A Short Review on Dye-Wastewater Valorization Using Up-Flow Anaerobic Sludge Blanket Reactors

Round 1

Reviewer 1 Report

This manuscript provides a review of the application of UASB in the stabilisation of dye wastewater, highlighting the potential for bioenergy and clean water in the treatment of dye-containing wastewater. This work is of some relevance to the development of the field, but as a review article, it is relatively weak and somewhat one-sided in its main content, and modest revisions are recommended. The main shortcomings are listed below:

Point 1:The abstract needs to be moderately revised. The main points about the contribution of UASB to the application of dye wastewater stabilisation and its relationship with environmental factors in different studies should be clearly presented.

Point 2:A deeper and speculative interpretation of the presented data is needed. The paper remains rather descriptive, and the results are not fully explored. A more quantitative understanding of the various regulating factors is largely lacking.

Point 3:Format and grammar mistakes need to be fixed.

Point 4:The author's views in sections 2 and 3 are not sufficiently distinctive.

Point 5:Recent advances in the theory of sludge granulation in UASB reactors in Section 2 can be further summarised.

Point 6:The main point of this paper is what is mentioned in lines 18-19: "However, data on full-scale UASB treating dye wastewater are missing.." However, the paper cited below is comprehensively summarised, thoroughly analysed and has sufficient data. Therefore, the purpose of writing this paper is not outstanding, and it is recommended to re-analyse and summarise the relevant issues in depth.

Point 7:Lines 124-126,”For optimal results, operating the UASB reactor at 30°C and 40°C, with HRT ranging from 5 to 20 hours, and an OLR of 2 to 15 kg COD m-3 d-1 is recommended.”Please provide relevant literature in support.

Point 8:Lines 188-189,”Other bench-scale studies reported biomethane production rates ranging from 0.36 to 2.7 L per day.”Literature cited is not sufficiently original, suggesting that more recent literature be added.

Point 9:The title of part 4 does not summarise the section very accurately and it is suggested that it be re-summarised.

Point 10:The discussion of the ultrafiltration nanofiltration section in section 5.2 is short, and it is recommended that more literature be cited to further summarise echoing the abstract.

It is recommended that the manuscript be carefully touched up and that grammatical, stylistic and spelling errors found in the manuscript be corrected.

Author Response

Responses to the comments of Reviewer #1

Reviewer #1: This manuscript provides a review of the application of UASB in the stabilisation of dye wastewater, highlighting the potential for bioenergy and clean water in the treatment of dye-containing wastewater. This work is of some relevance to the development of the field, but as a review article, it is relatively weak and somewhat one-sided in its main content, and modest revisions are recommended. The main shortcomings are listed below.

Response: The authors appreciate the reviewer's positive feedback and suggestions for enriching the manuscript.

Point 1: The abstract needs to be moderately revised. The main points about the contribution of UASB to the application of dye wastewater stabilisation and its relationship with environmental factors in different studies should be clearly presented.

Response: Thanks for your comment. Following the reviewer's suggestion, the abstract was revised to highlight the relevant issue of the present review paper.

"Dye-containing effluent generated in textile industries is polluting and complex wastewater. It should be managed adequately before the final destination. The up-flow anaerobic blanket (UASB) reactor application is an eco-friendly and cost-competitive treatment. The present study briefly reviews the UASB application for dye wastewater valorization. Bioenergy and clean water production potential during dye-containing wastewater treatment are emphasized to promote resource recovery in textile industries. Complex dye structures can hinder biomineralization. Pretreatment may be necessary to reduce dye concentration. Carbon source and redox mediators are added to the UASB reactor to expedite kinetic reactions. Hydraulic retention time (HRT), organic loading rate (OLR), pH, temperature, and hydraulic mixing influence sludge granulation, microbial activity, and dye removal. HRT and OLR range of 6 – 24 h and 1 – 12 kg m^-3 d^-1 of chemical oxygen demand (COD) at mesophilic temperature (30 – 40ºC) are recommended for efficient treatment. In these conditions, efficiencies of color and COD of 50–97% and 60–90% are reported in bench-scale UASB studies. A biogas yield of 1.48 – 2.70 L d^-1 in UASB, which treats dye-containing effluents, is documented. Cotreatment of dye wastewater and locally available substrate could increase biogas productivity in UASB reactors. Organic waste generated in the textile industry, such as dye sludge, cotton, and starch, is recommended to make cotreatment cost-competitive. Bioenergy production and water reuse allow environmental and economic benefits. However, data on full-scale UASB treating dye wastewater are missing. Studies on combined systems integrating UASB and membrane processes, such as ultrafiltration and nanofiltration, for the production of reusable water and pretreatment of wastewater and sludge for improvements in biogas production might realize the complete potential for resource recovery of UASB technology. UASB bioenergy usage for integrated treatment trains can reduce operating costs and assist process sustainability in the textile industry."

Point 2: A deeper and speculative interpretation of the presented data is needed. The paper remains rather descriptive, and the results are not fully explored. A more quantitative understanding of the various regulating factors is largely lacking.

Response: Thanks for your comment. Following the reviewer's comment on points #4, the authors elaborated on sections 3 and 4 to include quantitative aspects of various influencing factors for UASB operation and dye removal. In addition, data in section 4 were interpreted deeply according to subsequent comments.

"Based on data from Table 2, HRT and OLR ranges of 6 – 24 h and 1 – 12 kg COD m^-3 d^-1 at mesophilic temperature are recommended for efficient treatment. Operating conditions are similar to those previously discussed in the literature when treating diverse wastewater."

"On the other hand, amine byproducts have substituents with nitro and sulfonic groups, making their mineralization hamper in an aerobic environment. Romero-Soto et al. [50] investigated sequential UASB and electrochemical (EC) systems for Cong Red (CR) removal. COD and CR removals were >92% and >98% using UASB+electrocoagulation and >99% and >99% when UASB+electro-oxidation was employed. Results are promising to be used in dye wastewater treatment for removing byproducts that arise from UASB treatment. Still, despite the wide-range removal of pollutants, easy construction, and operating simplicity, technological developments of EC systems are needed to reduce energy consumption and electrode replacement in full-scale plants [54]."

Romero-Soto, I.C.; García-Gómez, C.; Álvarez-Valencia, L.H.; Meza-Escalante, E.R.; Leyva-Soto, L.A.; Camacho-Ruiz, M.A.; Concha-Guzmán, M.O.; Ulloa-Mercado, R.G.; Díaz-Tenorio, L.M.; Gortáres-Moroyoqui, P. Sequential Congo Red Elimination by UASB Reactor Coupled to Electrochemical Systems. Water 2021, 13, 3087, doi:10.3390/w13213087.

Akter, S.; Suhan, M.B.K.; Islam, M.S. Recent Advances and Perspective of Electrocoagulation in the Treatment of Wastewater: A Review. Environ. Nanotechnology, Monit. Manag. 2022, 17, 100643, doi:10.1016/j.enmm.2022.100643.

Point 3: Format and grammar mistakes need to be fixed.

Response: Thanks for your comment. The manuscript was revised for grammar and correction purposes.

Point 4: The author's views in sections 2 and 3 are not sufficiently distinctive.

Response: Thanks for your comment. After taking the reviewer's suggestions, sections 2 and 3 were complemented and extended, making them distinctive. In addition, sub-items were included in section 3 for clarity.

Point 5: Recent advances in the theory of sludge granulation in UASB reactors in Section 2 can be further summarised.

Response: Thanks for your comment. The authors included the recent advances in sludge granulation in UASB reactors accordingly.

"In contrast, developing anaerobic granular sludge requires 2 to 8 months, leading to an extended initiation phase for the bioreactor—a notable challenge inherent to UASB technology [18]. Hulshoff Pol et al. [19] thoroughly examined theories on sludge granulation within UASB reactors, ultimately discerning the pivotal role of incorporating inert support particles in conjunction with operational conditions in the genesis of granular sludge. Likewise, a hypothesis suggesting that granulation is an inherent defensive response of microorganisms against external stresses is presented in the literature [19]. Such stresses could be manipulated by regulating reactor operational conditions to stimulate the development of granules. It was reported that rapid growth of granules could be achieved through particle agglomeration of the flocculant sludge induced by hydraulic stress. In UASB, the up-flow liquid provides a selection pressure by washing out light and dispersed particles while retaining denser biomasses. Thus, controlling up-flow liquid velocity could be critical for granule formation [20].

As mentioned, the issue of sludge granulation relies on the extensive reactor's start-up time to develop granules. In this sense, one effective method for a rapid start-up is acquiring healthy granules from other reactors and using them as the inoculum. However, the availability of granular sludge may be limited, and the expenses for acquiring and transporting the granules can hamper it. Other possible ways to accelerate the start-up include supplementing chemicals and polymers or stressing the loading rate [20]. It was recently demonstrated that chemical addition could stimulate sludge granulation. Calcium sulfate (CaSO₄) and polymers were used to enhance granulation during the treatment of phenolic wastewater in UASB reactors. The CaSO₄ improved the granulation rate as nuclei and the subsequent dissolution of CaSO₄ improved methanogen activity. Utilization of CaSO4 and polymers enhanced the microbial diversity. The formed granules had a large particle size (> 0.25 mm), great settleability, and high methanogenic activity [21]."

Point 6: The main point of this paper is what is mentioned in lines 18-19: "However, data on full-scale UASB treating dye wastewater are missing." However, the paper cited below is comprehensively summarised, thoroughly analysed and has sufficient data. Therefore, the purpose of writing this paper is not outstanding, and it is recommended to re-analyse and summarise the relevant issues in depth.

Response: Thanks for your comment. The mentioned sentence was deleted from the abstract. The abstract was revised to highlight the relevant issue of the present review paper.

"Dye-containing effluent generated in textile industries is polluting and complex wastewater. It should be managed adequately before the final destination. The up-flow anaerobic blanket (UASB) reactor application is an eco-friendly and cost-competitive treatment. The present study briefly reviews the UASB application for dye wastewater valorization. Bioenergy and clean water production potential during dye-containing wastewater treatment are emphasized to promote resource recovery in textile industries. Complex dye structures can hinder biomineralization. Pretreatment may be necessary to reduce dye concentration. Carbon source and redox mediators are added to the UASB reactor to expedite kinetic reactions. Hydraulic retention time (HRT), organic loading rate (OLR), pH, temperature, and hydraulic mixing influence sludge granulation, microbial activity, and dye removal. HRT and OLR range of 6 – 24 h and 1 – 12 kg m^-3 d^-1 of chemical oxygen demand (COD) at mesophilic temperature (30 – 40ºC) are recommended for efficient treatment. In these conditions, efficiencies of color and COD of 50–97% and 60–90% are reported in bench-scale UASB studies. A biogas yield of 1.48 – 2.70 L d^-1 in UASB, which treats dye-containing effluents, is documented. Cotreatment of dye wastewater and locally available substrate could increase biogas productivity in UASB reactors. Organic waste generated in the textile industry, such as dye sludge, cotton, and starch, is recommended to make cotreatment cost-competitive. Bioenergy production and water reuse allow environmental and economic benefits. However, data on full-scale UASB treating dye wastewater are missing. Studies on combined systems integrating UASB and membrane processes, such as ultrafiltration and nanofiltration, for the production of reusable water and pretreatment of wastewater and sludge for improvements in biogas production might realize the complete potential for resource recovery of UASB technology. UASB bioenergy usage for integrated treatment trains can reduce operating costs and assist process sustainability in the textile industry."

Point 7: Lines 124-126," For optimal results, operating the UASB reactor at 30°C and 40°C, with HRT ranging from 5 to 20 hours, and an OLR of 2 to 15 kg COD m-3 d-1 is recommended." Please provide relevant literature in support.

Response: The reviewer is right in his/ her comment. References were included accordingly.

"For optimal results, operating the UASB reactor at 30°C and 40°C, with HRT ranging from 5 to 20 hours, and an OLR of 2 to 15 kg COD m-3 d-1 is recommended [10, 42]. Likewise, Mohan and Swathi [3] identified that optimal conditions for UASB for treating various types of wastewaters are HRT of 3–24 h, OLR of 1–15 kg COD m-3 d-1, and operational temperature in the mesophilic range (30–40ºC)."

van Lier, J.B.; van der Zee, F.P.; Frijters, CTMJ; Ersahin, M.E. Celebrating 40 Years Anaerobic Sludge Bed Reactors for Industrial Wastewater Treatment. Rev. Environ. Sci. Bio/Technology 2015, 14, 681–702, doi:10.1007/s11157-015-9375-5.

Haadel, A. van; Lubbe, J. van der UASB Reactor Design Guidelines. In Anaerobic Sewage Treatment: Optimization of Process and Physical Design of Anaerobic and Complementary Processes; van Haandel, A., van der Lubbe, J., Eds.; IWA Publishing, 2019; pp. 133–192 ISBN 9781780409627.

Mariraj Mohan, S.; Swathi, T. A Review on Upflow Anaerobic Sludge Blanket Reactor: Factors Affecting Performance, Modification of Configuration and Its Derivatives. Water Environ. Res. 2022, 94, 1–27, doi:10.1002/wer.1665.

Point 8: Lines 188-189," Other bench-scale studies reported biomethane production rates ranging from 0.36 to 2.7 L per day." Literature cited is not sufficiently original, suggesting that more recent literature be added.

Response: Thanks for your comment. Recent literature was added, and data were updated. In addition, the reviewer's comment allowed us to update Table 3 and elaborate on aspects of biogas production using UASB reactors.

"Katal et al. [58] conducted experiments using a lab-scale UASB reactor to treat textile effluent and measure the biogas production yield. They achieved maximum biogas productivity of 36 L per day at an HRT of 50 hours, with a biomethane content of 79%. Other bench-scale studies reported biomethane biogas production rates ranging from 1.48 to 2.7 L per day [41,59–61] (Table 3).

The cotreatment of actual dye wastewater and starch effluent indicated higher biogas production than solely dye-containing treatment in UASB. The literature reports a maximum biogas production range of 24.5 – 355 L d-1, cotreating dye and starch effluents [7,62,63]. Cotreatment using UASB reactors could be promising to increase biogas productivity; still, techno-economic analysis should be performed for adopting such a strategy since co-substrate availability and logistics can hamper implementation on a full scale [64]. Cotreatment is most cost-competitive when co-substrate is locally available and implemented on a large scale [65]. Based on this, organic waste generated in the textile industry, such as dye sludge, cotton, and starch, is suggested to increase biogas production outcomes."

Gnanapragasam, G.; Senthilkumar, M.; Arutchelvan, V.; Sivarajan, P.; Nagarajan, S. Recycle in Upflow Anaerobic Sludge Blanket Reactor on Treatment of Real Textile Dye Effluent. World J. Microbiol. Biotechnol. 2010, 26, 1093–1098, doi:10.1007/s11274-009-0275-0.

Senthilkumar, M.; Arutchelvan, V.; Kanakasabai, V.; Venkatesh, K.R.; Nagarajan, S. Biomineralisation of Dye Waste in a Two-Phase Hybrid UASB Reactor Using Starch Effluent as a Co-Substrate. Int. J. Environ. Waste Manag. 2009, 3, 354, doi:10.1504/IJEWM.2009.026351.

Volschan, I.; Cammarota, M.C.; Almeida, R. De; Lobato, L.C.S.; Aquino, S.F. de Part B: Sludge Sewage Pretreatment and Codigestion Technical Note 2 – Contributions about Sewage Sludge Pretreatment Techniques. Cad. Técnicos Eng. Sanitária e Ambient. 2022, 2, 13–22, doi:10.5327/276455760202002.

Yang, H.; Dou, X.; Pan, F.; Wu, Q.; Li, C.; Zhou, B.; Hao, L. Optimal Planning of Local Biomass-Based Integrated Energy System Considering Anaerobic Co-Digestion. Appl. Energy 2022, 316, 119075, doi:10.1016/j.apenergy.2022.119075.

Akter, S.; Suhan, M.B.K.; Islam, M.S. Recent Advances and Perspective of Electrocoagulation in the Treatment of Wastewater: A Review. Environ. Nanotechnology, Monit. Manag. 2022, 17, 100643, doi:10.1016/j.enmm.2022.100643.

Point 9: The title of part 4 does not summarise the section very accurately, and it is suggested that it be re-summarised.

Response: The reviewer is right in his/ her comment. The title was rewrite accordingly.

Point 10: The discussion of the ultrafiltration nanofiltration section in section 5.2 is short, and it is recommended that more literature be cited to further summarise echoing the abstract.

Response: Thanks for your comment. A table summarizing relevant studies on membrane-based methods for dye-containing effluent treatment for water reuse was added in the section 5.2. The discussion was elaborated accordingly.

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper elucidates the principle and the idea of the combination of dye waster water treatment with bioenergy production clearly, the paper itself is good.

The paper needs information on larger-scale experiments, for example, pilot projects from industrial areas. If possible, I would suggest the authors supply the content of feasibility in the industrial area, or the engineering balance such as cost, energy, and efficiency, and also, modify the economic possibility．

Author Response

Responses to the comments of Reviewer #2

Reviewer #2: The paper elucidates the principle and the idea of the combination of dye wasterwater treatment with bioenergy production clearly, the paper itself is good.

Response: The authors appreciate the positive reviewer feedback. The responses to the reviewer's comments are addressed below.

The paper needs information on larger-scale experiments, for example, pilot projects from industrial areas. If possible, I would suggest the authors supply the content of feasibility in the industrial area, or the engineering balance such as cost, energy, and efficiency, and also, modify the economic possibility．

Response: Thanks for your insight. Data on full-scale projects, energy, and economic aspects was added accordingly in section 5.1.

“Industrial treatment facilities have a high energy demand [67]; thus, UASB technology offers opportunities for reducing treatment costs while treating wastewater. Gadow and Li [47] showed that the UASB technology could be extended to full-scale applications for 2-Naphthol red removal with a bioenergy recovery of 139.6 MJ per m3 of effluent. A maximum methane yield of 13.3 mmol CH4 g^-1 COD d⁻¹ was obtained at an HRT of 6 h. In another work from the same research group, a similar methane yield of 13.18±0.64 CH4 g^-1 COD was recorded during the treatment of synthetic dye wastewater [68]. Apart from bioenergy recovery and related economic benefits, reducing greenhouse gas emissions is expected and could help boost the C-neutrality of wastewater treatment plants. Moreover, lower excess sludge is discharged from UASB reactors [69].

A recent study compared a pilot-scale UASB and anaerobic membrane bioreactor (AnMBR) treating domestic wastewater [66]. The UASB reactor produced 230±35 L of biogas daily (73±3% CH₄) at an HRT of 15 h. The UASB pilot plant demonstrated high stability and fewer technological requirements than AnMBR. Thus, it is a better candidate for decentralized treatment. It could also be integrated with other renewable energy alternatives for heat and electricity production.

A full-scale UASB reactor was operated for seven years for brewery effluent treatment in Korea. COD removal of the UASB reactor averaged over 80% throughout the period, incurring operating costs of 0.20 – 0.31 USD m^-3 [70]. In Brazil, the energy potential of biogas from sewage treatment using UASB reactors for wastewater and/or sludge valorization was estimated at 1.53 – 3.50 MJ m^-3. However, the energetic advantages of UASB have not been fully explored in the country [71]. In the Brazilian industry, biogas production was estimated at 0.7 billion Nm³ y^-1 in 2022, amounting to only 126 plants [72]. Data show much room for growth in the Brazilian market, and industries should further explore the techno-economic benefits of UASB technology.”

Rattier, M.; Jimenez, J.A.; Miller, M.W.; Dhanasekar, A.; Willis, J.; Keller, J.; Batstone, D. Long-Term Comparison of Pilot UASB and AnMBR Systems Treating Domestic Sewage at Ambient Temperatures. J. Environ. Chem. Eng. 2022, 10, 108489, doi:10.1016/j.jece.2022.108489.

Ahn, Y.-H.; Min, K.-S.; Speece, R.E. Full Scale UASB Reactor Performance in the Brewery Industry. Environ. Technol. 2001, 22, 463–476, doi:10.1080/09593332208618276.

Rosa, A.P.; Lobato, L.C.S.; Chernicharo, C.A.L. Mathematical Model to Predict the Energy Potential of UASB-Based Sewage Treatment Plants. Brazilian J. Chem. Eng. 2020, 37, 73–87, doi:10.1007/s43153-020-00012-2.

CIBiogas BIOGASMAP Available online: https://app.powerbi.com/view?r=eyJrIjoiNDZiYTYyNGQtYzliYS00NTMyLTk1Y2EtOWZmZjE4OTgwY2VkIiwidCI6ImMzOTg3ZmI3LTQ5ODMtNDA2Ny1iMTQ2LTc3MGU5MWE4NGViNSJ9 (accessed on 10 November 2023).

Author Response File: Author Response.pdf

Reviewer 3 Report

The English of the manuscript is not good and the authors should edit the entire manuscript.

Factors affecting UASB are not well described. The authors should describe better the effective factors.

Literature study is not strong. 

The process is not well described.

The removal mechanism of dyes is not explained. 

Tables and figures for a review study are not enough.

Therefore, the manuscript is rejected. However, my decision is subject to change if the authors try to improve the manuscript based on my comments.

Many editions are necessary to fix grammatical and typographical errors.

Author Response

Responses to the comments of Reviewer #3

The English of the manuscript is not good and the authors should edit the entire manuscript.

Response: Thanks for your comment. The manuscript was revised for grammar and correction purposes.

Factors affecting UASB are not well described. The authors should describe better the effective factors.

Response: Thanks for your comment. The manuscript was revised accordingly.

The UASB performance is influenced by hydraulic retention time (HRT), temperature, organic loading rate (OLR), hydraulic mixing, and sludge granulation. HRT affects treatment time and removal performance of pollution parameters. It is also linked with the up-flow velocity chosen for UASB operation. When up-flow velocity is higher than 1.5 m/h, sludge disintegration and biomass washout may occur, reducing the removal efficiency of chemical oxygen demand (COD) [16]. In addition, OLR impacts microbial activity and biodegradation performance. HRT range of 3 – 10 and OLR of 4 – 15 is recommended to achieve COD removal of 60 – 85% [8,10]. The thermophilic temperature (50 – 65ºC) assures higher process stability and biogas production [17]. Still, a temperature range of 30 – 40ºC effectively maintained methanogen activity and reactor stability [16]. The following section presents a comprehensive summary of influence parameters impacting dye removal in UASB.

Literature study is not strong.

Response: Thanks for your comment. After the reviewer's suggestions, more literature was added to the manuscript, including recent ones.

Akter, S.; Suhan, M.B.K.; Islam, M.S. Recent Advances and Perspective of Electrocoagulation in the Treatment of Wastewater: A Review. Environ. Nanotechnology, Monit. Manag. 2022, 17, 100643, doi:10.1016/j.enmm.2022.100643.

Cieślik, B.M.; Namieśnik, J.; Konieczka, P. Review of Sewage Sludge Management: Standards, Regulations and Analytical Methods. J. Clean. Prod. 2015, 90, 1–15, doi:10.1016/j.jclepro.2014.11.031.

Ćurić, I.; Dolar, D.; Karadakić, K. Textile Wastewater Reusability in Knitted Fabric Washing Process Using UF Membrane Technology. J. Clean. Prod. 2021, 299, 126899, doi:10.1016/j.jclepro.2021.126899.

Gnanapragasam, G.; Senthilkumar, M.; Arutchelvan, V.; Sivarajan, P.; Nagarajan, S. Recycle in Upflow Anaerobic Sludge Blanket Reactor on Treatment of Real Textile Dye Effluent. World J. Microbiol. Biotechnol. 2010, 26, 1093–1098, doi:10.1007/s11274-009-0275-0.

Gnanasekaran, G.; Sudhakaran, M.S.P.; Kulmatova, D.; Han, J.; Arthanareeswaran, G.; Jwa, E.; Mok, Y.S. Efficient Removal of Anionic, Cationic Textile Dyes and Salt Mixture Using a Novel CS/MIL-100 (Fe) Based Nanofiltration Membrane. Chemosphere 2021, 284, doi:10.1016/j.chemosphere.2021.131244.

Keskin, B.; Korkut, S.; Ormancı-Acar, T.; Turken, T.; Tas, C.E.; Menceloglu, Y.Z.; Unal, S.; Koyuncu, I. Pilot Scale Nanofiltration Membrane Fabrication Containing Ionic Co-Monomers and Halloysite Nanotubes for Textile Dye Filtration. Water Sci. Technol. 2023, 87, 1529–1541, doi:10.2166/wst.2023.081.

Nasr, R.A.; Ali, E.A. Polyethersulfone/Gelatin Nano-Membranes for the Rhodamine B Dye Removal and Textile Industry Effluents Treatment under Cost Effective Condition. J. Environ. Chem. Eng. 2022, 10, 107250, doi:10.1016/j.jece.2022.107250.

Rattier, M.; Jimenez, J.A.; Miller, M.W.; Dhanasekar, A.; Willis, J.; Keller, J.; Batstone, D. Long-Term Comparison of Pilot UASB and AnMBR Systems Treating Domestic Sewage at Ambient Temperatures. J. Environ. Chem. Eng. 2022, 10, 108489, doi:10.1016/j.jece.2022.108489.

Rendón-Castrillón, L.; Ramírez-Carmona, M.; Ocampo-López, C.; González-López, F.; Cuartas-Uribe, B.; Mendoza-Roca, J.A. Treatment of Water from the Textile Industry Contaminated with Indigo Dye: A Hybrid Approach Combining Bioremediation and Nanofiltration for Sustainable Reuse. Case Stud. Chem. Environ. Eng. 2023, 8, doi:10.1016/j.cscee.2023.100498.

Romero-Soto, I.C.; García-Gómez, C.; Álvarez-Valencia, L.H.; Meza-Escalante, E.R.; Leyva-Soto, L.A.; Camacho-Ruiz, M.A.; Concha-Guzmán, M.O.; Ulloa-Mercado, R.G.; Díaz-Tenorio, L.M.; Gortáres-Moroyoqui, P. Sequential Congo Red Elimination by UASB Reactor Coupled to Electrochemical Systems. Water 2021, 13, 3087, doi:10.3390/w13213087.

Sahinkaya, E.; Tuncman, S.; Koc, I.; Guner, A.R.; Ciftci, S.; Aygun, A.; Sengul, S. Performance of a Pilot-Scale Reverse Osmosis Process for Water Recovery from Biologically-Treated Textile Wastewater. J. Environ. Manage. 2019, 249, 109382, doi:10.1016/j.jenvman.2019.109382.

Senthilkumar, M.; Arutchelvan, V.; Kanakasabai, V.; Venkatesh, K.R.; Nagarajan, S. Biomineralisation of Dye Waste in a Two-Phase Hybrid UASB Reactor Using Starch Effluent as a Co-Substrate. Int. J. Environ. Waste Manag. 2009, 3, 354, doi:10.1504/IJEWM.2009.026351.

Volschan, I.; Cammarota, M.C.; Almeida, R. De; Lobato, L.C.S.; Aquino, S.F. de Part B: Sludge Sewage Pretreatment and Codigestion Technical Note 2 – Contributions about Sewage Sludge Pretreatment Techniques. Cad. Técnicos Eng. Sanitária e Ambient. 2022, 2, 13–22, doi:10.5327/276455760202002.

Yang, H.; Dou, X.; Pan, F.; Wu, Q.; Li, C.; Zhou, B.; Hao, L. Optimal Planning of Local Biomass-Based Integrated Energy System Considering Anaerobic Co-Digestion. Appl. Energy 2022, 316, 119075, doi:10.1016/j.apenergy.2022.119075.

Yin, H.; Qiu, P.; Qian, Y.; Kong, Z.; Zheng, X.; Tang, Z.; Guo, H. Textile Wastewater Treatment for Water Reuse: A Case Study. Processes 2019, 7, 34, doi:10.3390/pr7010034.

The process is not well described.

Response: Thanks for your comment. Following the suggestions of reviewer #1, the aspects related to UASB operation and influencing parameters were elaborated on. Thus, section 2 was complemented and expanded accordingly.

The removal mechanism of dyes is not explained.

Response: Thanks for your comment. Aspects of mechanisms of dye removal were extended and complemented.

"The dye removal process in UASB reactors involves two main mechanisms: abiotic adsorption and biotic biodegradation. The adsorption mechanism, facilitated by sludge granules, plays a significant role in decolorization. On the other hand, biodegradation occurs under anaerobic conditions and primarily focuses on azo dyes' biochemistry [23]. The primary degradation mechanism involves the cleavage of the azo bond (–N=N–) by extracellular azoreductase enzymes, which transfer four electrons (reducing equivalents) (Equation 1). The permeation of the azo dyes through the membrane of microbial cells acts as the principal rate-limiting factor for decolorization [24]. The generated hydrazo intermediates undergo reductive cleavage, resulting in uncolored aromatic amines as byproducts, as shown in Equation 2 [25].

It is important to note that produced aromatic amines are generally anaerobically recalcitrant and have higher toxicity than dye precursors. Consequently, anaerobically treated effluent needs further treatment. Biological sequential anaerobic-aerobic treatment has been used to remove azo dyes completely. Under low oxygen concentrations, facultative bacteria consume oxygen and introduce hydroxyl groups into polyaromatic compounds, facilitating biodegradation pathways. However, aromatic amines have substituents with nitro and sulfonic groups; these are highly recalcitrant for aerobic microorganisms, which prevents efficient contaminant mineralization [8,24]. Decolorization of azo dyes under anaerobic conditions is thought to be a relatively simple and non-specific process. Readers are guided toward the contribution of Saratale et al. [24] for further background information on dye decolorization using biological methods."

Tables and figures for a review study are not enough.

Response: Thanks for your comment. Following comment #9 of reviewer 1, a table summarizing relevant studies on membrane-based methods for dye-containing effluent treatment for water reuse was added in section 5.2. The discussion was elaborated accordingly.

Therefore, the manuscript is rejected. However, my decision is subject to change if the authors try to improve the manuscript based on my comments.

Response: We appreciate the reviewer's feedback. Reviewers' comments enabled us to improve the quality of our manuscript significantly. We hope the manuscript fits the Waste (ISSN 2813-0391) standards in its current form.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

I suggest to accept it.

Author Response

The authors appreciate the reviewer's feedback. Comments were highly insightful and enabled us to improve our manuscript's quality significantly.

Best regards,

The authors

Reviewer 3 Report

The manuscript seems seems to need minor corrections as follows:

The sentences related to dye contaminants in wastewater must be added to the manuscript. A table must be added to express different types of dyes and their standard amounts in water.

The English language of the manuscript must be carefully checked and revised to avoid any grammatical and typo mistakes.

The English language of the manuscript must be carefully checked and revised to avoid any grammatical and typo mistakes.

Author Response

Responses to the comments of Reviewer #3

The manuscript seems seems to need minor corrections as follows:

Response: The authors appreciate the reviewer's feedback for the manuscript improvements.

The sentences related to dye contaminants in wastewater must be added to the manuscript. A table must be added to express different types of dyes and their standard amounts in water.

Response: Thanks for your comment and suggestion. Discharge standards are essential, and it is crucial to note that permissible limits differ based on the regulations set by local authorities and municipalities. In textile wastewater, metal ions, dyes, and color are of the first concern due to their harmfulness to public health and the environment. This discussion was included to complement and extend the introduction section.

The present review article is focused on UASB application for dye wastewater valorization. To include specific data on disposal limits of dyes in water samples will not add to this work since this aspect is not further discussed in the manuscript. Besides, to our knowledge, the existing literature on specific dye concentrations in water is scarce; as mentioned above, permissible limits for disposal are based on standard wastewater parameters such as COD, BOD, and salts, among others.

"In textile wastewater, metal ions, dyes, and color are of the first concern due to their harmfulness to public health and the environment. Discharge standards vary according to the local regulatory agency and municipalities; Thus, it should be checked in each situation [1]. The recognition of the health hazards of dyes has highlighted the need to develop rapid and reliable analytical methods for detection and forced regulatory permissible limits in this respect. Twenty pharmacologically active dyes were quantified in water and industrial textile effluent samples. Dyes were found in two treated effluents. In one, rhodamine B was found at a concentration of 0.043 μg L-1, and the other one contained crystal violet, methyl violet 2B, and rhodamine B in 0.023, 0.017, and 0.027 μg L-1, respectively [2]." (Page 1-2, lines 40-49)

Holkar, C.R.; Jadhav, A.J.; Pinjari, D. V.; Mahamuni, N.M.; Pandit, A.B. A Critical Review on Textile Wastewater Treatments: Possible Approaches. J. Environ. Manage. 2016, 182, 351–366, doi:10.1016/j.jenvman.2016.07.090.

Tkaczyk-Wlizło, A.; Mitrowska, K.; Błądek, T. Quantification of Twenty Pharmacologically Active Dyes in Water Samples Using UPLC-MS/MS. Heliyon 2022, 8, e09331, doi:10.1016/j.heliyon.2022.e09331.

The English language of the manuscript must be carefully checked and revised to avoid any grammatical and typo mistakes

Response: Thanks for your comment. The manuscript was carefully checked for grammar and correction purposes.

Author Response File: Author Response.pdf