Master’s dissertation submitted in partial fulfilment of the requirements for the joint degree of

International Master of Science in Environmental Technology and Engineering an Erasmus Mundus Master Course jointly organized by UGent (Belgium), ICTP (Prague) and UNESCO‐IHE (the Netherlands)

Academic year 2013 – 2014

Use of a silicone bio-membrane for H2S removal from biogas Host University: Institute of Chemical Technology, Prague

Ana A. Alvarez da Costa Promotor: Prof. Ing. Pavel Jeniček, CSc. Co-promoter: Ing. Jan Bartáček, Ph.D.

This thesis was elaborated at Institute of Chemical Technology, Prague and defended at Institute of Chemical Technology, Prague within the framework of the European Erasmus Mundus Programme “Erasmus Mundus International Master of Science in Environmental Technology and Engineering " (Course N° 2011-0172) © 2014 Prague, Ana A. Alvarez da Costa, Ghent University, all rights reserved.

INSTITUTE OF CHEMICAL TECHNOLOGY, PRAGUE Faculty of Environmental Technology Department of Water Technology and Environmental Engineering

MASTER THESIS

Use of a silicone bio-membrane for H2S removal from biogas

Author:

Ana A. Alvarez da Costa

Supervisor: Consultant:

Prof. Ing. Pavel Jeníček, CSc. Ing. Jan Bartáček, PhD.

Study Program:

Environmental Technology Environmental Technology 2014

Subprogram: Year:

Engineering

and

Engineering

and

DECLARATION This thesis was written at the Department of Water Technology and Environmental Engineering of the Institute of Chemical Technology in Prague from February to September of 2014.

I hereby declare that this thesis is my own work. Where other sources of information have been used, they have been acknowledged and referenced in the list of used literature and other sources. I have been informed that the rights and obligations implied by Act No. 121/2000 Coll. on Copyright, Rights Related to Copyright and on the Amendment of Certain Laws (Copyright Act) apply to my work. In particular, I am aware of the fact that the Institute of Chemical Technology in Prague has the right to sign a license agreement for use of this work as school work under §60 paragraph 1 of the Copyright Act. I have also been informed that in the case that this work will be used by myself or that a license will be granted for its usage by another entity, the Institute of Chemical Technology in Prague is entitled to require from me a reasonable contribution to cover the costs incurred in the creation of the work, according to the circumstances up to the full amount. I agree to the publication of my work in accordance with Act No. 111/1998 Coll. on Higher Education and the amendment of related laws (Higher Education Act).

In Prague on 1st of September of 2014.

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SUMMARY The effectiveness of using a silicone bio-membrane for the removal of hydrogen sulfide from biogas was tested. A lab-scale bio-membrane unit (BMU) simulating the placement of the bio-membrane in the headspace of an anaerobic reactor was built. Changes in concentration of nitrogen (N2), oxygen (O2), methane (CH4), carbon dioxide (CO2) and hydrogen sulfide (H2S) were monitored and measured for three different configurations of the experimental setup and three different conditions in the surface of the membrane: dry surface, liquid layer and biofilm layer. The results showed that in all cases the silicone membrane allows the transfer of gases through it from biogas side to air side (and vice versa) as long as there is enough driving force (difference in partial pressures on each side of the membrane) to induce it. Regarding air and biogas composition, N2, CH4 and CO2 followed basically the same behavior in all configurations and conditions. Final concentration of these gases in the biogas was: 9% for N2, 59% for CH4 and 28% for CO2. The presence of the biofilm seemed to have a negligible effect in the transfer of these gases through the membrane. Final O2 concentration in the biogas side was much lower for phase III (1.5-1.8 mg L-1) than for phases I and II (3.5 mg L-1), suggesting that the O2 transferred was consumed by the biofilm on the surface of the membrane translating in a lower amount of O2 actually reaching the biogas. H2S concentration in the biogas decreased much faster with the bio-membrane (1.18 mg L-1 h-1) than in phases I and II, -0.4 and -0.5 mg L-1 h-1, respectively. The biomembrane showed higher H2S removal efficiency by reaching an overall removal efficiency of 55-70%, after 3 h of experiment achieving a final concentration in the biogas of 1.5-1.8 mg L-1 (initially 5 mg L-1). Light yellow deposits were observed on the surface of the biomembrane which might indicate the possible formation of elemental sulfur due to biological oxidation of H2S in the biofilm; however, no test was performed to corroborate this. Membrane characterization was done by determination of the permeability of all the gases in every configuration studied. The results show that the permeation of gases in decreasing order is as follows: H2S > CO2 > CH4 > O2 > N2.

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ACKNOWLEDGEMENT Thank you to Prof. Ing. Pavel Jeníček, CSc. for allowing me to participate in this project, and for your time and dedication throughout this whole process. Very special thanks to Ing. Jan Bartáček, Ph.D. for your patience, support and guidance; not only during the thesis but also during my whole stay in ICTP. Very special thanks to Bubu and Mamma Bear for their endless love and support. I could not have made it this far without you and I am proud to be your daughter. Thank you to all my IMETE classmates for becoming like a second family and making these last two years an experience that I will never forget. Thank you to my two favorite people in the lab, Lucie Krayzelová and Petr Dolejš, for your friendship, help and for all the good laughs we had even on the most stressful days. Thank you to Mrs. Helena Stará for your kindness and technical support in the analytical tests performed in the lab. Thank you to Ing. Aleš Pícha for his incredible work in building everything we needed for the experimental setup. Thank you the IMETE Management Board for creating this program and giving me the opportunity to participate in it. This research was financially supported by The Technology Agency of Czech Republic – project A03021413.

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TABLE OF CONTENTS 1. 2.

INTRODUCTION ......................................................................................................... 1 LITERATURE REVIEW .............................................................................................. 2 2.1 Anaerobic treatment ..................................................................................... 2 2.1.1 Microbiology and processes involved.................................................... 2 2.1.2 General composition and uses of biogas. ............................................... 3 2.2 The problem with hydrogen sulfide in biogas .............................................. 5 2.2.1 Methods for biogas desulfurization........................................................ 6 2.2.2 Microaeration ......................................................................................... 9 2.3 Application of membrane processes in environmental treatment technologies ................................................................................................................. 11 2.3.1 Use of bio-membranes in microaeration .............................................. 12 3. MATERIALS AND METHODS ................................................................................ 16 3.1 Experimental setup ..................................................................................... 16 3.1.1 Phase I: Gas - Gas ................................................................................ 18 3.1.2 Phase II: Gas - Liquid .......................................................................... 21 3.1.3 Phase III: Gas - Biofilm ....................................................................... 22 3.2 Analytical methods ..................................................................................... 25 3.2.1 Gas composition (CH4, CO2, O2, N2, H2) ............................................. 25 3.2.2 Hydrogen sulfide content in the gas..................................................... 25 3.2.3 Hydrogen sulfide recovered in the traps .............................................. 26 3.2.4 Dissolved Oxygen ................................................................................ 26 3.2.5 Sludge characterization ........................................................................ 26 3.2.6 Reject water characterization ............................................................... 28 4. RESULTS .................................................................................................................... 30 4.1 Intrinsic losses in the system ...................................................................... 30 4.2 Gas composition ......................................................................................... 31 4.2.1 Phase I: Gas-Gas .................................................................................. 31 4.2.2 Phase II: Gas-Liquid ............................................................................ 35 4.2.3 Phase III: Gas-Biofilm ......................................................................... 39 4.3 Membrane permeability.............................................................................. 43 4.4 Membrane flux ........................................................................................... 44 5. DISCUSSION .............................................................................................................. 48 5.1 Gas composition ......................................................................................... 48 5.1.1 Phase I: Gas-Gas .................................................................................. 48 5.1.2 Phase II: Gas-Liquid ............................................................................ 49 5.1.3 Phase III: Gas-Biofilm ......................................................................... 51 5.2 Membrane permeability.............................................................................. 54 5.2.1 Phase I: Gas-Gas .................................................................................. 56 5.2.2 Phase II: Gas-Liquid ............................................................................ 56 5.3 Flux of gases through the membrane.......................................................... 57 5.4 Implications of the results .......................................................................... 57 6. CONCLUSIONS ......................................................................................................... 59 7. BIBLIOGRAPHY ....................................................................................................... 60 APPENDIX A. Molar Balance .............................................................................................64 APPENDIX B. Effect of gas flow rate on permeability ......................................................74

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LIST OF FIGURES 3. MATERIALS AND METHODS Fig. 3.1. Schematic representation of the bio-membrane unit (BMU) ........................... 16 Fig. 3.2. Schematic representation of “open loop” configuration (Phase I) ................... 19 Fig. 3.3. Schematic representation of “closed loop” configuration (Phase I) ................. 20 Fig. 3.4. Schematic representation of “dead end” configuration (Phase I) ..................... 21 Fig. 3.5. Schematic representation of “closed loop” configuration (Phase II) ............... 22 Fig. 3.6. Schematic representation of “closed loop” configuration (Phase III) .............. 23 4. RESULTS Fig. 4.1. Intrinsic losses – Experimental data ................................................................. 31 Fig. 4.2. N2 composition – Biogas and Air side (Phase I) .............................................. 32 Fig. 4.3. O2 composition – Biogas and Air side (Phase I) .............................................. 33 Fig. 4.4. CH4 composition – Biogas and Air side (Phase I) ........................................... 33 Fig. 4.5. CO2 composition – Biogas and Air side (Phase I) ........................................... 34 Fig. 4.6. H2S composition – Biogas and Air side (Phase I) ............................................ 35 Fig. 4.7. N2 composition – Biogas and Air side (Comparison between phase I & II) .... 36 Fig. 4.8. O2 composition – Biogas and Air side (Comparison between phase I & II) .... 37 Fig. 4.9. CH4 composition – Biogas and Air side (Comparison between phase I & II) . 37 Fig. 4.10. CO2 composition – Biogas and Air side (Comparison between phase I & II) 38 Fig. 4.11. H2S composition – Biogas and Air side (Comparison between phase I & II) 38 Fig. 4.12. Biofilm and deposits formed on the surface of the membrane (Phase III) ..... 39 Fig. 4.13. N2 composition – Biogas and Air side (Experimental data: phase I, II & III) 40 Fig. 4.14. O2 composition – Biogas and Air side (Experimental data: phase I, II & III) 41 Fig. 4.15. CH4 composition – Biogas and Air side (Experimental data: phase I, II & III) ........................................................................................................................................ 42 Fig. 4.16. CO2 composition – Biogas and Air side (Experimental data: phase I, II & III) ........................................................................................................................................ 42 Fig. 4.17. H2S composition – Biogas and Air side (Experimental data: phase I, II & III) ........................................................................................................................................ 43

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Fig. 4.18. Flux of gases through the membrane (Phase I – Open Loop) ........................ 45 Fig. 4.19. Flux of gases through the membrane (Phase I – Closed Loop) ...................... 46 Fig. 4.20. Flux of gases through the membrane (Phase I – Dead End) .......................... 46 Fig. 4.21. Flux of gases through the membrane (Phase II) ............................................. 47 5. DISCUSSION Fig. 5.1. Rate of change in H2S concentration on biogas side (Comparison between phase I & II) .................................................................................................................... 50 Fig. 5.2. Rate of change in CO2 concentration on biogas side (Comparison between phase I & II) .................................................................................................................... 51 Fig. 5.3. Rate of change in H2S concentration on biogas side (Comparison between phases I, II & III) ............................................................................................................ 53 APPENDIX A. Molar balance Fig. A.1. Schematic representation of the flow of gases in each loop …………………64 Fig. A.2. Flows associated to the Intrinsic Losses mass balance ……………………...71 APPENDIX B. Effect of gas flow rate on permeability Fig. B.1. N2 composition – Biogas and Air side (Phase II) – Flow rate effect ………...75 Fig. B.2. O2 composition – Biogas and Air side (Phase II) – Flow rate effect ………...75 Fig. B.3. CH4 composition – Biogas and Air side (Phase II) – Flow rate effect ………76 Fig. B.4. CO2 composition – Biogas and Air side (Phase II) – Flow rate effect ………76 Fig. B.5. H2S composition – Biogas and Air side (Phase II) – Flow rate effect ………77

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LIST OF TABLES 2. LITERAURE REVIEW Table 2.1. Typical contents of biogas (Spellman, 2013) .................................................. 4 Table 2.2. Permeability of different gases in PDMS membranes ................................... 15 Table 2.3. Solubility of different gases in PDMS membranes ....................................... 15

3. MATERIALS AND METHODS Table 3.1 Specifications of the membrane...................................................................... 17 Table 3.2. Characterization of sludge for biofilm inoculum........................................... 24 Table 3.3. Characterization of reject water used in phase III ......................................... 24

4. RESULTS Table 4.1. Intrinsic H2S losses of each loop ................................................................... 30 Table 4.2. H2S recovered in the traps (open loop configuration) ................................... 35 Table 4.3. Permeability for each gas............................................................................... 44 Table 4.4. Adjustment parameter for each gas ............................................................... 44

5. DISCUSSION Table 5.1. Mole fraction solubility of gases in pure water (25 °C) (Lide, 2003) ........... 51 Table 5.2. Average permeability for each gas and configuration ................................... 55 APPENDIX B. Effect of gas flow rate on permeability Table B.1. Permeability for each gas (Phase II) – Flow rate effect ……………………74

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ABBREVIATIONS BG

Biogas

BMU

Bio-membrane unit

BOD

Biochemical oxygen demand

CHP

Combined heat and power unit (or co-generation unit)

COD

Chemical oxygen demand

DO

Dissolved oxygen

ICTP

Institute of Chemical Technology, Prague

ID

Inner diameter

MBR

Membrane bioreactor

OD

Outer diameter

P(I)

Phase I (Gas-Gas)

P(II)

Phase II (Gas-Liquid)

P(III)

Phase III (Gas-Biofilm)

PDMS

Poly dimethyl siloxane

SOB

Sulfur oxidizing bacteria

STP

Standard temperature (273.15 K) and pressure (1 atm).

TS

Total solids

TSS

Total suspended solids

VFA

Volatile fatty acids

VOC

Volatile organic compounds

VSS

Volatile suspended solids

WWTP

Wastewater treatment plant

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1. INTRODUCTION Anaerobic digestion is one of the most common technologies used for the treatment of high-strength wastewater and waste solids. Its main byproduct is biogas which consists mainly of methane and carbon dioxide and traces of other gases such as hydrogen, water vapor, hydrogen sulfide, nitrogen, among others. Because of its calorific value, biogas is a promising renewable energy source that can be used for many applications such as gas fuel for vehicles and heat and power generation, feedstock for chemical production, and natural gas replacement. However, the corrosive nature of hydrogen sulfide limits the use of biogas on any of these applications. Therefore, biogas desulfurization is of great importance. There are many technologies available for biogas desulfurization; based on biological, chemical or physicochemical principles. Nowadays, biological methods are becoming more important since they have limited or no consumption of toxic chemicals as additives/catalysts, have potentially low operation costs, and do not require the use of materials tolerant to high temperature and pressure. Microaeration is a novel biological desulfurization method that consists on the introduction of small amounts of oxygen into an anaerobic biochemical process to enable both anaerobic and aerobic biological activities to occur within a single bioreactor (Botheju & Bakke, 2011). For this, air is usually used because it is an accessible and low cost source of oxygen. Even though many studies have proved the effectiveness of microaeration in biogas desulfurization, a disadvantage of this method is the dilution of the biogas by the presence of surplus nitrogen from the dosed air. The use of a bio-membrane is a promising approach to microaeration because its selectivity could allow the control of gas flow (nitrogen and oxygen) to the biogas, as well as serve as support medium for biofilm growth. The main objective of this Master Thesis is to evaluate the effectiveness of using a silicone bio-membrane for the removal of hydrogen sulfide from biogas. To achieve this, it was necessary to design and perform several lab-scale experiments to: (1) characterize the mass transfer of gases through the silicone membrane, (2) determine the membrane permeability of the main components in air and biogas, and (3) evaluate the changes in hydrogen sulfide concentration in the biogas for different configurations.

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2. LITERATURE REVIEW 2.1 Anaerobic treatment Anaerobic treatment is a controlled process where the organic matter is degraded by a consortium of microorganisms (bacteria and archaea) in the absence of free oxygen. The overall result is a nearly complete conversion of the biodegradable materials into biogas (primarily methane and carbon dioxide with trace amounts of hydrogen, hydrogen sulfide and other minority components), and new biomass (Mudhoo, 2012; Nayono, 2009; Spinosa & Vesilind, 2001). The process is used mainly for the treatment of waste sludge and high strength wastewater; however, applications for dilute waste streams have also become more common (Metcalf & Eddy, 2003). Compared to aerobic technologies such as activated sludge, the anaerobic treatment technologies offer some advantages such as: (1) lower operation costs due to the absence of need for aeration; (2) the fact that the amount of produced waste biosolids (sludge) that must be handled, dewatered, and disposed of is less than that of aerobic systems by approximately a factor of ten; (3) production of methane, which is a valuable by-product that can be used as a source of energy to operate motors of pumps or for generating heat and power usable in the treatment facility, and (4) hygienization and stabilization of sludge in case of its digestion (Woodard & Curran, 2006; Spinosa & Vesilind, 2001; Lim & Wang, 2013). In contrast, some of the disadvantages of this treatment are: (1) longer retention times required to develop and maintain methane forming microorganisms, (2) relatively high capital costs due to all equipment required (circulation pumps, covered tanks, heat exchangers, compressors, gasholders, etc.), and (3) highly polluted supernatant in case of sludge digestion (Spinosa & Vesilind, 2001). Most anaerobic treatments take place in two specific temperature ranges: mesophilic and thermophilic. The temperature ranges are of the order of 30-38°C and 50-57 °C, respectively (Appels et al., 2008). 2.1.1 Microbiology and processes involved The whole process is commonly divided into four interrelated steps: hydrolysis, fermentation (also known as acidogenesis), acetogenesis and methanogenesis (Nayono, 2009). 2

Both hydrolysis and fermentation are performed by fermentative bacteria (Spinosa & Vesilind, 2001). During hydrolysis, these microorganisms use extra-cellular enzymes to convert complex organic polymers such as polysaccharides, proteins and lipids into soluble products (e.g. triglycerides, fatty acids, amino acids, and sugars). The size of these products must be small enough to allow their transport across the membrane of bacterial cells (Liu & Lipták, 2000; Nayono, 2010). After that, through fermentation (acidogenesis), the monomers produced during hydrolysis are degraded through many fermentative pathways. The degradation of these compounds results in the production of carbon dioxide, hydrogen gas, alcohols, organic acids, some organic-nitrogen compounds, and some organic-sulfur compounds (Nayono, 2010). In these first two stages no chemical oxygen demand (COD) or biochemical oxygen demand (BOD) reduction is carried out since complex organic molecules are merely converted into short-chain fatty acids, alcohols, and new bacterial cells, which exert an oxygen demand (Liu & Lipták, 2000). In acetogenesis low molecular weight volatile fatty acids (VFAs) produced during fermentation are converted by acetogenic bacteria into acetate, hydrogen gas and carbon dioxide (Nayono, 2010). Finally, during methanogenesis, methane gas is produced by a group of microorganisms collectively called methanogens (Metcalf & Eddy, 2003). Out of the methane produced, approximately 72% is formed by acetoclastic methanogens which split acetate into methane and carbon dioxide; while the rest (around 28%) is formed by hydrogen-utilizing methanogens which use hydrogen and carbon dioxide to produce methane (Metcalf & Eddy, 2003). Actual ratio depends on the composition of the degraded substrate. Even though the products of one step are required for the next one, all reactions happen simultaneously in a well-balanced system. 2.1.2 General composition and uses of biogas Biogas is the main byproduct generated by the consortia of microorganisms feeding off the biodegradable matter contained in the inlet of the reactor (Spellman, 2013). It consists mainly of methane (CH4) and carbon dioxide (CO2) with traces of other gases such as hydrogen (H2), water vapor, hydrogen sulfide (H2S), nitrogen (N2), volatile organic compounds (VOC), siloxanes, among others (Abbasi. et al., 2012). Methane

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content in biogas can range from 50-75% (by volume) but it is usually between 55 and 65% (Abbasi. et al., 2012). Table 1.1 presents the typical contents of biogas. Biogas yield and methane content varies for different substrates, biological consortia and digestion conditions (Abbasi et al., 2012). Longer digestion process yields higher methane content (Nijaguna, 2002). Moreover, methane content depends on the digestion temperature; low digestion temperature gives higher methane content, but less biogas is produced (Nijaguna, 2002). Typically, biogas has a calorific value of 35 - 44 kJ/g which is comparable to kerosene, petrol, diesel and LPG (butane) and higher than many solid fuels like coal, charcoal wood, among others (Basu et al., 2010). Additionally, the combustion of methane produces less carbon dioxide per unit of heat released than any other hydrocarbon fuel which makes it more environmentally friendly when referring to global warming (Lin et al., 2000). However, since CO2 is non-combustible gas, if the methane content is considerably lower than 50%, the biogas becomes no longer combustible (Nijaguna, 2002). Because biogas can be produced from wastes, residues and energy crops it could be considered as a renewable energy source that could play a vital role in future (Weiland, 2010). Table 2.1. Typical contents of biogas (Spellman, 2013)

Component

Percentage (%)

Methane

50-75

Carbon dioxide

25-50

Nitrogen

0-10

Hydrogen

0-1

Hydrogen sulfide

0-3

Oxygen

0-2

Biogas characteristics allow it to be used in many applications such as gaseous fuel for vehicles, feedstock for producing chemicals and materials, natural gas replacement (after conversion to biomethane), and as substitute of fossil fuels in power and heat production (Weiland, 2010).

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Traditionally, biogas is used as fuel in Combined Heat and Power (CHP) units (also known as co-generation units) as an efficient, clean and reliable approach to generating power and thermal energy from a single fuel source (Spellman, 2013). This type of power plants has an efficiency of up to 90% and produces approximately 35% of electricity and 65% of heat (Mudhoo, 2012). In addition to heat or power generation, several studies introduced biogas for microalgal growth, the process of which utilizes the carbon dioxide captured in the cultured media. The microalgal biomass is subsequently processed to produce biodiesel, dietary supplements, and animal feeds (Lin et al., 2013). Whatever the application, its quality and purity are crucial (Ramos et al., 2013). The presence of incombustible and acid gases, like CO2 and H2S, not only reduces its calorific value, but their corrosive nature also reduces the possibilities of biogas utilization in many applications (Basu et al., 2010; Lin et al., 2013). Therefore, biogas processing to remove these undesirable components is of great importance. 2.2 The problem with hydrogen sulfide in biogas Sulfur is a very important compound in the aquatic environment and it is mainly present as sulfate (SO42-) (Russell, 2006). When anaerobic conditions are present, sulfate reducing bacteria (SRB) use part of the hydrogen produced during anaerobic digestion to degrade S-containing compounds (mainly proteins) and/or reduce anionic species (particularly sulfate, SO42-) contained in the feedstock of the digester converting them to sulfide (S 2-) (Russell, 2006; Nayono, 2009; Ramos et al., 2013). The production range of hydrogen sulfide varies considerably from one process to another and depends on the amount of bioavailable sulfur compounds in the feedstock, the pH of the reactor, and the outcome of S-reducing microorganisms and methanogens, both competing for the same substrate (Díaz et al., 2011; Jansen et al., 2009). The concentration of H2S in the biogas from sludge digesters of municipal wastewater treatment plants varies considerably in the literature; however, concentrations below 50 mg m-3 or above 10000 mg m-3 are exceptional (Jenicek et al., 2013). H2S is both toxic and volatile and its concentration in both the biogas and the digester can cause many problems. Biologically speaking, if the concentration of soluble sulfides exceeds 200 mg L-1 then the metabolic activity of methanogens will be strongly

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inhibited, leading to process failure (Díaz et al., 2011). Actual value may vary depending on the pH in the system. Moreover, the presence of H2S has a great influence when the different uses of biogas are considered. It causes corrosion to metal parts and its effect on non-ferrous metals in components such as pressure regulators, gas meters, valves and mountings is even more serious (Nijaguna, 2002). Its chemical reaction and those of its combustion product, sulfur dioxide (SO2), quickly lead to severe corrosion and wear on engines, making it particularly harmful when biogas is used in internal combustion engines, such as the ones used in CHP units (Nijaguna, 2002). Therefore, the maximum permissible concentration when biogas is employed for CHP is between 100 and 500 mg/Nm3, depending on the manufacturer, and when utilized as fuel for vehicles the content must be lower than 5 mg/Nm3 (Díaz et al., 2011). Similarly low limits you can find for biomethane production. Nonetheless, H2S removal is also required for health and safety reasons. It causes an irritating, rotten-egg smell above 1 ppm (1.4 mg / m3), and at concentrations above 10 ppm the toxicological exposure limits are exceeded (Jansen et al., 1999). Also, SO2 produced during its combustion contributes to acid rain formation (Jansen et al., 1999). State laws and regulations have been issued in Europe to minimize its presence in all parts of the biogas plants, including bioreactors, gasholders, ignition and storage tanks, etc. (Ramos et al., 2013). Thus, appropriate biogas conditioning through desulfurization techniques can help reduce environmental and health issues, as well as save operational costs for CHP unit maintenance and also improve their efficiency and life span. 2.2.1 Methods for biogas desulfurization The removal of H2S is required for health, environmental and health reasons; and also to avoid operation and maintenance problems in installations intended for biogas use. Most of the commercial and well-established technologies used in full-scale applications rely on physical and/or chemical processes (Ramos et al., 2013; Díaz et al., 2011). Physical methods such as membrane separation, water scrubbing, and active carbon processes are fast and effective, but are not economical because of the frequent media replacement that is required when dealing with high H2S loading elimination. Chemical

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methods require the addition of chemicals (e.g. sodium hydroxide, chlorine, hypochlorite, chlorine dioxide, iron salts, etc.), which are costly, most of the time unhealthy, and produce secondary wastes (Jansen et al., 1999; Syed & Henshaw, 2003). Moreover, some chemical regeneration processes are dangerous (e.g. large amounts of heat produced during iron oxide regeneration) (Lin et al., 2013). Some methods, namely adsorption and absorption, combine a chemical reaction and physical absorption under high pressure in an amine or glycol solution to reduce H2S concentrations in the gas (Ramos et al., 2013; Jansen et al., 1999). Although physical and chemical treatments are rapid and efficient, their large investment and operational costs (e.g. high pressures, high temperatures or special chemicals), chemical requirement and production of secondary pollutants are unfavorable, especially for medium-low productions (Jansen et al., 1999; Díaz et al., 2011; Ho et al., 2013). Thus, the search for more economical methods has led to investigations into microbiological solutions (Jansen et al., 1999). Biological methods have shown great potential in this respect. Some of the significant advantages over conventional methods are: (1) potentially low operation costs; (2) limited or no consumption of toxic chemicals as additives/catalysts; and (3) materials tolerant to high temperature and pressure are not required (Ramos et al., 2013; Díaz et al., 2011; Syed & Henshaw, 2003). In general, it is considered that biological processes can achieve more deepness of H2S removal than physicochemical processes because of the extremely high substrate affinity of sulfur oxidizing bacteria (SOB) (Kobayashi et al., 2012). Nowadays, biological desulfurization takes place in additional units, represented mainly by biofilters, biotrickling filters, and bioscrubbers, In the aforementioned units, H2S is solubilized in a humid packed bed where SOB are immobilized and grown as a biofilm in the presence of oxygen (O2) (Ramos et al., 2013). Thiopaq (Paques) and Biopuric (Biothane Corporation) are the only two industrial biotechnologies that have been specifically developed for the H2S removal from biogas. Both of them combine a chemical scrubber and a bioreactor. Additionally, the Thiopaq process includes a settler to separate the formed solid (elemental sulfur, S0) from the liquid phase (Ramos et al., 2013).

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All these processes are based on the sulfur cycle, more specifically, in the H2S oxidation (Ramos et al., 2013). The biological removal is based on the utilization of SOB which are able to oxidize hydrogen sulfide to obtain energy when O2 is present as electron acceptor. This oxidation is proposed to take place in stages through several intermediates as shown in Eq. 2.1 (Díaz et al., 2011). SH-
S0
S2O32-
S4O62-
SO42-

Eq. 2.1

The predominance of elemental sulfur (S0) or sulfate (SO42-) as the final product of the oxidation depends on the oxygen availability; thus, in limited oxygen conditions, S0 is the main product (Díaz et al., 2011). Reaction of H2S with O2 for the production of S0 and SO42- are presented in Eq. 2.2 and 2.3, respectively (Díaz et al., 2010). H2S + 0.5 O2
S0 + H2O

∆G° = -201.4 kJ/reaction

Eq. 2.2

H2S + 2 O2
SO42- + 2 H+

∆G° = -798.2 kJ/reaction

Eq. 2.3

Biological processes that directly metabolize H2S into SO42- in an efficient and inexpensive way have been reported. However, the drop in pH caused by generation of SO42- has negative effects in the process (see Eq. 2.3) (Ho et al., 2013). The partial oxidation of H2S to S0 instead of SO42- has several advantages such as: (1) less oxygen demand; (2) possible S0 recovery for reuse; and (3) it is possible to prevent the formation of acidification-causing protons (Syed & Henshaw, 2003; Kobayashi et al., 2012). As a by-product, biological S0 is a thermodynamically stable, non-corrosive and nontoxic solid that is easy to handle and transport (Syed & Henshaw, 2003; Ramos et al., 2013; Henshaw & Zhu, 2001). Elemental sulfur is 3 to 8 times more valuable by mass than sulfuric acid (H2SO4) since it contains more sulfur per unit of mass than any other form (Henshaw et al., 1998). Its main uses include: feedstock for the chemical, fertilizer and materials manufacturing industries (Syed & Henshaw, 2003), bioleaching, and H2SO4 production (Ramos et al., 2013). Based on this, in recent years S0 formation has become a major focus on the biodesulfurization process. As sulfide oxidation level is regulated by the O2-H2S balance and S0 formation is favor by low O2 concentrations, some biogas plants have been using biological desulfurization by adding a small amount of air to the anaerobic reactor 8

(microaerobic conditions) to remove H2S from the biogas (Ramos et al., 2013; Kobayashi et al., 2012). 2.2.2 Microaeration The definition of “microaerobic” conditions is not fixed yet. The term has been used in several publications over the years, but its usage has become confusing due to the fact that different authors utilize it with different meanings in their particular perspectives (Botheju & Bakke, 2011; Jenicek et al., 2008). Depending on the particular research, microaerobic condition have been described as a limited concentration of dissolved oxygen below 1 mg/L, 0.2 mg/L or 0.05 mg/L (Jenicek et al., 2010). Nevertheless, “microaeration” can be defined as the introduction of small amounts of oxygen into an anaerobic biochemical process to enable both anaerobic and aerobic biological activities to occur within a single bioreactor (Botheju & Bakke, 2011). This process has been used conventionally for the desulfurization of biogas, and recently it has shown to be an alternative pretreatment to enhance hydrolysis of complex organic matter (Lim & Wang, 2013). Traditionally, it is believed that methanogens are strict anaerobes. However, several studies have shown that limited O2 supply to anaerobic digesters did not inhibit CH4 production both in granular and suspended sludge (Lim & Wang, 2013; Díaz et al., 2010; Díaz et al., 2011; Díaz et al., 2011a). The fact that methanogens are not greatly affected by the limited presence of O2 is due to the quick and full consumption of the oxygen in the reactor by the facultative microorganisms present in the mixed culture (anaerobic sludge) (Jenicek et al., 2008). Some studies reported an improvement in methanogenic activity (maximum methane production related to concentration of biomass) and methane yield under microaerobic conditions in comparison with purely anaerobic systems (Jenicek et al., 2010; Lim & Wang, 2013). Also, a decrease in the concentration of some recalcitrant compounds (e.g. AOX) was observed when combining anaerobic and microaerobic conditions (Jenicek et al., 2010). Full-scale experience of microaerobic desulfurization (liquid phase) in municipal wastewater treatment plants (mesophilic conditions) showed a removal efficiency of about 99% achievable at high initial concentrations (4000-8000 mg m-3) (Jenicek et al.,

9

2010; Jenicek et al., 2008). One research work reports that COD in the effluent is even lower after applying microaeration (Jenicek et al., 2010). An experiment with pilot-scale mesophilic anaerobic reactor under pure O2 microaeration (250 NmL O2 per L of feed sludge) showed a removal efficiency of 99.8%; initial H2S concentration was 14,437 ppmv while final concentration less than 30 ppmv (Díaz et al., 2011). Experiments with an external module for biodesulfurization under microaerobic conditions (using pure oxygen) of biogas from a pilot/scale anaerobic digester of sewage sludge showed removal efficiency higher than 94% (Ramos et al., 2013). In most of the applications for municipal wastewater treatment, the dose of oxygen range 1-2% of methane production (Jenicek et al., 2010). Although microaeration seems to be a promising technique for the desulfurization of biogas; the distrust of operators to the introduction of oxygen into the digester presents a drawback to the microaerobic modification of anaerobic digestion, even though the dose of oxygen is sufficiently low to keep the process absolutely safe (Jenicek et al., 2010). Another risk is the lack of long-term full scale experience. Long-term operation at higher extent of aeration could cause higher aerobic biomass production and deterioration of anaerobic granules, if this type of biomass is used (Jenicek et al., 2010). The microaerobic methods used for H2S removal vary in the technique used for the air dosing. In the first one air is introduced into the gas space of the reactor. The advantage of this method is that a lower air flow rate is required (Jenicek et al., 2008). On the other hand, SOB specifically develop in the headspace, and as a result, S0 deposition takes place in the top and walls of this area in the reactor (Ramos et al., 2013; Díaz et al., 2011). Periodic cleaning is required in order to prevent clogging problems, which in turn implies extra costs (Jenicek et al., 2008). The second method introduces air into the recirculation stream of the reactor. In such case, a bigger amount of air is required, as part of O2 can be consumed by other processes in addition to H2S oxidation (Jenicek et al., 2008). When O2 is injected in the biogas recirculation, H2S concentration is reduced in both gas and liquid; however, when injected in the sludge recirculation, it only reduces the

10

H2S concentration in the gas, because of poor contact between gas and liquid (Díaz et al., 2011). Even though pure oxygen can also be applied for microaeration, air is frequently used because it is a costless oxygen source which makes it effective and affordable (Díaz et al., 2010). A down side of injecting air into the bioreactor is the decrease in CH4 content in biogas, mostly due to the dilution effect created by the surplus of N2 from the dosed air (Jenicek et al., 2008). A possible solution could be the use of a bio-membrane which would serve as support for biofilm growth and, at the same time, allow microaeration without having direct contact between air and biogas. 2.3 Application of membrane processes in environmental treatment technologies A membrane can be simply defined as an interphase between two bulk phases (Javaid, 2005). In case of wastewater treatment, membranes have been used for more than 30 years for the liquid-solid separation in membrane bioreactor (MBR) technology (Hwang et al., 2009). The advantage is that they provide a small footprint alternative to conventional biological treatment methods, while producing a high-quality effluent at high organic loading rates (Brindle & Stephenson, 1996). Aside from MBRs, there are technologies that are intended to utilize membranes as biofilm supports, rather than liquid-solid separators (Hwang et al., 2009). Depending on the mechanism of pollutant removal, the systems can be classified as: extractive membrane bioreactors, membrane biofilters and membrane biofilm reactors. In extractive membrane bioreactors the contaminated liquid flows inside of the membrane and only selective contaminants are transported through the membrane for biodegradation on its surface or in the bulk solution (Hwang et al., 2009). Membrane biofilters refer to systems in which a waste gas diffuses through the membrane lumen and then it is biologically removed by the biofilm growing on the membrane surface (Hwang et al., 2009). Finally, membrane biofilm reactors are systems with biofilm growing on top of the membrane, where the pressurized gas diffuses through the membrane lumen, in order to oxidize or reduce the soluble constituents present outside the membrane lumen. Air,

11

oxygen, hydrogen and methane have been used as process gas depending on the treatment objectives (Hwang et al., 2009). Out of the three classifications, membrane biofilm reactors is the most interesting one because its working principle applied for biogas desulfurization through microaeration. A review of the short history of membrane biofilm reactors done by Hwang et al. (2009) shows that most of the research done so far is focused on nitrogen and BOD removal. However, some other applications such as autotrophic denitrification, ANAMOX, anaerobic digestion and phosphorus removal have also been studied. 2.3.1 Use of bio-membranes in microaeration Under the scope of microaeration processes, the working principle of a bio-membrane is quite simple: air could be blown into the membrane (air side) and on its surface (biogas side) there would be a biofilm of SOB. This biofilm would be supplied with O2 from membrane side an also with H2S from biogas. In the biofilm there would be optimal conditions for oxidation of H2S to S0. This effect has been observed in laboratory where silicone tubes are used for biogas or digestate. In these tubes S0 is produced because silicone is permeable for O2. Over the past two decades, membrane processes have gained a lot of attention for the separation of gases. They have been found to be very suitable for wide scale applications owing to their reasonable cost, low energy consumption, high compactness, good selectivity and easily engineered modules (Makaruk, et al., 2013; Basu et al., 2010). It has been shown in several scientific works that membrane separation processes are a suitable approach to biogas upgrading for the production of natural substitute. In this case, two types of membranes are used: glassy membranes and dense polymeric (rubbery) membranes (Makaruk, et al., 2013). Glassy membranes are effective for the separation of CO2 from CH4. Ammonia and water are also effectively separated from CH4 using these membranes. However, glassy membranes are not that effective in the separation of H2S from CH4 due to the relatively low H2S / CH4 selectivity, which is four to five times lower than CO2 / CH4 selectivity for typical glassy membranes like polyimides (Makaruk, et al., 2013).

12

In contrast, dense membranes exhibit higher H2S/CH4 selectivity than for CO2/CH4 representing and interesting alternative for the desulfurization of biogas (Makaruk, et al., 2013). Some of the advantages of this type of membrane are: (1) much cheaper than inorganic membranes, (2) able to be easily fabricated into commercially viable hollow fibers or flat sheets that can be processed into hollow fiber or spiral wound modules, (3) in advance stage development, (4) stable at high pressures, and (5) easily scalable (Basu et al., 2010). In dense polymeric materials, it is widely accepted that transmembrane flow is ruled by the solution-diffusion mechanism (Makaruk, et al., 2013). This mechanism is generally considered to be a three-step process. In the first step the gas molecules are absorbed by the surface of the membrane on the upstream end. This is followed by the diffusion of the gas molecules through the polymeric matrix. In the final step the gas molecules evaporate on the downstream end (Javaid, 2005; Sereda et al., 2003). The diffusion of gas through the membrane can be expressed by Fick’s first law, Eq. 2.4 (Javaid, 2005): 


= − 

Eq. 2.4

Where J is the flux of gas through the membrane, D, the diffusion coefficient, and dC/dx is the concentration gradient of the gas across the membrane. At steady state, the flux is a constant. If D is assumed to be constant, the previous equation can be integrated to give Eq. 2.5 (Javaid, 2005):
= 

 

Eq. 2.5



Where C0 and C1 are the concentrations of the gas on the upstream and downstream ends, respectively, and l is the thickness of the membrane. At low pressures, Henry’s law is often adequate to express the concentration of the gas, Eq. 2.6 (Javaid, 2005):  = 

Eq. 2.6

Where S is the solubility constant and p is the pressure of the gas. By substituting Eq. 2.6 into Eq. 2.5, Eq. 2.7 is obtained (Javaid, 2005):
= 

   

13

= 

   

Eq. 2.7

Where P is the permeability of the gas and it can be defined as shown in Eq. 2.8 (Javaid, 2005):  = 

Eq. 2.8

The separation efficiency, or selectivity, of a membrane reflects its capacity to isolate one molecule from another (Kraftschik et al., 2013). Based on single gas permeabilities of species “A” and “B”, ideal selectivity may be expressed as Eq. 2.9 (Javaid, 2005): / =





Eq. 2.9

Gas separation in membranes occurs due to differences in permeabilities of the species flowing through the membrane (Javaid, 2005). At the same time, the permeation of gas through rubbery polymers is ruled primarily by their solubility in the membrane polymer which depends mostly on the gas condensability, as well as polymer-gas interactions and the level of free volume in the membrane material (Kraftschik et al., 2013). Thus, H2S, a component with a relatively high critical temperature, is expected to permeate faster through rubbery polymers than CO2 (Makaruk, et al., 2013). Based on their properties, the order of permeabilities in polymeric membranes for the species in the biogas is H2S > CO2 > CH4 (Kraftschik et al., 2013). For solubility-based separations, polydimethyl siloxane (PDMS) (silicone rubber) is probably the most popular rubbery membrane material because it is highly permeable, vapor selective, and it has a flexible polymer chain backbone, as reflected in its very low glass transition temperature (Tg ~-120 °C) (Javaid, 2005; Raharjo et al., 2007; Makaruk, et al., 2013). For H2S/CH4 it presents a selectivity of around 10 (10.5), while for CO2/CH4 is around 3 (3.4). Certain works report that these selectivity values are lower when mixed gases are permeating (Makaruk, et al., 2013). PDMS also presents an interesting combination of properties such as: excellent thermal and oxidative stability, good resistance against wear and tear, a wide interval of temperatures of use, resistance to oil, solvents, acids, etc. (Díaz et al., 2011). Another important aspect of this type of rubber is the versatility of their synthesis process that allows the incorporation of different chemical structures in the chains of the polymer making it easier to change the properties of the elastomer to increase the selectivity of certain gases (Díaz et al., 2011).

14

Transport properties of PDMS have been reported previously. However, most studies report only pure gas sorption and transport properties. Mixture properties, which are important for estimating membrane separation performance, are less often reported (Raharjo et al., 2007). Tables 2.2 and 2.3 present permeability and solubility of different gases through PDMS membranes. Table 2.2. Permeability of different gases in PDMS membranes

Permeability [Barrer]a Gas

Javaid 2005 f

Tremblay et al. 2006 b

Basu et al. 2010 c

Merkel et al. 2000 d

Tremblay et al. 2006 e

N2

460

180

250

400

130-450

CH4

1452

90

800

1200

500-1600

CO2

-

1300

2700

3800

2800-5600

O2

-

500

800

-

a

-10

3

-2 -1

-1

Barrer = 10 cm (STP) cm cm s cmHg Measured at 28 °C. c Measured at 30 °C. d Measured at 35 °C. e Data gather from the literature review carried out by the authors. f Original data in 10-10 mol s-1 m-2 Pa-1. N2 = 0.00154; CH4 = 0.00486.

b

Table 2.3. Solubility of different gases in PDMS membranes

Solubility [cm3(STP) cm-3 s-1 atm-1] Gas

Tremblay et al. 2006 a

Tremblay et al. 2006 b

Merkel et al. 2000 c

N2

1.10

0.2-0.9

0.09

CH4

0.38

0.4

0.42

CO2

3.9

1.4

1.29

O2

-

-

0.18

a

Measured at 28 °C. Data gather from the literature review carried out by the authors. c Measured at 35 °C. b

15

3. MATERIALS AND METHODS 3.1 Experimental setup To properly characterize the membrane and determine the efficiency of hydrogen sulfide (H2S) removal from the biogas, the research was divided in three phases: gasgas, gas-liquid and gas-biofilm. Experiments of all phases were carried out using a biomembrane unit (BMU) built at laboratory scale, and additional equipment (e.g. pumps, air blowers, etc.) to adjust the overall setup to the needs of each of the phases. The module was designed to simulate the placement of the membrane in the headspace of the anaerobic reactor; therefore, in all cases the membrane was never submerged in liquid. A schematic representation of the BMU is presented in Fig. 3.1.

Fig. 3.1. Schematic representation of the bio-membrane unit (BMU)

In general, the unit consisted of an acrylic cylinder sealed at the top and bottom with acrylic plates. The bottom plate was permanently fixed with polymer glue, while the top one was sealed using a rubber line and tightening screws. The volume enclosed in the cylinder was 5 L. There were three nozzles located in the top plate: biogas outlet (N1), air inlet (N2), and liquid inlet (N3). Similarly, there were three nozzles located on the lower part of the cylinder: biogas inlet (N4), air outlet (N5), and liquid outlet (N6). Nozzles related to liquid were only required in phase II and III. Two additional nozzles

16

(S1 and S2) on the side were provided for the automatic sampling of biogas for H2S measurements. A silicone rubber tube was used as membrane for the gas exchange and as support for the biofilm growth in phase III. The membrane was fixed vertically inside the cylinder; its ends were connected to the air inlet (N2) and outlet (N5) nozzles at the top and bottom of the BMU. Table 3.1 presents the specifications of the membrane tube used. Table 3.1 Specifications of the membrane

Parameter

Unit

Value

-

Silicone rubber (polydimethyl siloxane)

Inner diameter (ID)

mm

10

Outer diameter (OD)

mm

12

Phase I

m

1

Phase II and III

m

0.9

m2

0.038

2

0.034

Material

Length

Surface area Phase I Phase II and III

m

As it can be seen by the placement of the biogas and air nozzles, the flow of gases was countercurrent. Biogas flowed inside the cylinder; air flowed inside the membrane. To reduce the entrance of air into the system as much as possible, low gas permeability Saint-Gobain Tygon® LFL or Norprene® A-60-G (for pumps) flexible tubing were used for all piping tubes in both gas circuits. Due to its relatively constant composition, a synthetic biogas with a volumetric composition of 64.1% of methane (CH4), 35.5% of carbon dioxide (CO2) and 5 mg L-1 (approximately 0.4%) of hydrogen sulfide (H2S) was used for all the experiments. This biogas was obtained by mixing these three gases from separate tanks to the desired composition using flow meters controlled by a mixing program developed in-house using National Instruments’ software LabVIEW 2012.

17

3.1.1 Phase I: Gas - Gas During this phase the transfer of gases through the membrane was studied without the interference of biomass or liquid. Both air and biogas were kept completely separated, each running in its own circuit; the only possible exchange of components was through the membrane. For this purpose, three different configurations were studied during this phase: open loop, closed loop, and dead end. The names of these configurations all refer to the air side (or “loop”) of the experiment. Biogas side operation was the same for all the configurations. At the start of each experiment fresh biogas from the mixing system was sent into the BMU. The biogas circuit was flushed with this mixture for 5 min to ensure that all the gas in the loop was refreshed. Then H2S content was measured automatically by an online sensor (AB), while samples taken through the sampling port (SPB) were analyzed offline. Once initial composition of the biogas had been established, the biogas dosing was stopped, and the circuit was closed to keep biogas recirculating during the whole experiment by means of a dual channel peristaltic pump (Watson Marlow, Sci-Q 400 series, 405U/L2). The biogas recirculation flow was 16.2 L h-1, and the whole volume of the circuit (including tubing) was 5.27 L. a) Open Loop In this configuration the air loop was kept open, i.e. there was fresh air flowing inside the membrane during the entire experiment (see Fig. 3.2). At the startup of the experiment air was sent to the membrane at a rate of 16.2 L h-1 using a dual channel peristaltic pump (Watson Marlow, Sci-Q 400 series, 405U/L2) for 10 min to refresh the gas in the whole loop. Afterwards, it was necessary to ensure that the air in the system was as free of CH4, CO2 and H2S as possible, so composition of the air was determined through online H2S measurement and offline analysis of the samples taken via sampling port (SPA). Then, the air outlet from the BMU was connected to the H2S traps and the experiment began. Total volume of air used was measured by a drum-type gas meter (Spectrum s.r.o, Skuteč, model G1) located after the traps. H2S traps were provided to precipitate the H2S in the air leaving the BMU since the concentration in it was under the detection limit of the sensor provided. Final amount of H2S removed was determined at the end of the experiment through analytical methods.

18

Fig. 3.2. Schematic representation of “open loop” configuration (Phase I)

The total length of the experiment was 4 h. The online measurement of H2S in the biogas and the sampling for offline composition analysis were carried out every hour. b) Closed Loop As opposed to the open loop configuration, in the closed loop configuration the same air volume was kept under recirculation throughout the whole length of the experiment (see Fig. 3.3). During startup of every experiment an air blower with a capacity of 216 L h-1 was used to pass fresh air through the system for 10 min to ensure that all the gas in the loop was refreshed. To establish initial concentration of the air, H2S content was measured automatically by an online sensor (AA), while the rest of the gas composition was analyzed offline using samples taken via sampling port (SPA). Afterwards, the fresh air flow was stopped and the circuit was closed to keep the air continuously recirculating for the rest of the experiment by means of a dual channel peristaltic pump (Watson Marlow, Sci-Q 400 series, 405U/L2) at a flow rate of 16.2 L h-1. The whole volume of the circuit was 1.45 L, including tubing and a glass bottle of 1.18 L used as an air reservoir to increase the total air volume in the system.

19

Fig. 3.3. Schematic representation of “closed loop” configuration (Phase I)

The movement of gases through the membrane generated changes in pressure on both loops which heavily affect the functioning of the H2S sensors. To keep atmospheric pressure on each side, atmospheric air was let inside the biogas loop through a needle1; excess of gas on the air side was released by means of a volumetric flow meter (Ritter, MGC-1 model) which kept the circuit under constant pressure while continuously measuring the amount of gas release as a function of time. The total length of the experiment was 4 h. For both air and biogas, online H2S measurement and sampling for offline composition analysis were carried out every hour. c)

Dead End

In this case, no pump was used to produce air flow. Gas movement on the air side was produced solely by diffusion due to difference in gas concentrations inside of the membrane and on the air reservoir. The general configuration is the same as the closed loop setup; however no recirculation pump for the air was necessary (see Fig. 3.4). Startup was carried out the same way as the one described for the closed loop configuration. After initial concentration of biogas was established, the loop was closed. 1

For details on the flows in and out of the loops for pressure control, refer to Appendix A.

20

In case of the air loop, the inlet of the membrane and the outlet of the air reservoir were closed using a metal clamp. The total length of the experiment was 4 h. The online measurement of H2S in the biogas and the sampling for offline composition analysis were carried out every hour.

Fig. 3.4. Schematic representation of “dead end” configuration (Phase I)

d) Intrinsic losses of the system The whole setup was not completely airtight. To quantify the H2S losses due to leakages, two simple experiments were performed. In the first one the membrane (with fresh air inside) was isolated by closing both of its ends using metal clamps. This experiment allowed determining the losses on the biogas loop. For the second test, the closed loop configuration was used but this time biogas was running inside both loops. Once the losses of the biogas side were known through the results of the first test, the value could be used in combination with the second test to determine the losses of the air loop. 3.1.2 Phase II: Gas - Liquid During this phase only the closed loop configuration was used. The configuration remained the same with the exception of an additional third loop included to study the effect that a liquid layer on the membrane’s surface has in the transfer of gases. Water was kept under constant recirculation at a flow rate of 1.33 L h-1 using a peristaltic

21

pump (Kouřil, PCD 81.2). The bottom of the BMU was used as a liquid reservoir (0.2 L); liquid level was 5 cm, just below the beginning of the membrane (see Fig. 3.5). Before being introduced in the BMU, demineralized water was sparged with nitrogen until dissolved oxygen concentration was as low as possible (0.15 g L-1). The startup and experimental procedures were the same as the ones described for the closed loop configuration in phase I. The length of the experiment and the sampling procedure were also the same.

Fig. 3.5. Schematic representation of “closed loop” configuration (Phase II)

3.1.3 Phase III: Gas - Biofilm In this phase only the closed loop configuration was used. The configuration remained the same as in phase II with some modifications done to the liquid circuit to allow biofilm growth on the surface of the membrane (see Fig. 3.6). A glass bottle of 1.18 L was added as a reservoir to be used for the sludge (inoculum for startup) and then for the reject water (experimental part). The liquid was kept at a constant temperature of 35°C (mesophilic conditions) by means of an electric hotplate (Heidolph, MR Hei-standard) which also provided constant stirring (magnetic, 250 rpm). Temperature control was done using a contact thermometer (Heidolph, EKT 3001T) submerged in the liquid reservoir and connected directly to the hotplate.

22

To maintain constant pressure on the biogas side, instead of letting ambient air in as it was done for the experiments on phase I and II, a nitrogen (N2) reservoir was added to limit the oxygen content in the BMU to only the one transferred through the membrane. For the N2 reservoir a Tedlar sample bag (SKC Inc.) of 25 L was used. Also a glass bottle of 300 mL was added to the system to allow draining the liquid from the bottom of the BMU when necessary. In this phase an initial startup period was required to allow biofilm growth. This period was followed by the experimental part; both are described below.

Fig. 3.6. Schematic representation of “closed loop” configuration (Phase III)

a) Startup – Biofilm growth The inoculum was done with 1.5 L of sludge from a mesophilic anaerobic stabilization tank from a municipal wastewater treatment plant (WWTP) in Česká Lípa (Czech Republic). The sludge was sieved to remove any large particles that could cause clogging problems in the system and then characterized based on the parameters presented in Table 3.2. After introducing the sludge in the system, it was left under recirculation for 10 days. H2S concentration on the biogas side was monitored constantly through the online

23

sensor (AB). Fresh biogas was introduced into the BMU periodically according to the H2S consumption as shown by the gather data. Table 3.2. Characterization of sludge for biofilm inoculum

Parameter

Units

Value

-

7.44

pH

-1

17.34

-1

0.94

TS

-1

gL

23.7

TSS

g L-1

22.1

DS

g L-1

1.6

VS

g L-1

13.2

% dry mass

4.78

Total COD

gL

Dissolved COD

Total Sulfur

gL

Following this period, the sludge was removed from the system. Reject water from the central municipal wastewater treatment plant in Prague (Czech Republic) was used as fluid in the liquid loop for the rest of startup and experimental stage. Characterization of the reject water is presented in Table 3.3. Table 3.3. Characterization of reject water used in phase III

Parameter

Units

Value

-

7.90

pH Total COD

0.89

-1

2.46

-1

1.08

-1

1.02

gL

Dissolved COD

gL

Total ammonia

gL

Dissolved ammonia Total sulfides

-1

gL

-1

mg L

11.2

b) Experiment The procedure used for the experimental stage was the same as the one used for the closed loop configuration in phase I. Only two additional steps were added: (1) liquid circulation was stopped before refreshing both air and biogas on the loops (start of experiment); and (2) liquid at the bottom of the BMU was drained, once experiment was over, liquid was put inside again to allow proper functioning of the liquid recirculation pump.

24

3.2 Analytical methods Physicochemical methods were used to determine several parameters required for the analysis of the data obtained. These methods are described in this section. 3.2.1 Gas composition (CH4, CO2, O2, N2, H2) Composition of each gas mixture (air and biogas) in terms of methane (CH4), carbon dioxide (CO2), oxygen (O2), nitrogen (N2) and hydrogen (H2) was determined by means of gas chromatography. Six samples of 1 mL were taken from each loop using the sampling ports and then injected into the chromatograph using a syringe. Samples were taken at the beginning of the experiment to establish an initial concentration and then hourly during the next four hours to study the variations in composition of both air and biogas. The results were normalized and then transformed and reported as partial pressures. The gas chromatograph used (CE Instruments Ltd, GC 8000 Top) was equipped with a thermal conductivity detector (CE Instruments Ltd, HWD 800) at 80 °C, filament at 240 °C and 8.0 V. The oven was used in isothermal condition (70 °C) for 2 minutes and equipped with two glass columns that use Argon (99.99%) as a carrier gas (100 mL min-1 for column 1 and 150 mL min-1 for column 2). The first column was packed with Porapak Q 80-100 mesh (Waters Corporation) and was used to determine air, CH4 and CO2; while the second one was packed with molecular sieve 5A (Restek Corporation) and was used to identify H2, O2 and N2. 3.2.2 Hydrogen sulfide content in the gas Hydrogen sulfide (H2S) concentration in the gas was monitored using an online electrochemical gas sensor (Membrapor H2S sensor type H2S/S-10000-S). Samples from both loops were taken automatically every hour using a dual channel peristaltic pump (Kouřil, PCD 81.2). Data storage and sampling control was done by a program developed in-house using National Instruments’ software LabVIEW 2012. When the sensors were not performing a measurement, they were constantly flushed with ambient air to keep H2S concentration as low as possible and avoid interference between measurements. Additionally, because H2S concentration on biogas side was much higher than on air side, the sensor dedicated to this loop was flushed with nitrogen just before performing the next measurement. To control all these flows into the

25

sensors, solenoid-operated valves were used (Bio-Chem ValveTM Pinch Valve, 100P2NO24-05B). 3.2.3 Hydrogen sulfide recovered in the traps H2S traps used in the open loop configuration contained a cadmium carbonate (CdCO3) solution prepared by mixing 30 mL of cadmium acetate (50 g L-1) and 2 mL of sodium carbonate (10 g L-1). When H2S reacts with CdCO3 in acid medium it produces cadmium sulfide (CdS) which precipitate as fine yellow particles. The amount of sulfide precipitated was determined using the iodometric method. H2S reacts with excess of iodine (0.1 M, 10 mL) in acid medium (HCl 2:1, 30 mL) and the remaining iodine was then determined by titration with sodium thiosulfate (0.1 M), using starch as an indicator (Kolthoff, et al. 1969). A blank was performed in the same way. Then, sulfide concentration was calculated using Eq. 3.1.   = 0.5 ∗ % − %&  ∗ '( ∗ ')

Eq 3.1.

Where:   : Mass of H2S recovered [mg]. % : Volume of sodium thiosulfate used for the titration (blank) [mL]. %& : Volume of sodium thiosulfate used for the titration (sample) [mL]. '( : Molar concentration of sodium thiosulfate [mol L-1]. '): Molecular weight of H2S [34.06 g mol-1]. 3.2.4 Dissolved Oxygen Dissolved oxygen (DO) concentration was determined using an optical probe (Hach LDO) connected to a single channel portable meter (Hach HQ30d flexi). 3.2.5 Sludge characterization To characterize the sludge used as inoculum for the biofilm in phase III the following tests were performed: pH, solids content, chemical oxygen demand (COD), and total sulfur content. A description of each of the test is presented below. In case of COD and solid content tests, analyses were carried out for both the homogenized sludge and the supernatant obtained after centrifugation (Sigma 3.16p, 13000 rpm, 10 min).

26

a) pH The pH value was determined electrochemically with a pH meter (InoLab WTW series, pH 730) using a combination electrode (Hamilton, Polyplast PRO). b) Solids content Analysis of total solids (TS), total suspended solids (TSS), and volatile suspended solids (VSS) were measured according to standard methods 2540 B-G of American Public Health Association (APHA, 1999). A 10 mL sample of both homogenized sludge and supernatant was evaporated in a weighed dish and dried to constant weight in an oven (Mermmet UNB 500) at 105°C. The increase in weight over that of the empty dish represents the total solids (homogenized sludge) and dissolved solids (supernatant). The residue was ignited in a muffle furnace at 550°C. The remaining solids represent the inorganic solids while the weight lost is the volatile solids in case of each sample. c) Chemical oxygen demand (COD) COD analyses were performed according to the standard method 5220-D “Closed Reflux, Colorimetric Method” of the American Public Health Association (APHA, 1999). Preliminary dilution of the samples was performed to adjust to the requirements of the spectrophotometer available in the laboratory. A 2.5 mL aliquot of the diluted sample was placed in a borosilicate tube; then 3.5 mL of a standard sulfuric acid (H2SO4) solution (with silver ions as a catalyst) and 1.5 mL of a standard potassium dichromate (K2CrO4) solution were added. The tube was closed with a TFE-lined screw cap and placed in a block heater (Hach, DRB 200) at 150 °C for 2 h. Afterwards, a spectrophotometer (Hach, DR 2500) operated at the required wavelength (420 nm) was used to quantify the COD content in the sample. The tests were performed in triplicates per sample and the results were reported in mg L-1. d) Total sulfur content Total sulfur content was determined by an elemental organic composition analysis. A sample of sludge was dried at 105 °C for 24 h, after that it was pulverized and homogenized until a 1 g sample was obtained. This sample was then analyzed by the Central Laboratory of the Institute of Chemical Technology in Prague (ICTP) for

27

processing by X-ray fluorescence analysis. The result was reported as percentage in dry weight. 3.2.6 Reject water characterization Reject water used in phase III was characterized in terms of COD, pH, ammonia content, and total sulfide content. COD and pH tests were performed the same way as described for the sludge characterization; the rest of the tests are described next. The ammonia test was carried out for both the homogenized sludge and the supernatant obtained after centrifugation (Sigma 3.16p, 13000 rpm, 10 min). a) Ammonia Ammonia (NH3) was determined for both the homogenized and centrifuged sample according to APHA 4500-NH3 B/C method. Samples (5 mL) were accomplished by raising the pH with sodium hydroxide 0.1 N to change the ammonium (NH4+) ion to ammonia (NH3). After that, the nitrogen was separated by distilling the ammonia in a distillation unit (Buchi K-355, Labortechnik AG); distillate was collected in a boric acid solution used as a trapping medium. Determination of the amount of nitrogen in the sample was done by titration of the ammonia with a standard solution of sulfuric acid 0.05 M in presence of a mixed indicator. Final ammonia concentration was determined using Eq. 3.2. *+ = 28 ∗ %. ∗ ' ∗ 1000⁄0

Eq. 3.2

Where: *+ : Concentration of NH3 in the reject water [mg L-1]. %. : Volume of acid used during titration [mL]. ': Molar concentration of sulfuric acid [mol L-1]. 0: Volume of sample [5 mL]. b) Total sulfide In this test 10 mL of the reject water were digested with 25 mL of sulfuric acid (9597%) during 30 min. Sulfides were swept with nitrogen gas and trap through bubbling in two consecutive vessels with 30 mL of sodium hydroxide solution (NaOH, 4 g L-1) each. The amount of sulfide trapped in the vessels was determined using the iodometric method. H2S reacts with excess of iodine (0.01 M, 5 mL) in acid medium (HCl 2:1, 10

28

mL) and the remaining iodine was then determined by titration with sodium thiosulfate (0.01 M), using starch as an indicator (Kolthoff, et al. 1969). A blank was performed in the same way using NaOH. Sulfide concentration was calculated using Eq. 3.3.   = 0.5 ∗ % − %&  ∗ '( ∗ ') ∗ 1000⁄0 Where:   : Concentration of H2S in the reject water [mg L-1]. % : Volume of sodium thiosulfate used for the titration (blank) [mL]. %& : Volume of sodium thiosulfate used for the titration (sample) [mL]. '( : Molar concentration of sodium thiosulfate [mol L-1]. '): Molecular weight of H2S [34.06 g mol-1]. 0: Volume of sample [10 mL].

29

Eq 3.3.

4. RESULTS 4.1 Intrinsic losses in the system As mentioned in section 3.1.1, two initial experiments were carried out to determine the losses of hydrogen sulfide (H2S) in the system due to leakages. In the first one (“Only membrane”) the membrane was isolated from the rest of the air system by means of two metallic clamps. H2S transfer through the membrane was slow; probably due to a decrease in the driving force created by the difference in partial pressure on both sides due to H2S accumulation inside the membrane. Under this configuration, the amount of H2S transferred through the membrane was considered negligible, so any decrease in H2S on the biogas side was assumed to be caused by leakages; thus, determining the losses of that side. The second experiment (“BG-BG closed loop”) was used to determine the losses on the air side. The closed loop configuration was used but this time biogas was introduced in both loops. Since the concentration of all components was the same on both loops, theoretically, there should be no transfer of gases through the membrane. Based on this assumption, any decrease in H2S concentration on each side would be only due to leakages. Using the losses for the biogas side calculated from the “only membrane” experiment and following the mass balance included in Appendix A, the losses on the air side were quantified. H2S concentration was measured for both experiments; a linear regression was applied to each data set to determine the rate at which the H2S concentration decreased (see Fig. 4.1). Table 4.1 shows calculated H2S losses for each loop. These values were used to correct the H2S data gathered in all the experiments. Therefore, variations on H2S concentrations presented from this point on were only due to the phenomena occurring inside the bio-membrane unit (BMU) (e.g. transport through membrane, oxidation, etc.). Table 4.1. Intrinsic H2S losses of each loop

Side Biogas Air

Unit mg L-1 h-1 mg L-1 h-1

30

Rate of losses 0.07 0.43

H2S concentration [mg/L]

5.0

4.8

4.6

4.4

4.2 y = -0.0713x + 4.7666 R² = 0.9993

y = -0.1487x + 4.9388 R² = 0.9818

4.0 0

1

2 Time [h]

Only membrane

3

4

BG-BG closed loop

Fig. 4.1. Intrinsic losses – Experimental data

4.2 Gas composition The composition in terms of methane (CH4), carbon dioxide (CO2), hydrogen sulfide (H2S), nitrogen (N2) and oxygen (O2) was measured both for the biogas and the air loop. This was performed for each of the phases and configurations of the experimental setup described in chapter 3. A model based on a molar balance of each component was carried out to study the change in its concentration on each side of the membrane, and to characterize the membrane in terms of permeability and flux. A detailed description of this balance is presented in Appendix A. 4.2.1 Phase I: Gas-Gas Following the experimental procedure described in section 3.1.1, five experiments were carried out in this phase: two for open loop configuration, two for closed loop configuration, and one for dead end configuration. Figures 4.2 to 4.6 show a comparison of both the data obtained for each experiment and the model developed using the molar balance for N2, O2, CH4, CO2, and H2S, respectively. Because the experimental data obtained during several runs of a same configuration was almost the same, only two sets for closed loop and open loop were included in the results. This is a good indication of the repeatability of the experiments with the BMU.

31

Data for gas composition on the air side was only available for the closed loop configuration, as it was not possible to measure these parameters for the open loop and dead end configurations. It was observed that N2 content in the biogas increased with time (Fig. 4.2). After 4 h, the final concentration reached was: 12% for open loop, 10% for closed loop, and 6% for dead end. On the air side N2 concentration, initially 79%, decreased with time reaching a final value of 59% after 4 h.

0.8

N2 Partial Pressure [atm]

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0

1

Open Loop A - BG Data Closed Loop B - BG Data Dead End - BG Data Closed Loop A - BG Model Closed Loop B - AIR Model

2 Time [h] Open Loop B - BG Data Closed Loop A - AIR Data Open Loop A - BG Model Closed Loop B - BG Model Dead End - BG Model

3

4

Closed Loop A - BG Data Closed Loop B - AIR Data Open Loop B - BG Model Closed Loop A - AIR Model

Fig. 4.2. N2 composition – Biogas and Air side (Phase I)

Similarly, the concentration of O2 (Fig. 4.3) on the biogas side increased with time. Highest final concentration was reached with the open loop configuration (3.1%), followed by closed loop (2.8%) and then dead end (1.4%). O2 content on air side decreased from 21% to 14% at the end of the experiment. On the biogas side, the decrease of CH4 content (Fig. 4.4) was almost the same for all configurations; going from 64% to 59-60%. In case of air side, CH4 content increased up to 12% by the end of the experiment.

32

0.25

O2 Partial Pressure [atm]

0.20

0.15

0.10

0.05

0.00 0

1

Open Loop A - BG Data Closed Loop B - BG Data Dead End - BG Data Closed Loop A - BG Model Closed Loop B - AIR Model

2 Time [h] Open Loop B - BG Data Closed Loop A - AIR Data Open Loop A - BG Model Closed Loop B - BG Model Dead End - BG Model

3

4

Closed Loop A - BG Data Closed Loop B - AIR Data Open Loop B - BG Model Closed Loop A - AIR Model

Fig. 4.3. O2 composition – Biogas and Air side (Phase I)

0.7

CH4 Partial Pressure [atm]

0.6 0.5 0.4 0.3 0.2 0.1 0.0 0

1

Open Loop A - BG Data Closed Loop B - BG Data Dead End - BG Data Closed Loop A - BG Model Closed Loop B - AIR Model

2 Time [h] Open Loop B - BG Data Closed Loop A - AIR Data Open Loop A - BG Model Closed Loop B - BG Model Dead End - BG Model

3

Closed Loop A - BG Data Closed Loop B - AIR Data Open Loop B - BG Model Closed Loop A - AIR Model

Fig. 4.4. CH4 composition – Biogas and Air side (Phase I)

33

4

Likewise, it was observed that CO2 concentration (Fig. 4.5) on the biogas side decreased with time. Lowest final concentration was obtained in the open loop configuration (25%), followed by the closed loop (27%). Dead end configuration showed the highest final concentration of CO2 (31%). On the air side, CO2 concentration increased to a final value of 15%.

CO2 Partial Pressure [atm]

0.4

0.3

0.2

0.1

0.0 0

1

Open Loop A - BG Data Closed Loop B - BG Data Dead End - BG Data Closed Loop A - BG Model Closed Loop B - AIR Model

2 Time [h] Open Loop B - BG Data Closed Loop A - AIR Data Open Loop A - BG Model Closed Loop B - BG Model Dead End - BG Model

3

4

Closed Loop A - BG Data Closed Loop B - AIR Data Open Loop B - BG Model Closed Loop A - AIR Model

Fig. 4.5. CO2 composition – Biogas and Air side (Phase I)

Finally, H2S content (Fig. 4.6) in the biogas side showed a drastic decrease in concentration for open loop and closed loop configurations; reaching a final concentration of 2.4 - 2.7 mg L-1 and 3.3 - 3.5 mg L-1, respectively. This represents a removal of H2S between 30 and 50%. In the dead end configuration the decrease in concentration was considerably lower with a final value of 4.4 mg L-1. Air side experienced an increase of H2S content up to 2.3 mg L-1 after 4 h. In case of the open loop, H2S was recovered through precipitation as indicated in section 3.1.1 and 3.2.3. Total amount of H2S recovered is presented in Table 4.2. By doing a simple mass balance between the initial and final amount of H2S in the biogas, it can be seen that 14% and 23% could not be recovered from experiments A and B, respectively.

34

Table 4.2. H2S recovered in the traps (open loop configuration)

Parameter

Unit

Open Loop A

Open Loop B

Initial amount of H2S in biogas c

mg

26.1

25.1

Final amount of H2S in biogas c

mg

0.01a

1b

Amount of H2S recovered (traps)

mg

22.3

18.3

Balance

mg

- 3.7

- 5.8

a

After 43 h. After 20 h. c Considering total volume of biogas in the loop. b

6

H2S concentration [mg L-1]

5

4

3

2

1

0 0

1 Open Loop A - BG Data Closed Loop B - BG Data Dead End - BG Data Closed Loop A - BG Model Closed Loop B - AIR Model

2 Time [h] Open Loop B - BG Data Closed Loop A - AIR Data Open Loop A - BG Model Closed Loop B - BG Model Dead End - BG Model

3

4

Closed Loop A - BG Data Closed Loop B - AIR Data Open Loop B - BG Model Closed Loop A - AIR Model

Fig. 4.6. H2S composition – Biogas and Air side (Phase I)

4.2.2 Phase II: Gas-Liquid In this phase only the closed loop configuration was used. Due to the similarity of the experimental data only two runs were included in the results. Data and results obtained from the model for each gas (N2, O2, CH4, CO2, and H2S) are presented in Fig. 4.7 to 4.11 including a comparison with the results from phase I (closed loop). As it can be seen in Fig. 4.7 to 4.10, the behavior of the concentration of N2, O2, CH4 and CO2 on each loop was the same as described in phase I. Content of N2 and O2 on the air side decreased with time; while increasing on the biogas side. N2 (Fig. 4.7) went

35

from 79 to 60% on the air side and from 2 to 11% on the biogas side. O2 (Fig. 4.8) content went from 21 to 14% on the air side and from 1 to 3% on the biogas side.

0.9

N2 Partial pressure [atm]

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0

1 P(I) A - Biogas - Data P(II) B - Biogas - Data P(II) A - Biogas - Model P(I) B - Air - Data P(I) A - Air - Model P(II) B - Air - Model

2 Time [h] P(I) B - Biogas - Data P(I) A - Biogas - Model P(II) B - Biogas - Model P(II) A - Air - Data P(I) B - Air - Model

3

4 P(II) A - Biogas - Data P(I) B - Biogas - Model P(I) A - Air - Data P(II) B - Air - Data P(II) A - Air - Model

Fig. 4.7. N2 composition – Biogas and Air side (Comparison between phase I & II)

Regarding CH4 and CO2, their concentration gradually decreased on the biogas side, while increasing in the air side. CH4 (Fig. 4.9) went from 0 to 11% on the air side and from 64 to 59% on the biogas side. CO2 (Fig. 4.10) content went from 0 to 15% on the air side and from 33 to 28% on the biogas side. In general, H2S concentration (Fig. 4.11) on both sides followed the same behavior; it decreased on the biogas side while increasing on the air side. Final value in biogas side was 3.5 mg L-1 for both phases, and 2.1 mg L-1 on the air side. However, the concentration on the biogas side in phase II decreased faster than in phase I. H2S content on the air side increased basically at the same rate for both phase I and II. The effect of gas flow rate on the permeability was also studied during this phase. Recirculation flow rate of both air and biogas was set to 20.4 L h-1 instead of the value used for all the rest of the experiments (16.2 L h-1). The results show that the gas flow rate has a negligible effect on the permeability values determined through the experiments. Details of these results are presented in Appendix B.

36

O2 Partial pressure [atm]

0.25

0.20

0.15

0.10

0.05

0.00 0

1

P(I) A - Biogas - Data P(II) B - Biogas - Data P(II) A - Biogas - Model P(I) B - Air - Data P(I) A - Air - Model P(II) B - Air - Model

2 Time [h] P(I) B - Biogas - Data P(I) A - Biogas - Model P(II) B - Biogas - Model P(II) A - Air - Data P(I) B - Air - Model

3

4

P(II) A - Biogas - Data P(I) B - Biogas - Model P(I) A - Air - Data P(II) B - Air - Data P(II) A - Air - Model

Fig. 4.8. O2 composition – Biogas and Air side (Comparison between phase I & II)

0.7

CH4 Partial pressure [atm]

0.6 0.5 0.4 0.3 0.2 0.1 0.0 0

1 P(I) A - Biogas - Data P(II) B - Biogas - Data P(II) A - Biogas - Model P(I) B - Air - Data P(I) A - Air - Model P(II) B - Air - Model

2 Time [h] P(I) B - Biogas - Data P(I) A - Biogas - Model P(II) B - Biogas - Model P(II) A - Air - Data P(I) B - Air - Model

3 P(II) A - Biogas - Data P(I) B - Biogas - Model P(I) A - Air - Data P(II) B - Air - Data P(II) A - Air - Model

Fig. 4.9. CH4 composition – Biogas and Air side (Comparison between phase I & II)

37

4

CO2 Partial pressure [atm]

0.4

0.3

0.2

0.1

0.0 0

1

P(I) A - Biogas - Data P(II) B - Biogas - Data P(II) A - Biogas - Model P(I) B - Air - Data P(I) A - Air - Model P(II) B - Air - Model

2 Time [h] P(I) B - Biogas - Data P(I) A - Biogas - Model P(II) B - Biogas - Model P(II) A - Air - Data P(I) B - Air - Model

3

4

P(II) A - Biogas - Data P(I) B - Biogas - Model P(I) A - Air - Data P(II) B - Air - Data P(II) A - Air - Model

Fig. 4.10. CO2 composition – Biogas and Air side (Comparison between phase I & II)

H2S Concentration [mg L-1]

6 5 4 3 2 1 0 0 P(I) A - Biogas - Data P(II) B - Biogas - Data P(II) A - Biogas - Model P(I) B - Air - Data P(I) A - Air - Model P(II) B - Air - Model

1

2 Time [h] P(I) B - Biogas - Data P(I) A - Biogas - Model P(II) B - Biogas - Model P(II) A - Air - Data P(I) B - Air - Model

3

4 P(II) A - Biogas - Data P(I) B - Biogas - Model P(I) A - Air - Data P(II) B - Air - Data P(II) A - Air - Model

Fig. 4.11. H2S composition – Biogas and Air side (Comparison between phase I & II)

38

4.2.3 Phase III: Gas-Biofilm During this phase only the closed loop configuration was used. The decrease in H2S concentration was much faster than in the other two phases; for this reason the experiments were run only for 3 h, instead of 4 h. After 37 d of biofilm growth it was observed that not all the surface area of the membrane was covered by it. Additionally, there were white or light yellow spots on the surface of the membrane, especially in the borders between the areas with biofilm coverage and bare membrane. It was not possible to performe analytical tests of the biofilm of the yellowish spots on the membrane, so there is no information available on the type of microorganisms present in the biofilm or confirmation that the yellow spots are indeed elemental sulfur. Figure 4.12 presents an image of this layer.

Fig. 4.12. Biofilm and deposits formed on the surface of the membrane (Phase III)2

Figures 4.13 to 4.17 present a comparison among the experimental data obtained for the closed loop configuration in each phase. A comparison among the results of the models is not presented since it was not possible to fit the data of phase III using the same molar balance approach used for the other two phases.

2

Photo taken after 10 d of biofilm growth.

39

0.9 0.8

N2 Partial pressure [atm]

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0

1

2

3

Time [h] P(I) A - Biogas

P(I) B - Biogas

P(II) A - Biogas

P(II) B - Biogas

P(III) A - Biogas

P(III) B - Biogas

P(I) A - Air

P(I) B - Air

P(II) A - Air

P(II) B - Air

P(III) A - Air

P(III) B - Air

Fig. 4.13. N2 composition – Biogas and Air side (Experimental data: phase I, II & III)

As shown in Fig. 4.13, the concentration of N2 basically followed the same trend in all phases. On the biogas side, N2 content increased from around 0% to 9%; while on the air side it decreased from 79% to around 64%. In case of O2 concentration (Fig. 4.14), it was observed that the tendency was the same for all phases; it increased on the biogas side, while decreasing on the air side. However, phase III shows a lower increase in the O2 concentration in the biogas side, as well as a lower decrease in concentration on the air side when compared with phases I and II. This means that final O2 content in the biogas was lower in phase III (1%), than for phase I and II (3%). No significant difference was observed in the behavior of CH4 (Fig. 4.15) and CO2 (Fig. 4.16) concentrations among the three phases. Their content in the biogas side decreased with time down to around 59% for CH4 and 27-31 % for CO2. In the air side, the content increased up to 9% for CH4 and 13% for CO2.

40

0.25

O2 Partial pressure [atm]

0.20

0.15

0.10

0.05

0.00 0

1

Time [h]

2

3

P(I) A - Biogas

P(I) B - Biogas

P(II) A - Biogas

P(II) B - Biogas

P(III) A - Biogas

P(III) B - Biogas

P(I) A - Air

P(I) B - Air

P(II) A - Air

P(II) B - Air

P(III) A - Air

P(III) B - Air

Fig. 4.14. O2 composition – Biogas and Air side (Experimental data: phase I, II & III)

The greatest difference between the results of phase I, II and III was observed in the H2S concentration (Fig. 4.17). It is clear that the decrease of H2S on the biogas side was much faster in phase III, than in any of the other two phases. Final concentration achieved after 3 h was 1.5-1.8 mg L-1 for phase III which translates to 55-70% removal; compared to a final concentration of 3.4 - 3.6 mg L-1 and removal of about 26-31% for phases I and II. Moreover, final concentration of H2S on the air side was much lower in phase III (0.05 mg L-1) than in phase I and II (2.1-2.2 mg L-1). The behavior was also different; in phase I and II the H2S concentration continuously increased with time while in phase III it increased to a maximum concentration of 0.28-0.35 mg L-1 and then started to decrease down until reaching the final concentration mentioned before.

41

0.7

CH4 Partial pressure [atm]

0.6 0.5 0.4 0.3 0.2 0.1 0.0 0

1

2

3

Time [h] P(I) A - Biogas

P(I) B - Biogas

P(II) A - Biogas

P(II) B - Biogas

P(III) A - Biogas

P(III) B - Biogas

P(I) A - Air

P(I) B - Air

P(II) A - Air

P(II) B - Air

P(III) A - Air

P(III) B - Air

Fig. 4.15. CH4 composition – Biogas and Air side (Experimental data: phase I, II & III)

0.40

CO2 Partial pressure [atm]

0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 0

1

Time [h]

2

3

P(I) A - Biogas

P(I) B - Biogas

P(II) A - Biogas

P(II) B - Biogas

P(III) A - Biogas

P(III) B - Biogas

P(I) A - Air

P(I) B - Air

P(II) A - Air

P(II) B - Air

P(III) A - Air

P(III) B - Air

Fig. 4.16. CO2 composition – Biogas and Air side (Experimental data: phase I, II & III)

42

6

H2S concentration [mg L-1]

5

4

3

2

1

0 0

1

2

3

Time [h] P(I) A - Biogas

P(I) B - Biogas

P(II) A - Biogas

P(II) B - Biogas

P(III) A - Biogas

P(III) B - Biogas

P(I) A - Air

P(I) B - Air

P(II) A - Air

P(II) B - Air

P(III) A - Air

P(III) B - Air

Fig. 4.17. H2S composition – Biogas and Air side (Experimental data: phase I, II & III)

4.3 Membrane permeability Based on the model developed (see Appendix A) and the data collected, the permeability of each gas through the membrane was determined for each of the experiments performed in phases I and II. It was not possible to fit the data of phase III with the molar balance proposed for phases I and II; therefore, permeability values for this phase I were not calculated. Table 4.3 presents the results. Additionally, Table 4.4 presents the values used for the adjustment parameter (S), to improve the fit between the experimental data and the model developed through the molar balance. It is thought that this parameter accounts for the effects of gas solubility in the membrane, losses of the compound due to potential chemical reactions (e.g. oxidation), and other losses in the system that were not possible to quantify.

43

Table 4.3. Permeability for each gas

Permeability [Barrer]a Case N2

O2

CH4

CO2

H2 S

Open Loop A

1340

1260

881

2600

7380

Open Loop B

1420

1590

795

3260

8130

Closed Loop A

221

475

795

2570

3650

Closed Loop B

206

526

807

2520

3170

Dead End

598

282

272

1030

1013

189

592

959

2670

3470

379

819

2650

3380

Phase I

Phase II Closed Loop A Closed Loop B a

Barrer = 10

-10

123 3

-2 -1

-1

cm (STP) cm cm s cmHg

Table 4.4. Adjustment parameter for each gas

Case

Adjustment parameter [cm3 (STP) cm-3memb. s-1] N2

O2

CH4

CO2

H2 S

Open Loop A

-

-

-

-

-

Open Loop B

-

-

-

5.47 10-5

-

Closed Loop A

-

-

1.59 10-8

1.08 10-6

3.72 10-8

Closed Loop B

-

-

1.30 10-6

-

3.69 10-8

Dead End

-

-

-

1.41 10-5

-

Closed Loop A

-

-

8.81 10-7

1.36 10-6

4.94 10-8

Closed Loop B

-

-

1.00 10-6

9.00 10-7

5.69 10-8

Phase I

Phase II

4.4 Membrane flux The flux of each component through the membrane was computed for each time step using Eq. 4.1. The rate at which the flux of each component changed is presented in Fig. 4.18 to 4.20 for phase I (open loop, closed loop and dead end, respectively), and Fig. 4.21 for phase II.

44


=

23 4 ∗ 5



Eq. 4.1

Where:
: Flux of gas (N2, O2, CH4, CO2 or H2S) through the membrane at a particular time step (i) [m3(STP) m-2 h-1]. %6 7 : Volume of gas (N2, O2, CH4, CO2 or H2S) that is transferred through the membrane at a particular time step (i), [m3(STP)]. 8: Surface area of the membrane, [m2].

9: Time or duration of the time step [h]. When compared, the results of phase I showed that open loop configuration presented the highest flux for all the gases, followed by closed loop and then dead end. Closed loop (Fig. 4.19) configurations exhibited a fast decline in the flux with time, especially for CO2, which flux declined 50% by the end of the experiment. Open loop (Fig. 4.18) and dead end (Fig. 4.20) configurations showed a relatively constant flux. Moreover, the order of gases according to decreasing flux is different among the configurations. For close loop the order of the gases is CO2 > CH4 > N2 > O2 > H2S; while for open loop and dead end the order is N2 > CO2 > CH4 > O2 > H2S.

0.003

Flux [m3(STP) m-2 h-1]

0.0025

0.002

0.0015

0.001

0.0005

0 0

1 P(I) Open Loop A - N2 P(I) Open Loop A - CO2 P(I) Open Loop B - O2 P(II) Open Loop B - H2S

2 Time [h] P(I) Open Loop A - O2 P(I) Open Loop A - H2S P(I) Open Loop B - CH4

3 P(I) Open Loop A - CH4 P(I) Open Loop B - N2 P(I) Open Loop B - CO2

Fig. 4.18. Flux of gases through the membrane (Phase I – Open Loop)

45

4

0.0025

Flux [m3(STP) m-2 h-1]

0.002

0.0015

0.001

0.0005

0 0

1 P(I) Closed Loop A - N2 P(I) Closed Loop A - CO2 P(I) Closed Loop B - O2 P(II) Closed Loop B - H2S

2 Time [h] P(I) Closed Loop A - O2 P(I) Closed Loop A - H2S P(I) Closed Loop B - CH4

3

4

P(I) Closed Loop A - CH4 P(I) Closed Loop B - N2 P(I) Closed Loop B - CO2

Fig. 4.19. Flux of gases through the membrane (Phase I – Closed Loop)

0.0016 0.0014

Flux [m3(STP) m-2 h-1]

0.0012 0.001 0.0008 0.0006 0.0004 0.0002 0 0

1

2

3

Time [h] P(I) Dead End - N2 P(I) Dead End - CO2

P(I) Dead End - O2 P(I) Dead End - H2S

P(I) Dead End - CH4

Fig. 4.20. Flux of gases through the membrane (Phase I – Dead End)

46

4

The comparison between phases I and II (Fig. 4.21) showed that all gases flow a similar behavior between configurations with a fast flux decline. There is no significant difference in the flux of CO2; however, the flux for the rest of the gases (N2, O2, CH4 and H2S) seems to be lower in phase II than in phase I. The order of the gases in terms of decreasing flow was the same for both phases (N2 > CO2 > CH4 > O2 > H2S).

0.0025

Flux [m3(STP) m-2 h-1]

0.002

0.0015

0.001

0.0005

0 0

1

2

3

4

Time [h] P(II) A - N2 P(II) A - H2S P(II) B - CO2

P(II) A - O2 P(II) B - N2 P(II) B - H2S

P(II) A - CH4 P(II) B - O2

Fig. 4.21. Flux of gases through the membrane (Phase II)

47

P(II) A - CO2 P(II) B - CH4

5. DISCUSSION 5.1 Gas composition Since the membrane allows mass transfer of gases between the two loops, as it was expected, gas composition changed with time for all experimental phases and configurations. These changes were presented in section 4.2. 5.1.1 Phase I: Gas-Gas During phase I, three configurations were used: open loop, closed loop and dead end. Open loop configuration could be considered as a realistic approach since it is probable that in full-scale applications the air side of the membrane will have a constant flow of fresh air (steady state) instead of air recirculation; however, this approach proved to be harder for the control and measurement of all parameters required for the molar balance. The closed loop configuration was a less realistic approach but allowed easier control and measurement of the variables in play. The purpose of the dead end configuration was to study the permeation through the membrane when the movement of gases on the air side was only affected by diffusion and not by external forces (e.g. pumps). From the results, it is clear that the change in concentration was faster for all gases when the open loop configuration was used, and slower for dead end configuration. Flux through the membrane is affected by the difference of partial pressures of a particular gas on both sides of the membrane (see Eq. 2.7). The higher the difference between these partial pressures is, the higher the flux will be; therefore, a faster decrease/increase in concentration will be observed. In the open loop configuration, the constant flow of fresh air on the air side affected the partial pressure of all gases on this side. Partial pressures of nitrogen (N2) and oxygen (O2) were kept to the maximum possible value (atmospheric composition); while methane (CH4), carbon dioxide (CO2) and hydrogen sulfide (H2S) concentrations were kept at the lowest possible value (0%) preventing their buildup in the air loop, and particularly on the surface of the membrane. Thus, keeping the difference in partial pressures for each component at its maximum; i.e. the same as the ones at the beginning of the experiment. The exact opposite was observed in the dead end configuration. The lack of flow on the air side contributed to a faster increase in the concentration of CH4, CO2 and H2S in the

48

surface of the membrane, and a faster decrease in case of N2 and O2 content. This caused a reduction in the difference of partial pressures for all gases; therefore, transfer was much slower. A slow rate of transfer through the membrane (e.g. dead end), translates into a lower amount of the component moved from one loop to the other; and therefore a smaller change in the concentration of that particular component on each loop. The opposite also applies. These differences in the rates of gas transfer through membrane among the experiment configurations explains why the final concentration of CH4, CO2 and H2S on the biogas side was much lower in case of open loop than for any other configuration, and why it was the largest for the dead configuration. It also explains why final concentration of N2 and O2 on biogas side was higher for the open loop configuration and the lowest for dead end. Additionally, it was observed that in the closed loop configuration the concentration of H2S on each side tends to approach the same value after 4 h of experiment; between 2.4 and 3.5 mg L-1. This equalization of concentrations on both sides of the membrane is also due to the changes in partial pressure experience by each gas. The closed loop configuration presents a middle point situation when compared to the open loop and dead end configurations. Initially, the fresh air in the loop provides a high driving force for the transfer of H2S through the membrane; inducing a decrease of its concentration on the biogas side and an increase on the air side. With time, these concentrations will start to become closer to each other; thus, decreasing the difference in partial pressures. Hence, the transfer through the membrane will virtually stop. 5.1.2 Phase II: Gas-Liquid The biogas contained in the headspace of an anaerobic reactor is saturated with water vapor. If a bio-membrane were to be placed in this headspace a thin liquid layer will be formed on the surface of the membrane. During phase II the effect of this liquid layer on the permeability of gases through the silicone membrane was studied. This was achieved by introducing water at the top of the bio-membrane unit (BMU) to keep the surface of the membrane wet. As shown in Fig. 4.7 to 4.10 in section 4.2.2, the behavior and final concentration of N2, O2, CH4 and CO2 was basically unchanged when compared to the results of the closed

49

loop configuration in phase I. However, there was a distinct difference in the behavior of H2S concentration but only on the biogas side. By approximating the change in H2S concentration to a first order equation and applying a linear fit, the rate at which its content decreased was determined. In phase I the decrease in concentration was slower (-0.4 mg L-1 h-1) than in phase II (-0.5 mg L-1 h-1), as shown in Fig. 5.1. This difference could be attributed to the absorption of H2S in water, which was only present on the biogas side. H2S has a high solubility in (pure) water compared to the rest of the gases (see Table 5.1). When water is present in the system, H2S in the biogas is first absorbed in the water and then transferred through the membrane. This creates the appearance of a faster transfer rate.

6

H2S Concentration [mg L-1]

5 4 3 2 1

y = -0.5057x + 5.0774 R² = 0.9949

y = -0.4223x + 4.7021 R² = 0.9903 0 0

1

2

3

4

Time [h] P(I) A - Biogas - Data

P(II) B - Biogas - Data

P(I) A - Biogas - Model

P(II) B - Biogas - Model

Fig. 5.1. Rate of change in H2S concentration on biogas side (Comparison between phase I & II)

CO2 also presents a high solubility in water, second to H2S; therefore, a similar result was expected for this component. However, by simple inspection of Fig. 4.10, it seems that its behavior was the same for phase I and II. Moreover, when applying the same linear fit approach as with H2S, the rates of change obtained can be considered the same for phase I (-0.016 atm h-1) and phase II (-0.015 atm h-1) as it is shown in Fig. 5.2.

50

Table 5.1. Mole fraction solubility of gases in pure water (25 °C) (Lide, 2003)

Gas

Solubility in pure water [mol gas mol solution-1]

N2

1.183 10-5

O2

2.293 10-5

CH4

2.552 10-5

CO2

6.15 10-4

H2S

1.85 10-3

CO2 Partial pressure [atm]

0.4

0.3

0.2

0.1 y = -0.0164x + 0.3308 R² = 0.9913

y = -0.0145x + 0.323 R² = 0.9917

0.0 0

1

2 Time [h]

P(I) A - Biogas - Data P(I) A - Biogas - Model Lineal (P(I) A - Biogas - Model)

3

4

P(II) B - Biogas - Data P(II) B - Biogas - Model Lineal (P(II) B - Biogas - Model)

Fig. 5.2. Rate of change in CO2 concentration on biogas side (Comparison between phase I & II)

5.1.3 Phase III: Gas-Biofilm In this last experimental phase, the effectiveness of a bio-membrane in the removal of H2S from the biogas was studied. A biofilm was grown on the surface of the membrane. After 37 d, the biofilm had covered great part of the surface of the membrane; however, bare membrane areas were still visible. This lack of full coverage of the biofilm could be attributed to two main factors: (1) the lack of sufficient time for development of a 51

mature biofilm, and (2) lack of uniformity in the wetting of the membrane due to limitations in the design of the BMU. No significant difference was observed in the behavior and final concentrations of N2, CH4 and CO2 in each loop for phases I, II and III. It seems that the presence of an additional layer in the membrane (biofilm) presents very little effect in the transfer of these gases through the membrane, probably because they are basically not being used by the communities of microorganisms in the biofilm. In case of O2, the tendency of concentration increase in the biogas and decrease in the air side was consistent for all phases. However, it is clear that the changes in concentration on the biogas side are much lower in phase III than in phase I and II. After 3 h of experiment, final O2 content in the biogas (1%) was around 50% lower than in phases I and II (3%). This represents an advantage for the bio-membrane since low oxygen concentrations on the biogas side are desirable for full-scale applications because it lowers the risk of explosion and risk of affecting methanogenic activity. Moreover, the concentration of O2 in the air side was higher for phase III than for phase I and II, implying that less oxygen was transferred from one loop to the other. It is believed that the decline observed for the O2 concentration in the biogas side was caused by the biofilm. O2 diffused through the membrane from the air side; when it reached the biofilm layer in the other side (biogas) the sulfur oxidizing bacteria (SOB) possibly present in the biofilm consumed the O2 to oxidize the H2S present in the biogas. By consuming part of the O2 that permeates through the membrane, the biofilm actually lowers the amount of O2 that ends up in the biogas. This seems to implicate that the biofilm creates an additional resistance to the transfer of O2, exerting a control on the amount of O2 that passes through the membrane depending on the needs of the communities of microorganisms present in the biofilm. The experimental data for the H2S concentration on the biogas side was approximated to a first order equation; a linear fit was applied to determine the rate at which it changed with time (see Fig. 5.3), the results obtained are as follow: -0.4 mg L-1 h-1 for phase I, 0.5 mg L-1 h-1 for phase II, and -1.2 mg L-1 h-1.

52

6

H2S concentration [mg L-1]

5

4

3

2

1 y = -0.4148x + 4.7181 R² = 0.8553

y = -1.1842x + 5.1295 R² = 0.9978

y = -0.5235x + 4.9648 R² = 0.8982

0 0

1

2

3

Time [h] P(I) B - Biogas

P(II) B - Biogas

P(III) A - Biogas

Lineal (P(I) B - Biogas)

Lineal (P(II) B - Biogas)

Lineal (P(III) A - Biogas)

Fig. 5.3. Rate of change in H2S concentration on biogas side (Comparison between phases I, II & III)

It is indisputable that the rate at which the concentration decreased in the biogas side was much faster in phase III than in any other phase. Because this rate was much higher in phase III, the percentage of removal after 3 h of experiment was also much higher for this phase (55-70%) than phases I and II (26-31%). Moreover, the presence of light yellow spots on the surface of the membrane appears to indicate that indeed, the H2S in the biogas is been oxidized to elemental sulfur (S0) by the biofilm. The first observation confirms that the bio-membrane is capable of a much faster removal of H2S from the biogas than the bare silicone membrane. Furthermore, the second observation seems to indicate the removal mechanism is biological oxidation performed by the biofilm, thus resulting in sulfur deposits on the surface of the membrane. Further tests are recommended in future research to confirm that the deposits formed consist of S0 and to ratify the presence of SOB in the biofilm. Additionally, H2S concentration in the air side was also much lower in phase III (0.05 mg L-1) than in phases I and II (2.1 mg L-1). This result was expected since the oxidation of H2S by the biofilm on the biogas side would lower the amount of the gas that actually reaches the other side of the membrane.

53

It was also observed that although H2S concentration in the air side continuously increased in phases I and II; while in phase III the concentration increased up to a maximum of 0.27-0.35 mg L-1 and then started to go down for the rest of the duration of the experiment. This change in behavior could be explained by two phenomena: biological and chemical oxidation. At the beginning of the experiment, the amount of H2S in the biogas is higher than what the biofilm can oxidize, so part of the H2S that is not transformed reaches the air side causing an increase in concentration. At the same time, chemical oxidation of the H2S can be occurring on the air side due to the presence of O2. The rate of chemical oxidation seem to be smaller than the rate of transfer of H2S through the membrane; hence, the increase in concentration on the air side. As time passes, the concentration of H2S on the biogas side decreases but the biofilm keeps oxidizing the H2S available, so the amount that permeates to the other side gradually declines. Since chemical oxidation is still happening (there is still O2 present on the air side), then the concentration of H2S goes down because there is very little being transferred through the membrane and the amount that is already present in the loop is being consumed. 5.2 Membrane permeability Table 5.2 presents average permeability for phases I & II and their associated standard deviation expressed as their variation. The removal of H2S with the bio-membrane is more complex than in the other two phases due to the interaction between the physicochemical and biological phenomena happening at once. The lack of information regarding all these processes is the main reason why it was not possible to adapt the model used for the first to phases to describe the results of the last one. Therefore, the permeability of the gases for the bio-membrane was not determined. The following discussion addresses only the results of phases I and II. In general, the order of permeability of the components was H2S > CO2 > CH4 > O2 > N2; with H2S being the fastest component to move through the membrane and N2 the slowest. An exception was observed only on the open loop configuration in phase I where the last three components changed order, H2S > CO2 > O2 > N2 > CH4, being CH4 the slowest to permeate.

54

Table 5.2. Average permeability for each gas and configuration

Permeability [Barrer]a Case Phase I Open Loop Closed Loop Dead End Phase II Closed Loop a

Barrer = 10

-10

N2

O2

CH4

CO2

H2 S

1380±57 214±11 598

1425±233 501±36 282

838±61 801±8 272

2930±467 2545±35 1030

7755±530 3410±339 1013

156±47

486±151

889±99

2660±14

3425±64

3

-2 -1

-1

cm (STP) cm cm s cmHg

For all experiments, the permeation order of H2S > CO2 > CH4 coincides with the findings reported by Kraftschik et al. (2013). Moreover, permeability values for N2, O2, CH4 and CO2 reported in the literature (see Table 2.2) follow the same permeation order (CO2 > CH4 > O2 > N2), with the exception of Tremblay et al. (2006) which reports a higher permeation for N2 than for CH4. As shown in Table 2.2, values for the permeability of gases through silicone rubber vary greatly from one research to another. When comparing the results from this experimental work with the ones reported in the literature, the following can be concluded for each gas: N2: permeability results closed loop in phase I and II are similar to those reported by Basu et al. (2010) (250 barrer) and Tremblay et al. (2006) (130-450 barrer). Results from the open loop and dead end configuration are out of range when compared to any of the results reported in the literature reviewed. O2: calculated permeability values for the closed loop configuration in phase I and II are close to the one reported by Basu et al. (2010) (500 barrer). Results from the open loop and dead end configuration are out of range when compared to any of the results reported in the literature reviewed. CH4: results obtained for closed loop are similar to those reported by Basu et al. (2010) (800 barrer) and Tremblay et al. (2006) (500-1600 barrer).The rest of the literature reviewed do not match the experimental results from the present work. CO2: data of closed loop configuration in phase I and II is similar to the one reported by Basu et al. (2010) (3800 barrer) and Tremblay et al. (2006) (2800-5600 barrer). The rest of the results are out of range when compared to the results reported by other authors.

55

H2S: there were no permeability values for H2S reported in the literature reviewed for the present research work. It has been reported that transport properties of a membrane can change depending on whether the experiment was carried out with a pure gas or a mixture of two or more gases (Raharjo et al., 2007). This fact could have caused the difference observed between the permeability values reported in previous works and the ones obtained in this one; calculations in this work were done based on the behavior of a mixture of gases, while in the rest of the literature reviewed the values were calculated experimenting with mostly pure gases. 5.2.1 Phase I: Gas-Gas Dead end configuration showed the lowest permeability values for all components when compared to the results obtained for the other two configurations. Permeability values for N2, O2 and H2S were larger in case of open loop than for closed loop. In case of CH4 and CO2, these values are similar for both configurations. Values obtained for N2 and O2 present the largest variability among configurations. Since the permeability of the membrane should not be affect by the configuration of the experimental setup in this phase, it is reasonable to assume that the lack of control and measurement of gas composition on the air side on the open loop (and dead end) led to less accurate results. Because of this, closed loop configuration was the one chosen to carry on with the experiments on phases II and III. 5.2.2 Phase II: Gas-Liquid In the molar balance for the model, the water layer was considered as part of the membrane, so the concentrations used to calculate the transfer of each gas were still its partial pressure on each loop. Based on this, it was expected that some permeability values determined during phase II would be different than those from phase I, especially for H2S and CO2 because of their high solubility in water (see Table 5.1). CO2 also presents a high solubility in water, second to H2S; therefore, a similar result was expected for this component. However, by simple inspection of Fig. 4.10, it seems that its behavior was the same for phase I and II. Moreover, when applying the same

56

linear fit approach as with H2S, the rates of change obtained can be considered the same for phase I (-0.016 atm h-1) and phase II (-0.015 atm h-1) as it is shown in Fig. 5.2. However, this was not the case. As shown in Table 5.2, results from closed loop configuration of both phases can be considered the same. This could be attributed to the fact that it was not possible to uniformly wet the whole surface of the membrane with the current design of the BMU; there were random spots of the membrane that remained dry. 5.3 Flux of gases through the membrane The variation of flux through the membrane for each component was presented in section 4.4. As described before, the flow of fresh air had a great impact on the partial pressure of all components on the air side. This can also be observed in the variations of flux for all the configurations in phase I. In the open loop, the flux of most of the components was constant throughout the whole duration of the experiment, thanks to the presence of a constant high driving force (difference in partial pressures). On the contrary, in the closed loop configuration there was a fast decline in the flux of all components with time, especially for CH4 and CO2, because of a buildup in concentration of these components on the air side which caused and increase in their partial pressures on that side. Additionally, it was observed that CO2 had the highest flux followed by CH4 > N2 > O2 > H2S. This result demonstrates that even though some components had higher permeability than others, its difference in partial pressures on each side of the membrane plays and important role on how much of the component is being transferred (see Eq. 2.7 and 4.1); e.g. H2S has the highest permeability but the lowest flux of all components because of low difference in partial pressures due to its low concentration when compared to the overall composition of the gas on each side. 5.4 Implications of the results The flux values calculated for phase II can be used to estimate the membrane area required for achieving the delivery of the amount of O2 necessary to oxidize the H2S present in the biogas. As mentioned before, the open loop configuration presents the closest resemblance to the possible configuration of this bio-membrane process in full-

57

scale application; however, these values present some errors so the flux at the beginning of the closed loop experiments will be used instead. Considering the chemical reaction for the oxidation of H2S to S0 presented in Eq. 5.1, according to the stoichiometry, 0.33 L of O2 are required to oxidize 1 g of H2S. H2S + 0.5 O2
S0 + H2O

Eq. 5.1

Based on this, 1 m3 h-1 of biogas production with a concentration of 3000 mg m-3 would require about 1 L h-1 of O2 to oxidize all the H2S. This amount is very low. Using the calculated flux values the area required to achieve this O2 transfer is 3.7 m2. With the membrane area set, the possible losses of CH4 and CO2 from the biogas can also be computed, providing an estimate of 5.2 L h-1 of CH4 and 8 L h-1 of CO2 in losses per m3 of biogas. Based on the experimental data gathered for phase III, it is probable that the membrane area required in case of desulfurization through the bio-membrane will be smaller than the one required in phases I and II, so it is probable the amount of CH4 and CO2 losses will be also smaller. The results from this research show that combination of microaeration with a biomembrane process for biogas desulfurization is promising. However, before the process can be taken to full-scale verification, further research is recommended to fully understand the processes involved. Moreover, many researchers have identified biofilm control as the most challenging aspect of operating applications using bio-membranes. Excessive biofilm growth will not only cause non-uniform flow distribution and channeling, but also the inhibition of substrate or gas diffusion, eventually deteriorating the system performance (Hwang et al., 2009). Therefore, it is also necessary to run long term experiments to determine the effect of a thicker biofilm layer or sulfur accumulation in the membrane over the transfer of gases and the overall H2S removal from the biogas. Control of the elemental sulfur deposited in the membrane and its harvesting for future use is also another challenge for research.

58

6. CONCLUSIONS The effectiveness of using a silicone bio-membrane for the removal of hydrogen sulfide from biogas was tested using a lab-scale bio-membrane unit (BMU) which simulated the placement of the bio-membrane in the headspace of an anaerobic reactor. The results obtained can be summarized as follows: 

The structure of the silicone membrane allows the transfer of gases through it from biogas side to air side (and vice versa) as long as there is enough driving force (difference in partial pressures on each side of the membrane) to induce it.



Changes in composition followed by nitrogen (N2), methane (CH4) and carbon dioxide (CO2) are basically the same for all the phases of the experiment. The presence of the biofilm seems to have a negligible effect in the transfer of these gases through the membrane.



Final oxygen (O2) concentration in the biogas side was much lower for phase III than for phases I and II, suggesting that the O2 transferred was consumed by the biofilm on the surface of the membrane.



H2S concentration in the biogas decreased much faster in phase III (biofilm stage) than in any other phase. After 3 h of experiment, the percentage of H2S removed from the biogas was 55-70%. Final concentration achieved in the biogas was 1.51.8 mg L-1 (initially 5 mg L-1). This demonstrates a higher efficiency of H2S removal for the bio-membrane than for the simple silicone membrane.



The permeability of all gases was determined for all configurations in phases I and II. The permeation of gases in decreasing order is as follows: H2S > CO2 > CH4 > O2 > N2. Flux calculations based on permeability values allowed estimating possible losses of CH4 and CO2 during microaeration as well as O2 consumption. Volume of gas losses and O2 requirement are very low.



The membrane works as a support for the attachment and growth of the biofilm. During phase III, light yellow deposits were observed on the surface of the membrane as well, indicating possible formation of elemental sulfur due to biological oxidation of H2S.

59

7. BIBLIOGRAPHY Abbasi, T., Tauseef, S.M. & Abbasi, S.A. (2012) “Biogas Energy”, Springer Science+Business Media, LLC. American Public Health Association (APHA) (1999), Standard Methods for the Examination of Water and Wastewater (20th Edition). American Public Health Association, American Water Works Association and Water Environment Federation, Washington D.C., USA. Appels, L., Baeyens, J., Degreve, J. & Dewil, R. (2008) Principles and potential of the anaerobic digestion of waste-activated sludge, Process in Energy and Combustion Science, vol. 34, pp. 755-781. Basu, S., Khan, A.L., Cano-Odena, A., Liu, C. & Vankelecom, I.F.J. (2010) Membranebased technologies for biogas separations, Chemical Society Reviews, vol. 39, pp. 750768. Botheju, D. & Bakke, .R. (2011) Oxygen effects in anaerobic digestion – A review, The Open Waste Management Journal, vol. 4, pp. 1-19. Brindle, K. & Stephenson, T. (1996) The Application of Membrane Biological Reactors for the Treatment of Wastewaters, Biotechnology and Bioengineering, vol. 49, pp. 601610. Diaz, I., Lopes, A.C., Pérez, S.I & Fdz-Polanco, M. (2010) Performance evaluation of oxygen, air and nitrate for the microaerobic removal of hydrogen sulfide in biogas from sludge digestion, Bioresource Technology, vol. 101, pp. 7724-7730. Diaz, I., Pérez, S.I., Ferrero, E.M & Fdz-Polanco, M (2011) Effect of oxygen dosing point and mixing on the microaerobic removal of hydrogen sulphide in sludge digesters, Bioresource Technology, vol. 102, pp. 3768-3775. Diaz, I., Donoso-Bravo, A. & Fdz-Polanco, M. (2011a) Effect of microaerobic conditions on the degradation kinetics of cellulose, Bioresource Technology, vol. 102, pp. 10139-10142. Henshaw, P.F., Bewtra, J.K. & Biswas, N. (1998) Hydrogen sulphide conversion to elemental sulphur in a suspended-growth continuous stirred tank reactor using Chlorobium limicola, Water resources, vol. 32 (6), pp. 1769-1778. Henshaw, P.F. & Zhu, W. (2001) Biological conversion of hydrogen sulphide to elemental sulphur in a fixed-film continuous flow photo-reactor, Water Research, vol. 35 (15), pp. 3605-3610. Ho, K.L., Lin, W.C., Chung, Y.C., Chen, Y.P. & Tseng, C.P. (2013) Elimination of high concentration hydrogen sulfide and biogas purification by chemical-biological process, Chemosphere, vol. 92, pp. 1396-1401. Hwang, J.H., Cicek, N. & Oleszkiewicz, J.A. (2009) Membrane biofilm reactors for nitrogen removal: state-of-the-art and research needs, Water Science & Technology, vol. 660 (11), pp. 2739-2747. Janssen, A.J.H., Lettinga, G. & de Keizer, A. (1999) Removal of hydrogen sulphide from wastewater and waste gases by biological conversion to elemental sulphur.

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Colloidal and interfacial aspects of biologically produced sulphur particles, Colloids and Surfaces:Physicochemical And Engineering Aspects, vol. 151, pp. 389-397. Janssen, A.J.H., Lens, P.N.L., Stams, A.J.M., Plugge, C.M., Sorokin, D.Y., Muyzer, G., Dijkman, H., Van Zessen, E., Luimes, P., & Buisman, C.J.N (2009) Application of bacteria involved in the biological sulfur cycle for paper mill effluent purification, Science of the Total Environment, vol. 407, pp. 1333-1343. Javaid, A. (2005) Membranes for solubility-based gas separation applications, Chemical Engineering Journal, vol. 112, pp. 219-226. Jenicek, P., Keclik, F., Maca, J. & Bindzar, J. (2008) Use of microaerobic conditions for the improvement of anaerobic digestion of solid wastes, Water Science & Technology, vol. 58 (7), pp. 1491-1496. Jenicek, P., Koubova, J., Bindzar, J. & Zabranska, J. (2010) Advantages of anaerobic digestion of sludge in microaerobic conditions, Water Science & Technology, vol. 62 (2), pp. 427-434. Jenicek, P., Celis, C., Picha, A. & Pokorna, D. (2013) Influence of Raw Sludge Quality on the Efficiency of Microaeration Sulfide Removal during Anaerobic Digestion of Sewage Sludge, Journal of Residuals Science & Technology, vol. 10 (1), pp. 11-16. Kobayashi, T., Li. Y., Kubota, K., Harada, H., Maeda, T. & Yu, H. (2012) Characterization of sulfide-oxidizing microbial mats developed inside a full-scale anaerobic digester employing biological desulfurization, Applied Microbiology and Biotechnology, vol. 93, pp. 847-857. Kolthoff, I.M., Sandell, E.B., Meehan, E.J., & Bruckenstein, S. (1969) Quantitative Chemical Analysis (4th Edition), Macmillan Pub. Co., New York. Kraftschik, B., Koros, W.J., Johnson, J.R. & Karvan, O. (2013) Dense film polyimide membranes for aggressive sour gas feed separations, Journal of Membrane Science, vol. 428, pp. 608-619. Lide, D.R. (ed) (2003) CRC Handbook of Chemistry and Physics, 84th Ed., CRC Press, USA. Lim, J. W. & Wang, J.Y. (2013) Enhanced hydrolysis and methane yield by applying microaeration pretreatment to the anaerobic co-digestion of brown water and food waste, Waste Management, vol. 33, pp. 813-819 Lin, W.C., Chen, Y.P. & Tseng, C.P. (2013) Pilot-chemical-biological system for efficient H2S removal from biogas, Bioresource Technology, vol. 135, pp. 283-291. Liu, D. H. F. & Lipták, B. G. (2000) “Environmental Engineers’ Handbook, 2nd edition, CRC Press LLC, Boca Raton, U.S.A. Makaruk, A., Miltner, M. & Harasek, M. (2013) Biogas desulfurization and biogas upgrading using a hybrid membrane system – modeling study, Water Science & Technology, vol. 67 (2), pp. 326-332. Merkel, T.C., Bondar, V.I., Nagai, K., Freeman, B.D. & Pinnau, I. (2000) Gas sorption, diffusion, and permeation in poly(dimethylsiloxane), Journal of Polymer Science. Part B: Polymer Physics, vol. 38 (3), pp. 415-434. Metcalf & Eddy, Inc. (2003). Wastewater Engineering – Treatment and Reuse. McGraw-Hill. China.

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Mudhoo, A. (ed.) (2012) “Biogas production. Pretreatment Methods in Anaerobic Digestion”, Scrivener Publishing LLC, Canada. Nayono, S.E. (2009) “Anaerobic Digestion of organic solid waste for energy production”, KIT Scientific Publishing, Germany. Nijaguna, B.T (2002) “Biogas Technology”, New Age International Ltd, Publishers, New Delhi. Raharjo, R.D., Freeman, B.D., Paul, D.R., Sarti, G.C. & Sanders, E.S. (2007) Pure and mixed gas CH4 and n-C4H10 permeability and diffusivity in poly(dimethylsiloxane), Journal of Membrane Science, vol. 306, pp. 75-92. Ramos, I., Pérez, R. & Fdz-Polanco, M. (2013) Microerobic desulphurization unit: A new biological system for the removal of H2S from biogas, Bioresource Technology, vol. 142, pp. 633-640. Russell, D.L. (2006) “Practical Wastewater Treatment”, John Wiley & Sons, Inc., New Jersey. Sereda, L., López-González, M.M., Visconte, L.L.Y., Nunes, R.C..R., Guimaraes, C.R. & Riande, E. (2003) Influence of silica and black rice husk ash fillers on the diffusivity and solubility of gases in silicone rubbers, Polymer, vol. 44, pp. 3085-3093. Spellman, F.R. (2013) “Water & Wastewater Infrastructure. Energy Efficiency and Sustainability”, CRC Press, Boca Raton, Florida. Spinosa, L & Vesilind, P.A. (ed.) (2001) “Sludge into Biosolids. Processing, Disposal, Utilization”, IWA Publishing, London. Syed, M.A. & Henshaw, P.F (2003) Effect of tube size on performance of a fixed-film tubular bioreactor for conversion of hydrogen sulfide to elemental sulfur, Water Research, vol. 37, pp. 1932-1938. Tremblay, P., Savard, M.M., Vermette, J. & Paquin, R. (2006) Gas permeability , diffusivity and solubility of nitrogen, helium, methane, carbon dioxide and formaldehyde in dense polymeric membrane using a new on-line permeation apparatus, Journal of Membrane Science, vol. 282, pp. 245-256. Weiland, P. (2010) Biogas production: current state and perspectives, Applied Microbiology and Biotechnology, vol. 85, pp. 849-860. Woodard & Curran, Inc. (2006) “Industrial Waste Treatment Handbook”, 2nd edition, Elsevier Inc., United States of America.

62

APPENDIX A Molar Balance The permeability of each of the gases through the silicone membrane was determined using a model obtained by performing a molar balance. This balance takes into account all the flows in and out of each loop and some special considerations that are particular to each configuration of the experimental setup. A.1. Phase I: Gas-Gas. A.1.1. Closed loop. A general representation of the flows of methane (CH4), carbon dioxide (CO2), hydrogen sulfide (H2S), nitrogen (N2) and oxygen (O2) is presented in Fig. A.1. Detailed description of each flow and the molar balance associated to each component is presented below.

Fig. A.1. Schematic representation of the flow of gases in each loop The flow of components through the membrane was induced due to the difference of partial pressures of the components on each side. Additionally, because these flows were not equal, a change in the total pressure of each loop would develop with time. There would be a pressure buildup on the air side and a pressure decrease on the biogas side. To keep a constant atmospheric pressure on each

63

loop, gas was constantly released on the air side and ambient air was let inside the biogas loop. For calculation purposes, these volumes were considered to be equal. By keeping constant pressure (1 atm) and temperature (25 °C) the total gas volume of each loop was considered constant throughout the experiment. These values were 5.27 L for the biogas side and 1.45 L for the air side. Both biogas and air side were kept under constant recirculation and no fresh gas was added during the experiment. Therefore, the composition was changing with time (transient state). For that reason, the molar balance was applied to small time steps (6 min) in which the partial pressure, and thus the concentration, of the gases were considered to be constant. The change in moles during a particular time step determined the partial pressure of the gas for the next step. a) Flow of nitrogen and oxygen Partial pressure of N2 and O2 was higher on the air side than on the biogas side. This difference in partial pressures created a flow through the membrane to the biogas side; thus, producing a decrease in the concentration of these gases on the air side and an increase on the biogas side. In general, the change in molar mass of each component during a time step can be expressed as function of the number of moles there are in the loop at the beginning of the time step, the moles that are transferred through the membrane, and the moles that are released (air side) or incorporated (biogas side) to compensate the pressure. The relationship between these parameters is presented in Eq. A.1 and A.2 for air side and biogas, respectively. *:: 7;( = *:: 7 − *6 7 − *< 7 − 

Eq. A.1

*:: 7;( = *:: 7 + *6 7 + * 7

Eq. A.2

Where: *:: 7 : Moles of gas (N2 or O2) inside the loop at the beginning of a particular time step (i), [mol]. *:: 7;( : Moles of gas (N2 or O2) inside the loop at the end of a particular time step (i), [mol]. *6 7 : Moles of gas (N2 or O2) that pass through the membrane in that particular time step (i), [mol].

64

*< 7 : Moles of gas (N2 or O2) that are released from the air side in that particular time step (i), [mol]. * 7 : Moles of gas (N2 or O2) from the environment that go into the biogas side in that particular time step (i), [mol].  : Adjustment parameter, [mol]. An additional parameter (S) was added to quantify the effect of gas solubility in the membrane, losses of the compound due to possible chemical reactions (e.g. oxidation), and other losses in the system that were not possible to quantify. The number of moles of every component present inside each loop was determined using the Ideal Gas Law as shown in Eq. A.3. *:: 7 =

4 ∗ 2 > ?∗.

Eq. A.3

Where: 7 : Partial pressure of the gas (N2 or O2) inside the loop at the beginning of a particular time step (i), [atm]. %:: : Total gas volume in the loop, [m3]. @ : Ideal gas constant, [0.00008205 m3 atm mol-1 K-1]. A : Ambient temperature, [298.15 K]. The moles of each component that were released to the environment from the air side were also determined using the Ideal Gas Law as Eq. A.4 *< 7 =

4 ∗ 2B 4 ?∗.

Eq. A.4

Where: %< 7 : Volume of gas released from the air side in a particular time step (i), [m3]. Similarly, the moles of ambient air introduced on the biogas side to compensate the pressure drop were calculated using the same principle (see Eq. A.5), and assuming that this volume was equal to the volume of gas released from the air side. Partial pressure of N2 and O2 were assumed equal to atmospheric. * 7 =

4 ∗ 2B 4 ?∗.

Eq. A.5

The moles transferred through the membrane were calculated using Eq. A.6. *6 7 = 3600 ∗

∗ E

65

∗ FG 7 ( −  7 ( H ∗ 9

Eq. A.6

Where: : Permeability of the gas (N2 or O2) through the membrane, [mol m m-2 s-1 Pa-1]. 8: Surface area of the membrane, [m2]. I: Thickness of the membrane, [m]. G 7 (: Partial pressure of the gas (N2 or O2) on the feed side, in this case the air side, in the previous time step (i) [Pa].  7 ( : Partial pressure of the gas (N2 or O2) on the permeate side, in this case the biogas side, in the previous time step (i) [Pa]. 9: Time or duration of the time step [h]. Equations A.1 through A.6 provide the changes in molar mass for a particular component in a time step. After substituting all these parameters in Eq. A.1, the amount of each component left in the loop at the end of the time step was determined. Then, using Eq. A.7, the new partial pressure for that component was computed and the whole molar balance was repeated again for the next time step. 7;( =

J >

4KL

∗ ? ∗ .

2 >

Eq. A.7

Where: 7;( : Partial pressure of the gas (N2 or O2) for the next time step (i + 1) [Pa]. @ : Ideal gas constant, [0.00008205 m3 atm mol-1 K-1]. To adjust this model to the experimental data, arbitrary values were required for the permeability of the gas (P) and the adjustment parameter (S). The least squares method was the approach used to find the best fit of the model to the experimental data. First, keeping S = 0, the permeability was determined. Afterwards, with the value of P that provided the best fit possible, S was determined. b) Flow of methane and carbon dioxide Partial pressure of CH4 and CO2 was higher on the biogas side than on the air side. In this case, the flow of components through the membrane was from biogas side to air side; thus, producing a decrease in the concentration of these gases on the biogas side and an increase on the air side. The change in molar mass of each component was computed using Eq. A.8 (air side) and A.9 (biogas side).

66

*:: 7;( = *:: 7 + *6 7 − *< 7

Eq. A.8

*:: 7;( = *:: 7 − *6 7 − 

Eq. A.9

Where: *:: 7 : Moles of gas (CH4 or CO2) inside the loop at the beginning of a particular time step (i), [mol]. *:: 7;( : Moles of gas (CH4 or CO2) inside the loop at the end of a particular time step (i), [mol]. *6 7 : Moles of gas (CH4 or CO2) that pass through the membrane in that particular time step (i), [mol]. *< 7 : Moles of gas (CH4 or CO2) that are released from the air side in that particular time step (i), [mol].  : Adjustment parameter, [mol]. Parameter (S) quantifies the effect of gas solubility in the membrane and other possible losses that could not be experimentally quantified. The amount of moles of every component present inside each loop (Nloopi), and the one released to the environment from the air side (NLi) was determined using Eq. A.3 and A.4, respectively. The moles transferred through the membrane were calculated using the same Eq. A.6. However, feed side would be the biogas side while air side was considered as the permeate side. New partial pressure for next time step was computed by means of Eq. A.7. P and S for CH4 and CO2 were also determined using the least squares method following the approach described before. c) Flow of hydrogen sulfide Just like with CH4 and CO2, partial pressure of H2S was higher on the biogas side than on the air side. Flow through the membrane was from biogas side to air side; thus, producing a decrease in the concentration on the biogas side and an increase on the air side. Because the exchange of H2S through the membrane depends on the partial pressure on each side, it was necessary to convert the experimental data (concentration values) gathered for each loop. For each time step, the moles of H2S in the system were

67

determined first based in Eq. A.10. Then, Eq. A.11 was used to determine the total number of moles of gas (considering all components) present in the loop. Afterwards, H2S partial pressure was determined computing the ratio of both as shown in Eq. A.12. *:: 7 =

4 ∗ 2 > MN

*._:: 7 = *P& 7 + *J 7 + *Q 7 + *PR 7 + *Q 7 :: 7 =

J > ∗  4

JS_ >

Eq. A.10 Eq. A.11

Eq. A.12

4

Where: *:: 7 : Moles of H2S inside the loop at the beginning of a particular time step (i), [mol]. 7 ∶ H2S concentration in the loop at the beginning of a particular time step (i), [mg L-1]. ') : Molecular weight of H2S, [34.08 g mol-1]. *._:: 7 : Total number of moles of gas in the loop at a particular time step (i), [mol]. *P& 7 : Moles of H2S present in the loop at a particular time step (i), [mol]. *J 7 : Moles of N2 present in the loop at a particular time step (i), [mol]. *Q 7 : Moles of O2 present in the loop at a particular time step (i), [mol].

*PR 7 : Moles of CH4 present in the loop at a particular time step (i), [mol]. *Q 7 : Moles of CO2 present in the loop at a particular time step (i), [mol].

:: 7 : Partial pressure of H2S in the loop at a particular time step (i), [atm].

: Absolute pressure of the gas inside the loop, [1 atm].

The amount of H2S transferred through the membrane was calculated for each time step using Eq. A.6; taking into account that biogas was the feed side and air was the permeate side. The change in molar mass can be expressed using Eq. A.8 (air side) and A.9 (biogas side). The losses on the air side due to pressure increase (*< 7 ) were calculated following Eq. A.4. Finally, the concentration at the end of the time step was defined using Eq. A.13. 7;( =

J >

4KL

∗ MN

2 >

68

Eq. A.13

Where: 7;( ∶ H2S concentration in the loop at the end of a particular time step (i), [mg L-1]. P and S were also determined using the least squares method following the approach described before. A.1.2. Open loop. In general, the molar balance used for the open loop configuration was the same as the one used for the closed loop; however, the following assumptions were made: 1. Flow of fresh air through the system was high enough to considered that its composition was constant (steady state) and equal to that of atmospheric air. 2. Because of the constant flow of fresh air, there was no increase in concentration of CH4, CO2 and H2S on the air side; therefore, their partial pressure on this loop was always considered equal to zero (0). A.2. Phase II: Gas-Liquid. During this phase, the only configuration used was closed loop. Balances performed were the same as described for phase I. A.3. Phase III: Gas-Biofilm. The molar balance presented above was intended to be used for this phase; however, when applied to the experimental data, it was not possible to find a good fit. Because of this, permeability and flux values of this phase were not possible to calculate. A.4. Intrinsic losses. This section contains the mass balances used to determine the losses of H2S due to leakages on each loop of the experimental setup. The mass balances are presented according to the experiments described in sections 3.1.1 and 4.1. The schematic representation used for the flows in these balances is presented in Fig A.2.

69

Fig. A.2. Flows associated to the Intrinsic Losses mass balance a) “Only membrane” experiment This configuration was used to determine the losses of H2S on the biogas side. By isolating the membrane, it was assumed that the transfer of gases through it was negligible; therefore, the observed decreased in H2S concentration was considered to be only due to leakages. The change in H2S mass on the biogas side can be expressed as shown in Eq. A.14. ML 5

= − '( + '+ 

Eq. A.14

Where: ML 5

: Rate of change in the mass of H2S inside the biogas side, [mg h-1].

'( : Mass flow of H2S leaving the biogas side due to losses, [mg h-1]. '+ : Mass flow of H2S transferred through the membrane, [mg h-1]. Knowing that the mass of H2S can be expressed as a function of the total volume of the biogas loop (constant) and the concentration of the gas (see Eq. A.15), and assuming that the transfer through the membrane in negligible ('+ = 0), Eq. A.14 could be rewritten as Eq. A.16. ' =  ∗ % L 5

= −

70

ML 2

Eq. A.15 Eq. A.16

Where: ': Mass of H2S, [mg]. : Concentration of H2S, [mg L-1]. %: Volume of the biogas loop, [L]. L 5

: Rate of change in the concentration of H2S inside the biogas side, [mg L-1 h-1].

By making a linear regression on the H2S data obtained experimentally,

L 5

was

determined. This value directly represents the losses of H2S in terms of decrease in concentration per unit of time. b) “BG-BG closed loop” experiment The change in H2S mass can be expressed as shown in Eq. A.17 and A.18 for biogas and air sides, respectively. ML

= − '( + '+ 

Eq. A.17

MU

= '+ − '

Eq. A.18

5

5

Where: ML 5

MU 5

: Rate of change in the mass of H2S inside the biogas side, [mg h-1]. : Rate of change in the mass of H2S inside the air side, [mg h-1].

'( : Mass flow of H2S leaving the biogas side due to losses, [mg h-1]. ' : Mass flow of H2S leaving the air side due to losses, [mg h-1]. '+ : Mass flow of H2S transferred through the membrane, [mg h-1]. In this case the membrane was not isolated, so '+ had to be taken into account. By substituting Eq. A.15 in Eq. A.17 and A.18 and rearranging Eq. A.19 and A.20 were obtained. L

= −

U

=

5

5

ML ; MV  2

MV MU  2

Eq. A.19

Eq. A.20

Where: L 5

: Rate of change in the concentration of H2S inside the biogas side, [mg L-1 h-1]. 71

U 5

: Rate of change in the concentration of H2S inside the air side considering both

losses and transfer through the membrane, [mg L-1 h-1]. % : Volume of the air loop, [L]. % : Volume of the biogas loop, [L]. By making a linear regression on the H2S data obtained experimentally for each loop, L 5



and 5U were determined.

'( was calculated from Eq. A.16 from the “only membrane” experiment and used in Eq. A.19 to determine the flow through the membrane ('+ ). Afterwards, this flow was used in Eq. A.20. to compute the value for ' . Finally, the rate of change of the concentration of H2S in the biogas was determined by means of Eq. A.21. U W 5

M

= 2 U 

Eq. A.21

Where: U W

: Rate of change in the concentration of H2S inside the air side due to losses, [mg L-1 h-1]. 5

72

APPENDIX B Effect of gas flow rate on permeability During phase II, besides the two experiments performed using the closed loop configuration, an additional experiment was carried out to determine if the calculated values for gas permeability obtained were independent of the gas flow on the surface of the membrane. The setup had the exact configuration as the rest of the experiments in this phase with the only difference being the increase in the recirculation flow from 16.2 to 20.4 L h-1. Changes in gas composition and results from the model were compared to those obtained for the experiments carried out using a lower flow rate. Figures B.1 to B.5 present the comparison of the results for N2, O2, CH4, CO2 and H2S, respectively. It was observed that the behavior of all gases in terms of rate of change in concentration and final concentration achieved on each loop was practically the same for both flow rates. Additionally, calculated permeability values for each gas could be considered the same among the three experiments; with the exception of O2 which was lower when the recirculation flow was increased. Based on these results, it can be concluded that the gas flow rate has a negligible effect over the permeability of gases in this experimental setup. Table B.1. Permeability for each gas (Phase II) – Flow rate effect Permeability [Barrer]a Case N2

O2

CH4

CO2

H2 S

Closed Loop A

189

592

959

2670

3470

Closed Loop B

123

379

819

2650

3380

236

834

2760

3500

Phase II

190

Flow rate effect a

Barrer = 10

-10

3

-2 -1

-1

cm (STP) cm cm s cmHg

73

N2 Partial pressure [atm]

1.0

0.8

0.6

0.4

0.2

0.0 0

1

P(II) A - Biogas - Data P(II) A - Biogas - Model P(II) A - Air - Data P(II) A - Air - Model

2 Time [h] P(II) B - Biogas - Data P(II) B - Biogas - Model P(II) B - Air - Data P(II) B - Air - Model

3

4

P(II) Flow - Biogas - Data P(II) Flow - Biogas - Model P(II) Flow - Air - Data P(II) Flow - Air - Model

Fig. B.1. N2 composition – Biogas and Air side (Phase II) – Flow rate effect

O2 Partial pressure [atm]

0.25

0.20

0.15

0.10

0.05

0.00 0

1

P(II) A - Biogas - Data P(II) A - Biogas - Model P(II) A - Air - Data P(II) A - Air - Model

2 Time [h] P(II) B - Biogas - Data P(II) B - Biogas - Model P(II) B - Air - Data P(II) B - Air - Model

3

P(II) Flow - Biogas - Data P(II) Flow - Biogas - Model P(II) Flow - Air - Data P(II) Flow - Air - Model

Fig. B.2. O2 composition – Biogas and Air side (Phase II) – Flow rate effect 74

4

0.7

CH4 Partial pressure [atm]

0.6 0.5 0.4 0.3 0.2 0.1 0.0 0

1

2 Time [h]

3

4

P(II) A - Biogas - Data

P(II) B - Biogas - Data

P(II) Flow - Biogas - Data

P(II) A - Biogas - Model P(II) A - Air - Data P(II) A - Air - Model

P(II) B - Biogas - Model P(II) B - Air - Data P(II) B - Air - Model

P(II) Flow - Biogas - Model P(II) Flow - Air - Data P(II) Flow - Air - Model

Fig. B.3. CH4 composition – Biogas and Air side (Phase II) – Flow rate effect

CO2 Partial pressure [atm]

0.4

0.3

0.2

0.1

0.0 0

1

2

3

4

Time [h] P(II) A - Biogas - Data P(II) A - Biogas - Model P(II) A - Air - Data P(II) A - Air - Model

P(II) B - Biogas - Data P(II) B - Biogas - Model P(II) B - Air - Data P(II) B - Air - Model

P(II) Flow - Biogas - Data P(II) Flow - Biogas - Model P(II) Flow - Air - Data P(II) Flow - Air - Model

Fig. B.4. CO2 composition – Biogas and Air side (Phase II) – Flow rate effect

75

6

H2S Concentration [mg L-1]

5

4

3

2

1

0 0

1

2

3

4

Time [h] P(II) A - Biogas - Data P(II) A - Biogas - Model P(II) A - Air - Data P(II) A - Air - Model

P(II) B - Biogas - Data P(II) B - Biogas - Model P(II) B - Air - Data P(II) B - Air - Model

P(II) Flow - Biogas - Data P(II) Flow - Biogas - Model P(II) Flow - Air - Data P(II) Flow - Air - Model

Fig. B.5. H2S composition – Biogas and Air side (Phase II) – Flow rate effect

76