Bio-Based Processes for Material and Energy Production from Waste Streams under Acidic Conditions
, Merve Atasoy 1,2, Adam Cenian 3, Gaweł Sołowski 3, Janja Trček 4,5, Aysenur Ugurlu 6 and Jana Sedlakova-Kadukova 7Round 1
Reviewer 1 Report
Dear Authors,
I revised the manuscript “Bio-based Production From Waste Streams Under Acidic Conditions” submitted to the Fermentation Journal. The paper is interesting. The article discusses the issue of obtaining, in acidic processes, valuable chemical compounds such as: organic acids, biomethane, biohydrogen and some metals. These are very important processes when it comes to environmental protection and sustainable development.
However, I have some concerns, which need to be addressed.
2.2. Aerobic volatile fatty acid production by pure culture fermentation
Line. 228-229. Please specify the filtration parameters (pore size, efficiency, type of membrane or filter, etc.).
Line 237-239. Please explain what the two-stage oxygen supply method is.
2.3. Biomethane
Line. 257-258. I am asking for the given strains resistant to pH 4.5.
Line. 259-260. Please provide the time of acclimatization of methanogens.
2.4. Biohydrogen
Line. 315. Figure S1 is missing from the article. Please add it.
Line. 320. Figure S2 is not included in the text. Please add it.
Line. 334-335. Please explain what is the difference between milling and comminution and grinding.
Line 354. Please give the chemical formula of these iron salts.
Line. 363-364. On Line 357-358 it is already written, don't repeat it.
5. Conclusions
The conclusions are correctly formulated and related to the research findings.
Author Response
General Response:
The comments and suggestions of the reviewers are much appreciated by the authors. These comments lead to interesting discussions among the authors. All comments have been given serious consideration and served as guidelines for revising the manuscript.
The authors are now of the view that the manuscript, revised in accordance with comments received has been significantly improved and it is hoped that the revised version meets the expectations of the journal for publication.
Reviewer 1
2.2. Aerobic volatile fatty acid production by pure culture fermentation
Comment 1.1:
Line. 228-229. Please specify the filtration parameters (pore size, efficiency, type of membrane or filter, etc.).
Response 1.1: This information has now been added.
Comment 1.2: Line 237-239. Please explain what the two-stage oxygen supply method is.
Response 1.2: The two stage oxygen supply has been now explained by adding: In this process, the aeration rate 0.1 vvm was for 9 hours and then raised to 0.15 vvm. In this way, a 20.78% higher production rate was achieved than at the 0.1 vvm one-stage aeration process.
2.3. Biomethane
Comment 1.3: Line. 257-258. I am asking for the given strains resistant to pH 4.5.
Response 1.3 Although the optimum pH for growth of Methanosarcina sp. is near neutrality, it was shown that some of its strains Methanosarcina barkeri and Methanosarcina vacuolata which were acidophilic could withstand pH as low as 4.3 while growing best at pH 5 [53]. Methanobacterium sp. has ability to grow at a pH as low as 3.8 by using H2 and CO2 as sole source of carbon and energy (Kotsyurbenko et al., 2007).
Kotsyurbenko, O. R. ; Friedrich, M. W.; Simankova, M. V.; Nozhevnikova, N.; Golyshin, N.; Timmis, K. N.; Conrad, R. Shift from Acetoclastic to H2-Dependent Methanogenesis in a West Siberian Peat Bog at Low pH Values and Isolation of an Acidophilic Methanobacterium Strain. Applied and Environmental Microbiology 2007,73(7):2344-8, DOI: 10.1128/AEM.02413-06
Comment 1.4: Line. 259-260. Please provide the time of acclimatization of methanogens.
Response 1.4 It has been demonstrated that acclimation of methanogens under a sudden shock of low pH could enrich a hydrogenotrophic methanogen-dominant culture, whereas mixed acetotrophic and hydrogenotrophic methanogens enriched under prolonged and gradual decrease of pH.
Wang, C; Li, Y., Sun,Y. Acclimation of Acid-Tolerant Methanogenic Culture for Bioaugmentation: Strategy Comparison and Microbiome Succession, ACS Omega, 2020. 5, 11, 6062–6068, doi:10.1021/acsomega.9b03783
2.4. Biohydrogen
Comment 1.6: Line. 315. Figure S1 is missing from the article. Please add it.
Line. 320. Figure S2 is not included in the text. Please add it.
Response 1.6:
Figure S1 and S2 have been added in Supplement
Comment 1.7: Line. 334-335. Please explain what is the difference between milling and comminution and grinding
Response 1.7:
The milling and grinding are processes usually similar to corn milling or coffee grinding but comminution is more general term related also to processes using e.g. high-pressure waterjets. We propose to change the sentence in line 333-335 to
“The authors consider various lignocellulose pretreatment methods including physical (using grinder or comminution e.g. by waterjets, etc.), … “
Comment 1.8: Line 354. Please give the chemical formula of these iron salts.
Response 1.8:
Cieciura-Włoch et al. (2020) studied influence of FeSO4, FeCl3 salts. We added to the text:
“… whereas iron salts (FeSO4, FeCl3) …”
Comment 1.9: Line. 363-364. On Line 357-358 it is already written, don't repeat it.
Response 1.9:
It was a summary statement but we deleted as you advised.
- Conclusions
Comment 1.10: The conclusions are correctly formulated and related to the research findings.
Response 1.10: The comment is highly appreciated and the conclusion part is merged to future perspective where the author had provided research findings. The revised version based on the reviewer’s comment as given below:
Industrial scale biobased chemical production is compulsory for a sustainable and environmentally friendly chemical industry. Furthermore, waste streams offer a valuable feedstock for biobased chemical production to sustain circular economy objectives. Nevertheless, the bioproduction process must be optimized for efficient and economical production and acidic conditions have several advantages for biobased chemical production depending on the product type: In biohydrogen production and bioleaching, acidic conditions are the force majeure. In organic acid production, acidic pH provides easier operation and less chemical consumption for pH regulations. This review paper aimed to provide a comprehensive perspective for acidic conditions on the production of the most promising biobased chemicals.
Organic acids, specifically VFA, have gained worldwide attention as promising biobased products for resource recovery approaches from waste streams. It is expected to increase considerably in the next decade due to the objectives of both the Horizon Europe 2030 and the EU Green Deal – as well as national programs and agreements. From the view of VFA production from waste streams, acidogenic conditions have importance regarding process efficiency and product profile. Despite that VFA cannot be produced under pH 4.5 by mixed culture, pH 5 enhance VFA production for long term reactor operation and it provides chain elongation conditions. Nevertheless, further studies are required for process optimization to increase production efficiency and to produce target products by tailor-made process design.
Aerobic production of acetic acid by pure cultures is a promising alternative to mixed cultures, due to acido-tolerant mechanisms present in certain bacterial species of the genera Komagataeibacter and Acetobacter. Moreover, some strains even increase acetic acid production rate at lower pH. However, further technological improvements are needed for this microbial process, especially due to necessity of constant oxygen supply to bacteria producing acetic acid.
Inoculating certain microorganisms for bioaugmentation will improve the process efficiency of anaerobic digestion by increasing the resistance of microorganisms to organic or hydraulic overloading conditions, and/or improving methane production and decreasing the start-up period of a bioreactor. Application of pretreatment and bioaugmentation in combination with lignocellulosic substrates can enhance methane yields significantly. Besides, injecting hydrogen gas into the reactor for biogas upgrading can also enrich hydrogenotrophic methanogens which also prevents reactor failure during instabilities.
Biohydrogen could play important role in future green energetics as well as the basic substrate for the chemical industry. Before one should solve the problem of stable operation of the dark fermentation process. However, it was already confirmed that some biogas plants (with acidic pretreatment) might be easily transformed for the efficient production of cheap hydrogen. In order to stabilize the hydrogen production in industrial installations, low pH control during process operation and an efficient hydrogen release system are necessary. Also, further research on optimal fermentation conditions for various available biowastes is recommended.
The exploitation of bacterial leaching is growing in the metal recycling sector owing to its low energy and capital input, mild reaction conditions and environmentally friendly operations. It offers a competitive alternative for recycling complex and refractory metal-bearing solid wastes. a Based on the process advantages, metal bioleaching from secondary resources is considered a significant area of research for securing critical metals for the European market and developing processes adopting circular economy principles, especially in combination with other biological processes for metal extraction from solutions such as bioaccumulation or biosorption. However, there are still several limitations that need to be solved such as the longer time necessary for the process compared to the conventional processes, lower efficiency due to metal toxicity to bacteria etc. Application of thermophilic bacteria, indirect bioleaching using spent bioleaching media or application of voltage can increase the efficiency considerably and shorten the time required for the process.
Author Response File: 
Author Response.docx
Reviewer 2 Report
The manuscript entitled “Bio-based production from waste streams under acidic conditions” reviews selected bioprocesses (organic acid production, biomethane, biohydrogen and metal leachates) carried out at low pH values, focusing also on waste streams as fermentation substrate. The manuscript is in general well-written and structured. However, the main point/scope of the review is unclear. Is it in general fermentations carried out at low pH? Why the selection of this specific process parameter? What are the economics behind it?
In the Introduction section the authors have stated the following: “Acidity is one of the major factors… as well as laboratory scale”. In my opinion, this sentence contains the essence of this manuscript and the authors should elaborate on this point in the main body of the text.
Why were these specific products selected?
In the title the terms “products” or “energy” or “materials” should be also added because “bio-based production” is too general.
There are several publications dealing with succinic acid and lactic acid production under moderate acidic conditions that should be included. Some examples are the following:
https://doi.org/10.1002/jctb.6587
https://doi.org/10.1021/acssuschemeng.1c03447
https://doi.org/10.1002/bit.26745
https://doi.org/10.1016/j.biortech.2019.121540
https://doi.org/10.1002/bit.22859
https://doi.org/10.1016/j.ymben.2017.06.007
Why these organic acids were not included?
Lignocellulosic biomass is currently the main alternative, renewable source under study. The author should discuss the possibility an potential to exploit this abundant waste stream under acidic conditions.
What is the main conclusion on this manuscript? Are there some bioprocesses that present real advantages at acidic conditions? A Table summarizing product yields and probably some economics would be valuable. What is the actual gain in the framework of bioeconomy and circular economy?
Author Response
General Response:
The comments and suggestions of the reviewers are much appreciated by the authors. These comments lead to interesting discussions among the authors. All comments have been given serious consideration and served as guidelines for revising the manuscript.
The authors are now of the view that the manuscript, revised in accordance with comments received has been significantly improved and it is hoped that the revised version meets the expectations of the journal for publication.
Reviewer 2
The manuscript entitled “Bio-based production from waste streams under acidic conditions” reviews selected bioprocesses (organic acid production, biomethane, biohydrogen and metal leachates) carried out at low pH values, focusing also on waste streams as fermentation substrate. The manuscript is in general well-written and structured.
Comment 2.1: However, the main point/scope of the review is unclear. Clarify the scope of both organic acids, biohydrogen and bioleaching.
Response 2.1: For acetic acid the following has been added: The global acetic acid market size was valued at USD 8.92 billion in 2019 and is expected to grow at a compound annual growth rate (CAGR) of 5.2% from 2020 to 2027.
The global bioleaching market has reached 15 million in 2020 and is expected to grow at 23 million in period 2021-2027 and reach CAGR at 4.9% during this period.
Hydrogen is a prime raw material for many industrial processes, and a possible energy storage in ‘green economy’. In 2020, roughly 87 million tons of hydrogen was produced worldwide; the production may increase up to 240 million tons in 2050.
Comment 2.2: Is it in general fermentations carried out at low pH? Why the selection of this specific process parameter? What are the economics behind it?
Response 2.2: Thank you for the comment. In general, the pH range for fermentation depends on the bioproduct type. The lower pH (pH 5) has a positive effect on the yield of volatile fatty acids, whereas, it is necessary for biohydrogen production and bioleaching processes. The following section was added into the manuscript to make this part complete:
“In biohydrogen production and bioleaching, acidic conditions are the force majeure. In organic acid production, acidic pH provides easier operation and less chemical consumption for pH regulations.”
Comment 2.3: In the Introduction section the authors have stated the following: “Acidity is one of the major factors… as well as laboratory scale”. In my opinion, this sentence contains the essence of this manuscript and the authors should elaborate on this point in the main body of the text.
Response 2.3: We provided this information based on the comments in Comment 2.2.
“In biohydrogen production and bioleaching, acidic conditions are the force majeure. In organic acid production, acidic pH provides easier operation and less chemical consumption for pH regulations.”
Comment 2.4: Why were these specific products selected?
Response 2.4: The following sentence is added to the manuscript to elucidate the selection of bioproducts.
“These products are chosen based on high transferability potential for biobased products, technology readiness level (TRL) of biobased production processes, and market demand for the products.”
Comment 2.5: In the title the terms “products” or “energy” or “materials” should be also added because “bio-based production” is too general.
Response 2.5: The title is revised according to the comment. The revised title:
Bio-based Processes for Material and Energy Production From Waste Streams Under Acidic Conditions
Comment 2.6: There are several publications dealing with succinic acid and lactic acid production under moderate acidic conditions that should be included. Some examples are the following:
https://doi.org/10.1002/jctb.6587
https://doi.org/10.1021/acssuschemeng.1c03447
https://doi.org/10.1002/bit.26745
https://doi.org/10.1016/j.biortech.2019.121540
https://doi.org/10.1002/bit.22859
https://doi.org/10.1016/j.ymben.2017.06.007
Why these organic acids were not included?
Response 2.6: The comment is very appreciated. As described in the manuscript (also as given below), lactic acid and succinic acid are very important groups of organic acids. Nevertheless, these acids are produced majorly by biobased methods. Other organic acids, especially volatile fatty acids, are mainly (more than 90%) produced by petrol-based methods. Hence, this manuscript focuses on organic acids with high potential for industrial-scale biobased products that are yet to be revealed.
“Organic acids production (specifically of short chain fatty acids) that is a typical example of the first group may be covered by bio-based methods, which fully produce some organics acids such as lactic, itaconic, citric and gluconic acids. However, more than 90% of other acids are still produced by petrol-based methods [8].”
Comment 2.7: Lignocellulosic biomass is currently the main alternative, renewable source under study. The author should discuss the possibility an potential to exploit this abundant waste stream under acidic conditions.
Response 2.7: The comment is very appreciated. The manuscript is revised by adding following parts:
VFA production:
“Another promising waste stream for VFA production is lignocellulosic biomass wastes due to their valuable composition and enormous production amount (Clauser et al., 2021). According to Sun et al. (2021), pH is a critical operational parameter in VFA production from lignocellulosic biomass wastes. They stated that depending on the fermentation pathway, pH directly affects the acid profile (Sun et al., 2021).”
Biomethane:
The lignocellulosic material such as agricultural residues and microalgae contains polymers like cellulose, hemicellulose and lignin has great biogas production potential. Since the cellulose is strongly cross linked and shielded by lignin, the accessibility of enzymes and microorganisms to the cellulose is hindered [61]. This recalcitrant characteristic of the lignocellulosic biomass makes it resistant to physical, chemical, or biological degradation. Therefore, its breakdown is usually the major limiting factor for biogas yield and requires an affective pretreatment to improve the performance of anaerobic digestion. Application of physical, chemical or biological pretreatment methods or their combinations can help disruption of this rigid cell wall to improve biogas production. The high energy cost and residence time and the production of inhibitory products during processing make pretreatment a challenging task. Biological pretreatment methods are considered as a more sustainable alternative to energy-intensive physicochemical pretreatments.
Biohydrogen:
“Biohydrogen is produced during dark fermentation, a type of anaerobic digestion process (Figure 2), which converts various substrates (including lignocellulosic, municipal solid, agricultural and food industry waste, sewage sludge, etc.) into hydrogen, carbon dioxide, and low organic acids, e.g., butyric, acetic, propionic, caproic, or lactic (Sarkar et al., 2021). In the case of lignocellulosic waste a proper pretreatment method is needed, as discussed below. … “
Comment 2.8: What is the main conclusion on this manuscript? Are there some bioprocesses that present real advantages at acidic conditions? A Table summarizing product yields and probably some economics would be valuable. What is the actual gain in the framework of bioeconomy and circular economy?
Response 2.8: The authors are appreciated those comments. The conclusion is merged to Future Perspective where the authors have provided main conclusion points for each process. The most important conclusion is:
….., the bioproduction process must be optimized for efficient and economical production and acidic conditions have several advantages for biobased chemical production depending on the product type: In biohydrogen production and bioleaching, acidic conditions are the force majeure. In organic acid production, acidic pH provides easier operation and less chemical consumption for pH regulations.
In addition, the authors aimed to provide a table that summarizes operational conditions, product yield and profile, market demand and size, for each bioproduct and bioprocess however such a table did not seem realistic in light of the number of published studies.
Round 2
Reviewer 1 Report
Dear Authors,
I have revisited the manuscript “Bio-based Processes for Material and Energy Production From Waste Streams Under Acidic Conditions” submitted to the Fermentation Journal. The work has been improved according to my recommendations. The article is fully suitable for publication in Fermentation Journal.
I only have one comment.
2.2. Aerobic volatile fatty acid production by pure culture fermentation
Line. 244. Correct citation according to journal guidelines.
Reviewer 2 Report
The authors have revised their manuscript adequately.
