Over the past two decades, the use of microbes to remove pollutants from contaminated air streams has become a widely accepted and efficient alternative to the classical physical and chemical treatment technologies. This book focuses on biotechnological alternatives, looking at both the optimization of bioreactors and the development of cleaner biofuels. It is the first reference work to give a broad overview of bioprocesses for the mitigation of air pollution. Essential reading for researchers and students in environmental engineering, biotechnology, and applied microbiology, and industrial and governmental researchers.
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Table of Contents
List of Contributors
Part I: Fundamentals and Microbiological Aspects
Chapter 1: Introduction to Air Pollution
1.2 Types and sources of air pollutants
1.3 Air pollution control technologies
Chapter 2: Biodegradation and Bioconversion of Volatile Pollutants
2.2 Biodegradation of volatile compounds
2.3 Mass balance calculations
2.4 Bioconversion of volatile compounds
Chapter 3: Identification and Characterization of Microbial Communities in Bioreactors
3.2 Molecular techniques to characterize the microbial communities in bioreactors
3.3 The link of microbial community structure with ecological function in engineered ecosystems
Part II: Bioreactors for Air Pollution Control
Chapter 4: Biofilters
4.2 Historical perspective of biofilters
4.3 Process fundamentals
4.4 Operation parameters of biofilters
4.5 Design considerations
4.6 Start-up of biofilters
4.7 Parameters affecting biofilter performance
4.8 Role of microorganisms and fungal growth in biofilters
4.9 Dynamic loading pattern and starvation conditions in biofilters
4.10 On-line monitoring and control (intelligent) systems for biofilters
4.11 Mathematical expressions for biofilters
4.12 Artificial neural network-based models
4.13 Fuzzy logic-based models
4.14 Adaptive neuro-fuzzy interference system-based models for biofilters
Chapter 5: Biotrickling Filters
5.2 Main characteristics of BTFs
5.3 Pressure drop and clogging
5.4 Full-scale applications and scaling up
Chapter 6: Bioscrubbers
6.2 General approach of bioscrubbers
6.3 Operating conditions
6.4 Removing families of pollutants
6.5 Treatment of by-products generated by bioscrubbers
6.6 Conclusions and trends
Chapter 7: Membrane Bioreactors
7.2 Membrane basics
7.3 Reactor configurations
7.5 Performance of membrane bioreactors
7.6 Membrane bioreactor modeling
7.7 Applications of membrane bioreactors in biological waste-gas treatment
7.8 New applications: sequestration
7.9 Future needs
Chapter 8: Two-Phase Partitioning Bioreactors
8.2 Features of the sequestering phase—selection criteria
8.3 Liquid two-phase partitioning bioreactors (TPPBs)
8.4 Solids as the partitioning phase
Chapter 9: Rotating Biological Contactors
9.2 The rotating biological contactor
9.3 Studies on removal of dichloromethane in modified RBCs
Chapter 10: Innovative Bioreactors and Two-Stage Systems
10.2 Innovative bioreactor configurations
10.3 Two-stage systems for waste gas treatment
Part III: Bioprocesses for Specific Applications
Chapter 11: Bioprocesses for the Removal of Volatile Sulfur Compounds from Gas Streams
11.2 Toxicity of VOSCs to animals and humans
11.3 Biological formation of VOSCs
11.4 VOSC-producing and VOSC-emitting industries
11.5 Microbial degradation of VOSCs
11.6 Treatment technologies for gas streams containing volatile sulfur compounds
11.7 Operating experience from biological gas treatment systems
11.8 Future developments
Chapter 12: Bioprocesses for the Removal of Nitrogen Oxides
12.2 NOx and N2O emissions at wastewater treatment plants (WWTPs)
12.3 Recent developments in bioprocesses for the removal of nitrogen oxides
12.4 Challenges in NOx treatment technologies
Chapter 13: Biogas Upgrading
13.2 Biotechnologies for biogas desulphurization
13.3 Removal of mercaptans
13.4 Removal of ammonia and nitrogen compounds
13.5 Removal of carbon dioxide
13.6 Removal of siloxanes
13.7 Comparison between biological and non-biological methods
Part IV: Environmentally-Friendly Bioenergy
Chapter 14: Biogas
14.2 Anaerobic digestion
14.6 Environmental impact of biogas
Chapter 15: Biohydrogen
15.2 Environmental impacts of biohydrogen production
15.3 Properties and production of hydrogen
15.4 Potential applications of hydrogen as a zero-carbon fuel
15.5 Policies and economics of hydrogen production
15.6 Issues and barriers
15.7 Future prospects
Chapter 16: Catalytic Biodiesel Production
16.2 Trends in biodiesel production
16.3 Challenges for biodiesel production at industrial scale
Chapter 17: Microalgal Biodiesel
17.2 Wild versus modified microalgae
17.3 Lipid extraction and purification
17.4 Lipid transesterification
17.5 Economic considerations
17.6 Environmental considerations
17.7 Final considerations
Chapter 18: Bioethanol
18.2 Fermentation of lignocellulosic saccharides to ethanol
18.3 Syngas conversion to ethanol—biological route
18.4 Demonstration projects
18.5 Comparison of conventional fuels and bioethanol (corn, cellulosic, syngas) on air pollution
18.6 Key problems and future research needs
Part V: Case Studies
Chapter 19: Biotrickling Filtration of Waste Gases from the Viscose Industry
19.1 The waste-gas situation in the viscose industry
19.2 Biological and oxidation
19.3 Case study of biological waste-gas treatment in the casing industry
Chapter 20: Biotrickling Filters for Removal of Volatile Organic Compounds from Air in the Coating Sector
20.2 Case study 1: VOC removal in a furniture facility
20.3 Case study 2: VOC removal in a plastic coating facility
Chapter 21: Industrial Bioscrubbers for the Food and Waste Industries
21.2 Food industry emissions
21.3 Bioscrubbing treatment of gaseous emissions from waste composting
21.4 Conclusions and perspectives
Chapter 22: Desulfurization of biogas in biotrickling filters
22.2 Microbiology and stoichiometry of sulfide oxidation
22.3 Case study background and description of biotrickling filter
22.4 Operational aspects of the full-scale biotrickling filter
22.5 Economic aspects of desulfurizing biotrickling filters
Chapter 23: Full-Scale Biogas Upgrading
23.2 Case 1: Zalaegerszeg, PWS system with car fuelling station
23.3 Case 2: Zwolle, PWS system with gas grid injection
23.4 Case 3: Wijster, PWS system with gas grid injection
23.5 Case 4: Poundbury, MS system with gas grid injection
23.6 Configuration overview and evaluation
23.7 Capital and operational expenses
This edition first published 2013
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Library of Congress Cataloging-in-Publication Data
Air pollution prevention and control : bioreactors and bioenergy / edited by Christian Kennes and M. C. Veiga.
Includes bibliographical references and index.
ISBN 978-1-119-94331-0 (cloth)
1. Air– Pollution. 2. Air– Purification. 3. Bioreactors. 4. Biomass energy. I. Kennes, C. II. Veiga, M. C.
A catalogue record for this book is available from the British Library.
Planet Earth is made up of three major natural compartments: air, water and soil. Pollution of those compartments will negatively affect human beings, as well as other living organisms and ecosystems. Therefore, air pollution has become an ever-increasing concern over recent decades. The metabolic activity and healthy development of most mammals relies on the availability of clean air. Oxygen—one of the major components of air—is necessary in the breathing process. The presence of pollutants in the atmosphere, such as carbon monoxide, may inhibit the role of oxygen in metabolic processes, while other pollutants, either organic or inorganic, may exhibit toxic and carcinogenic properties in humans. Plants, microorganisms, as well as buildings are all susceptible to the presence and undesirable effects of volatile pollutants. Two major ways to reduce and control such pollution problems are, on one side, the development of treatment technologies allowing the removal of pollutants from the atmosphere or even from anaerobic gases, and, on the other side, the use of cleaner (bio)fuels. This book focuses on biotechnological alternatives to deal with air pollution problems based on the optimization of bioreactors for pollution control (as end-of-pipe treatment technologies) and on the development of biofuels with reduced environmental impact (as a more preventive alternative). This is the first reference work offering a comprehensive overview of those different aspects.
In Part I, fundamental and microbiological aspects are addressed. Chapter 1 describes the major different types of volatile pollutants, their characteristics and environmental impact, as well as the major emitting sources. Biological and nonbiological treatment technologies are briefly discussed as well as some aspects related to bioenergy. Chapter 2 discusses biodegradation and biotransformation processes. It first deals with the main biodegradation processes of pollutants described in Chapter 1, then focuses on the possibilities of bioconversion—rather than biodegradation—of some of those volatile pollutants to useful products, which is a quite new approach in the field of air pollution and could improve cost-effectiveness. Chapter 3 addresses basic microbiological aspects useful in the field of air pollution prevention and control. Molecular techniques and methods for the quantification of microbial populations and microbial diversity are explained.
Part II gives an overview of all major bioreactors currently available for air pollution control. It does largely focus on bioreactors that are already being used in field applications and have proven their efficiency, but it also describes some bioreactors that are still in the development stage and that have, so far, been studied at only the laboratory- or pilot- scale. Biofilters, which represent one of the most extensively used bioreactors for air pollution control, are described in details in Chapter 4, focusing largely on recent information and data that have not previously been reviewed. Chapter 5 explains the fundamentals of biotrickling filters and offers an overview of recent aspects studied and reported in the literature over the past decade, as well as information on full-scale reactors based on the authors' practical experience. Bioscrubbers, detailed in Chapter 6, are hybrid processes combining a scrubber, as a first stage, with a bioreactor, in a second stage. Important aspects of both the biological and nonbiological steps are described. Membrane bioreactors (Chapter 7) and two-phase partitioning bioreactors (Chapter 8) have not yet been implemented in full-scale applications for waste-gas treatment, contrary to the bioreactors explained in Chapters 4–6. The basic principles of operation of those systems are detailed as well as aspects related to different membranes and liquid- and solid-partitioning phases that can be used in those applications. A very limited number of pilot- and full-scale plants have been built in the case of rotating biological contactors (RBCs) (Chapter 9). So far, the latter are not very popular in the field of air pollution control. An overview of recent research on RBCs is presented. Chapter 10 deals, on one side, with two-stage and hybrid systems and, on the other side, with innovative bioreactors. Innovative systems have mainly been studied at the research and/or early development stage. However, hybrid and two-stage bioreactors have already been used in the field. They allow tackling challenging aspects that cannot easily be solved with more conventional systems. They may present some advantages for specific applications where the performance reached in single reactors would not be sufficient enough. Multistage processes may combine several biological reactors or may, otherwise, combine both biological and nonbiological techniques, similarly as in the case of bioscrubbers described in Chapter 6.
Part III focuses on two specific applications in Chapters 11 and 12, namely, the removal of sulphur compounds and nitrogen compounds from gases, mainly SOx and other organic or inorganic sulphur compounds (Chapter 11) as well as NOx (Chapter 12).
Biofuels production and their environmental impact represent the topics of Part IV. Several different (bio)fuels are currently being considered as interesting alternatives to the conventional energy sources produced in fossil fuel industries. Not all different known biofuels could be included in this book; but the best known and most common ones are described, and whenever possible we offer information on their environmental impact, mainly in terms of air pollution, as well as data on economic aspects. In many cases, biofuels can be obtained from waste, pollutants or renewable resources. Biogas production is addressed in Chapter 14. Different feedstock and bioreactor configurations suitable for biogas production are described, as well as that biofuel's environmental impact. Chapter 15 deals with biohydrogen production and the role of bacteria and algae in different biotransformation reactions for hydrogen production, addressing mainly photo-fermentation and dark fermentation technologies. Chapters 16 and 17 both focus on biodiesel, obtained either through catalytic processes (Chapter 16) or through bioprocesses by means of microalgae (Chapter 17). The last chapter of Part IV describes cellulosic bioethanol production through pre-treatment and subsequent fermentation of lignocellulosic material, and on the anaerobic bioconversion of waste gases and synthesis gas into ethanol. Information is also given on demonstration projects and recent full-scale applications, as well as on economic and environmental aspects.
Part V concludes the book with applied information and the description of some case studies. The number of case studies presented here had to be limited for this one-volume book to remain a reasonable length. Pilot-scale and full-scale bioreactors are described in details.
Finally, Christian Kennes would like to take advantage in this introduction to thank the Wiley team for inviting him to prepare this book and for their very efficient assistance and support in this joint endeavour. Acknowledgments are also due to the different agencies and industries collaborating in our research on air pollution control and bioenergy; and more specifically to the Spanish Ministry of Science and Innovation (Project CTM2010-15796 to CK) and European FEDER funds for providing financial support. Publication of this reference work would not have been possible without the efficient contribution and thorough collaboration of many colleagues and friends who agreed to write excellent chapters.
Christian Kennes María C. VeigaLa Coruña, Spain, 2012
Christian Kennes and María C. Veiga
Department of Chemical Engineering, University of La Coruña, Spain
This book describes the different biodegradation processes and bioreactors available for air pollution control as well as other alternatives for reducing air pollution, mainly by using more environmentally friendly fuels and biofuels, such as ethanol, hydrogen, methane or biodiesel. Only the bioreactors and (bio)fuels most widely used or studied over the past decade are reviewed in this book. Bioreactors, for which not much significant research or many new developments have occurred over the past decade, have been described in other book chapters  and are not included in this book.
Two major groups of pollutants can be considered in terms of air pollution: particulate matter and gaseous pollutants. The latter may be subdivided into volatile organic compounds (VOCs) and volatile inorganic compounds (VICs). The best available treatment technology will depend on the composition and other characteristics of the emissions to be treated. The most significant contaminants and their origin are shown in Figure 1.1, in terms of emission percentages, in 2006 by source category for the 27 member states of the European Union. The member states are (year of entry in brackets) Austria (1995), Belgium (1952), Bulgaria (2007), Cyprus (2004), Czech Republic (2004), Denmark (1973), Estonia (2004), Finland (1995), France (1952), Germany (1952), Greece (1981), Hungary (2004), Ireland (1973), Italy (1952), Latvia (2004), Lithuania (2004), Luxembourg (1952), Malta (2004), The Netherlands (1952), Poland (2004), Portugal (1986), Romania (2007), Slovakia (2004), Slovenia (2004), Spain (1986), Sweden (1995) and the United Kingdom (1973).
Figure 1.1Distribution of EU-27 total emission estimates for different pollutants, by source category, in 2006.
Table 1.1 and Table 1.2 compare the annual emission estimates for both the European Union (EU-27) and the United States, considering anthropogenic land-based sources only . Natural sources of emission and other possible sources such as navigation have not been included, as comparable information for Europe (EU-27) and the United States often could not be obtained. Although some recent data were sometimes not available for the United States and needed to be extrapolated , it is still possible and accurate to conclude that the results follow in both cases a similar trend for the different pollutants, in terms of both the relative total emission of each pollutant and the source of pollution. However, some differences may still be found when analysing the tables in detail, mainly in the case of carbon monoxide (CO) emission. For example, in Europe, almost 43% of CO emissions come from mobile sources (vehicles and transportation in general), while this represents as much as 85% in the United States. Conversely, CO from combustion sources represents about 44% in Europe, while it is only 7% in the United States.
Table 1.12006 emission estimates for different pollutants, by source category, in the European Union (EU-27) (106 kg yr−1). Reprinted under the terms of the STM agreement from  Copyright (2012) Elsevier Ltd.
Table 1.22006 emission estimates for different pollutants, by source category, in United States ( kg yr−1). Reprinted under the terms of the STM agreement from  Copyright (2012) Elsevier Ltd.
Particulate matter can be defined as a small solid or liquid mass in suspension in the atmosphere. Primary particles are directly emitted from a polluting source, while secondary particles are formed in the atmosphere as a result of reactions or interactions between pollutants and/or compounds present in the atmosphere, usually volatile organic compounds, nitrogen oxides or sulphur oxides as well as water. A water droplet of acid rain, carrying sulphuric acid () or nitric acid () produced from nitrogen oxides (NOx) or sulphur oxides (SOx), would be classified as particulate matter. Different terms can be used for particles (e.g. dust, smoke, mist or aerosol) depending on their nature and characteristics.
Although many particles are not spherical, for the sake of simplicity and for engineering calculations, nonspherical particles are often assimilated to spheres of the same volume as the original particle. Particle size, then, refers to the corresponding particle diameter.
Typically, the size (diameter) of particulate matter found in polluted air or waste gases may vary between about and a few hundreds of micrometres (), although smaller and larger particles may also be found. Larger particles do, however, settle quite fast, and in that way are quickly eliminated from the atmosphere. In order to give an idea of the scale, is a common size for viruses, while coal particles, flour or cement dust may be around 10+2 µm. The sizes of the latter may, however, vary considerably, between only a few micrometres and about 1 mm. The same is true for water droplets, for example mist or raindrops, with sizes ranging between a few micrometres up to more than 1 mm. Particles of are considered large particles. Particulate matter is classified as for sizes up to , and for smaller sizes up to .
The effect of particles on health is more important in the case of smaller particles, for instance those below , as they will more easily reach the lungs than larger particles. Some particles may carry heavy metals and carcinogenic molecules. They can also cause disorders of the respiratory system, asthma, bronchitis and even heart problems. Besides, particles can reduce visibility and be involved in acid precipitations, or acid rain, described later in this chapter.
According to data of the European Environment Agency and the US Environmental Protection Agency (EPA), the highest emission of gaseous pollutants to the atmosphere corresponds to emissions of CO, in both the European Union and the United States (Table 1.1 and Table 1.2). Close to 50%, or somewhat more, of the total anthropogenic emission of pollutants corresponds to CO. Large amounts may be produced by natural sources as well. On average, mobile sources account for about 85% of the total CO emissions in the United States. It reaches 42.6% in Europe; another 43.7% come from combustion processes in stationary sources (Figure 1.1). Considering that a large part of mobile sources are vehicles such as cars and trucks, it becomes obvious that CO pollution will be more significant in urban areas. As mentioned, the second largest source of CO emission, after motor vehicle exhaust, corresponds to stationary combustion processes and other industrial production processes. Its main origin is the incomplete combustion of fossil fuels or other materials such as wood. Combustion is the result of a reaction between oxygen and a fuel. Carbon dioxide, water and heat will be produced if the reaction is complete and if the fuel contains only carbon, hydrogen and, eventually, oxygen atoms, such as in the example of methane (a major chemical present in natural gas):
Carbon monoxide, instead of carbon dioxide, will be formed when the combustion is not complete, as shown in this reaction:
Several reasons may be involved in this incomplete reaction. The most important ones are the amount of available oxygen, temperature, reaction time and turbulence. The theoretical amount of oxygen needed for complete combustion can be calculated from the stoichiometric equation. However, some excess air is generally recommended for ensuring complete oxidation, but not too much, since excess air needs to be heated as well. Increasing the temperature and residence time in the combustor will be favourable to complete combustion, as well as increasing turbulence in order to achieve intimate mixing between the oxygen and fuel.
Carbon monoxide is not of significant concern in terms of its impact on the environment, but it is flammable and, above all, highly toxic when inhaled. It is an odourless and colourless gas. Therefore, its presence is difficult to detect in closed environments. Prolonged exposure to concentrations above 50–100 ppmv will cause fatigue, nausea and headache, while several hours of exposure to concentrations exceeding 400–500 ppmv will gradually lead to dizziness and death. Carbon monoxide combines with haemoglobin (Hb) in the blood and in that way prevents haemoglobin from transporting oxygen from the respiratory organs to the tissues. The affinity between CO and Hb is much stronger than that between Hb and oxygen.
Similarly to CO, the major source of carbon dioxide () is combustion. It is only recently that has been considered a compound of environmental concern. Pollutants such as , methane and nitrogen oxides are all greenhouse gases, supposed to play a key role in global temperature changes. The average residence time of at concentrations typically found in the atmosphere is about 15 years, while it is about 10 weeks for the much more reactive CO molecule. The concept of residence time of a species in the atmosphere is similar to the residence time of a molecule in a continuous reactor. It is the average time that species spends in the atmosphere before disappearing, for example through chemical or photochemical reactions. Such difference in residence time between CO and justifies the negligible increase of the CO concentration in the air over the past century compared to . Actually, one major product formed from atmospheric CO is . The normal concentration of in nonpolluted air currently is around 380 ppmv, whereas it hardly reached 300 ppmv a century ago.
Sulphur oxides (SOx) include both sulphur dioxide () and sulphur trioxide (). Sulphur dioxide appears in larger amounts than in combustion gases and is largely released during the combustion of fossil fuels, mainly coal, in stationary sources, according to the following equations:
As shown in Figure 1.1, stationary combustion processes represent by far the major source of , with around either 80% or 90% of the total SOx emissions, respectively, in the United States and EU-27. The average residence time of in the atmosphere is about 5–6 weeks. That pollutant is largely generated at electric power plants. Its concentration in combustion gases will depend on the amount of sulphur present in the original fuel, which usually does not exceed 3–4% by weight but may occasionally reach 10%. Oil does, in most cases, contain higher amounts of sulphur than coal, while sulphur content is basically negligible in natural gas. Part of that sulphur may be removed from the fuel to reduce emissions during combustion.
In terms of environmental impact, can react with moisture in the air to form , leading to acid precipitation commonly known as acid rain. In terms of health effects, can cause respiratory disorder and lung diseases.
A small amount of the formed during combustion may be further oxidized to SO3, usually not more than 5%. Its concentration will increase at higher temperature and in the presence of excess oxygen. has a much higher corrosion potential than . It is important to prevent its condensation in the plants. Sulphur trioxide may react with water vapour to produce . Besides, has been reported to be 10 times more toxic than , mainly for the respiratory system .
Among the different oxides of nitrogen (nitric oxide (NO), nitrogen dioxide (), nitrate (), nitrous oxide (), dinitrogen trioxide (), dinitrogen tetroxide () and dinitrogen pentoxide ()), the symbol NOx refers to the sum of NO and which are considered to be the major relevant contaminants of that group in the atmosphere. NO and have average residence times in the air close to one day. Other oxides of nitrogen generally appear only at very low concentrations in NOx-polluted environments. On reaction with atmospheric moisture, NOx form small particles. The environmental impact of has also been discussed, although, for historical reasons, that compound is not included in the group of NOx as such. is a major greenhouse gas, similarly to methane and , but with a global warming potential almost 300 times higher than that of . The global warming potential of a pollutant is an estimation of its ability to trap heat or infrared radiation reflected by the Earth's surface. Agriculture is a major source of anthropogenic emissions to the atmosphere, through nitrification of ammonium-containing fertilizers and animal waste or denitrification of in soils. Contrary to NOx, is not directly a product of fuel combustion.
NOx react with hydrocarbons and oxygen in the presence of ultraviolet (UV) radiation to produce photochemical smog, mainly in urban areas. They can cause eye and skin irritation and have adverse effects on the respiratory system and on plants. In the atmosphere, NO generated during combustion will eventually be converted to . Besides, reacts with the hydroxyl radical from water to form . It is then eliminated from the atmosphere by either dry deposition or wet deposition, resulting in the latter case in acid rain, as summarized in the following reactions:
As shown in the reactions in Equations (1.6), (1.7) and (1.8), leads to the formation of unwanted ground-level ozone, in the presence of UV light and volatile organic compounds, in the lower atmosphere. The reactions are temperature dependent, and more ozone is detected in the air at higher temperature (i.e. during the day and in the summer period).
Part of the ozone formed from is removed through a reaction of with NO. This suggests that other mechanisms or compounds must be involved in ozone accumulation in the atmosphere. It will be shown, in Section 1.2.5, that VOCs also play a key role in the overall process.
As much as about 90% of NOx are emitted into the atmosphere during combustion processes, from either mobile sources or stationary sources. Nitrogen oxides are formed from both nitrogen naturally present in combustion air (in which case it is called thermal NOx) and nitrogen compounds found in the fuels. The reaction between nitrogen and oxygen is significant only at high temperatures. The influence of temperature on the rate of NOx formation is, however, highly variable and depends on the source of nitrogen. Its formation may also sometimes depend on the involvement of hydrocarbons in the reaction. Depending on its origin, fuel oil generally does not contain more than 0.5% nitrogen by weight. Conversely, coal may contain up to 2–3% nitrogen by weight, where it is mainly combined with carbon in the form of polycyclic aromatic rings. Those CN bonds are more stable than CC bonds and need high temperatures to be converted to NOx. It was mentioned in this chapter that natural gas contains hardly any sulphur. The same is true for nitrogen. Natural gas is thus a quite clean fuel in terms of NOx and SOx emissions. Besides, natural gas emits virtually no particulate matter compared to coal and oil. The products of its combustion are mainly water vapour and , if complete oxidation takes place. It is estimated that the world would run out of coal in about 200–300 years based on current consumption estimations, while the sources of oil and natural gas would presumably get exhausted before the end of this century. It is worth mentioning that the use of hydrogen as a source of energy would theoretically not produce any NOx, according to the following reaction:
However, this is generally not totally true in practice. In the presence of air, hydrogen may even produce more NOx than during natural gas combustion, as a result of the reaction involving nitrogen and oxygen naturally present in air.
Ammonia () is another important nitrogen-bearing pollutant (see also Chapter 2). In most developed countries, more than 80% of emission originates from agricultural activities, mainly livestock and poultry operations on the one hand and the use of fertilizers on the other. Farm animals used to produce milk, meat or eggs are often fed food with high protein content. A nonnegligible fraction of it is not metabolized and is excreted in the urine and faeces, which represent the main components of manure. Ammonia is released during the microbial decomposition of manure. The other major source of ammonia emission is through the use of mineral fertilizers. Only about 50% of the nitrogen (mainly urea) present in high-nitrogen fertilizers is taken up by crops and forage species. A large part of the nitrogen is lost mainly through gaseous emissions, but also through leaching, erosion or runoff.
There are different definitions of VOCs, but most of them generally agree on some specific aspects. VOCs are organic compounds or, in other words, carbon-containing molecules that also contain other species, such as H or O. It is worth recalling that hydrocarbons and VOCs are not the same. Some hydrocarbons are indeed VOCs, composed of only C and H atoms, but not all VOCs are hydrocarbons. Some organic molecules have traditionally been excluded from this definition used in organic chemistry and are not included in the list of VOCs, such as carbon monoxide, carbon dioxide, carbonates, carbides and cyanides, which are classified as inorganic compounds. VOCs are volatile. The vapour pressure at room temperature or, otherwise but less frequently, the boiling point at atmospheric pressure is often considered to decide whether a compound is volatile or not. According to the European EC-Directive 1999/13/EC, VOCs have a vapour pressure of kPa or more at 293.15 K (20 °C). VOCs are molecules with a boiling temperature usually below about 240–250 °C at 101 325 Pa (normal atmospheric pressure). However, the border between volatile and nonvolatile organic compounds is somewhat arbitrary. Additionally, some definitions add that VOCs should participate in photochemical reactions in the atmosphere. However, not all VOCs, as defined here based on their vapour pressure or boiling point, exhibit significant photochemical reactivity. As shown in Figure 1.1, the major sources of VOCs are industries, which represent about 50% of the overall emission estimates. VOCs may exert highly different and variable effects on human health. Depending on their nature, they may irritate the eyes, nose and throat, or act as central nervous system depressants. Some VOCs are carcinogenic. They are also frequently found in indoor air, as they are present in most paints, varnishes, waxes, glues, cleansers and cosmetics, among others. In terms of environmental impact, VOCs play a key role in the formation of ozone in the lower atmosphere.
It was shown in Section 1.2.4 (Equation (1.8)) that part of the ozone formed in the troposphere is later destroyed through a reaction with nitric oxide.
However, NO will also react with VOC in the air. This will reduce the amount of available NO that could potentially react with ozone. The latter will then accumulate in the atmosphere. Although the overall process is quite complex and involves several reactions, the effect of VOC (here: ROO·) on NO removal can be summarized in the following reaction
It is interesting to observe both the elimination of NO in that reaction and the formation of , which is one of the precursors of ozone formation.
Among the many different VOCs, methane () is known for its impact on global warming, with a global-warming potential 25 times higher than that of . This means that its impact on temperature over a 100-year period will be 25 times that of the same amount of . Some of the main sources of methane are agriculture, ruminants, wetlands and landfills. The typical concentration of methane in clean air is currently around 1800 ppb. Methane is sometimes excluded from the list of VOCs. Those pollutants are then divided into methane VOCs as a specific pollutant and nonmethane volatile organic compounds (NMVOCs). Other examples of VOCs are listed in Table 1.3. This is not an exhaustive list, since a very wide range of different VOCs is known.
Table 1.3Common groups and examples of VOC pollutants.
Group of VOCs
Pollutants containing only C, H and/or O
Methane, butane, hexane
Acetone, methylethyl ketone (MEK) Methyl iso butyl ketone (MIBK)
Formaldehyde, acetaldehyde, benzaldehyde
Methanol, ethanol, propanol
Acetic acid, butyric acid, formic acid
Aromatic compounds (usually monocyclic)
Benzene, toluene, xylenes, ethylbenzene, styrene, α-pinene
Pollutants including species other than C, H and/or O
Tetrachloroethylene (PCE), trichloroethylene (TCE), vinyl chloride
Halogenated aromatic compounds (monocyclic)
Carbon disulphide, (di)methyl (di)sulphide, ethanethiol
In the case of air pollution, the word odour refers to the ability of a pollutant or mixture of pollutants to activate the sense of smell. Odour nuisance is generally the result of a nasty smell. A pleasant odour may become nasty at high concentrations or after long-term exposure. Both organic and inorganic compounds may lead to odour nuisance. Two common examples are hydrogen sulphide (, a VIC) and butyric acid (, a VOC). Hydrogen sulphide smells like rotten eggs. Its odour threshold value is . Butyric acid has a sweet rancid odour and an odour threshold of about .
The intensity of odour nuisance is expressed in terms of odour units per cubic metre . The European odour unit is the amount of pollutant that, when diluted in of an inert gas (generally pure air) under standard conditions, leads to the same physiological response from a panel (detection threshold) as a European reference odour mass (EROM) in gas. One EROM is equivalent to -butanol neutral gas.
Ozone formation in the lower atmosphere, called the troposphere (i.e. between ground level and about 10–12 km), is known as a secondary contaminant. Secondary contaminants are pollutants formed from a reaction between other compounds in the atmosphere; this is contrary to primary contaminants, as described in the previous sections of this chapter, which are directly released from emission sources. As mentioned, ground-level ozone has adverse effects on human health and the environment. It may cause asthma and other respiratory problems. It plays a key role in photochemical smog, and it damages plants and agricultural crops. Conversely, ozone in the upper atmosphere (stratosphere) is desirable as it helps to filter UV radiation. Exposure to excess UV light is considered to cause skin cancer and other related problems.
The concentration of a given VOC or VIC in polluted air or waste gases is often expressed in . Those units can be converted to ppb or ppmv (sometimes simply called ppm). The abbreviation ppb means parts per billion. In the case of volatile pollutants present in the gas phase, 1 ppb corresponds to one volume of pollutant diluted in volumes of gas, that is, pollutant air (or waste gas). Similarly, 1 ppmv will be 1 volume of pollutant diluted in volumes of gas.
Concentrations expressed in can easily be converted to ppmv and vice versa. Under normal conditions of pressure and temperature (101 325 Pa, 273 K):
where MW stands for the molecular weight of the pollutant and 22.4 is the molar volume of the pollutant at such pressure and temperature. At 298 K (25 °C), the molar volume would be 24.5 instead of 22.4 .
In other words, in order to convert the concentration of, for example, 1 mg methane to ppmv, at and 1 atm, one should do the following calculation:
Thus, and are the same.
Bioreactors are not suitable for the removal of particulate matter from waste gases. The only biological alternative suitable to efficiently reduce the amount of particulate matter emitted into the atmosphere would be the use of more environmentally friendly fuels or biofuels generating less particles upon combustion. Whenever the effluent contains both volatile pollutants and particles, the latter will need to be eliminated first if one plans to remove the volatile pollutants in a bioreactor, above all in the case of using packed-bed bioreactors. Otherwise, particles would end up clogging the packed bed.
The five most common groups of processes used for the removal of particles from waste gases are gravity settling, cyclone collection, filtration, electrostatic precipitation and absorption . Some information on their principles and basic characteristics are summarized in Table 1.4. More detailed information can be found in specialized literature.
Table 1.4Characteristics of technologies suitable for the removal of particulate matter.
Methods used for the removal of volatile pollutants are based on either mass transfer or chemical or biological reactions. In the case of mass transfer, the pollutant is transferred from one phase (gas) to another phase. This is the case in absorption and adsorption processes. It means that such technologies may sometimes allow for the recovery of the pollutant, if needed. This is important and useful if the pollutant, for example an organic solvent, is expensive and/or if it needs to be reused. Conversely, if it does not need to be recovered, its transfer from the gas phase to another phase (e.g. liquid or solid) will generally lead to the contamination of that new phase. It will be necessary to treat or dispose of that new, polluted phase. Methods based on some kind of reaction transform the pollutant into other products, which are usually innocuous if the process has been optimized. This is the case for bioreactors and nonbiological oxidation processes. Bioreactors are used for the biodegradation or bioconversion of pollutants, while incineration relies on nonbiological reactions of oxidation.
In waste-gas treatment, absorption is a gas–liquid mass transfer process and may be used for the removal of both particulate matter and volatile pollutants. The most common absorption columns are spray chambers, sieve trays or plate columns and packed-bed columns; the latter are the most popular configuration for waste-gas treatment. The fluids (gas and liquid) may flow through the column co-currently, counter-currently or in a cross-flow mode. In the first case, both fluids enter through the top of the tower. In counter-current operation, which is the most common design, the liquid is introduced at the top and the gas at the bottom of the column. In cross-flow, the gas follows a horizontal path through the absorption column, while the liquid flows vertically through the packed bed, as a result of the effect of gravity. Absorption may be due to mass transfer only or it may, simultaneously, involve a chemical reaction. The main interest of a chemical reaction is to increase the solubility of the compound to be absorbed in the liquid phase, thus improving the efficiency of the process. Several factors will affect the rate and efficiency of pollutants removal. The mass transfer rate is directly related to the difference between the concentration of the pollutant in the gas phase and in the liquid phase, also called the driving force, and expressed as follows:
where is the transfer rate of pollutant P (kmoles ), is the overall gas mass transfer coefficient and () is the driving force with yP being the gas mole fraction of P in the bulk gas phase and being the gas mole fraction of that would be in equilibrium with (i.e. the mole fraction of in the bulk liquid phase) (Figure 1.2).
Figure 1.2Transfer of a pollutant from a gas phase to a liquid phase (two-liquid-film theory).
A similar equation can be written in terms of the overall liquid mass transfer coefficient, , yielding the same value:
where is the driving force with being the liquid mole fraction of in the bulk liquid phase and being the liquid mole fraction of that would be in equilibrium with (i.e. the mole fraction of in the bulk gas phase).
In air pollution, the concentration of pollutant in the gas phase is generally low. Under such conditions, the equilibrium relationship between the pollutant concentration in the gas phase and in the liquid phase is generally linear.
Some key parameters to be chosen that allow optimization of the process are the packing and the nature and hydrodynamic characteristics of the liquid. Whenever possible, water will be used as the liquid phase, as it is cheap and readily available in large quantities. It presents several other advantages: it is inert, nontoxic and noncorrosive and has a low volatility. The liquid flow rate should be optimized and depends, among others, on the packing and on the gas flow rate. The process should be designed in such a way that it avoids reaching flooding conditions. The latter corresponds to a situation where the gas velocity will hold up the liquid in such a way that it will no longer be able to flow through the column. Too high liquid flow rates will lead to high pressure drops and may also result in flooding conditions, as it may be difficult for such high volumes of liquid to flow through the column. A homogeneous gas and liquid distribution of both fluids through the column is of prime importance to work under optimal conditions. For high towers, it may be necessary to collect and redistribute the liquid at a given packing height, depending on the packing characteristics.
The packing is another key parameter in absorption towers. Packings can be divided into two groups: on the one hand structured packings and on the other packings introduced randomly in the column. The most common materials are plastic, ceramic and stainless steel. Some important characteristics of packings are the specific surface area and the void space. A higher surface area will improve mass transfer. A higher void space will minimize pressure drop, as the fluids will have more space to flow freely through the column.
Adsorption is a fluid–solid mass transfer technology, in which the pollutant to be adsorbed is called the adsorbate and the adsorbing solid is called the adsorbent. The fluid is a gas in the case of gas treatment. The adsorbate adheres to the solid surface either through physical adsorption or through chemisorption. In physical adsorption, weak physical bonding forces are created, mainly van der Waals forces, while stronger chemical bonds are formed in chemisorption. The most common adsorbent, in air or gas pollution control, is activated carbon (AC). It is used in the form of pellets or granules of sizes generally ranging from less than 1 mm to a few millimetres. AC is quite efficient for the removal of many VOCs, though not all pollutants will be retained with the same efficiency. Inorganic compounds such as H2S are also well adsorbed onto AC. The latter is produced through a heat treatment of carbonaceous materials (e.g. coal, wood, shells and peat) in the absence of oxygen followed by steam treatment, resulting in a solid with a very high surface area, often close to about . The adsorbent is usually introduced in a fixed-bed column. The contaminated gas phase flows through the bed and the pollutants are retained inside the bed, while clean gas is released to the atmosphere.
Spent AC can be regenerated and reused, once it is exhausted and its surface is saturated with pollutants or other adsorbates. This avoids costly disposal. Carbon regeneration is generally done through thermal reactivation with steam, at temperatures in the range of 100–140 °C. Desorbed gases can be recovered and reused.
The equilibrium relationship is expressed mathematically by means of adsorption isotherms, representing the amount of adsorbate adsorbed on the solid material versus its concentration—or equilibrium partial pressure (atm)—in the gas phase. Different mathematical equations can accurately represent the equilibrium data, at a given temperature, depending on parameters such as the type of pollutant or the adsorbent. The equilibrium relationship is generally not linear. The most common equilibrium equations are the Langmuir isotherm, Freundlich isotherm and Brunauer–Emmett–Teller (BET) equation.
In oxidizers, volatile pollutants react with oxygen at high temperature—as a result of the combustion of a fuel—to form, mainly, and water, if complete degradation takes place. It is worth mentioning that fuel combustion contributes to the release of high amounts of , besides the amount of released from the oxidation of the volatile pollutants themselves. Almost any type of natural gas can be considered as suitable fuel. Other products, such as acids, may also be formed when halogenated pollutants are present in the waste gas. Hydrogen chloride in oxidizers treating chlorinated compounds needs to be removed and may be eliminated, for example, in a scrubber. Free chlorine is sometimes also released during such oxidation process. will be produced when sulphur-containing VOCs are oxidized. may also appear in the combustion gases, if the pollutants contain or as a result of the oxidation of nitrogen present in the air used for combustion, above all at high temperatures.
Besides the amount of available oxygen, three other key factors allow optimization of the oxidation process, namely, temperature, time and turbulence.
The minimum amount of oxygen (air) needed for complete oxidation to take place can be calculated from the stoichiometric equation of the oxidation reaction. A lack of oxygen will produce some CO instead of as a combustion product. Therefore, some oxygen in excess is recommended. However, too much excess oxygen should be avoided, as excess air will take away part of the heat needed for the oxidation of the waste gas.
Either thermal oxidizers or catalytic oxidizers may be used. In thermal oxidation, operating temperatures are commonly in the range of 700–1000 °C. Higher temperatures, exceeding 1000 °C, may be needed for specific applications or if high destruction efficiencies (usually ) must be reached. In catalytic oxidation, the flue gas is preheated and then exposed to a catalyst. The presence of a catalyst increases the reaction rate and allows catalytic oxidizers to be operated at lower temperatures, typically around 300–500 °C. Thus, they require substantially less fuel, reducing in that respect the operation costs. However, the need for costly catalysts will increase the investment costs. Nevertheless, lower temperatures can be applied, resulting in reduced construction costs, which contribute to the investment costs as well. The catalyst contains either noble metals, such as platinum or palladium, or metal oxides. It often undergoes gradual deterioration and needs to be regenerated or replaced after 2–5 years.
Residence times typically applied in thermal and catalytic oxidizers are in the range of 0.1–2.0 seconds. Similarly as for temperature, a higher residence time should improve the destruction efficiency. Thus, either temperature or residence time—or both—may be increased in order to improve the removal of pollutants. A higher temperature will increase the operation costs, while the investment costs will be more if a longer residence time (i.e. larger oxidizer volumes) needs to be applied.
Good mixing will improve the efficiency of the oxidation process. A high turbulence will optimize the reaction between the fuel and oxygen from air (i.e. the fuel combustion). It will also ensure good mixing between the fuel combustion gas and the waste gas that will better reach its combustion temperature. Introducing baffles in the oxidizer helps create a higher turbulence, although it may also result in the presence of unwanted dead zones.
Most of the bioprocesses and bioreactors suitable for air pollution prevention and control are described further in this book, focusing on both biodegradation and bioconversion technologies. One of the oldest bioprocesses, developed in the mid-20th century, for waste-gas treatment is the conventional open biofilter, which is used mainly to solve odour problems at wastewater treatment plants and composting facilities (Chapter 4) . Sulphur compounds, such as , are typical odorous pollutants in such waste gases, and their biological removal has been extensively studied and optimized in full-scale processes. Gas-loading rates treated originally in such conventional biofilters were quite low. In the 1980s, significant research and improvements were made. Later, closed biofilters were developed as well as biotrickling filters (Chapter 5). Laboratory-scale studies were then performed that studied the potential removal of VOCs typically found in industrial waste gases from process industries. However, applied research and the number of research groups focusing on studies with biotrickling filters became internationally significant only in the 1990s . The biotrickling filtration technology was gradually and efficiently implemented at industrial sites, mainly over the past two decades. The first full-scale bioscrubbers were probably installed in the 1970s in German industries (Chapter 6). Activated sludge diffusion is another technology that is suitable for air pollution control and has been used for several decades, mainly at wastewater treatment plants. It has been extensively described elsewhere . That topic is not addressed in this book because little research or new developments have been made over the past 10 years. Membrane bioreactors, which are already used in other areas such as wastewater treatment, are described in Chapter 7. They have, however, not yet been implemented at full scale for waste-gas treatment. Other innovative approaches have been suggested recently such as the two-liquid-phase bioreactors, rotating biological contactors or multistage processes, among others (Chapter 8, Chapter 9 and Chapter 10). Interestingly, for some very important pollutants such as CO or NOx, research with bioreactors has been done for several years already at the lab scale, but the technology has hardly been scaled up so far [6–8]. There is certainly room for further studies in that direction.
Fossil fuels used in combustion processes in either stationary or mobile sources of pollution are known to emit high amounts of particulate matter and volatile organic and inorganic pollutants to the atmosphere (Figure 1.1). This has led to the search for alternative, more environmentally friendly fuels and sources of energy to drive vehicles. The European Emission standards for new gasoline and diesel passenger cars are summarized in Table 1.5, for different directives approved since 1970, showing that overall the emissions have significantly decreased over the past four decades. All these exhaust gases are released through the tailpipes and significantly affect our environment. One alternative to running fossil fuel–powered cars is electric vehicles. The latter do not have any tailpipe. This does, however, not mean that they are not polluting at all, since the production of electric power required to charge the cars' batteries depends on air-polluting processes. Some other drawbacks of electric vehicles should be considered as well, such as their limited driving range, as batteries need to be recharged about every 300–400 km in the best case, based on the presently available technologies.
Table 1.5European emission standards, in g km−1, for gasoline and diesel passenger cars (vehicles of , laden).
Other options include using more environmentally friendly (bio)fuels, such as ethanol, methanol, biodiesel, hydrogen and biogas (methane), among others. Several chapters in this book focus on such fuels (Chapter 13, Chapter 14, Chapter 15, Chapter 16, Chapter 17 and Chapter 18). They will thus not be described in detail here. As a general rule, emissions from such fuels are often lower than in the case of fossil fuels, although this is not always the case. One major advantage is that they can often be produced from renewable resources or from waste, wastewater or waste gases.
The most common air pollutants are particulate matter, carbon monoxide, ammonia, nitrogen oxides, sulphur oxides, and volatile organic compounds. They are emitted from either stationary sources or mobile sources. Carbon monoxide is, by far, the most abundant air pollutant, and it is mainly released during combustion processes. Bioprocesses are not suitable for dealing with particulate matter pollution, although several other efficient, nonbiological alternatives are available. All other pollutants can be eliminated biologically or through other physical and chemical techniques. Some of those techniques simply transfer the pollutant from one phase (air or gas) to another (absorption and adsorption, mainly), while others destroy the pollutants (biological and nonbiological degradation or oxidation processes). Actually, bioprocesses can be used both for destroying the pollutant and for their bioconversion into other useful products, which is a quite more recently developed option.
1. C. Kennes, M.C. Veiga. Bioreactors for waste gas treatment. Dordrecht: Kluwer Academic Publishers, (2001).
2. G. Pouliot, T. Pierce, H. Denier van der Gon, M. Schaap, M. Moran, U. Nopmongcol. Comparing emission inventories and model-ready emission datasets between Europe and North America for the AQMEII project. Atmospheric Environment,53:4–14 (2012).
3. R. Kikuchi. Environmental management of sulphur trioxide emission: impact of SO3 on human health. Environmental Management,27:837–44 (2001).
4. C. Kennes, M.C. Veiga. Technologies for the abatement of odours and volatile organic and inorganic compounds. Chemical Engineering Transactions,23:1–6 (2010).
5. C. Kennes, E.R. Rene, M.C. Veiga. Bioprocesses for air pollution control. Journal of Chemical Technology and Biotechnology,84:1419–36 (2009).
6. H.N. Abubackar, M.C. Veiga, C. Kennes. Biological conversion of carbon monoxide rich syngas or waste gases to bioethanol. Biofuel Bioproducts and Biorefining,5:93–114 (2011).
7. R. Jiang, S. Huang, J. Yang, K. Deng, Z. Liu. Field applications of a bio-trickling filter for the removal of nitrogen oxides from flue gas. Biotechnology Letters,31:967–73 (2009).
8. Y. Jin, M.C. Veiga, C. Kennes. Bioprocesses for the removal of nitrogen oxides from polluted air, Journal of Chemical Technology and Biotechnology,80:483–94 (2005).
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