Advances in Biogas Desulfurization Printed Edition of the Special Issue Published in ChemEngineering www.mdpi.com/journal/ChemEngineering Martín Ramírez Edited by Advances in Biogas Desulfurization Advances in Biogas Desulfurization Special Issue Editor Mart ́ ın Ram ́ ırez MDPI • Basel • Beijing • Wuhan • Barcelona • Belgrade • Manchester • Tokyo • Cluj • Tianjin Special Issue Editor Mart ́ ın Ram ́ ırez University of C ́ adiz Spain Editorial Office MDPI St. Alban-Anlage 66 4052 Basel, Switzerland This is a reprint of articles from the Special Issue published online in the open access journal ChemEngineering (ISSN 2305-7084) (available at: https://www.mdpi.com/journal/ ChemEngineering/special issues/Biogas Desulfurization). For citation purposes, cite each article independently as indicated on the article page online and as indicated below: LastName, A.A.; LastName, B.B.; LastName, C.C. Article Title. Journal Name Year , Article Number , Page Range. ISBN 978-3-03928-660-7 (Pbk) ISBN 978-3-03928-661-4 (PDF) c © 2020 by the authors. Articles in this book are Open Access and distributed under the Creative Commons Attribution (CC BY) license, which allows users to download, copy and build upon published articles, as long as the author and publisher are properly credited, which ensures maximum dissemination and a wider impact of our publications. The book as a whole is distributed by MDPI under the terms and conditions of the Creative Commons license CC BY-NC-ND. Contents About the Special Issue Editor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii Preface to ”Advances in Biogas Desulfurization” . . . . . . . . . . . . . . . . . . . . . . . . . . . ix Mart ́ ın Ram ́ ırez Special Issue “Advances in Biogas Desulfurization” Reprinted from: ChemEngineering 2020 , 4 , 17, doi:10.3390/chemengineering4010017 . . . . . . . . 1 Antonio Velasco, Mariana Franco-Morgado, Sergio Revah, Luis Alberto Arellano-Garc ́ ıa, Mat ́ ıas Manzano-Zavala and Armando Gonz ́ alez-S ́ anchez Desulfurization of Biogas from a Closed Landfill under Acidic Conditions Deploying an Iron-Redox Biological Process Reprinted from: ChemEngineering 2019 , 3 , 71, doi:10.3390/chemengineering3030071 . . . . . . . . 5 Fernando Almenglo, Mart ́ ın Ram ́ ırez and Domingo Cantero Application of Response Surface Methodology for H 2 S Removal from Biogas by a Pilot Anoxic Biotrickling Filter Reprinted from: ChemEngineering 2019 , 3 , 66, doi:10.3390/chemengineering3030066 . . . . . . . . 15 Samir Prioto Tayar, Renata de Bello Solcia Guerrero, Leticia Ferraresi Hidalgo and Denise Bevilaqua Evaluation of Biogas Biodesulfurization Using Different Packing Materials Reprinted from: ChemEngineering 2019 , 3 , 27, doi:10.3390/chemengineering3010027 . . . . . . . . 27 Oseweuba Valentine Okoro and Zhifa Sun Desulphurisation of Biogas: A Systematic Qualitative and Economic-Based Quantitative Review of Alternative Strategies Reprinted from: ChemEngineering 2019 , 3 , 76, doi:10.3390/chemengineering3030076 . . . . . . . . 39 Sylvie Le Borgne and Guillermo Baquerizo Microbial Ecology of Biofiltration Units Used for the Desulfurization of Biogas Reprinted from: ChemEngineering 2019 , 3 , 72, doi:10.3390/chemengineering3030072 . . . . . . . . 69 v About the Special Issue Editor Mart ́ ın Ram ́ ırez (Ph.D.) has been an Associate Professor in the Faculty of Science of the University of Cadiz (UCA, Spain) since 2017. He obtained a B.S. in Chemical Engineering in 2002 and M.S. and Ph.D. degrees in the Chemical Science and Technology program at the same institution in 2007. He was the head of the chemical engineering section in a technology driven spin-off for 3 years (2008–2011). His research activities have mainly focused on effluent gases biofiltration, such as air (odor removal), biogas (desulfurization and upgrading), and bioremediation of high value metals. Currently, he is a partner team leader of a LIFE EU project (Biogasnet) and project leader of two projects related to the bioremediation of platinum group metals from three-way catalysts. He is co-author of 28 scientific contributions to ISI journals (H-index of 14, Scopus databases), 2 patents, 7 books chapters, more than 40 conference proceedings in international congresses; he has supervised five M.S. students, four Ph.D. theses, and four Ph.D. theses are in progress. M.R. belongs to the Editorial Board of ChemEngineering and serves as Guest Editor for ChemEnginering in this Special Issue “Advances in Biogas Desulfurization”. He is regularly involved in the peer-review in ISI journals (more than 100 since 2009) and he has participated in the peer-reviewing processes of technical and scientific projects (NCSTE of Kazakahstan and UEFISCDI of Romania). vii Preface to ”Advances in Biogas Desulfurization” The environmental impacts of non-renewable energies and crude oil depletion have increased the use of biogas. Biogas is a renewable energy source produced under anaerobic conditions by the degradation of organic matter. However, before its valorization, it needs to be desulfurized (H 2 S removal) and/or upgraded (CO 2 removal). The main biogas uses are heat and power production, injection into the natural gas grid, fuel for solid oxide fuel cells, biogas reforming, and vehicle fuel. In all these applications, biogas needs to be desulfurized because H 2 S causes corrosion and sulfur oxides emissions during biogas combustion. In the last 15 years, the number of desulfurization technologies have increased and their performance has improved. However, shortcomings remain, such as elemental sulfur accumulation avoidance (packed bed bioreactors) and its separation (suspension biomass bioreactors). In addition, these technologies need to be extended to demonstration and industrial scales. This Special Issue shows some advances in biogas desulfurization. Velasco et al. (Desulfurization of Biogas from a Closed Landfill under Acidic Conditions Deploying an Iron-Redox Biological Process) removed H 2 S using a system composed of an absorption bubble column and a biotrickling filter. Almenglo et al. (Application of Response Surface Methodology for H 2 S Removal from Biogas by a Pilot Anoxic Biotrickling Filter) studied and modeled H 2 S removal using an anoxic biotrickling filter. Tayar et al. (Evaluation of Biogas Biodesulfurization Using Different Packing Materials) researched the effect of four packing materials for an anoxic biotrickling filter. Okoro and Sun (A Systematic Qualitative and Economic-Based Quantitative Review of Alternative Strategies) reviewed the current state of biogas desulfurization technologies and the annual operation and annualized capital cost per unit of volume. Le Borgne and Baquerizo (Microbial Ecology of Biofiltration Units Used for the Desulfurization of Biogas) reviewed the microbial ecology of biofiltration units. The main characteristics of sulfur-oxidizing chemotrophic bacteria are presented. As the Guest Editor, I hope that the readership will find the information published in this Special Issue about biogas desulfurization useful. Finally, I would like to thank the effort contributed by all the authors, the publisher and the reviewers that allowed this book to be published. Mart ́ ın Ram ́ ırez Special Issue Editor ix ChemEngineering ȱ 2020 , ȱ 4 , ȱ 17 ȱ www.mdpi.com/journal/chemengineering ȱ Editorial ȱ Special ȱ Issue ȱ “Advances ȱ in ȱ Biogas ȱ Desulfurization” ȱ Martín ȱ Ramírez ȱ Department ȱ of ȱ Chemical ȱ Engineering ȱ and ȱ Food ȱ Technologies, ȱ Wine ȱ and ȱ Agrifood ȱ Research ȱ Institute ȱ (IVAGRO), ȱ Faculty ȱ of ȱ Sciences, ȱ University ȱ of ȱ Cadiz, ȱ Puerto ȱ Real, ȱ 11510 ȱ Cádiz, ȱ Spain; ȱ martin.ramirez@uca.es; ȱ Tel.: ȱ +34 Ȭ 956 Ȭ 01 Ȭ 6286 ȱ Received: ȱ 5 ȱ March ȱ 2020; ȱ Accepted: ȱ 5 ȱ March ȱ 2020; ȱ Published: ȱ 9 ȱ March ȱ 2020 ȱ Abstract: ȱ This ȱ Special ȱ Issue ȱ contains ȱ three ȱ articles ȱ and ȱ two ȱ reviews. ȱ The ȱ biological ȱ reactors ȱ used ȱ in ȱ the ȱ studies ȱ were ȱ fed ȱ with ȱ real ȱ biogas ȱ from ȱ Landfill ȱ or ȱ STPs. ȱ One ȱ research ȱ article ȱ concerns ȱ the ȱ use ȱ of ȱ a ȱ pilot ȱ scale ȱ plant ȱ with ȱ a ȱ combined ȱ process ȱ with ȱ a ȱ chemical ȱ and ȱ biological ȱ system. ȱ The ȱ other ȱ two ȱ studies ȱ concern ȱ anoxic ȱ biotrickling ȱ filters, ȱ with ȱ one ȱ study ȱ focused ȱ on ȱ the ȱ study ȱ of ȱ variable ȱ operation ȱ and ȱ its ȱ optimization ȱ through ȱ the ȱ response ȱ surface ȱ methodology, ȱ and ȱ the ȱ other ȱ focused ȱ on ȱ the ȱ selection ȱ of ȱ packing ȱ material. ȱ The ȱ reviews ȱ concern ȱ the ȱ current ȱ state ȱ of ȱ biogas ȱ desulfurization ȱ technologies, ȱ including ȱ an ȱ economic ȱ analysis, ȱ and ȱ the ȱ microbial ȱ ecology ȱ in ȱ biofiltration ȱ units. ȱ This ȱ Issue ȱ highlights ȱ some ȱ of ȱ the ȱ most ȱ relevant ȱ aspects ȱ about ȱ biogas ȱ desulfurization. ȱ Keywords: ȱ hydrogen ȱ sulfide; ȱ biogas; ȱ desulfurization; ȱ biotrickling ȱ filter; ȱ anoxic; ȱ response ȱ surface ȱ methodology; ȱ microbial ȱ ecology; ȱ sulfur Ȭ oxidizing ȱ bacteria; ȱ packing ȱ material; ȱ anaerobic ȱ digestion ȱ 1. Introduction This ȱ Special ȱ Issue ȱ contains ȱ the ȱ invited ȱ submissions ȱ to ȱ a ȱ Special ȱ Issue ȱ of ȱ ChemEngineering ȱ on ȱ the ȱ topic ȱ “Advanced ȱ Biogas ȱ Desulfurization” ȱ [1–5]. ȱ Three ȱ research ȱ articles ȱ [1–3] ȱ and ȱ two ȱ reviews ȱ [4,5] ȱ have ȱ been ȱ published. ȱ Biogas ȱ is ȱ a ȱ renewable ȱ energy ȱ source ȱ produced ȱ by ȱ the ȱ biodegradation ȱ of ȱ organic ȱ matter ȱ under ȱ anaerobic ȱ conditions. ȱ The ȱ use ȱ of ȱ renewable ȱ energies ȱ is ȱ increasing ȱ due ȱ to ȱ global ȱ warming ȱ and ȱ the ȱ increasing ȱ price ȱ of ȱ fossil ȱ fuels. ȱ However, ȱ biogas ȱ needs ȱ to ȱ be ȱ desulfurized ȱ prior ȱ to ȱ use. ȱ The ȱ biogas ȱ composition ȱ mainly ȱ depends ȱ on ȱ the ȱ feedstock ȱ (sludge ȱ from ȱ sewage ȱ treatment ȱ plants ȱ (STPs), ȱ waste ȱ from ȱ the ȱ agri Ȭ food ȱ industry, ȱ the ȱ organic ȱ fraction ȱ of ȱ municipal ȱ solid ȱ waste, ȱ livestock ȱ manure, ȱ etc.), ȱ with ȱ the ȱ main ȱ components ȱ being ȱ methane ȱ (45%–75%) ȱ and ȱ carbon ȱ dioxide ȱ (20%–50%). ȱ However, ȱ hydrogen ȱ sulfide ȱ (H 2 S) ȱ leads ȱ to ȱ corrosion ȱ and ȱ the ȱ combustion ȱ of ȱ non Ȭ desulfurized ȱ biogas ȱ produces ȱ the ȱ emission ȱ of ȱ SOx ȱ in ȱ flue ȱ gases. ȱ The ȱ H 2 S ȱ concentration ȱ can ȱ range ȱ from ȱ a ȱ few ȱ ppm V ȱ (0.5– 700 ȱ ppm V ȱ in ȱ landfill ȱ gas) ȱ up ȱ to ȱ more ȱ than ȱ 30,000 ȱ ppm V ȱ in ȱ the ȱ plant’s ȱ pulp Ȭ making ȱ process; ȱ and ȱ biogas ȱ flow ȱ rates ȱ can ȱ be ȱ in ȱ the ȱ range ȱ from ȱ several ȱ hundred ȱ cubic ȱ meters ȱ per ȱ hour ȱ (usually ȱ in ȱ STPs) ȱ to ȱ several ȱ thousand ȱ cubic ȱ meters ȱ per ȱ hour ȱ (usually ȱ in ȱ landfills). ȱ The ȱ applications ȱ of ȱ biogas ȱ are ȱ also ȱ wide Ȭ ranging, ȱ with ȱ the ȱ most ȱ common ȱ being ȱ burning ȱ in ȱ motors ȱ to ȱ produce ȱ electricity ȱ or ȱ electricity ȱ and ȱ heat. ȱ However, ȱ biogas ȱ can ȱ also ȱ be ȱ used ȱ as ȱ a ȱ fuel ȱ for ȱ solid ȱ oxide ȱ fuel ȱ cells ȱ or ȱ for ȱ hydrogen ȱ production ȱ by ȱ biogas ȱ reforming. ȱ Moreover, ȱ in ȱ the ȱ case ȱ of ȱ upgrading ȱ (CO 2 ȱ removal) ȱ biogas ȱ can ȱ be ȱ injected ȱ into ȱ the ȱ natural ȱ gas ȱ grid ȱ or ȱ used ȱ as ȱ fuel ȱ for ȱ vehicles. ȱ This ȱ wide ȱ range ȱ of ȱ biogas ȱ flow ȱ rates, ȱ H 2 S ȱ concentrations, ȱ types ȱ of ȱ applications ȱ and ȱ purification ȱ requirements, ȱ as ȱ well ȱ as ȱ biogas ȱ sources, ȱ results ȱ in ȱ a ȱ wide ȱ variety ȱ of ȱ technologies, ȱ which ȱ can ȱ be ȱ subdivided ȱ into ȱ those ȱ that ȱ involve ȱ physicochemical ȱ phenomena ȱ and ȱ those ȱ that ȱ involve ȱ biological ȱ processes. ȱ This ȱ Special ȱ Issue ȱ aims ȱ to ȱ bring ȱ together ȱ the ȱ scientific/technical ȱ advances ȱ on ȱ physicochemical ȱ and/or ȱ biological ȱ processes ȱ for ȱ biogas ȱ desulfurization. ȱ Biogas ȱ desulfurization ȱ is ȱ considered ȱ to ȱ be ȱ essential ȱ by ȱ many ȱ stakeholders ȱ (biogas ȱ producers, ȱ suppliers ȱ of ȱ biogas ȱ upgrading ȱ devices, ȱ gas ȱ traders, ȱ researchers, ȱ etc.) ȱ around ȱ the ȱ world, ȱ as ȱ the ȱ importance ȱ of ȱ biogas ȱ desulfurization ȱ to ȱ allow ȱ its ȱ valorization ȱ is ȱ well ȱ understood. ȱ 1 ChemEngineering ȱ 2020 , ȱ 4 , ȱ 17 ȱ 2 ȱ of ȱ 4 ȱ 2. ȱ Brief ȱ Overview ȱ of ȱ the ȱ Contributions ȱ to ȱ This ȱ Special ȱ Issue ȱ Velasco ȱ et ȱ al. ȱ [1] ȱ carried ȱ out ȱ the ȱ desulfurization ȱ of ȱ landfill ȱ biogas ȱ (‘Prados ȱ de ȱ la ȱ Montaña’, ȱ Mexico) ȱ at ȱ the ȱ pilot ȱ scale ȱ by ȱ a ȱ combined ȱ process ȱ involving ȱ chemical ȱ and ȱ biological ȱ treatments. ȱ In ȱ this ȱ study, ȱ an ȱ Absorption ȱ Bubble ȱ Column ȱ (ABC) ȱ was ȱ used ȱ in ȱ which ȱ H 2 S ȱ was ȱ absorbed ȱ and ȱ oxidized ȱ to ȱ elemental ȱ sulfur ȱ by ȱ ferric ȱ sulfate. ȱ A ȱ biotrickling ȱ filter ȱ (BTF) ȱ was ȱ employed ȱ for ȱ ferric ȱ sulfate ȱ regeneration ȱ (oxidation ȱ of ȱ Fe 2+ ȱ produced ȱ in ȱ the ȱ ABC ȱ to ȱ Fe 3+ ) ȱ by ȱ an ȱ enriched ȱ acidophilic ȱ mineral Ȭ oxidizing ȱ bacteria ȱ consortium ȱ (AMOB). ȱ The ȱ first ȱ reported ȱ application ȱ of ȱ an ȱ iron Ȭ based ȱ process ȱ and ȱ biological ȱ regeneration ȱ was ȱ reported ȱ in ȱ 1984 ȱ by ȱ Barium ȱ Chemical ȱ Ltd. ȱ and ȱ this ȱ approach ȱ is ȱ known ȱ as ȱ the ȱ Bio Ȭ SR ȱ process ȱ using ȱ Acidithiobacillus ȱ ferrooxidans ȱ and ȱ a ȱ jet Ȭ scrubber ȱ for ȱ H 2 S ȱ oxidation. ȱ Since ȱ then, ȱ numerous ȱ configurations ȱ have ȱ been ȱ published, ȱ but ȱ the ȱ main ȱ drawbacks ȱ are ȱ related ȱ to ȱ the ȱ elemental ȱ sulfur ȱ separation ȱ and ȱ jarosite ȱ formation. ȱ Jarosite ȱ helps ȱ to ȱ develop ȱ the ȱ biofilm ȱ growth ȱ but ȱ it ȱ reduces ȱ the ȱ amount ȱ of ȱ Fe 3+ ȱ and, ȱ therefore, ȱ its ȱ formation ȱ must ȱ be ȱ controlled, ȱ usually ȱ by ȱ controlling ȱ the ȱ pH. ȱ The ȱ effect ȱ of ȱ no ȱ pH ȱ control ȱ and ȱ no ȱ forced ȱ convection ȱ of ȱ air ȱ on ȱ the ȱ iron ȱ oxidation ȱ rates ȱ and ȱ H 2 S ȱ removal ȱ were ȱ studied. ȱ In ȱ this ȱ study, ȱ removal ȱ efficiencies ȱ (REs) ȱ higher ȱ than ȱ 99.5% ȱ were ȱ achieved ȱ for ȱ H 2 S ȱ concentrations ȱ in ȱ the ȱ range ȱ 120–250 ȱ ppm V ȱ (Empty ȱ Bed ȱ Residence ȱ Time ȱ (EBRT) ȱ of ȱ 4.5 ȱ min ȱ in ȱ the ȱ ABC). ȱ The ȱ system ȱ was ȱ successfully ȱ operated ȱ for ȱ around ȱ seven ȱ months ȱ with ȱ minimal ȱ energy ȱ input ȱ and ȱ a ȱ metastable ȱ operation ȱ the ȱ zone ȱ between ȱ 2FeOH 2+ ȱ and ȱ Fe 2+ ȱ However, ȱ elemental ȱ sulfur ȱ was ȱ accumulated ȱ on ȱ the ȱ packed ȱ bed ȱ of ȱ the ȱ BTF ȱ and ȱ this ȱ was ȱ not ȱ recovered ȱ in ȱ the ȱ settler ȱ due ȱ to ȱ its ȱ colloidal ȱ nature. ȱ Almenglo ȱ et ȱ al. ȱ [2] ȱ studied ȱ the ȱ H 2 S ȱ removal ȱ from ȱ biogas ȱ produced ȱ in ȱ an ȱ STP ȱ (‘Bahía ȱ Gaditana’, ȱ Cádiz, ȱ Spain) ȱ by ȱ an ȱ anoxic ȱ BTF ȱ at ȱ the ȱ pilot ȱ scale ȱ (packed ȱ bed ȱ volume ȱ of ȱ 0.167 ȱ m 3 ). ȱ The ȱ effect ȱ of ȱ the ȱ biogas ȱ flow ȱ rate, ȱ trickling ȱ liquid ȱ velocity ȱ (TLV) ȱ and ȱ nitrate ȱ concentration ȱ on ȱ the ȱ H 2 S ȱ RE ȱ and ȱ elimination ȱ capacity ȱ (EC) ȱ were ȱ studied ȱ using ȱ a ȱ full ȱ factorial ȱ design ȱ (3 3 ). ȱ Anoxic ȱ biofiltration ȱ is ȱ a ȱ promising ȱ technology ȱ for ȱ biogas ȱ desulfurization ȱ because ȱ it ȱ avoids ȱ biogas ȱ dilution ȱ and ȱ reduces ȱ the ȱ risk ȱ of ȱ explosion ȱ when ȱ compared ȱ to ȱ aerobic ȱ BTFs. ȱ In ȱ fact, ȱ in ȱ the ȱ past ȱ seven ȱ years, ȱ there ȱ has ȱ been ȱ a ȱ significant ȱ increase ȱ in ȱ the ȱ number ȱ of ȱ published ȱ studies ȱ in ȱ this ȱ area. ȱ In ȱ this ȱ study, ȱ the ȱ highest ȱ H 2 S ȱ RE ȱ values ȱ were ȱ obtained ȱ at ȱ a ȱ TLV ȱ of ȱ 15.27 ȱ m ȱ h ƺ 1 , ȱ with ȱ RE ȱ values ȱ of ȱ 99.53, ȱ 97.65 ȱ and ȱ 92.13% ȱ for ȱ EBRTs ȱ of ȱ 600, ȱ 200 ȱ and ȱ 120 ȱ s, ȱ respectively. ȱ Therefore, ȱ the ȱ maximum ȱ and ȱ critical ȱ ECs ȱ were ȱ 158.83 ȱ gS ȱ m ƺ 3 ȱ h ƺ 1 ȱ (RE ȱ 92.13%) ȱ and ȱ 34.93 ȱ gS ȱ m ƺ 3 ȱ h ƺ 1 ȱ (RE ȱ 99.53%), ȱ respectively. ȱ Higher ȱ values ȱ can ȱ be ȱ found ȱ in ȱ the ȱ literature ȱ for ȱ laboratory ȱ scale ȱ systems ȱ with ȱ a ȱ higher ȱ height:diameter ȱ ratio, ȱ but ȱ this ȱ pilot ȱ plant ȱ was ȱ one ȱ of ȱ the ȱ first ȱ anoxic ȱ BTFs ȱ to ȱ be ȱ installed ȱ in ȱ an ȱ STP, ȱ thus ȱ demonstrating ȱ the ȱ feasibility ȱ of ȱ this ȱ technology ȱ under ȱ real ȱ operating ȱ conditions ȱ (fluctuations ȱ in ȱ the ȱ biogas ȱ composition, ȱ weather, ȱ etc.). ȱ Moreover, ȱ experimental ȱ data ȱ were ȱ adjusted ȱ using ȱ Ottengraf’s ȱ model, ȱ which ȱ allows ȱ the ȱ H 2 S ȱ concentration ȱ along ȱ the ȱ bed ȱ to ȱ be ȱ predicted ȱ in ȱ a ȱ simple ȱ way. ȱ In ȱ contrast, ȱ dynamic ȱ models ȱ have ȱ been ȱ published ȱ previously ȱ by ȱ the ȱ same ȱ authors ȱ and ȱ this ȱ provided ȱ a ȱ better ȱ understanding ȱ of ȱ the ȱ process—although ȱ these ȱ models ȱ are ȱ more ȱ complex ȱ and ȱ are ȱ seldom ȱ used. ȱ Tayar ȱ et ȱ al. ȱ [3] ȱ studied ȱ different ȱ packing ȱ materials ȱ for ȱ an ȱ anoxic ȱ BTF ȱ (packed ȱ bed ȱ volume ȱ of ȱ 3 ȱ L). ȱ One ȱ of ȱ the ȱ main ȱ drawbacks ȱ of ȱ BTFs, ȱ both ȱ aerobic ȱ and ȱ anoxic, ȱ is ȱ the ȱ clogging ȱ of ȱ the ȱ packed ȱ bed. ȱ Clogging ȱ can ȱ be ȱ caused ȱ by ȱ the ȱ accumulation ȱ of ȱ elemental ȱ sulfur ȱ and/or ȱ biomass ȱ growth. ȱ However, ȱ in ȱ most ȱ cases ȱ it ȱ is ȱ due ȱ to ȱ elemental ȱ sulfur ȱ accumulation, ȱ since ȱ the ȱ biofilm ȱ growth ȱ rate ȱ is ȱ low. ȱ It ȱ can ȱ be ȱ seen ȱ from ȱ the ȱ literature ȱ that ȱ elemental ȱ sulfur ȱ production ȱ can ȱ be ȱ controlled ȱ by ȱ increasing ȱ the ȱ electron ȱ acceptor ȱ feed ȱ (nitrate, ȱ nitrite ȱ or ȱ oxygen) ȱ but ȱ sulfur ȱ formation ȱ is ȱ unavoidable. ȱ In ȱ this ȱ respect, ȱ the ȱ selection ȱ of ȱ the ȱ packing ȱ material ȱ is ȱ critical ȱ as ȱ it ȱ will ȱ affect ȱ the ȱ amount ȱ of ȱ sulfur ȱ and ȱ biomass ȱ that ȱ can ȱ attach ȱ to ȱ the ȱ packing ȱ in ȱ the ȱ bed. ȱ In ȱ this ȱ study, ȱ four ȱ packing ȱ materials ȱ were ȱ tested: ȱ strips ȱ of ȱ polyvinyl ȱ chloride ȱ (PVC), ȱ polyethylene ȱ terephthalate ȱ (PET), ȱ polytetrafluoroethylene ȱ (Teflon ® ) ȱ and ȱ open Ȭ pore ȱ polyurethane ȱ foam ȱ (OPUF). ȱ PVC ȱ was ȱ chosen ȱ due ȱ to ȱ the ȱ high ȱ concentration ȱ of ȱ biomass, ȱ although ȱ it ȱ was ȱ lower ȱ than ȱ for ȱ OPUF, ȱ and ȱ its ȱ low ȱ cost. ȱ The ȱ BTF ȱ performance ȱ showed ȱ high ȱ H 2 S ȱ removal ȱ (95.72%) ȱ and ȱ EC ȱ (98 ȱ gS ȱ m ƺ 3 ȱ h ƺ 1 ), ȱ with ȱ values ȱ similar ȱ to ȱ those ȱ obtained ȱ in ȱ previous ȱ studies ȱ carried ȱ out ȱ with ȱ OPUF. ȱ Therefore, ȱ PVC ȱ could ȱ be ȱ a ȱ potential ȱ low Ȭ cost ȱ packing ȱ material ȱ for ȱ use ȱ in ȱ BTFs. ȱ Okoro ȱ and ȱ Sun ȱ [4] ȱ submitted ȱ an ȱ interesting ȱ review ȱ about ȱ the ȱ current ȱ state ȱ of ȱ biogas ȱ desulfurization ȱ technologies: ȱ physicochemical, ȱ biological, ȱ in ȱ situ, ȱ and ȱ post Ȭ biogas ȱ desulfurization ȱ 2 ChemEngineering ȱ 2020 , ȱ 4 , ȱ 17 ȱ 3 ȱ of ȱ 4 ȱ strategies. ȱ Moreover, ȱ a ȱ review ȱ of ȱ the ȱ annual ȱ operation ȱ and ȱ annualized ȱ capital ȱ cost ȱ per ȱ unit ȱ volume ȱ was ȱ carried ȱ out. ȱ To ȱ perform ȱ a ȱ cost ȱ analysis ȱ is ȱ a ȱ challenging ȱ undertaking ȱ and ȱ there ȱ are ȱ very ȱ few ȱ published ȱ studies. ȱ Biogas ȱ stakeholders ȱ could ȱ make ȱ decisions ȱ about ȱ the ȱ best ȱ technology ȱ based ȱ on ȱ these ȱ results. ȱ However, ȱ there ȱ are ȱ numerous ȱ factors ȱ that ȱ make ȱ the ȱ comparison ȱ complex: ȱ lack ȱ of ȱ data ȱ from ȱ companies, ȱ differences ȱ between ȱ studies ȱ (scale, ȱ source ȱ of ȱ biogas, ȱ etc.), ȱ location ȱ in ȱ different ȱ countries, ȱ supplies ȱ (prices ȱ of ȱ electricity, ȱ chemicals, ȱ etc.), ȱ etc. ȱ In ȱ this ȱ review, ȱ the ȱ authors ȱ carried ȱ out ȱ a ȱ thorough ȱ review; ȱ for ȱ instance, ȱ studies ȱ from ȱ member ȱ countries ȱ of ȱ the ȱ OECD ȱ and ȱ uncertainties ȱ about ȱ the ȱ 50%–150% ȱ variation ȱ in ȱ the ȱ cost ȱ were ȱ considered. ȱ The ȱ study ȱ shows ȱ that ȱ in ȱ situ ȱ chemical ȱ dosing ȱ is ȱ the ȱ cheapest ȱ biogas ȱ desulfurization ȱ technique, ȱ although ȱ limitations ȱ were ȱ identified ȱ in ȱ terms ȱ of ȱ the ȱ system ȱ control ȱ costs ȱ and ȱ environmental ȱ impact ȱ due ȱ to ȱ the ȱ continuous ȱ chemical ȱ supply. ȱ Moreover, ȱ the ȱ integration ȱ of ȱ several ȱ technologies ȱ could ȱ be ȱ of ȱ interest ȱ to ȱ reduce ȱ the ȱ weaknesses ȱ of ȱ each ȱ desulfurization ȱ strategy. ȱ Le ȱ Borgne ȱ and ȱ Baquerizo ȱ [5] ȱ present ȱ a ȱ review ȱ on ȱ the ȱ microbial ȱ ecology ȱ of ȱ biofiltration ȱ units ȱ for ȱ biogas ȱ desulfurization. ȱ Moreover, ȱ a ȱ review ȱ of ȱ the ȱ biofiltration ȱ technologies ȱ is ȱ included: ȱ conventional ȱ biofilter, ȱ BTF ȱ and ȱ bioscrubbers. ȱ Biological ȱ H 2 S ȱ oxidation ȱ can ȱ be ȱ carried ȱ out ȱ under ȱ aerobic ȱ or ȱ anoxic ȱ conditions. ȱ Therefore, ȱ the ȱ main ȱ chemotrophic ȱ Sulfur ȱ Oxidizing ȱ Bacteria ȱ (SOB) ȱ will ȱ depend ȱ on ȱ the ȱ final ȱ electron ȱ acceptor. ȱ The ȱ review ȱ shows ȱ the ȱ microbial ȱ ecology ȱ in ȱ aerobic ȱ and ȱ anoxic ȱ BTFs ȱ through ȱ molecular ȱ techniques ȱ such ȱ as ȱ fingerprint ȱ methods ȱ (PCR Ȭ DGGE, ȱ T Ȭ RFLP), ȱ fluorescence ȱ in ȱ situ ȱ hybridization ȱ (FISH), ȱ or ȱ next ȱ generation ȱ sequencing ȱ technologies ȱ (450 Ȭ pyrosequencing ȱ or ȱ Illumina ȱ platforms). ȱ As ȱ one ȱ would ȱ expect, ȱ the ȱ environmental ȱ conditions ȱ had ȱ a ȱ direct ȱ impact ȱ on ȱ the ȱ diversity ȱ of ȱ bacterial ȱ communities ȱ and ȱ their ȱ structure ȱ and ȱ dynamics. ȱ However, ȱ not ȱ enough ȱ is ȱ currently ȱ known ȱ about ȱ the ȱ role ȱ of ȱ the ȱ main ȱ populations ȱ in ȱ these ȱ bioreactors. ȱ 3. ȱ Gaps ȱ in ȱ Biogas ȱ Desulfurization ȱ In ȱ the ȱ past ȱ 15 ȱ years, ȱ there ȱ have ȱ been ȱ significant ȱ advances ȱ in ȱ the ȱ development ȱ of ȱ biological ȱ desulfurization ȱ processes. ȱ However, ȱ there ȱ are ȱ still ȱ shortcomings ȱ that ȱ require ȱ further ȱ investigation. ȱ To ȱ my ȱ mind, ȱ one ȱ of ȱ the ȱ main ȱ issues ȱ is ȱ the ȱ formation ȱ of ȱ elemental ȱ sulfur ȱ in ȱ bioreactors ȱ with ȱ packing ȱ material, ȱ such ȱ as ȱ BTFs, ȱ although ȱ there ȱ has ȱ been ȱ a ȱ significant ȱ advance ȱ in ȱ the ȱ increase ȱ of ȱ the ȱ oxygen ȱ mass ȱ transfer ȱ in ȱ aerobic ȱ bioreactors, ȱ such ȱ as ȱ aerobic ȱ BTFs. ȱ In ȱ the ȱ case ȱ of ȱ anoxic ȱ bioreactors, ȱ it ȱ is ȱ possible ȱ to ȱ feed ȱ the ȱ system ȱ with ȱ high ȱ nitrate ȱ or ȱ nitrite ȱ concentrations, ȱ although ȱ this ȱ entails ȱ a ȱ significant ȱ cost ȱ (environmental ȱ and ȱ economic) ȱ in ȱ terms ȱ of ȱ the ȱ use ȱ of ȱ chemical ȱ compounds. ȱ However, ȱ the ȱ feasibility ȱ of ȱ feeding ȱ nitrified ȱ effluent ȱ from ȱ ammonium Ȭ rich ȱ wastewater ȱ has ȱ been ȱ demonstrated, ȱ thus ȱ avoiding ȱ the ȱ above ȱ impact. ȱ In ȱ any ȱ case, ȱ elemental ȱ sulfur ȱ formation ȱ is ȱ unavoidable ȱ and ȱ further ȱ research ȱ is ȱ needed ȱ to ȱ prevent ȱ its ȱ accumulation. ȱ A ȱ possible ȱ solution ȱ would ȱ be ȱ the ȱ use ȱ of ȱ suspended ȱ biomass ȱ bioreactors, ȱ in ȱ which ȱ clogging ȱ would ȱ be ȱ avoided. ȱ In ȱ these ȱ bioreactors, ȱ a ȱ new ȱ issue ȱ would ȱ be ȱ the ȱ separation ȱ of ȱ elemental ȱ sulfur ȱ in ȱ an ȱ economic ȱ way. ȱ Another ȱ important ȱ gap ȱ is ȱ to ȱ determine ȱ the ȱ role ȱ of ȱ the ȱ key ȱ populations ȱ to ȱ avoid ȱ operational ȱ outages ȱ in ȱ the ȱ bioreactors. ȱ Likewise, ȱ progress ȱ can ȱ be ȱ made ȱ in ȱ the ȱ control ȱ systems ȱ by ȱ improving ȱ the ȱ mathematical ȱ model ȱ using ȱ the ȱ latest ȱ advances ȱ in ȱ microsensors. ȱ A ȱ microsensor ȱ has ȱ been ȱ developed ȱ to ȱ measure ȱ pH ȱ and ȱ O 2 ȱ profiles ȱ in ȱ the ȱ biofilm ȱ and, ȱ in ȱ this ȱ respect, ȱ it ȱ would ȱ be ȱ interesting ȱ to ȱ develop ȱ new ȱ microsensors ȱ to ȱ measure ȱ sulfide, ȱ nitrate ȱ and ȱ nitrite ȱ profiles. ȱ A ȱ high ȱ priority ȱ area ȱ is ȱ the ȱ scaling Ȭ up ȱ of ȱ desulfurization ȱ technologies ȱ to ȱ the ȱ demonstration ȱ or ȱ industrial ȱ scales. ȱ Many ȱ of ȱ these ȱ systems ȱ have ȱ only ȱ reached ȱ the ȱ pilot ȱ scale, ȱ so ȱ it ȱ is ȱ necessary ȱ to ȱ develop ȱ larger ȱ plants ȱ in ȱ order ȱ to ȱ obtain ȱ long Ȭ term ȱ operational ȱ data, ȱ to ȱ determine ȱ the ȱ operational ȱ limits ȱ and ȱ to ȱ evaluate ȱ economic ȱ and ȱ environmental ȱ impacts. ȱ In ȱ this ȱ regard, ȱ some ȱ studies ȱ have ȱ already ȱ been ȱ carried ȱ out, ȱ but ȱ they ȱ are ȱ very ȱ scarce, ȱ and ȱ greater ȱ effort ȱ is ȱ needed ȱ in ȱ this ȱ direction. ȱ In ȱ other ȱ cases, ȱ such ȱ as ȱ aerobic ȱ BTFs, ȱ all ȱ of ȱ this ȱ information ȱ is ȱ available ȱ and ȱ a ȱ better ȱ dissemination ȱ in ȱ companies ȱ is ȱ necessary ȱ in ȱ order ȱ to ȱ increase ȱ the ȱ number ȱ of ȱ industrial ȱ plants. ȱ All ȱ of ȱ this ȱ information ȱ will ȱ allow ȱ the ȱ evaluation ȱ of ȱ these ȱ technologies ȱ and ȱ enable ȱ the ȱ installation ȱ of ȱ new ȱ plants. ȱ It ȱ is ȱ also ȱ necessary ȱ to ȱ reduce ȱ the ȱ gas ȱ residence ȱ time ȱ in ȱ order ȱ to ȱ design ȱ smaller ȱ equipment, ȱ minimize ȱ energy ȱ consumption ȱ and ȱ integrate ȱ desulfurization ȱ systems ȱ in ȱ plants ȱ to ȱ avoid ȱ the ȱ consumption ȱ of ȱ chemical ȱ reagents. ȱ Finally, ȱ it ȱ would ȱ be ȱ interesting ȱ to ȱ look ȱ for ȱ new ȱ biological ȱ processes ȱ for ȱ biogas ȱ revalorization. ȱ For ȱ instance, ȱ other ȱ value Ȭ 3 ChemEngineering ȱ 2020 , ȱ 4 , ȱ 17 ȱ 4 ȱ of ȱ 4 ȱ added ȱ products ȱ could ȱ be ȱ obtained ȱ from ȱ the ȱ methane ȱ and ȱ carbon ȱ dioxide ȱ present ȱ in ȱ the ȱ biogas ȱ that ȱ can ȱ be ȱ integrated ȱ into ȱ the ȱ biological ȱ desulfurization ȱ processes. ȱ Acknowledgments: ȱ I ȱ would ȱ like ȱ to ȱ thank ȱ the ȱ publisher ȱ and ȱ the ȱ Editorial ȱ staff ȱ Team, ȱ especially ȱ Joqeen ȱ Meng, ȱ for ȱ inviting ȱ me ȱ to ȱ be ȱ Guest ȱ Editor ȱ of ȱ this ȱ Special ȱ Issue. ȱ Moreover, ȱ I ȱ would ȱ like ȱ to ȱ thank ȱ all ȱ of ȱ the ȱ contributors ȱ and ȱ reviewers ȱ for ȱ their ȱ efforts. ȱ Conflicts ȱ of ȱ Interest: ȱ The ȱ author ȱ declares ȱ no ȱ conflict ȱ of ȱ interest ȱ References ȱ 1. Velasco, ȱ A.; ȱ Franco Ȭ Morgado, ȱ M.; ȱ Revah, ȱ S.; ȱ Arellano Ȭ García, ȱ L.; ȱ Manzano Ȭ Zavala, ȱ M.; ȱ González Ȭ Sánchez, ȱ A. ȱ Desulfurization ȱ of ȱ Biogas ȱ from ȱ a ȱ Closed ȱ Landfill ȱ under ȱ Acidic ȱ Conditions ȱ Deploying ȱ an ȱ Iron Ȭ Redox ȱ Biological ȱ Process. ȱ Chemengineering ȱ 2019 , ȱ 3 , ȱ 71. ȱ 2. Almenglo, ȱ F.; ȱ Ramírez, ȱ M.; ȱ Cantero, ȱ D. ȱ Application ȱ of ȱ Response ȱ Surface ȱ Methodology ȱ for ȱ H 2 S ȱ Removal ȱ from ȱ Biogas ȱ by ȱ a ȱ Pilot ȱ Anoxic ȱ Biotrickling ȱ Filter. ȱ Chemengineering ȱ 2019 , ȱ 3 , ȱ 66. ȱ 3. Tayar, ȱ S.; ȱ de ȱ Guerrero, ȱ R.; ȱ Hidalgo, ȱ L.; ȱ Bevilaqua, ȱ D. ȱ Evaluation ȱ of ȱ Biogas ȱ Biodesulfurization ȱ Using ȱ Different ȱ Packing ȱ Materials. ȱ ChemEngineering ȱ 2019 , ȱ 3 , ȱ 27. ȱ 4. Okoro, ȱ O.; ȱ Sun, ȱ Z. ȱ Desulphurisation ȱ of ȱ Biogas: ȱ A ȱ Systematic ȱ Qualitative ȱ and ȱ Economic Ȭ Based ȱ Quantitative ȱ Review ȱ of ȱ Alternative ȱ Strategies. ȱ Chemengineering ȱ 2019 , ȱ 3 , ȱ 76. ȱ 5. Le ȱ Borgne, ȱ S.; ȱ Baquerizo, ȱ G. ȱ Microbial ȱ Ecology ȱ of ȱ Biofiltration ȱ Units ȱ Used ȱ for ȱ the ȱ Desulfurization ȱ of ȱ Biogas. ȱ Chemengineering ȱ 2019 , ȱ 3 , ȱ 72. ȱ ȱ © ȱ 2020 ȱ by ȱ the ȱ authors. ȱ Submitted ȱ for ȱ possible ȱ open ȱ access ȱ publication ȱ under ȱ the ȱ terms ȱ and ȱ conditions ȱ of ȱ the ȱ Creative ȱ Commons ȱ Attribution ȱ (CC ȱ BY) ȱ license ȱ (http://creativecommons.org/licenses/by/4.0/). ȱ ȱ 4 chemengineering Article Desulfurization of Biogas from a Closed Landfill under Acidic Conditions Deploying an Iron-Redox Biological Process Antonio Velasco 1 , Mariana Franco-Morgado 2 , Sergio Revah 3, *, Luis Alberto Arellano-Garc í a 4 , Mat í as Manzano-Zavala 3 and Armando Gonz á lez-S á nchez 2, * 1 Departamento de Biotecnolog í a, Universidad Aut ó noma Metropolitana-Unidad Iztapalapa, Iztapalapa, 09340 Mexico City, Mexico 2 Instituto de Ingenier í a, Universidad Nacional Aut ó noma de Mexico, Circuito Escolar, Ciudad Universitaria, 04510 Mexico City, Mexico 3 Departamento de Procesos y Tecnolog í a, Universidad Aut ó noma Metropolitana-Unidad Cuajimalpa, Cuajimalpa, 09340 Mexico City, Mexico 4 C á tedras CONACYT-Centro de Investigaci ó n y Asistencia en Tecnolog í a y Diseño del Estado de Jalisco, Unidad de Tecnolog í a Ambiental. Av. Normalistas 800, Guadalajara, 44270 Jalisco, Mexico * Correspondence: srevah@correo.cua.uam.mx (S.R.); agonzalezs@iingen.unam.mx (A.G.-S.) Received: 30 April 2019; Accepted: 5 August 2019; Published: 7 August 2019 Abstract: Desulfurization processes play an important role in the use of biogas in the emerging market of renewable energy. In this study, an iron-redox biological process was evaluated at bench scale and pilot scale to remove hydrogen sulfide (H 2 S) from biogas. The pilot scale system performance was assessed with real biogas emitted from a closed landfill to determine the desulfurization capacity under outdoor conditions. The system consisted of an Absorption Bubble Column (ABC) and a Biotrickling Filter (BTF) with useful volumes of 3 L and 47 L, respectively. An acidophilic mineral-oxidizing bacterial consortium immobilized in polyurethane foam was utilized to regenerate Fe(III) ion, which in turn accomplished the continuous H 2 S removal from inlet biogas. The H 2 S removal e ffi ciencies were higher than 99.5% when H 2 S inlet concentrations were 120–250 ppmv, yielding a treated biogas with H 2 S < 2 ppmv. The ferrous iron oxidation rate (0.31 g · L − 1 · h − 1 ) attained when the system was operating in natural air convection mode showed that the BTF can operate without pumping air. A brief analysis of the system and the economic aspects are briefly analyzed. Keywords: biogas; hydrogen sulfide; removal process 1. Introduction The use of biogas from municipal landfills to obtain energy (electricity generation) is a growing trend worldwide as part of the quest for clean energy alternatives to the traditional fossil fuels [ 1 ]. Landfill biogas is produced by the anaerobic digestion of organic wastes, and its composition depends on the type and age of digested organic matter. Typically, landfill biogas is composed of methane (CH 4 ) 50% v, carbon dioxide (CO 2 ) 45% v, alkanes / alkenes (C 7 H 8 –C 16 H 34 ) 0.1–85.3 mg · m − 3 , chlorides (CCl 4 –C 2 HCl 3 ) 0.14–4.52 mg · m − 3 , mercury compounds (CH 3 Hg–(CH 3 )2Hg) 1–91 μ g m − 3 , siloxanes 1–17 mg · m − 3 , volatile organic compounds (VOCs) (benzenes, isopropyl benzene, halogenated compounds) 5–85 mg · m − 3 and hydrogen sulfide 0.005–2% v [ 2 ]. The H 2 S content depends on the composition and age of the waste disposed in the landfill besides the protein content in organic waste [3,4]. Hydrogen sulfide must be removed from biogas due to technical problems related to corrosion in pipes, pumps, engines, gas storage tanks and electric power plants, as well as the fact of it being ChemEngineering 2019 , 3 , 71 www.mdpi.com / journal / chemengineering 5 ChemEngineering 2019 , 3 , 71 a potential pollutant when it is combusted, producing sulfur dioxide (SO 2 ) [ 3 ]. This gas is further oxidized, which promotes acid rain containing sulfuric acid (H 2 SO 4 ) [ 5 ]. Additionally, it causes a bad odor at very low concentrations due to its low odor threshold (1 ppbv) [6]. The final use of biogas, composition and flow variability, concentration of H 2 S, and the absolute quantity of H 2 S to be removed define the requirements of the desulfurization technique to be deployed [ 4 ]. To remove the H 2 S content in a biogas stream, there are several physicochemical technologies with good removal e ffi ciencies ( > 99%) [ 3 ]. LO-CAT ® technology is an example of a physicochemical technology that has been applied in more than 120 plants around the world [ 7 ]. The removal mechanism is based on a series of chemical reactions of H 2 S with iron chelating agents under slightly alkaline conditions [ 8 , 9 ]. The products, after the chemical H 2 S removal, are elemental sulfur and ferrous ion, the former being recovered by sedimentation, while ferrous ion is continuously oxidized into ferric ion using an inlet air stream [ 7 ]. Moreover, under acid conditions (pH < 2) the chemical H 2 S reactions with ferric ions can be carried out without chelating agents because iron (both Fe(II) and Fe(III)) remain soluble without sulfide iron precipitation; however, under acidic conditions, the oxidation rate of ferrous ion by molecular oxygen is slow [ 7 ]. Certain bacteria, such as Acidithiobacillus ferrooxidans, play an important role in increasing the rate of ferrous iron oxidation into ferric iron. Meruane and Vargas [ 10 ] showed that at a low pH (pH < 5), the rate of bacterial oxidation of ferrous iron is about 10 4 times larger than the corresponding rate of chemical oxidation. This result indicates that acidophilic iron-oxidizing bacteria, such as A. ferrooxidans, are a promising microorganism for usage in desulfurization processes, regenerating Fe(III) biologically [ 7 ]. In combination with physicochemical oxidation, A. ferrooxidans can act as catalyst of reoxidation of ferrous ions to achieve the removal of H 2 S from biogas with lower operational and environmental costs compared with a sole physicochemical technology [ 9 ]. Nowadays, biological desulfurization treatments have gained attention due to the achieved removal e ffi ciencies ( > 99%) and are competitive with physicochemical methods. Some documented examples of biodesulfurization processes, including biological ferric ion regeneration, are biofilters, biotrickling filters, Biogas Cleaner ® , Biopuric ® , DMT filter ® , LO-CAT ® and SulFerox ® among others [ 4 , 7 , 11 – 13 ]. However, challenges remain in the scaling up of these technologies in terms of the consumption of chelated iron, pH control, and overall economic balance of the process [14]. The aim of this work was to present the experimental performance of an on-site chemical-biological desulfurization system removing H 2 S from biogas generated at a closed landfill. The e ff ects of no pH control and no forced convection of air on the iron oxidation rates and H 2 S removal were evaluated. 2. Materials and Methods 2.1. Microorganisms An enriched acidophilic mineral-oxidizing bacterial consortium (AMOB), obtained from the sediments and soil of an acid mine drainage in Taxco Guerrero Mexico, was used as inoculum for the biological oxidation of the ferric ion. The AMOB was grown in medium 9K [ 15 ] containing (g · L − 1 ): 3.0 (NH 4 ) 2 SO 4 , 0.5 MgSO 4 · 7H 2 O, 0.5 K 2 HPO 4 , 0.1 KCl, 0.01 Ca(NO 3 ) 2 and 44.8 FeSO 4 · 7H 2 O (corresponding to 9.9 g Fe(II) L − 1 ); the pH was adjusted to 1.6 with H 2 SO 4 2.2. Prototype Experimental System The prototype system was previously tested in lab conditions, feeding controlled H 2 S concentrations in defined air flow rates, which were made by mixing fresh air with a controlled flow of pure H 2 S. Further details can be found elsewhere [9]. Figure 1 shows the prototype system installed in the closed landfill “Prados de la Montaña” in the western part of Mexico City. The landfill was closed in 1992; however, it continues to produce biogas, and further details can be found elsewhere [ 16 ]. The prototype system was connected to a venting-outlet of the landfill through a peristatic pump that supplied the sour gas at a flow of 6 ChemEngineering 2019 , 3 , 71 960 L · d − 1 . The prototype experimental system called Hybrid System at Pilot Scale (HSPS) consisted of two columns: an absorption bubble column (ABC) and a biotrickling filter (BTF) with useful volumes of 3 L and 47 L, respectively, and interconnected by a recycled aqueous stream. The 960 L · d − 1 of sour biogas were fed at the bottom of the ABC co-currently with a 777 L · d − 1 stream of 9K medium coming from the bottom of the BTF. The desulfurized biogas stream obtained from the top of the ABC was captured for a posterior composition analysis. The BTF was packed with polyurethane foam (EDT, Germany) with a specific area of 600 m 2 · m − 3 , a density of 35 kg · m − 3 and a porosity of 0.97. The BTF was inoculated with the aforementioned AMOB. To keep aerobic conditions in the BTF, either a forced or a natural convective flow of air was implemented by pumping air at a flow of 82,000 L · d − 1 to the BTF or just by keeping two air vents at extreme opposed sides of the BTF open, respectively. The forced and natural convective airflow tests allowed for the evaluation of the re oxidizing rates of ferrous ions with a minimum input of energy for aeration. The 9K medium was trickled from the top of the BTF with a flow of 3740 L · d − 1 . The pH was maintained at 1.2 without an automatic control, and the temperature oscillated between 5 and 30 ◦ C due to the outdoor conditions prevailing in Mexico City. The water evaporation was compensated daily with fresh water, while the 9K medium was renewed every 3 months. The HSPS was operated continuously for around 7 months. Mineral medium Fresh water pH ORP DO Elemental S 0 purge Setting tank Absoption bubble column for oxidation of H 2 S Treated biogas Biotrickling filter for Fe 3+ regeneration Sour biogas from landfill Gas stream Liquid stream Solid purge Level balance tank Figure 1. Desulfurization Hybrid System at Pilot Scale (HSPS) installed on a landfill cover. The Fe(III) ion regeneration rate in the BTF was evaluated under a batch operation, with an initial Fe(II) concentration of around 4.5 L · d − 1 under the continuous recycling of the 9K aqueous medium at the conditions described above. The predominance zones diagrams for the stable iron and sulfur species with water under the experimental conditions were calculated with the software HSC Chemistry ® Version 4.1 (Outokumpu Research Oy, Pori, Finland). The software computes the predominance zones in the pH vs. Oxidation-Reduction Potential (ORP) graph (also called Pourbaix diagram) at equilibrium. The software inputs are the total molal sulfur and iron concentrations in the aqueous phase contained in the HSPS, as well as the system conditions (temperature and pressure). 7 ChemEngineering 2019 , 3 , 71 2.3. Analytical Methods During the operation of the desulfurization system, the gas phase H 2 S concentrations were continuously measured using an Odalog sensor with a range of 1–1000 ppm (App-Tek, distributed by Detection Instruments, Phoenix, AZ), which included a temperature sensor. In the aqueous phase, the total iron concentration in the 9K recycling medium was measured by titration with potassium dichromate and barium diphenylamine-sulfonate according to the method reported by Vogel [ 17 ]. Samples were collected from the bottom of the BTF and in the ABC. The ORP was measured with a polished platinum probe, using an Ag / AgCl electrode as a reference (EW-27018-40, Cole Parmer, Vernon Hills, IL, USA). The dissolved oxygen (DO) was monitored through a polarographic probe (Hanna Instruments, Woonsocket, RI, USA), and both ORP and DO were recorded online by means of a personal computer. 3. Results and Discussion 3.1. Oxidation of Ferrous Iron in the BTF Figure 2A shows the depletion of Fe(II) in the culture medium under continuous forced air supply to the BTF. In this experiment, the Fe(II) oxidation rate was around 8.16 g · L − 1 · d − 1 , which, compared with the value of 0.19 g · L − 1 · h − 1 reported by Daoud and Karamanev [ 18 ], it shows that ferrous iron-oxidizing bacteria consortia used in our study have an adequate response to the reactor fixed conditions (pH, nutrients, airstream, flow recirculation, etc.). Moreover, Figure 2B shows that Fe(II) oxidation in the BTF was e ff ective both under air forced convection (initial 250 min of experiment) and under the natural convection mode (final 130 min). The ferrous iron oxidation rate was calculated as 7.44 g · L − 1 · d − 1 in the natural convection mode, being similar to the rate obtained with the forced air convection (9.12 g · L − 1 · d − 1 ). These results suggested that the BTF can operate under a natural convection mode to accomplish the biological oxidation of Fe(II). The dissolved oxygen concentration in the trickling liquid remained constant around 0.0065 g · L − 1 for both assays, confirming that for this BTF the oxygen mass transfer did not limit the Fe(II) biological oxidati