Optimization of Anaerobic Digestion Systems for Biomethane Recovery from Septic Tank Sludge

Abstract

This study presents a process design, simulation, and optimization framework for converting septic sludge into biomethane using Aspen Plus^®. The sludge was characterized, revealing carbon, hydrogen, and volatile matter contents of 33.80, 5.86, and 34.86 wt.%, respectively. The developed Aspen Plus^® model was validated against three literature datasets, achieving percentage errors below unity. Optimization using Response Surface Methodology-Central Composite Design (RSM-CCD) showed that the maximum biomethane yield was 58.227 vol% under optimal conditions: 25 °C hydrolysis temperature, 60 °C digester temperature, 35 days hydraulic retention time (HRT), and an organic loading rate (OLR) of kg·VS·m⁻³·day⁻¹, with a desirability score of 1.0. A techno-economic evaluation using the Aspen Process Economic Analyser (APEA) demonstrated the system’s economic feasibility, with a total capital investment of USD 3.19 million, an annual operating cost of USD 1.29 million, and a payback period of approximately 3.8 years. The optimized system achieved a net energy gain of 82.6%, IRR of 16.6%, and NPV of $4.64 M, confirming strong economic viability. Sensitivity analysis further revealed that CAPEX, OPEX, feedstock cost, and upgrading energy demand significantly influence system profitability, emphasizing the importance of process optimization and energy-efficient upgrading strategies. Environmental assessment showed that the optimized system improved methane recovery efficiency to 98.7% and achieved a CO₂ emission reduction potential of 0.49 kg CO₂-eq/kg CH₄, demonstrating strong greenhouse gas mitigation potential. Overall, the findings establish anaerobic digestion of septic sludge as a sustainable and cost-effective waste-to-energy pathway suitable for decentralized urban wastewater management, supporting circular economy and clean energy objectives in developing regions.

Keywords

Anaerobic digestion / Bio-methane / Energy recovery / Optimization / Circular economy / Waste-to-energy

Statement of the Use of Generative AI and AI-Assisted Technologies in the Writing Process

The authors acknowledge the use of digital tools, Aspen Plus and Design Expert Software as well as ChatGPT (OpenAI) and Grammarly, to enhance the clarity, grammar, and readability of this manuscript. All contents generated with these tools were critically reviewed, verified, and edited by the authors to ensure accuracy, originality and compliance with the journal standard. The authors take full responsibility for the final version of the manuscript.

Acknowledgments

The authors gratefully acknowledge the Green Engineering Research Group for providing the essential simulation tools and technical support required for this study.

Author Contributions

A.M.I. and E.K.T.: Conceptualization, methodology, formal analysis, writing original draft. E.K.T., U.M.A. and S.R.: supervision, resources. M.A.A., U.S.I., M.I and A.Y.M.: writing—review and editing, project administration. A.M.I., E.K.T., V.O.F., M.I. and S.R.: writing—review and editing, validation. All authors have read and agreed to the published version of the manuscript.

Ethics Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data will be available on request.

Funding

This research received no external funding.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References

[1] Olabi AG, Abdelkareem MA. Renewable energy and climate change. Renew. Sustain. Energy Rev. 2022, 158, 112111. DOI:10.1016/j.rser.2022.112111

[2] Kandpal V, Jaswal A, Santibanez Gonzalez EDR,Agarwal N. Challenges and Opportunities for Sustainable Energy Transition and Circular Economy. In Sustainable Energy Transition: Circular Economy and Sustainable Financing for Environmental, Social and Governance (ESG) Practices; Springer: Cham, Switzerland, 2024; pp. 307-324. DOI:10.1007/978-3-031-52943-6_11

[3] Li X, Wang Z, He Y, Wang Y, Wang S, Zheng Z, et al. A Comprehensive Review of the Strategies to Improve Anaerobic Digestion: Their Mechanism and Digestion Performance. Methane 2024, 3, 227-256. DOI:10.3390/methane3020014

[4] Marconi P, Rosa L. Role of biomethane to offset natural gas. Renew. Sustain. Energy Rev. 2023, 187, 113697. DOI:10.1016/j.rser.2023.113697

[5] Magro AD, Lovarelli D, Bacenetti J, Guarino M. The potential of insect frass for sustainable biogas and biomethane production: A review. Bioresour. Technol. 2024, 412, 131384. DOI:10.1016/j.biortech.2024.131384

[6] Shekhar Bose R, Zakaria BS, Kumar Tiwari M, Ranjan Dhar B. High-rate blackwater anaerobic digestion under septic tank conditions with the amendment of biosolids-derived biochar synthesized at different temperatures. Bioresour. Technol. 2021, 331, 125052. DOI:10.1016/j.biortech.2021.125052

[7] Capodaglio AG. Biorefinery of Sewage Sludge: Overview of Possible Value-Added Products and Applicable Process Technologies. Water 2023, 15, 1195. DOI:10.3390/w15061195

[8] Pratap V, Kumar S, Yadav BR. Sewage sludge management and enhanced energy recovery using anaerobic digestion: An insight. Water Sci. Technol. 2024, 90, 696-720. DOI:10.2166/wst.2024.269

[9] Zhou Y, Liu Y, Qiao Y, Li B, Chen H, Lv W. Unlocking the potential: Sustainable land use of urban septic tank slurry as granules with emphasis on soil improvement and safety consideration. J. Environ. Chem. Eng. 2024, 12, 113227. DOI:10.1016/j.jece.2024.113227

[10] Chandana N, Rao B. A critical review on sludge management from onsite sanitation systems: A knowledge to be revised in the current situation. Environ. Res. 2022, 203, 111812. DOI:10.1016/j.envres.2021.111812

[11] Mo R, Guo W, Batstone D, Makinia J, Li Y. Modifications to the anaerobic digestion model no. 1 (ADM1) for enhanced understanding and application of the anaerobic treatment processes—A comprehensive review. Water Res. 2023, 244, 120504. DOI:10.1016/j.watres.2023.120504

[12] Piadeh F, Offie I, Behzadian K, Rizzuto JP, Bywater A, Córdoba-Pachón JR, et al. A critical review for the impact of anaerobic digestion on the sustainable development goals. J. Environ. Manag. 2024, 349, 119458. DOI:10.1016/j.jenvman.2023.119458

[13] Long S, Yang J, Hao Z, Shi Z, Liu X, Xu Q, et al. Multiple roles of humic substances in anaerobic digestion systems: A review. J. Clean. Prod. 2023, 418, 138066. DOI:10.1016/j.jclepro.2023.138066

[14] Paranjpe A, Saxena S, Jain P. A Review on Performance Improvement of Anaerobic Digestion Using Co-Digestion of Food Waste and Sewage Sludge. J. Environ. Manag. 2023, 338, 117733. DOI:10.1016/j.jenvman.2023.117733

[15] Ojo OM. Anaerobic Digestion for Wastewater Treatment: A Review of Principles, Processes and Performance. NIPES-J. Energy Technol. Environ. 2025, 7, 52-60. DOI:10.5281/zenodo.15015847

[16] ASTM D2216-19; Standard Test Methods for Laboratory Determination of Water (Moisture) Content of Soil and Rock by Mass. Annual Book of ASTM Standards; ASTM International: West Conshohocken, PA, USA, 2010.

[17] Zhang W, Xu Y, Dong B, Dai X. Characterizing the sludge moisture distribution during anaerobic digestion process through various approaches. Sci. Total Environ. 2019, 675, 184-191. DOI:10.1016/j.scitotenv.2019.04.095

[18] Raveendravarrier B, Palatel A, Chandrasekharan M. Physico-chemical Characterization of sewage sludge for thermochemical conversion processes. Energy Sources Part A Recovery Util. Environ. Eff. 2025, 47, 2641-2665. DOI:10.1080/15567036.2020.1841858

[19] Septiariva IY, Suryawan IWK, Zahra NL, Putri YNK, Sarwono A, Qonitan FD, et al. Characterization Sludge from Drying Area and Sludge Drying Bed in Sludge Treatment Plant Surabaya City for Waste to Energy Approach. J. Ecol. Eng. 2022, 23, 268-275. DOI:10.12911/22998993/150061

[20] Kunh SS, Tavares MHF, da Silva EA, de Oliveira RS, Bittencourt PRS, Damaceno FM, et al. Briquette production from a mixture of biomass: Poultry slaughterhouse sludge and sawdust. Environ. Sci. Pollut. Res. 2022, 29, 64192-64204. DOI:10.1007/s11356-022-20218-w

[21] Gajewska T, Malinowski M, Szkoda M. The Use of Biodrying to Prevent Self-Heating of Alternative Fuel. Materials 2019, 12, 3039. DOI:10.3390/ma12183039

[22] Liu K. Effects of sample size, dry ashing temperature and duration on determination of ash content in algae and other biomass. Algal Res. 2019, 40, 101486. DOI:10.1016/j.algal.2019.101486

[23] Baffoe EE, Otoo SL, Kareem S, Dankwah JR. Evaluation of initial pH and urea hydrogen peroxide (UHP) co-pretreatment on waste-activated sludge. Environ. Res. 2024, 246, 118155. DOI:10.1016/j.envres.2024.118155

[24] Wu R, Shen R, Liang Z, Zheng S, Yang Y, Lu Q, et al. Improve Niche Colonization and Microbial Interactions for Organohalide-Respiring-Bacteria-Mediated Remediation of Chloroethene-Contaminated Sites. Environ. Sci. Technol. 2023, 57, 17338-17352. DOI:10.1021/acs.est.3c05932

[25] Miao H, Wang S, Zhao M, Huang Z, Ren H, Yan Q, et al. Codigestion of Taihu blue algae with swine manure for biogas production. Energy Convers. Manag. 2014, 77, 643-649. DOI:10.1016/j.enconman.2013.10.025

[26] Vale GB, Scalize PS, Tonetti AL, Ruggeri Junior HC. Cost-effectiveness study of septic tank management in rural communities. Int. J. Environ. Sci. Technol. 2024, 21, 4599-4610. DOI:10.1007/s13762-023-05299-5

[27] Maqbool W, Biller P, Anastasakis K. A kinetic process model for sewage sludge hydrothermal liquefaction in Aspen Plus: Model validation with pilot-plant data and scale up. Energy Convers. Manag. 2024, 302, 118136. DOI:10.1016/j.enconman.2024.118136

[28] Ajala OO, Odejobi OJ. Modelling, simulation and optimization of domestic and agricultural wastes-based anaerobic digestion using Aspen Plus. Biomass Conv. Bioref. 2025, 15, 2817-2833. DOI:10.1007/s13399-023-05128-2

[29] Inuwa AM, Oluwafemi Fatokun V, Kweinor Tetteh E, Rathilal S, Aliyu UM. Energy Recovery and Techno-Economic Analysis of Hydrothermal Carbonization and Anaerobic Digestion of Food Waste. Clean Technol. 2026, 8, 57 DOI:10.3390/cleantechnol8020057

[30] Ochieng R, Gebremedhin A, Sarker S. A comparative assessment of sewage sludge valorization via anaerobic digestion and supercritical water gasification: A techno-economic case study in Norway. J. Water Process Eng. 2024, 66, 106016. DOI:10.1016/j.jwpe.2024.106016

[31] Jana AK. Process Simulation and Control Using ASPEN; PHI Learning Pvt. Ltd.: New Delhi, India, 2012.

[32] Ras M, Lardon L, Bruno S, Bernet N, Steyer JP. Experimental study on a coupled process of production and anaerobic digestion of Chlorella vulgaris. Bioresour. Technol. 2011, 102, 200-206. DOI:10.1016/j.biortech.2010.06.146

[33] Inayat A, Raza M, Ghenai C, Shanableh A, Said Z, Samman S, et al. Simulation of Anaerobic Co-Digestion Process for the Biogas Production using ASPEN PLUS. In Proceedings of the 2019 Advances in Science and Engineering Technology International Conferences (ASET), Dubai, United Arab Emirates, 26 March-10 April 2019; pp. 1-5. DOI:10.1109/ICASET.2019.8714403

[34] Anaya Menacho W, Mazid AM, Das N. Modelling and analysis for biogas production process simulation of food waste using Aspen Plus. Fuel 2022, 309, 122058. DOI:10.1016/j.fuel.2021.122058

[35] Ugwu SN, Enweremadu CC. Optimization of iron-enhanced anaerobic digestion of agro-wastes for biomethane production and phosphate release. Environ. Technol. 2023, 44, 721-738. DOI:10.1080/09593330.2022.2061379

[36] Melas GA, Habtu NG, Worku AK, Getahun E. Recent Progresses and Future Perspective of Biogas-Upgrading Techniques. Bioenerg. Res. 2025, 18, 80. DOI:10.1007/s12155-025-10875-3

[37] Karimi M, Siqueira RM, Shirzad M, Ferreira AFP, Rodrigues AE, Silva JAC. Integrated experimental, process simulation, and techno-economic assessment of biogas upgrading via pressure/vacuum swing adsorption. Sep. Purif. Technol. 2026, 386, 136571. DOI:10.1016/j.seppur.2025.136571

[38] Hurtig O, Buffi M, Besseau R, Scarlat N, Carbone C, Agostini A. Mitigating biomethane losses in European biogas plants: A techno-economic assessment. Renew. Sustain. Energy Rev. 2025, 210, 115187. DOI:10.1016/j.rser.2024.115187

[39] Díaz-Herrera PR, Vega E, Villanueva-Estrada RE, Rocha-Miller R. Techno-economic analysis of solvent-based biogas upgrading technologies for vehicular biomethane production: A case study in Prados de la Montaña landfill, Mexico City. Sustain. Energy Technol. Assess. 2023, 60, 103542. DOI:10.1016/j.seta.2023.103542

[40] Sun X, Liu T, Zhai Y, Zhang Y, Shi H. The Impact of Fossil Energy Prices on Carbon Emissions: The Dual Mediation of Energy Efficiency and Renewable Energy. Energies 2025, 18, 6186. DOI:10.3390/en18236186

[41] Fikru MG, Belaïd F, Ma H. Carbon capture and renewable energy policies: Could policy harmonization be a puzzle piece to solve the electricity crisis? Energy Econ. 2024, 136, 107753. DOI:10.1016/j.eneco.2024.107753

[42] Chowdhury TH. Technical-economical analysis of anaerobic digestion process to produce clean energy. Energy Rep. 2021, 7, 247-253. DOI:10.1016/j.egyr.2020.12.024

[43] Richardson JF. Coulson and Richardson’s Chemical Engineering; Elsevier: Gurgaon, India, 2002.

[44] Panda S, Jain MS. Assessment of the biomethane potential of commingled farm residues with sewage sludge and its techno-economic viability for rural application. Biomass Conv. Bioref. 2025, 15, 5141-5154. DOI:10.1007/s13399-024-05457-w

[45] Vinardell S, Feickert Fenske C, Heimann A, Cortina JL, Valderrama C, Koch K. Exploring the potential of biological methanation for future defossilization scenarios: Techno-economic and environmental evaluation. Energy Convers. Manag. 2024, 307, 118339. DOI:10.1016/j.enconman.2024.118339

[46] Zheng Q, Ni L. Analysis of the effect of intrinsic sludge properties on sludge drying characteristics from both sludge composition and type scales. Waste Manag. 2024, 183, 278-289. DOI:10.1016/j.wasman.2024.05.020

[47] Zheng Y, Wang P, Yang X, Lin P, Wang Y, Cheng M, et al. Process Performance and Microbial Communities in Anaerobic Co-digestion of Sewage Sludge and Food Waste with a Lower Range of Carbon/Nitrogen Ratio. Bioenerg. Res. 2022, 15, 1664-1674. DOI:10.1007/s12155-021-10357-2

[48] Dhabu FI, Shukla V, Agarwal Y. Cadmium Sulfide Nanoparticles as Corrosion Inhibitors for Sulfate-Reducing Bacteria. BioNanoScience 2025, 15, 580. DOI:10.1007/s12668-025-02215-8

[49] Kiran S, Ghaffar A, Iqbal S, Javed S, Aslam N, Rafique MA, et al. Characterization and Valorization of Sludge from Textile Wastewater Plant for Positive Environmental Applications. In Handbook of Biomass Valorization for Industrial Applications; Wiley: Hoboken, NJ, USA, 2022; pp. 465-489. DOI:10.1002/9781119818816.ch20

[50] Appels L, Baeyens J, Degrève J, Dewil R. Principles and potential of the anaerobic digestion of waste-activated sludge. Prog. Energy Combust. Sci. 2008, 34, 755-781. DOI:10.1016/j.pecs.2008.06.002

[51] Cazaudehore G, Guyoneaud R, Lallement A, Gassie C, Monlau F. Biochemical methane potential and active microbial communities during anaerobic digestion of biodegradable plastics at different inoculum-substrate ratios. J. Environ. Manag. 2022, 324, 116369. DOI:10.1016/j.jenvman.2022.116369

[52] Wang M, Wang Y, Peng J, Wang L, Yang J, Kou X, et al. A comparative study on Mesophilic and thermophilic anaerobic digestion of different total solid content sludges produced in a long sludge-retention-time system. Results Eng. 2023, 19, 101228. DOI:10.1016/j.rineng.2023.101228

[53] Basinas P, Rusín J, Chamrádová K. Assessment of high-solid mesophilic and thermophilic anaerobic digestion of mechanically-separated municipal solid waste. Environ. Res. 2021, 192, 110202. DOI:10.1016/j.envres.2020.110202

[54] Yenigün O, Demirel B. Ammonia inhibition in anaerobic digestion: A review. Process Biochem. 2013, 48, 901-911. DOI:10.1016/j.procbio.2013.04.012

[55] Lanko I, Hejnic J, Říhová-Ambrožová J, Ferrer I, Jenicek P. Digested Sludge Quality in Mesophilic, Thermophilic and Temperature-Phased Anaerobic Digestion Systems. Water 2021, 13, 2839. DOI:10.3390/w13202839

[56] Wang T, Chen J, Shen H, An D. Effects of total solids content on waste activated sludge thermophilic anaerobic digestion and its sludge dewaterability. Bioresour. Technol. 2016, 217, 265-270. DOI:10.1016/j.biortech.2016.01.130

[57] Mächtig T, Moschner CR, Hartung E. Monitoring the efficiency of biogas plants—Correlation between gross calorific value and anaerobically non-degradable organic matter of digestates. Biomass Bioenergy 2019, 130, 105389. DOI:10.1016/j.biombioe.2019.105389

[58] Cheng S, Yang S, Huang J, Liu F, Shen F. Investigation of the Mechanism for Removal of Typical Pathogenic Bacteria from Three-Compartment Septic Tanks under Low Temperature Conditions. Processes 2023, 12, 87. DOI:10.3390/pr12010087

[59] Wardhani WK, Soedjono ES, Titah HS, Mardyanto MA. Pharmaceutical emerging micropollutants potential in septic tanks: Its fate and transport study in Indonesia—A literature review. Environ. Qual. Mgmt 2024, 34, e22176. DOI:10.1002/tqem.22176

[60] Yusuf HH, Pan X, Ye ZL, Cai G, Appels L, Cai J, et al. Revolutionizing sanitation: Valorizing fecal slags through co-digesting food waste at high-solid content and dosing metallic nanomaterials for anaerobic digestion stability. J. Environ. Manag. 2024, 353, 120177. DOI:10.1016/j.jenvman.2024.120177

[61] Sayin A. Dissertation, New York, NY, Implementation of Acid/Gas Digestion Approach to Maximize Volatile Fatty Acids Production from Municipal Sludge and a Techno-Economic Feasibility Study. Ph.D. The City College of New York, USA, 2024.

[62] Shen D, Zhang P, Wu SL, Long Y, Wei W, Ni BJ. Enhanced biomethane production from waste activated sludge anaerobic digestion by ceramsite and amended Fe₂O₃ ceramsite. J. Environ. Manag. 2024, 351, 119973. DOI:10.1016/j.jenvman.2023.119973

[63] de la Cruz-Azuara JE, Ruiz-Marin A, Canedo-Lopez Y, Aguilar-Ucan CA, Ceron-Breton RM, Ceron-Breton JG, et al. Biomethane Production from the Two-Stage Anaerobic Co-Digestion of Cow Manure: Residual Edible Oil with Two Qualities of Waste-Activated Sludge. Energies 2024, 17, 2848. DOI:10.3390/en17122848

[64] Nauman M, Tayyab M, Faheem M, Ikram K, Akram MW, Asif M, et al. Designing and performance evaluation of continuously stirring anaerobic batch reactor for biomethane production from biowaste. Biomass Conv. Bioref. 2024, 14, 18065-18078. DOI:10.1007/s13399-023-04203-y

[65] Al-Rubaye H, Karambelkar S, Shivashankaraiah MM, Smith JD. Process Simulation of Two-Stage Anaerobic Digestion for Methane Production. Biofuels 2019, 10, 181-191. DOI:10.1080/17597269.2017.1309854

[66] Bijos JCBF, Pessoa RWS, Queiroz LM, Oliveira-Esquerre KPS. Methane liquid-gas phase distribution during anaerobic sludge digestion: A thermodynamic approach. Chemosphere 2022, 298, 134325. DOI:10.1016/j.chemosphere.2022.134325

[67] Djimtoingar SS, Derkyi NSA, Kuranchie FA, Yankyera JK. A review of response surface methodology for biogas process optimization. Cogent Eng. 2022, 9, 2115283. DOI:10.1080/23311916.2022.2115283

[68] Rasouli M, Ataeiyan B. Investigation and Optimization of Operational Conditions of Anaerobic Digestion Process for Enhanced Biogas Production Yield in a CSTR Using RSM. Int. J. Energy Res. 2024, 2024, 9158477. DOI:10.1155/2024/9158477

[69] Pratap V, Kumar S, Yadav BR. Optimization of biogas production from thermal-alkali pre-treated sludge using response surface methodology and random forest regressor model. J. Taiwan Inst. Chem. Eng. 2025, 177, 105571. DOI:10.1016/j.jtice.2024.105571

[70] Islam Siddique MN, Khalid ZB, Ibrahim MZB. Effect of additional nutrients on bio-methane production from anaerobic digestion of farming waste: Feasibility & Fertilizer recovery. J. Environ. Chem. Eng. 2020, 8, 103569. DOI:10.1016/j.jece.2019.103569

[71] Khan N, Khan MD, Sabir S, Nizami AS, Anwer AH, Rehan M, et al. Deciphering the effects of temperature on bio-methane generation through anaerobic digestion. Environ. Sci. Pollut. Res. 2020, 27, 29766-29777. DOI:10.1007/s11356-019-07245-w

[72] Parajuli A, Khadka A, Sapkota L, Ghimire A. Effect of Hydraulic Retention Time and Organic-Loading Rate on Two-Staged, Semi-Continuous Mesophilic Anaerobic Digestion of Food Waste during Start-Up. Fermentation 2022, 8, 620. DOI:10.3390/fermentation8110620

[73] Nkuna R, Roopnarain A, Rashama C, Adeleke R. Insights into organic loading rates of anaerobic digestion for biogas production: A review. Crit. Rev. Biotechnol. 2022, 42, 487-507. DOI:10.1080/07388551.2021.1942778

[74] Casallas-Ojeda M, Soto-Paz J, Alfonso-Morales W, Oviedo-Ocaña ER, Komilis D. Optimization of Operational Parameters during Anaerobic Co-digestion of Food and Garden Waste. Environ. Process. 2021, 8, 769-791. DOI:10.1007/s40710-021-00506-2

[75] Steiniger B, Hupfauf S, Insam H, Schaum C. Exploring Anaerobic Digestion from Mesophilic to Thermophilic Temperatures—Operational and Microbial Aspects. Fermentation 2023, 9, 798. DOI:10.3390/fermentation9090798

[76] He H, Chen X, Peng Z, Zhang Z, Ren X, Ma S, et al. A review of temperature and key parameters influencing the hydrolysis-methanogenesis balance in anaerobic digestion. Fuel 2025, 394, 134927. DOI:10.1016/j.fuel.2025.134927

[77] Yang Z, Yang D, Hua Y, Chen X, Wang X, Gong H, et al. Dual optimization in anaerobic digestion of rice straw: Effects HRT and OLR coupling on methane production in one-stage and two-stage systems. J. Environ. Manag. 2024, 370, 123041. DOI:10.1016/j.jenvman.2024.123041

[78] Hassan GK, Hemdan BA, El-Gohary FA. Utilization of food waste for bio-hydrogen and bio-methane production: Influences of temperature, OLR, and in situ aeration. J. Mater. Cycles Waste Manag. 2020, 22, 1218-1226. DOI:10.1007/s10163-020-01014-5

[79] Hosseini Koupaie E, Lin L, Bazyar Lakeh AA, Azizi A, Dhar BR, Hafez H, et al. Performance evaluation and microbial community analysis of mesophilic and thermophilic sludge fermentation processes coupled with thermal hydrolysis. Renew. Sustain. Energy Rev. 2021, 141, 110832. DOI:10.1016/j.rser.2021.110832

[80] Hangri S, Derbal K, Benalia A, Policastro G, Panico A, Pizzi A. Enhancing Biomethane Yield from Microalgal Biomass via Enzymatic Hydrolysis: Optimization and Predictive Modeling Using RSM Approach. Processes 2025, 13, 2086. DOI:10.3390/pr13072086

[81] Ahmad RM, Javied S, Aslam A, Alamri S, Zaman QU, Hassan A, et al. Optimizing Biogas Production and Digestive Stability through Waste Co-Digestion. Sustainability 2024, 16, 3045. DOI:10.3390/su16073045

[82] Mahmod SS, Jahim JM, Abdul PM, Luthfi AAI, Takriff MS. Techno-economic analysis of two-stage anaerobic system for biohydrogen and biomethane production from palm oil mill effluent. J. Environ. Chem. Eng. 2021, 9, 105679. DOI:10.1016/j.jece.2021.105679

[83] Swinbourn R, Li C, Wang F. A Comprehensive Review on Biomethane Production from Biogas Separation and its Techno‐Economic Assessments. ChemSusChem 2024, 17, e202400779. DOI:10.1002/cssc.202400779

[84] Ghafoori MS, Loubar K, Marin-Gallego M, Tazerout M. Techno-economic and sensitivity analysis of biomethane production via landfill biogas upgrading and power-to-gas technology. Energy 2022, 239, 122086. DOI:10.1016/j.energy.2021.122086

[85] Khan EU, Mainali B, Martin A, Silveira S. Techno-economic analysis of small scale biogas based polygeneration systems: Bangladesh case study. Sustain. Energy Technol. Assess. 2014, 7, 68-78. DOI:10.1016/j.seta.2014.03.004

[86] Wu N, Moreira CM, Zhang Y, Doan N, Yang S, Phlips EJ, et al. Techno-economic analysis of biogas production from microalgae through anaerobic digestion. In Anaerobic Digestion; IntechOpen: London, UK, 2019.

[87] Scholz M, Frank B, Stockmeier F, Falß S, Wessling M. Techno-economic Analysis of Hybrid Processes for Biogas Upgrading. Ind. Eng. Chem. Res. 2013, 52, 16929-16938. DOI:10.1021/ie402660s

[88] Teghammar A, Forgács G, Sárvári Horváth I, Taherzadeh MJ. Techno-economic study of NMMO pretreatment and biogas production from forest residues. Appl. Energy 2014, 116, 125-133. DOI:10.1016/j.apenergy.2013.11.053

[89] Götze U, Northcott D, Schuster P. Discounted Cash Flow Methods. In Investment Appraisal: Methods and Models; Springer: Berlin/Heidelberg, Germany, 2015; pp. 47-83. DOI:10.1007/978-3-662-45851-8_3

[90] Obileke K, Mukumba P. Techno‐Economic Evaluation of Wind and Bio‐Energy Systems for Sustainable Development: A Systematic Review. Energy Sci. Eng. 2025, 13, 2179-2202. DOI:10.1002/ese3.70025

[91] Sganzerla WG, da Rosa RG, Barroso TLCT, Castro LEN, Forster-Carneiro T. Bioenergy, and Techno-Economic Assessment of On-Site Production of Biomethane, Fertilizer from Small-Scale Anaerobic Digestion of Jabuticaba By-Product. Methane 2023, 2, 113-128. DOI:10.3390/methane2020009

[92] Tolessa A, Louw TM, Goosen NJ. Probabilistic techno-economic assessment of anaerobic digestion predicts economic benefits to smallholder farmers with quantifiable certainty. Waste Manag. 2022, 138, 8-18. DOI:10.1016/j.wasman.2021.11.004

[93] Elhaus N, Volkmann M, Kolb S, Schindhelm L, Herkendell K, Karl J. Techno-economic evaluation of anaerobic digestion and biological methanation in Power-to-Methane-Systems. Energy Convers. Manag. 2024, 315, 118787. DOI:10.1016/j.enconman.2024.118787

[94] Bruno M, Marchi M, Ermini N, Niccolucci V, Pulselli FM. Life Cycle Assessment and Cost-Benefit Analysis as Combined Economic-Environmental Assessment Tools: Application to an Anaerobic Digestion Plant. Energies 2023, 16, 3686. DOI:10.3390/en16093686

[95] Gbadeyan OJ, Muthivhi J, Linganiso LZ, Deenadayalu N, Alabi OO. Biogas production and techno‐economic feasibility studies of setting up household biogas technology in Africa: A critical review. Energy Sci. Eng. 2024, 12, 4788-4806. DOI:10.1002/ese3.1887

[96] Taramasso MA, Motaghi M, Casasso A. A techno-economic feasibility analysis of solutions to cover the thermal and electrical demands of anaerobic digesters. Renew. Energy 2024, 236, 121485. DOI:10.1016/j.renene.2024.121485

[97] Arias A, Feijoo G, Moreira MT. Benchmarking environmental and economic indicators of sludge management alternatives aimed at enhanced energy efficiency and nutrient recovery. J. Environ. Manag. 2021, 279, 111594. DOI:10.1016/j.jenvman.2020.111594

[98] Anh LTL, Le Luu T. Greenhouse gas emissions from municipal wastewater treatment: Global insights and Vietnam’s approach. Curr. Opin. Environ. Sci. Health 2025, 47, 100655. DOI:10.1016/j.coesh.2025.100655

[99] Chen S, Liu H. Achieving low-carbon and sustainable wastewater treatment by controlling the flow of pollutants: A pilot scale investigation on reducing greenhouse gas emissions and enhancing resource recovery. J. Water Process Eng. 2024, 64, 105665. DOI:10.1016/j.jwpe.2024.105665

[100] Carmona-Martínez AA, Bartolomé C, Jarauta-Córdoba CA. The Role of Biogas and Biomethane as Renewable Gases in the Decarbonization Pathway to Zero Emissions. Energies 2023, 16, 6164. DOI:10.3390/en16176164

[101] Farghali M, Osman AI, Umetsu K, Rooney DW. Integration of biogas systems into a carbon zero and hydrogen economy: A review. Environ. Chem. Lett. 2022, 20, 2853-2927. DOI:10.1007/s10311-022-01468-z