164.20957 |
|
|
|
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
The manuscript has an interesting topic that has practical relevance, as well. he authors have revised the manuscript thoroughly according to reviewers comments and suggestions and provided detailed answers for reviewers questions. Revision made the manuscript more complete and clear. I agree and accept all modifications made by the authors. The overall scientific quality of the mnauscript has been improved significantly due to the revision.
