THE OPEN UNIVERSITY OF TANZANIA FACULTY OF SCIENCE, TECHNOLOGY AND ENVIRONMENTAL STUDIES OEV 205 (3 UNIT COURSE) FUNDAMENTALS OF ENVIRONMENTAL CHEMISTRY JOSEPHAT ALEXANDER SARIA PREFACE Environmental chemistry is becoming an increasingly popular subject in tertiary education. All over the world, all Universities, courses in chemistry, environmental science, civil engineering, environmental studies, public health and environmental engineering all have to include environmental chemistry in their syllabuses to a greater or lesser extent. Many textbooks have appeared in recent years aiming to fulfill this requirement; however, most of these deal mainly with theoretical aspects of the subject. This manual aims to supplement the existing environmental textbooks by providing detailed, in understanding the chemistry of the environment. This module is about environmental issues (atmospheric, water, soil etc) and the chemistry behind them. It aims to apply knowledge of chemistry to understand and solve environmental issues. Treatment of general and analytical chemistry was considered to be outside the scope of this manual. It is assumed that the student would be familiar with basic chemical theory. Anyway, many good textbooks dealing with these topics are available and the student is referred to these books in the text where appropriate. It is my assumption you will enjoy reading this book and good-luck in your examinations. Learning Outcomes On completion of this course, students should have knowledge and understand: Inter-relationships between water, soil and atmospheric environments, their physical, chemical and biochemical properties. The chemistry that govern environment degradation. Application of chemistry in solving environmental problems. Types and sources of air, soil and ground and surface water pollution. Mode of Assessment Timed tests 30% Final examination 70% Table of Content 9 List of Figures ………………………………………………………………………………..vi List of Tables ………………………………………………………………………………. vii 1: LECTURE ONE 1 1.1: The Meaning of Environmental Chemistry 1 2: LECTURE TWO 4 2.1: Science of the Surrounding of the Earth 4 2.2: Earth – Air – Water 4 2.1: Science of the Surrounding of the Earth 5 2.2.1:Water and Hydrosphere 5 2.2.2:Earth’s Surface 6 2.2.3:Earth’s Water 7 3: LECTURE THREE 8 3.1: The Chemistry of the Light and Atmosphere 3.1: Light 8 4: LECTURE FOUR 12 4.1: Atmospheric Structure 4.2: Troposphere 13 4.3: Stratosphere 13 4.4: Mesosphere 14 4.5: Thermosphere 14 4.5: Exosphere 14 8 12 5: LECTURE FIVE 17 5.1: Ozone layer 17 5.2: Ozone Formation 18 5.3: Importance of Ozone to our Life 18 6: LECTURE SIX 20 6.1: Destruction of Ozone Layer and Mechanisms of Depletion 6.2: Destruction of Ozone Layer 20 6.3: Mechanism of Ozone Destruction 22 7: LECTURE SEVEN 25 7.1: Ozone Layer Depletion 25 7.2: Consequences of ozone Layer Depletion 20 25 8: LECTURE EIGHT 32 8.1: Climate: Global Warming, Green House Effect and Radiation Balance 8.2: Greenhouse Gases 32 10 32 9: LECTURE NINE 36 9.1: The Consequences of Green House Gases Imbalance 9.2: Global Warming 36 10: LECTURE TEN 40 10.1:How Does the Earth Balance the Radiations 10.2:Radiation Balance 40 36 40 11: LECTURE ELEVEN 44 11.1:Components of the Atmosphere: Clouds and Aerosols44 11.2: Aerosols and Clouds 45 11.2.1: Clouds 45 11.2.2:Aerosols 47 11.3: The importance of Clouds and Aerosols to Climate Change 47 11.4: Healthy Hazards of the Aerosols 49 12: LECTURE TWELVE 51 12.1:Air Pollution and Management 51 12.2: Air Pollutants 51 12.2.1: Air Major Primary Pollutants 12.2.2:Air Major Secondary Pollutants 12.2.3:Air Minor pollutants 56 52 54 13: LECTURE THIRTEEN 57 13.1: Water Pollution and Management 57 13.2:Water in the Atmosphere 57 13.2:Physical Properties of Water in the Atmosphere 13.3: Chemical Properties of Water in the Atmosphere 14: LECTURE FOURTEEN 61 14.1: Acid rain and the Effect to the Environment 61 14.2: Acid Rain and its Source 62 14.2.1: Human Activity 63 14.2.2: Chemical Processes 63 14.3: The Effect of Acid Rain in the Environment 63 14.3.1: Effect on Surface Waters and Aquatic Animals 14.3.2: Effect on Soils 64 14.3.4: Effect on Human Health 65 14.3.5: Other Adverse Effects 65 15: 15.1: 15.2: 15.3: 15.3.1: 15.3.2: 58 59 64 LECTURE FIFTEEN 67 Soil and Ground Water Pollution Management 67 Soil and Groundwater 67 Contamination of Water and Soil 69 Groundwater Pollution 69 Surface and Ground Water Quality Situation: Tanzania Case 11 70 15.3.3: Pollution of Water Resources 71 15.3.4: Organic Water Pollutants: 72 15.3.5: Inorganic Water Pollutants: 72 15.3.6: Macroscopic Pollution 72 15.3.7: Thermal Pollution 73 15.7: Waste Management as Source of water Pollution Tanzania Case 73 15.4: Soil Pollution 75 16: Lecture 16 77 16.1: Health Problems of Water Pollution 77 16.2: Unsafe Water a Source of Waterborne Diseases 77 17: LECTURE SEVENTEEN 82 17.1: Water Treatment 82 17.2:Various Processes Used in Drinking Water Treatment 17.3: Sewage Treatment 83 17.3.1: Pre-treatment 85 17.3.3: Primary Treatment - Sedimentation 86 17.3.4: Secondary Treatment 86 17.3.5: Activated Sludge 87 17.3.6: Surface Aerated Basins 87 17.3.7: Filter Beds (Oxidizing Beds) 87 17.3.8: Biological Aerated Filters 88 17.3.9: Membrane Bioreactors88 17.4: Secondary Treatments 89 17.4:1: Secondary Sedimentation 89 17.4.3: Tertiary Treatment 90 17.4.4: Filtration 90 17.4.7: Nitrogen Removal 91 17.4.8: Phosphorus Removal 91 17.4.8: Disinfection 92 17.4.9: Package Plants and Batch Reactors 93 17.4.10: Electricity Use 94 17.4.11: Sludge Treatment and Disposal 94 17.4.12: Anaerobic Digestion 94 17.4.13: Aerobic Digestion 94 17.4.14: Composting 95 17.4.15: Sludge Disposal 95 18: 19: 82 Additional Practice Questions and Model Answers …………………………………….97 Additional Reading List 103 12 LIST OF FIGURES Fig. 1: Water Cycle ……………………………………………………………………..6 Fig. 2: The Different Regions of Earth’s Atmosphere ……………………………… 12 Fig 3: Position of Ozone Layer ……………………………………………………. 19 Fig 4: Chemical Mechanism of the Bromine ……………………………………… 21 Fig. 5: Depth to which UV Radiation Penetrates in Human Skin …………………… 25 Fig. 6: Picture of Skin Cancer ………………………………….………………... 28 Fig. 7: The use of Pesticide in Tanzania ………………………………………...… 29 Fig. 8: Radiation Balance…………………………………………….……..... 42 Fig. 9: Picture of the Clouds ……………………………………………………….. 44 Fig. 10: Earth's Radiation Balance Between Incoming and Outgoing Radiation …… 48 Fig. 11: Air Pollution …………………………………………………………………. 51 Fig. 12: Effect of Acid Rain on a Forest, Jizera Mountains, Czech Republic ………… 64 Fig. 13: Waste Management Structure of the Dar es Salaam Health Department …… Fig. 14: Trash Closer to Living Premises with Colored Discharge Find its Way to 74 Water/Soil System ……………………………………………………. 75 Fig. 15: Process Flow Diagram for a Typical Large-Scale Treatment Plant …………. 85 LIST OF TABLES 13 Table 1: Types of Water Sources in Tanzania ……………………………………….... 70 Table 2: Protozoal Infections …………………………………………………………. 78 Table 3: Parasitic Infections (Kingdom Animalia) …………………………………..... 78 Table 4: Bacterial Infections ………………………………………………................... 79 Table 5: Viral Infections ………………………………………………………………. 80 14 1: 1.1: LECTURE ONE The Meaning of Environmental Chemistry We shall begin this lecture by defining the term chemistry, environment and then the environmental chemistry according to views of various scholars. From these definitions characteristics of the environment will be summarized. What does the term chemistry mean to you as an environmentalist? Most of you will refer back in class room “form one” and define chemistry as a branch of science which deals with composition and decomposition of matter. That is very right, because Chemistry is the scientific study of interaction of chemical substances that are constituted of atoms or the subatomic particles: protons, electrons and neutrons. Atoms combine to produce molecules or crystals. In The American heritage Dictionary they define chemistry as the science of the composition, structure, properties and reaction of matter, especially the atomic and molecular system. Chemistry is often called "the central science" because it connects the other natural sciences such as astronomy, physics, material science, biology, and geology. Holderness and Lambert they define chemistry as a study of the nature and properties of all forms of matter-the substances which makeup our environment – and various changes which these substances undergo in different conditions. Therefore the chemistry is related with what happen to the environment. Now, what is the environment? According to the American heritage dictionary defines the environment as the totality of circumstances surrounding the organism or group of organisms, especially the combination of external physical conditions affecting the nature of organism. From this we can now define the environmental study as the systematic study of human interaction with their environment. That’s where the science/chemistry comes. This is a broad field of study that includes the natural environment, built environments, social environments, organizational environments, and the sets of relationships between them. The discipline encompasses study in the basic principles of ecology and environmental sciences as well as the associated subjects such as policy, politics, law, economics, social aspects, planning, pollution control, natural resources, and the interactions of human beings and nature. Current environmental problems have evolved into a complex set of interdisciplinary issues involving ecological, political, economic, social, as well as physical and 15 biological considerations. Modern environmental studies must include the study of the urban environment as well as the natural environment. What is the environmental chemistry then? Mahan in 2000, tell us that in order to understand environmental chemistry it is better to understand first environmental science. He defines this as “the study of the earth, air, water, and living environments, and the effects of technology thereon”. In the actual sense environmental science in its broadest sense in the science of the complex interactions that occur among the terrestrial, atmospheric, aquatic, living, and anthropological environments. It includes all the disciplines, such as chemistry, biology, ecology, sociology, and government that affect or describe these interactions. Manahan define the environmental chemistry as “the study of the sources, reactions, transport, effects, and fates of chemical species in water, soil, air, and living environments, and the effects of technology thereon”. This should not be confused with green chemistry, which aims to reduce potential pollution at its source. Environmental chemistry is an interdisciplinary science that includes atmosphere, aquatic and soil chemistry, as well as heavily relying on analytical chemistry and being related to environmental and other areas of science. Environmental chemistry involves first understanding how the uncontaminated environment works, which chemicals in what concentrations are present naturally, and with what effects. Without this it would be impossible to accurately study the effects humans have on the environment through the release of chemicals. In the past decades, massive amounts of chemicals have given humankind an unprecedented standard of living and quality of life. However, this has also exerted a price of environmental degradation. You can't go far without being reminded about pollution, global warming and the ozone layer. Chemistry plays an intrinsic part in issues surrounding the environment; some people would even blame “chemistry” for most of what's going wrong. From all this we can think of what happens to the chemicals in an industrial cleaner after someone pour it into the sink? When you see black smoke heavy out of the pipe (chimney) at an industrial complex, what impact is it having on the atmosphere? These are the types of questions environmental chemists seek to answer. 16 The fate of chemicals in the environment and their effects are matters of increasing concern to specialists in environmental management. "The outcome" involves studying where chemicals show up in streams, rivers, and air. Such pollution/contamination contains molecules that have not been removed in water treatment plants, caught by the filters in industrial smokestacks, disposed of properly, or successfully sealed in containers. As concerns about geochemistry and the natural environment increase, environmental chemists also study the processes that affect chemicals in the environment. Gases emitted by a pine forest may create a mist when mixed with car exhaust, for example. In other instances, the environment may have effects on chemicals that can be toxic. Environmental chemists examine the ways both chemicals and the environment are changed by interacting. Study questions 1. What is the chemistry of the environment? 2. Who is the environmentalist? 3. What is the relation ship of atom, proton, electron and neutron? Class Activity Chemists are not the only one affect the natural environment. Discuss this statement by citing with vivid examples like what biologists have done to environment. References 1. The American Heritage Dictionary of the English Language, 4th ed Houghton Mifflin Company, 2006. 2. Holderness, A. and Lambert, J. (1987), A New Certificate Chemistry, Heinemann Education 3. Manahan S. E. (2000), Environmental Chemistry, 7th ed Lewis Publisher. 17 2: 2.1: LECTURE TWO Science of the Surrounding of the Earth Introduction The earth is the third planet from the sun and the largest of the other planets in the solar system in terms of diameter, mass and density. It is also referred to as the World. According to the American heritage dictionary earth which is revolving around the sun in a distance of about 149 million km has an average radius of 6,378 km and a mass of approximately 5.974 x 1024 kg. Let us think again the definitions given in Lecture 1, it is now possible to consider environmental chemistry from the viewpoint of the interactions among water, air, earth and organism life in relation to the surrounding. These environmental “spheres” and the interrelationships among them are summarized in this Lecture. Earth science generally recognizes four spheres, the lithosphere, the hydrosphere, the atmosphere, and the biosphere. These correspond to rocks, water, air, and life. Some practitioners include the cryosphere (ice) as a distinct portion of the hydrosphere and the pedosphere (soil) as an active, intermixed sphere as part of Earth's spheres. We might prefer the word ecosphere, as all encompassing of both biological and physical components of the planet. So it is combining gasses, water, life and earth's crust. You have the ecosystem which is a natural unit consisting of all plants, animals and micro-organisms in an area functioning together with all the non-living physical factors of the environment. 2.2: Earth – Air – Water The environment encompasses three main zones or “spheres”: the earth's crust or simply “earth”, the hydrosphere or the water on the surface of the earth like oceans, lakes, rivers and groundwater and the atmosphere or the air around us. These three zones and their interactions with one another are collectively “the environment”. So the chemistry of the environment is the chemistry of what goes on in the three areas and their interfaces. 18 2.1: Science of the Surrounding of the Earth 2.2.1: Water and Hydrosphere The term hydrosphere means the water vapor in the earth’s atmosphere. Water, with a very simple chemical formula of H2O, is a vitally important substance in all parts of the environment. Water covers about 71% of Earth’s surface. It occurs in all spheres of the environment in the oceans as a vast reservoir of saltwater, on land as surface water in lakes and rivers, underground as groundwater, in the atmosphere as water vapor, in the polar icecaps as solid ice (top of mountain Kilimanjaro), and in many segments of the anthrosphere such as in boilers or municipal water distribution systems. Water is an essential part of all living systems and is the medium from which life evolved and in which life exists. On earth, it is found mostly in oceans and other large water bodies, with 1.6% of water below ground in aquifers and 0.001% in the air as vapor, clouds (formed of solid and liquid water particles suspended in air), and precipitation, hold 97% of surface water, glaciers and polar ice caps 2.4%, and other land surface water such as rivers, lakes and ponds 0.6%. A very small amount of the Earth's water is contained within biological bodies and manufactured products. Other water is trapped in ice caps, glaciers, aquifers, or in lakes, sometimes providing fresh water for life on land. Water moves continually through a cycle of evaporation or transpiration (evapo-transpiration), precipitation, and runoff, usually reaching the sea. Winds carry water vapor over land at the same rate as runoff into the sea. Over land, evaporation and transpiration contribute to the precipitation over land. Figure 1 below shows the water circle. 19 Fig. 1: Water Cycle Water has three states (solid, liquid and gaseous). Below freezing water is a solid (ice or snowflakes), between freezing and boiling water is a liquid, and above its boiling point water is a gas. Water changing from solid to liquid is said to be melting. When it changes from liquid to gas it is evaporating. Water changing from gas to liquid is called condensation (An example is the 'dew' that forms on the outside of a glass of cold soda). Frost formation is when water changes from gas directly to solid form. When water changes directly from solid to gas the process is called sublimation. 2.2.2: Earth’s Surface Geochemists study the chemistry of the earth's crust. They define earths crust as a thin layer of rock, some 40 km thick that covers the whole surface of the planet. In terms of elements, the crust is 46% oxygen, 28% silicon, 8% aluminium, and 5% iron with smaller amounts of other elements. These elements are combined into mainly silicon dioxide (silica) based rocks. The earth is divided into layers, including the solid, iron-rich inner core, molten outer core, mantle, and crust. Environmental science is most concerned with the lithosphere, which consists of the outer mantle and the crust. The latter is the earth’s outer skin that is accessible to humans. It is extremely thin compared to the diameter of the earth, ranging from 5 to 40 km thick. As such, it pertains mostly to the solid mineral portions of Earth’s crust. But it must also consider water, which is involved in weathering rocks and 20 in producing mineral formations; the atmosphere and climate, which have philosophical effects on the geosphere and interchange matter and energy with it; and living systems, which largely exist on the geosphere and in turn have significant effects on it. Geological science uses chemistry in the form of geochemistry to explain the nature and behavior of geological materials, physics to explain their mechanical behavior, and biology to explain the mutual interactions between the geosphere and the biosphere. Modern technology, for example the ability to move massive quantities of dirt and rock around, has a profound influence on the geosphere. The most important part of the geosphere for life on earth is soil formed by the disintegrative weathering action of physical, geochemical, and biological processes on rock. It is the medium upon which plants grow, and virtually all terrestrial organisms depend upon it for their existence. The productivity of soil is strongly affected by environmental conditions and pollutants. That’s where the environmental chemistry and environmental science plays. 2.2.3: Earth’s Water The hydrosphere encompasses the water gathered in different places on the surface of the earth. The huge majority of this water is found in the oceans and trapped beneath them in sediments. Only about 0.5% of water is found on the continents in rivers, lakes, glaciers and ground waters. The oceans play an important part in absorbing a transporting solar energy which controls climate. They also are involved in the evaporation-precipitation cycles which so affect life on the planet. Water is the universal solvent, therefore the chemistry of water is a huge subject and its ability as a solvent is the basis of all life on earth. Study Questions: 1) What is the difference between these terms: atmosphere, hydrosphere, geosphere, and biosphere 2) How can you distinguish between evapo-transpiration and sublimation? References 1. The American Heritage Dictionary of the English Language, 4th ed Houghton Mifflin Company, 2006. 2. Water cycle http://ga.water.usgs.gov/edu/watercycleprint.html 3. Manahan, S. E. (2000), Environmental Chemistry, 7th ed Lewis Publisher. 21 3: 3.1: LECTURE THREE The Chemistry of the Light and Atmosphere Introduction The chemistry composition of the atmosphere is of importance for several reasons, but primarily because of the interactions between the atmosphere and living organisms. The composition of the Earth's atmosphere has been changed by human activity and some of these changes are harmful to human health, crops and ecosystems. Examples of problems which have been addressed by atmospheric chemistry include acid rain, photochemical smog and global warming. Atmospheric chemistry seeks to understand the causes of these problems, and by obtaining a theoretical understanding of them, allow possible solutions to be tested and the effects of changes in government policy evaluated. Instead of concentrating on atmospheric chemistry in isolation the focus is now on seeing it as one part of a single system with the rest of the atmosphere, biosphere and geosphere. An especially important driver for this is the links between chemistry and climate such as the effects of changing climate on the recovery of the ozone hole and vice versa but also interaction of the composition of the atmosphere with the oceans and terrestrial ecosystems. 3.1: Light Light plays an important role in atmospheric chemistry. The energy of the light in the Earth’s atmosphere is often needed to initiate reactions. For example, without the energy of light coming from the Sun, there would be no stratospheric ozone. The critical feature of light as a reactant is its wavelength. Thus: E = hυ = hC λ Where: E = Energy in Joules h = Planck’s Constant (6.63 x 10-34Js/molecule) υ = Frequency of light in s-1 22 C = Speed of light (3 x 108 m/s) λ = Wavelength (nm) The following are the examples of electromagnetic radiation: microwaves, infrared and ultraviolet light, X-rays and gamma-rays. Hotter, more energetic objects and events create higher energy radiation than cool objects. Only extremely hot objects or particles moving at very high velocities can create high-energy radiation like X-rays and gamma-rays. These types of radiation in the electromagnetic spectrum can be arranged from the one with lowest energy to highest as follows: Radio wave: This is the same kind of energy that radio stations emit into the air for your radio to capture. But radio waves are also emitted by other things such as stars and gases in space. They have a frequency ranging from 104 to 1010 s-1 Microwaves: They are used in cooking just in a few minutes. Also astronomers in space are using to learn about the structure of nearby galaxies. They have a frequency ranging from 109 to 1011 s-1 Infrared: Our skin emits infrared light, which is why we can be seen in the dark by someone using night vision goggles. In space, IR light maps the dust between stars. They have a frequency ranging from 1011 to 1015 s-1 Visible: This is the part that our eyes see. Visible radiation is emitted by everything from fireflies to light bulbs to stars ... also by fast-moving particles hitting other particles. They have wavelength ranging from 390 nm to 760 nm. 23 Ultraviolet: We know that the Sun is a source of ultraviolet (or UV) radiation, because it is the UV rays that cause our skin to burn! Stars and other "hot" objects in space emit UV radiation. They have a frequency ranging from 1014 to 1017 s-1 X-rays: Used by medical doctors to look at your bones and dentist to look at your teeth. Hot gases in the Universe also emit X-rays. They have a frequency ranging from 1017 to 1021 s-1 Gamma-rays: Radioactive materials (some natural and others made by man in things like nuclear power plants) can emit gamma-rays. Big particle accelerators that scientists use to help them understand what matter is made of can sometimes generate gamma-rays. But the biggest gamma-ray generator of all is the Universe! It makes gamma radiation in all kinds of ways. They have a frequency ranging from 1019 to 1024 s-1 24 Study Question and Model Answer: A wavelength of 242.4 nm is the longest wavelength that will bring about the photo-dissociation of O2. What is the energy of (a) one photon and (b) a mole of photon of this light? 2.998 x1010 ms −1 Answer: (a) We know that : υ = = = 1.237 x1015 s −1 −9 λ 242.4 x10 m C E = hυ = 6.626 x 10-34 Js/photon x 1.237 x 1015 s-1 = 8.196 x 10-19J/photon (b) E = 8.196 x 10-19J/photon x 6.022 x 1023 photon/mol = 4.936 x 105J/mol Practice Question The energy of a photon is higher when the frequency of the associated wave is higher (and the wavelength shorter). Discuss. Activity: If gaseous dihydrogen (H2) at low pressure is subjected to electrical discharge, the molecules are broken down into energetically excited atoms (H). These atoms emit visible radiation which, when passed through a prism, show a discrete set of wavelengths an atomic spectrum is observed. The emitted radiation is not a continuous spectrum. Draw the Emission spectrum of hydrogen showing the position of distinct colors in respect of the wavelength of each. References: 1. Chang, R. (1991), Chemistry, 4th ed McGraw-Hill, Inc 2. Manahan S. E. (2000), Environmental Chemistry, 7th ed Lewis Publisher 3. Wright, J. (2005), Environmental Chemistry, Taylor & Francis e-Library. NY. 25 4: 4.1: LECTURE FOUR Atmospheric Structure Introduction The gaseous area surrounding the planet is divided into several concentric spherical strata separated by narrow transition zones. The upper boundary at which gases disperse into space lies at an altitude of approximately 1000 km above sea level. More than 99% of the total atmospheric mass is concentrated in the first 40 km from Earth's surface. Atmospheric layers are characterized by differences in chemical composition that produce variations in temperature. 100 Thermosphere N2, O2+, NO+, O2 Mesopause 80 Mesosphere [N2, O2, O2+, NO+] Stratopause Altitude (Km) 60 Stratosphere [N2, O2, O3] 40 Ozone Layer 20 Tropospause Troposphere [N2, O2, H2O, Ar] CO2] -80 -60 -40 -20 0 20 40 Temperature (oC)- Fig. 2: The Different Regions of Earth’s Atmosphere (Chang, R. 1991 pg 584) 26 4.2: Troposphere The troposphere is the atmospheric layer closest to the planet and contains the largest percentage of the mass of the total atmosphere. It is characterized by the density of its air and an average vertical temperature change of 6oC/km.Temperature and water vapor content in the troposphere decrease rapidly with altitude. Water vapor plays a major role in regulating air temperature because it absorbs solar energy and thermal radiation from the planet's surface. The troposphere contains 99 % of the water vapor in the atmosphere. Water vapor concentrations vary with latitudinal position. They are greatest above the tropics, where they may be as high as 3 %, and decrease toward the polar regions. All weather phenomena occur within the troposphere, although turbulence may extend into the lower portion of the stratosphere. Troposphere means "region of mixing" and is so named because of vigorous convective air currents within the layer. The upper boundary of the layer ranges in height from 8 Km in high latitudes, to 18 Km above the equator. Its height also varies with the seasons; highest in the summer and lowest in the winter. A narrow zone called the tropopause separates the troposphere from the next highest layer called the stratosphere. Air temperature within the tropopause remains constant with increasing altitude. 4.3: Stratosphere The stratosphere is the second major strata of air in the atmosphere. It resides between 10 and 50 km above the planet's surface. The air temperature in the stratosphere remains relatively constant up to an altitude of 25 km. Then it increases gradually to 200-220 degrees Kelvin (K) at the lower boundary of the stratopause (~50 Km), which is marked by a decrease in temperature. Because the air temperature in the stratosphere increases with altitude, it does not cause convection and has a stabilizing effect on atmospheric conditions in the region. Ozone plays the major role in regulating the thermal regime of the stratosphere, as water vapor content within the layer is very low. Temperature increases with ozone concentration. Solar energy is converted to kinetic energy when ozone molecules absorb ultraviolet radiation, resulting in heating of the stratosphere. The ozone layer (27k jpeg) is located at an altitude between 20-30 km. Approximately 90% of the ozone in the atmosphere resides in the stratosphere. Ozone concentration in the region is about 10 27 parts per million by volume as compared to approximately 0.04 parts per million by volume in the troposphere. Ozone absorbs the bulk of solar ultraviolet radiation in wavelengths from 290 nm - 320 nm. These wavelengths are harmful to life because they can be absorbed by the nucleic acid in cells. Increased penetration of ultraviolet radiation to the planet's surface would damage plant life and have harmful environmental consequences. Appreciably large amounts of solar ultraviolet radiation would result in a host of biological effects, such as a dramatic increase in cancers. Meteorological conditions strongly affect the distribution of ozone. Most ozone production and destruction occurs in the tropical upper stratosphere, where the largest amounts of ultraviolet radiation are present. Dissociation takes place in lower regions of the stratosphere and occurs at higher latitudes than does production. 4.4: Mesosphere The mesosphere, a layer extending from approximately 50 km to 80 km, is characterized by decreasing temperatures, which reach 190-180 K at an altitude of 80 km. In this region, concentrations of ozone and water vapor are negligible. Hence the temperature is lower than that of the troposphere or stratosphere. With increasing distance from Earth's surface the chemical composition of air becomes strongly dependent on altitude and the atmosphere becomes enriched with lighter gases. At very high altitudes, the residual gases begin to stratify according to molecular mass, because of gravitational separation. 4.5: Thermosphere The thermosphere is located above the mesosphere and is separated from it by the mesopause transition layer. The temperature in the thermosphere generally increases with altitude up to 10001500 K. This increase in temperature is due to the absorption of intense solar radiation by the limited amount of remaining molecular oxygen. At an altitude of 100-200 km, the major atmospheric components are still nitrogen and oxygen. At this extreme altitude gas molecules are widely separated. 4.5: Exosphere The exosphere is the most distant atmospheric region from Earth's surface. The upper boundary of the layer extends to heights of perhaps 960 to 1000 km and is relatively undefined. The exosphere is 28 a transitional zone between Earth's atmosphere and interplanetary space. Atmospheric chemists are interested in understanding the chemical composition of the natural atmosphere, the way gases, liquids, and solids in the atmosphere interact with each other and with the earth's surface and associated biota, and how human activities may be changing the chemical and physical characteristics of the atmosphere. This latter question is currently a driving force behind the growing need for atmospheric chemists in the world. There are a number of critical environmental issues associated with a changing atmosphere, including photochemical smog, global climate change, toxic air pollutants, acidic deposition, and stratospheric ozone depletion. All of these issues affect the world, and a great deal of research and development activity aimed at understanding and hopefully solving some of these problems is underway. Much of the anthropogenic (human) impact on the atmosphere is associated with our increasing use of fossil fuels as an energy source - for things such as heating, transportation, and electric power production. Photochemical smog/tropospheric ozone is one serious environmental problem associated with burning fossil fuels. The importance of Atmospheric Chemistry has been acknowledged by The Royal Swedish Academy of Sciences awarding the Nobel Prize in Chemistry 1995 to the Atmospheric Scientists P. Crutzen, M. Molina and F. S. Rowland particularly concerning the formation and decomposition of ozone. 29 Study question and model answer: Flight engineers know much the use of Figure 2. They know that for more fuel efficiency the airlines cannot reach the stratosphere. Discuss this in detail. Answer: Commercial airliners typically sail at altitudes of 9–12 km in temperate latitudes, in the lower reaches of the stratosphere. They do this to optimize jet engine fuel burn, mostly thanks to the low temperatures encountered near the tropopause. It also allows them to stay above any hard weather, and avoid atmospheric turbulence from the convection in the troposphere. Turbulence experienced in the cruise phase of flight is often caused by convective overshoot from the troposphere below. Although a few gliders have achieved great altitudes in the powerful thermals in thunderstorms, this is dangerous. Most high altitude flights by gliders use lee waves from mountain ranges and were used to set the current record of 15,447m. Class activity: Why the pressure on this mountain is very low and the temperature is colder than sea level. Suppose the summit of Mountain Kilimanjaro is flat; is it possible to play football on top for 90 minutes? References: 1. Chang, R. (1991), Chemistry, 4th ed McGraw-Hill, Inc. 2. Seinfeld, J. H., and Pandis, S. N. (2006), Atmospheric Chemistry and Physics: From Air Pollution to Climate Change 2nd ed, Wiley, New Jersey. 30 5: 5.1: LECTURE FIVE Ozone layer Introduction Ozone or trioxygen (O3) is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic O2. Ground-level ozone is an air pollutant with harmful effects on the respiratory systems of animals. The ozone layer in the upper atmosphere filters potentially damaging ultraviolet light from reaching the Earth's surface. It is present in low concentrations throughout the Earth's atmosphere. It has many industrial and consumer applications. The chemistry of ozone can be considered as a powerful oxidizing agent, far better than dioxygen. It is also unstable at high concentrations, decaying to ordinary diatomic oxygen (in about half an hour in atmospheric conditions): 2 O3 → 3 O2. Also ozone can combine with metal or non metal to give oxygen as shown here: Ozone will oxidize metals (except gold, platinum, and iridium) to oxides of the metals in their highest oxidation state: 2 Cu+ (aq) + 2 H3O+ (aq) + O3 (g) → 2 Cu2+ (aq) + 3 H2O (l) + O2 (g) or for non metal Ozone reacts with carbon to form carbon dioxide, even at room temperature: C + 2 O3 → CO2 + 2 O2 or another metal like potassium 2 KOH + 5 O3 → 2 KO3 + 5 O2 + H2O. Example of non-metal is the reaction of ozone with ammonia. Ozone does not react with ammonium salts but it reacts with ammonia to form ammonium nitrate: 2NH3 + 4O3 → NH4NO3 + 4O2 + H2O The question is oxygen is very important in life as every one knows. What is the problem of having this big source of oxygen? First we have to discuss the formation of this ozone. What is the importance of Ozone to our life? 31 5.2: Ozone Formation When fossil fuels (e.g., gasoline) are burned, a variety of pollutants are emitted into the earth's troposphere, i.e., the region of the atmosphere in which we live - from ground level up to about 15 km. Two of the pollutants that are emitted are hydrocarbons (e.g., unburned fuel) and nitric oxide (NO). When these pollutants build up to sufficiently high levels, a chain reaction occurs from their interaction with sunlight in which the NO is converted to nitrogen dioxide (NO2). NO2 is a brown gas and at sufficiently high levels can contribute to urban haze. However, a more serious problem is that NO2 can absorb sunlight and break apart to produce oxygen atoms that combine with the O2 in the air to produce ozone (O3). Oxygen also decomposes with the solar radiation at a wavelength shorter than 260 and in effect can form ozone as shown in equations below: O2 (g) + hυ → O(g) + O(g) O(g) + O2 → O3(g) 5.3: Importance of Ozone to our Life The largest quantity of ozone is used in the preparation of pharmaceuticals, synthetic lubricants, as well as many other commercially useful organic compounds, where it is used to sever carboncarbon bonds. It can also be used for bleaching substances and for killing microorganisms in air and water sources. Many municipal drinking water systems kill bacteria with ozone instead of the more common chlorine. Ozone has a very high oxidation potential. Ozone does not form organochlorine compounds, nor does it remain in the water after treatment. Where electrical power is abundant, ozone is a cost-effective method of treating water, since it is produced on demand and does not require transportation and storage of hazardous chemicals. Once it has decayed, it leaves no taste or odor in drinking water. Ozone is also called earth’s blanket. About 25 km above the earth is a layer of this gas ozone. Ozone is produced naturally in the atmosphere. The ozone layer is very important because it stops too many of the sun’s ‘ultra-violet rays’ (UV rays) getting through to the Earth these are the rays that cause burn to our skin especially people with low melanin. Too much UV can cause skin cancer and will also harm all plants and animals. The following figure can summarize the lecture. 32 Fig 3: Position of Ozone Layer Class activity Life on Earth could not exist without the protective shield of the ozone layer. Discuss this statement in depth and give relevant consequences. References 1. Chang, R. (1991), Chemistry, 4th Ed, McGraw-Hill, Inc. 2. Shriver, D. and Atkins, P. (2003), 3rd Ed. Inorganic Chemistry, W. H. Freeman Company, NY. 3. http://science.howstuffworks.com/global-warming.htm. 33 6: 6.1: LECTURE SIX Destruction of Ozone Layer and Mechanisms of Depletion Introduction We have learned from previous lecture that the stratosphere is the layer of our atmosphere just above the one we breathe which includes a thin layer of ozone. This layer is thicker over the poles than the equator. It might seem insignificant compared to the depth of the rest of the atmosphere, but it does a very important job. Example of the job is to prevents much of the sun's ultraviolet (UV) light from reaching the earth. UV light can cause skin cancer, cataracts and other disorders. Ozone protects us from the sun by interacting with light. When ultraviolet light hits oxygen molecules (O2) in the stratosphere, it splits the molecules into two atoms of oxygen (O). When this atom encounters another oxygen molecule, the two combine to make ozone (O3). Ultraviolet light also breaks ozone back down into an oxygen molecule and an oxygen atom. We can term this process as ozone-oxygen cycle, and it converts UV radiation into heat, protecting the Earth. In this lecture we will lean what destruct this important layer and disturb the circle so that our life is in danger? 6.2: Destruction of Ozone Layer Ozone is a powerful oxidizing agent, and a toxic gas. It is believed that the natural level of ozone in the clean troposphere is 10 to 15 parts-per-billion (ppb). Because of increasing concentrations of hydrocarbons and NO in the atmosphere, scientists have found that ozone levels in "clean air" are now approximately 30 ppb. A principal activity of atmospheric chemists is to study and determine how we might reverse this trend. Ozone depletion describes two distinct, but related observations: a slow, steady decline of about 4 percent per decade in the total volume of ozone in Earth's stratosphere (ozone layer) since the late 1970s, and a much larger, but seasonal, decrease in stratospheric ozone over Earth's polar regions during the same period. The latter phenomenon is commonly referred to as the ozone hole. In addition to this well-known stratospheric ozone depletion, there are also tropospheric ozone depletion events, which occur near the surface in polar regions during spring. I know for us the 34 word spring could be very confusing because we are in Africa very close to equator where the seasons are not very distinct. For those in Europe or America they can tell the differences, that we start with winter (Very cold we can see snow here) to spring and then summer (very hot). Now we know spring is not hot not cold. During this time in the Polar Regions, unique photochemistry converts inert halide salt ions (e.g. Br-) into reactive halogen species (e.g. Br atoms and BrO) that deplete ozone in the boundary layer to near zero levels. Since their discovery in the late 1980s, research on these ozone depletion events (ODEs) has shown the central role of bromine photochemistry. Due to the autocatalytic nature of the reaction mechanism, it has been called bromine explosion. The following is the bromine explosion which shows what we have just learned as the reaction mechanism. Fig 4: Chemical mechanism of the bromine explosion (Adopted from http://en.wikipedia.org/wiki/Tropospheric_ozone_depletion_events on 1st August 2009) It is still not fully understood how salts are transported from the ocean and oxidized to become reactive halogen species in the air. Other halogens (chlorine and iodine) are also activated through mechanisms coupled to bromine chemistry. The main consequence of halogen activation is chemical destruction of ozone, which removes the primary precursor of atmospheric oxidation, and generation of reactive halogen atoms/oxides that become the primary oxidizing species. The different reactivity of halogens as compared to OH and ozone has broad impacts on atmospheric chemistry, including near complete removal and deposition of mercury, alteration of oxidation fates 35 for organic gases, and export of bromine into the free troposphere. Recent changes in the climate of the Arctic and state of the Arctic sea ice cover are likely to have strong effects on halogen activation and ODEs. 6.3: Mechanism of Ozone Destruction The detailed mechanism by which the polar ozone holes form is different from that for the midlatitude thinning, but the most important process in both trends is catalytic destruction of ozone by atomic chlorine and bromine. The main source of these halogen atoms in the stratosphere is photodissociation of chlorofluorocarbon (CFC) compounds, commonly called freons, and of bromofluorocarbon compounds known as halons. These compounds are transported into the stratosphere after being emitted at the surface. Both ozone depletion mechanisms strengthened as emissions of CFCs and halogens increased. CFCs and other contributory substances are commonly referred to as ozone-depleting substances (ODS). Since the ozone layer prevents most harmful UVB wavelengths (270–315 nm) of ultraviolet light (UV light) from passing through the Earth's atmosphere, observed and projected decreases in ozone have generated worldwide concern leading to adoption of the Montreal Protocol banning the production of CFCs and halons as well as related ozone depleting chemicals such as carbon tetrachloride and trichloroethane. It is suspected that a variety of biological consequences such as increases in skin cancer, damage to plants, and reduction of plankton populations in the ocean's photic zone may result from the increased UV exposure due to ozone depletion. Oxygen are involved in the ozone-oxygen cycle: oxygen atoms (O or atomic oxygen), oxygen gas (O2 or diatomic oxygen), and ozone gas (O3 or triatomic oxygen). Ozone is formed in the stratosphere when oxygen molecules photodissociate after absorbing an ultraviolet photon whose wavelength is shorter than 240 nm. This produces two oxygen atoms. The atomic oxygen then combines with O2 to create O3. Ozone molecules absorb UV light between 310 and 200 nm, following which ozone splits into a molecule of O2 and an oxygen atom. The oxygen atom then joins up with an oxygen molecule to regenerate ozone. This is a continuing process which terminates when an oxygen atom "recombines" with an ozone molecule to make two O2 molecules: O + O3 → 2 O2 36 The overall amount of ozone in the stratosphere is determined by a balance between photochemical production and recombination. Ozone can be destroyed by a number of free radical catalysts, the most important of which are the hydroxyl radical (OH·), the nitric oxide radical (NO·) and atomic chlorine (Cl·) and bromine (Br·). All of these have both natural and anthropogenic (manmade) sources; at the present time, most of the OH· and NO· in the stratosphere is of natural origin, but human activity has dramatically increased the levels of chlorine and bromine. These elements are found in certain stable organic compounds, especially chlorofluorocarbons (CFCs), which may find their way to the stratosphere without being destroyed in the troposphere due to their low reactivity. Once in the stratosphere, the Cl and Br atoms are liberated from the parent compounds by the action of ultraviolet light, e.g. ('h' is Planck's constant, 'ν' is frequency of electromagnetic radiation) CFCl3 + hν → CFCl2 + Cl The Cl and Br atoms can then destroy ozone molecules through a variety of catalytic cycles. In the simplest example of such a cycle, a chlorine atom reacts with an ozone molecule, taking an oxygen atom with it (forming ClO) and leaving a normal oxygen molecule. The chlorine monoxide (i.e., the ClO) can react with a second molecule of ozone (i.e., O3) to yield another chlorine atom and two molecules of oxygen. The chemical shorthand for these gas-phase reactions is: Cl + O3 → ClO + O2 ClO + O3 → Cl + 2 O2 The overall effect is a decrease in the amount of ozone. More complicated mechanisms have been discovered that lead to ozone destruction in the lower stratosphere as well. A single chlorine atom would keep on destroying ozone (thus a catalyst) for up to two years (the time scale for transport back down to the troposphere) were it not for reactions that remove them from this cycle by forming reservoir species such as hydrogen chloride (HCl) and chlorine nitrate (ClONO2). On a per atom basis, bromine is even more efficient than chlorine at destroying ozone, but there is much less bromine in the atmosphere at present. As a result, both chlorine and bromine contribute significantly to the overall ozone depletion. Laboratory studies have shown that fluorine and iodine atoms participate in analogous catalytic cycles. However, in the Earth's stratosphere, 37 fluorine atoms react rapidly with water and methane to form strongly-bound HF, while organic molecules which contain iodine react so rapidly in the lower atmosphere that they do not reach the stratosphere in significant quantities. Furthermore, a single chlorine atom is able to react with 100,000 ozone molecules. This fact plus the amount of chlorine released into the atmosphere by chlorofluorocarbons (CFCs) yearly demonstrates how dangerous CFCs are to the environment. Study Question 1. What do you understand by the term ozone hole? Why do you think slow or steady state is not referred as ozone hole? 2. Which one is most harmful UVA or UVB? Give reason. 3. What can you say about tropospheric ozone depletion events, which occur near the surface in polar regions during spring. To assist you please refer reference 2and 3 below. References: 1. Simpson, W. R. , von Glasow, R. , Riedel, Anderson, K. , Ariya, P. , P. (2007), Halogens and their role in polar boundary-layer ozone depletion, atmos. Chem. Phys., 7, 4375-4418. 2. http://ozonewatch.gsfc.nasa.gov/facts/hole.html 3. http://www.wunderground.com/education/ozone_skeptics.asp 38 7: 7.1: LECTURE SEVEN Ozone Layer Depletion Introduction We have seen in the previous lecture that life on Earth depends in part on a thin shell of gaseous ozone that stretches from about 10 to 25 miles above our heads, encompassing the planet like an invisible, protective shield. At this altitude, it lies well above the height at which normal commercial aircraft fly, and far beneath the orbital paths of spacecraft. The ozone layer is the main barrier between us and the hazardous ultraviolet radiation that streams toward the Earth, day in and day out, from the burning surface of the Sun. Ozone--a form of oxygen--is selective in what it takes from sunlight: screening out, through a process of atomic absorption, only the more energetic ultraviolet rays while allowing the visible light and the warm infrared to pass through, untouched. The remaining question is what are the scientific consequences of this layer exhausted? Let us start by looking on the issues may happen. 7.2: Consequences of ozone Layer Depletion Let us start this lecture by using the following figure: Fig. 5: Depth to which UV Radiation Penetrates in Human Skin (adopted from http://www.gcrio.org/CONSEQUENCES/ summer95/fig2-2.html) 39 On figure 5, we can find UVA, UVB and UVC. They seem to be confusing. What are these? The answer is simple because they are based on the wavelength. • UVC - 100 to 290 nm • UVB - 290 to 320 nm • UVA - 320 to 400 nm UVA UVA was once thought to have a minor effect on skin damage, but now studies are showing that UVA is a major contributor to skin damage. UVA penetrates deeper into the skin and works more efficiently. The intensity of UVA radiation is more constant than UVB without the variations during the day and throughout the year. UVA is also not filtered by glass. UVB UVB affects the outer layer of skin, the epidermis, and is the primary agent responsible for sunburns. It is the most intense between the hours of 10:00 am and 2:00 pm when the sunlight is brightest. It is also more intense in the summer months accounting for 70% of a person's yearly UVB dose. UVB does not penetrate glass. UVC UVC radiation is almost completely absorbed by the ozone layer and does not affect the skin. UVC radiation can be found in artificial sources such as mercury arc lamps and germicidal lamps. Since the ozone layer absorbs UVB ultraviolet light from the Sun, ozone layer depletion is expected to increase surface UVB levels, which could lead to damage, including increases in skin cancer. Although decreases in stratospheric ozone are well-tied to CFCs and there are good theoretical reasons to believe that decreases in ozone will lead to increases in surface UVB, there is no direct observational evidence linking ozone depletion to higher incidence of skin cancer in human beings. This is partly due to the fact that UVA, which has also been implicated in some forms of skin cancer, is not absorbed by ozone, and it is nearly impossible to control statistics for lifestyle changes in the populace. 40 While a minority constituent in the earth's atmosphere, is responsible for most of the absorption of UVB radiation. The amount of UVB radiation that penetrates through the ozone layer decreases exponentially with the slant-path thickness/density of the layer. Correspondingly, a decrease in atmospheric ozone is expected to give rise to significantly increased levels of UVB near the surface. Because it is this same UV radiation that creates ozone in the ozone layer from O2 (regular oxygen) in the first place, a reduction in stratospheric ozone would actually tend to increase photochemical production of ozone at lower levels (in the troposphere), although the overall observed trends in total column ozone still show a decrease, largely because ozone produced lower down has a naturally shorter photochemical lifetime, so it is destroyed before the concentrations could reach a level which would compensate for the ozone reduction higher up. The main public concern regarding the ozone hole has been the effects of surface UV on human health. So far, ozone depletion in most locations has been typically a few percent and, as noted above, no direct evidence of health damage is available in most latitudes. Were the high levels of depletion seen in the ozone hole ever to be common across the globe, the effects could be substantially more dramatic. As the ozone hole over Antarctica has in some instances grown as large as to reach southern parts of Australia and New Zealand, environmentalists have been concerned that the increase in surface UV could be significant. UVB (the higher energy UV radiation absorbed by ozone) is generally accepted to be a contributory factor to skin cancer. In addition, increased surface UV leads to increased tropospheric ozone, which is a health risk to humans. The increased surface UV also represents an increase in the vitamin D synthetic capacity of the sunlight. The cancer preventive effects of vitamin D represent a possible beneficial effect of ozone depletion. In terms of health costs, the possible benefits of increased UV irradiance may outweigh the burden. Below is the picture of skin cancer adopted from About.com Health's Disease and Condition content is reviewed by the Medical Review Board. 41 Fig. 6: Picture of Skin Cancer Not only that Ozone depletion has only effect on human beings. A decrease in the amount of ozone in the upper atmosphere results an increase in the amount of ultraviolet (UV) radiation at the Earth's surface. Additional UV-B (the most harmful wavelengths) is expected to have negative effects on human health, health services, farm animals, crop production, forest production, fisheries and tourist industries. Examples of likely effects include: • Reduced immune responses, which may increase the incidence of infectious disease and reduce the effectiveness of vaccination programmes; • Disrupted growth processes in some plants, leading to reduced yields for certain crops and forest trees; and hence hunger, death, poverty etc. • Disrupted development in fish, estimated to reduce ocean fish stocks by several million tones per annum. In agriculture we can discuss this better by giving up a story, especially those who have being using pesticides to spray on their crops. The figure below shows the use of DDT by farmer from Kilimanjaro, Old Moshi Tella, spray on his coffee plantation. 42 Fig. 7: The use of Pesticide in Tanzania Most of the commonly used chemical especially in coffee is the pesticide. One of the main threats to the Earth's ozone layer today is a powerful pesticide called methyl bromide that plays a pivotal role in Southern cash-crop economies. This type was used mainly in Zimbabwe. Methyl bromide is a highly toxic fungicide and the second most widely applied pesticide in the world. It is primarily used on "high value" export crops such as tomatoes, peppers, grapes, strawberries, tobacco and flowers. Methyl bromide is also used to protect stored grains. The pesticide is effective against a wide range of pests including insects, worms, and pathogenic microorganisms. However, methyl bromide has a dramatic environmental impact. After being sprayed on crops, the pesticide drifts into the upper atmosphere where it damages the ozone layer, which blocks ultraviolet (UV) rays from reaching the Earth's surface. Although a shorter-lived substance than chlorofluorocarbons (CFCs) — a better-known family of ozone-depleting compounds — methyl bromide destroys ozone molecules at 50 times the rate of CFCs. In a 1994 scientific assessment, the World Meteorological Organization concluded that phasing out this chemical is the single largest step that governments can take to protect the ozone layer. The question is what should we use to kill pests in our home or in the farm? 43 Study Questions and Model Answers: 1. What is the ozone layer and why is it important? Answer: The ozone layer is a concentration of ozone molecules in the stratosphere. About 90% of the planet's ozone is in the troposphere layer. The stratosphere, the next higher layer, extends about 10-50 kilometers above the Earth's surface. Stratospheric ozone is a naturally-occurring gas that filters the sun's ultraviolet (UV) radiation. A diminished ozone layer allows more radiation to reach the Earth's surface. For people, over-exposure to UV rays can lead to skin cancer, cataracts, and weakened immune systems. Increased UV can also lead to reduced crop yield and disruptions in the marine food chain. 2. How does ozone depletion occur? Answer:It is caused by the release of chlorofluorocarbons (CFCs), hydrofluorocarbons (HCFCs), and other ozone-depleting substances (ODS), which were used widely as refrigerants, insulating foams, and solvents. Although CFCs are heavier than air, they are eventually carried into the stratosphere in a process that can take as long as 2 to 5 years. When CFCs and HCFCs reach the stratosphere, the ultraviolet radiation from the sun causes them to break apart and release chlorine atoms which react with ozone, starting chemical cycles of ozone destruction that deplete the ozone layer. One chlorine atom can break apart more than 100,000 ozone molecules. Other chemicals that damage the ozone layer include methyl bromide (used as a pesticide), halons (used in fire extinguishers), and methyl chloroform (used as a solvent in industrial processes for essential applications). As methyl bromide and halons are broken apart, they release bromine atoms, which are 60 times more destructive to ozone molecules than chlorine atoms. 3. Will the ozone layer recover? Can we make more ozone to fill in the hole? Provided that we stop producing ozone-depleting substances, ozone will be created through natural processes that should return the ozone layer to normal levels by about 2050. It is very important that the world comply with the Montreal Protocol; delays in ending production could result in additional damage and prolong the ozone layer's recovery. More detail on these questions is provided here. 44 4. How do we know the amount of CO2 in the atmosphere is linked to human Activity? Answer: One way we know this is linked to human activity is by looking at historical records of human activity. Fossil fuels have been burnt since the start of the industrial revolution, along with forested land being cleared at unprecedented rates. These processes have been turning the organic compounds into Carbon Dioxide. Enough carbon dioxide has been produced to raise the atmospheric content to 500ppm. Fortunately, these levels haven't been reaching because the oceans and biosphere have the ability to absorb some of this CO2. Unfortunately, because we are producing CO2 faster than the oceans and biosphere can absorb them, we see the increased levels. Another way, we know that the fossil fuels are causing CO2 increase in the atmosphere is by analyzing the carbon isotopes, similar to carbon dating. More information can be found at Real Climate. Class Activity 1. In late 80’s the government of Tanzania burn the use of pesticides. There was a pile of stock in the country. Where are they now? Who destroy them? How did they be destroyed? 2. What are the contents of the chemicals your relatives or your self have been spraying on the plant like maize, coffee or any agricultural products? References: 1. Gruijl, F. R. (1995), Impacts of a Projected Depletion of the Ozone Layer, The Nature and Implications of Environmental Changes, V 1 &2. 2. http://www.gcrio.org/CONSEQUENCES/summer95/fig2-2.html. 3. http://archive.idrc.ca/books/reports/1997/14-02e.html 45 8: 8.1: LECTURE EIGHT Climate: Global Warming, Green House Effect and Radiation Balance Introduction We have talked about the atmosphere and the consequences of the destruction of the normal setup on our life. Now it is time to discuss on these changes. We will start with the commonly used term climate. You may be familiar with this word but probably you don’t know exactly what does this mean. The word climate comes from the Greek word klima, meaning leaning or inclination. The term is commonly defined as the weather averaged over a long period of time. The standard averaging period is 30 years, but other periods may be used depending on the purpose. Climate also includes statistics other than the average, such as the magnitudes of day-to-day or year-to-year variations. According to Intergovernmental Panel on Climate Change (IPCC) they define the climate as the "average weather," or more rigorously, as the statistical description in terms of the mean and variability of relevant quantities over a period of time ranging from months to thousands or millions of years. The classical period is 30 years, as defined by the World Meteorological Organization (WMO). These quantities are most often surface variables such as temperature, precipitation, and wind. Climate in a wider sense is the state, including a statistical description, of the climate system. Some people tend to use interchangeably the climate and weather. However these two terms are different. The difference between climate and weather is usefully summarized by the popular phrase "Climate is what you expect, weather is what you get." The climate is controlled by different factors. One of the factor controlling the climate change is the green house gas. In this lecture we will discuss the usefulness of this green house gas and the effect of missing these gases to our life. 8.2: Greenhouse Gases Greenhouse gases are gases in an atmosphere that absorb and emit radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. Common greenhouse gases in the Earth's atmosphere include water vapor, carbon dioxide, methane, nitrous oxide, ozone, and chlorofluorocarbons. 46 In our solar system, the atmospheres of Venus, Mars and Titan also contain gases that cause greenhouse effects. Greenhouse gases, mainly water vapor, are essential to helping determine the temperature of the earth; without them this planet would likely be so cold as to be uninhabitable. Although many factors such as the sun and the water cycle are responsible for the Earth's weather and energy balance, if all else was held equal and stable, the planet's average temperature should be considerably lower without greenhouse gases. In the absence of the greenhouse effect and an atmosphere, the Earth's average surface temperature of 14 °C could be as low as −18 C. Could we survive at this very low temperature? How do we counter balance the greenhouse gases? We will come to this after learning the mechanism used by the earth to control his temperature. We have learned that the energy is coming from the sun. The Earth receives energy from the Sun mostly in the form of visible light. The atmosphere is almost transparent to visible light, so that about 50% of the sun's energy reaches the Earth and is absorbed by the surface. Like all bodies with a temperature above absolute zero the Earth's surface radiates energy in the infrared range. Greenhouse gases absorb infrared radiation and pass the absorbed heat to other atmospheric gases through molecular collisions. The greenhouse gases also radiate in the infrared range. Radiation is emitted both upward, with part escaping to space, and downward toward Earth's surface. The surface and lower atmosphere are warmed by the part of the energy that is radiated downward, making our life on earth possible. Earth's radiation balance is the equation of the incoming and outgoing thermal radiation which can be illustrated in terms of mathematical model. Though it is complicated and beyond the scope of this module, but it worth for understanding. That the incoming solar radiation is short wave, therefore the equation below is called the short wave radiation balance Qs: Qs = G - R = D + H - R or depending on the albedo (back-reflection to space): = G (1 - a) • G = global radiation • D = direct radiation • H = diffuse radiation • R = reflected portion of global radiation (ca. 4%) • a = albedo 47 The Earth's surface and atmosphere emits heat radiation (in the infrared spectrum). There is little overlap between this and the solar spectrum. Since this is long wave radiation, this formula also is known as the long wave radiation balance (Ql): Ql = AE = AO - AG • AE = effective radiation • AO = radiation of the Earth's surface • AG = trapped radiation (radiation forcing, also known as the so called greenhouse effect) From those two equations for incoming and outgoing radiation, the total amount of energy now can be calculated (total radiation balance (Qt), net radiation): Qt = Qs - Ql = G - R - AE The difficulty is to accurately quantify a range of internal and external factors influencing the radiation balance. Internal factors are all mechanisms affecting atmospheric composition (volcanism, biological activity, land use change, human activities etc.). The main external factor is solar radiation. It is interesting to note in this context that over its lifetime the sun's average luminosity to date has increased by ca. 25%. Human activities have an impact upon the levels of greenhouse gases in the atmosphere, which has other effects upon the system, with their own possible repercussions. The 2007 assessment report compiled by the IPCC observed that "changes in atmospheric concentrations of greenhouse gases and aerosols, land cover and solar radiation alter the energy balance of the climate system", and concluded that "increases in anthropogenic greenhouse gas concentrations is very likely to have caused most of the increases in global average temperatures since the mid-20th century" Recent increases in atmospheric carbon dioxide (CO2). Monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere. 48 The greenhouse effect was discovered by Joseph Fourier in 1824 and first investigated quantitatively by Svante Arrhenius in 1896. It is the process by which absorption and emission of infrared radiation by atmospheric gases warm a planet's lower atmosphere and surface. Existence of the greenhouse effect as such is not disputed even by those who do not agree that the modern temperature increase is attributable to human activity. The question is instead of the importance of the greenhouse gas human activity increases the atmospheric concentrations of greenhouse gases. Does this have problem? Practice Question: List all examples of gases considered to be green house gases. Do we consider O3 to be one of green house gas? Explain. Class Activity The external and internal factors are closely interconnected in disturbing the earth radiation balance. Discuss this statement by giving vivid examples. References 1. Fredlund, D.G.; Rahardjo, H. (1993), Soil Mechanics for Unsaturated Soils. WileyInterscience. 2. Thornthwaite, C. W. (1948), An Approach Toward a Rational Classification of Climate, Geographical Review, 38:55-94. 49 9: 9.1: LECTURE NINE The Consequences of Green House Gases Imbalance Introduction Sometimes we will use the word "greenhouse effect". However the term can be a source of confusion as actual greenhouses do not function by the same mechanism the atmosphere does. Various materials at times imply incorrectly that they do, or do not make the distinction between the processes of radiation and convection. By definition from The American Heritage Dictionary, indicated this as the phenomenon whereby the earth’s atmosphere traps solar radiations, caused by the presence in the atmosphere gases such as carbon dioxide, water vapor, and methane that allow incoming sun light to pass through but absorb heat radiated back from the earth’s surface. The term 'greenhouse effect' originally came from the greenhouses used for gardening, but as mentioned the mechanism for greenhouses operates differently. Many sources make the "heat trapping" analogy of how a greenhouse limits convection to how the atmosphere performs a similar function through the different mechanism of infrared absorbing gases. From this definition we can now discuss the consequences of disturbing the natural balance of these gases. One of the biggest problem is what it causes the global warming. 9.2: Global Warming Naturally occurring greenhouse gases have a mean warming effect of about 33 °C, without which Earth would be uninhabitable. The major greenhouse gases are water vapor, which causes about 36– 70 percent of the greenhouse effect (not including clouds); carbon dioxide (CO2), which causes 9– 26 percent; methane (CH4), which causes 4–9 percent; and ozone, which causes 3–7 percent. We have discussed earlier that human activity is one of the internal factor which increase the levels of the green house gases. The question is what is the problem? We know the bigger the blanket the efficiency is the protection. Let us answer this question in detail. 50 While the greenhouse effect is an essential environmental prerequisite for life on Earth, there really can be too much of a good thing. The problems begin when human activities distort and accelerate the natural process by creating more greenhouse gases in the atmosphere than are necessary to warm the planet to an ideal temperature. The following are some of the human activities which accelerate levels of these green house gases: • Burning natural gas, coal and oil —including gasoline for automobile engines—raises the level of carbon dioxide in the atmosphere. • Some farming practices and land-use changes increase the levels of methane and nitrous oxide. Taking practices of burning bushes during farming at your home. • Many factories produce long-lasting industrial gases that do not occur naturally, yet contribute significantly to the enhanced greenhouse effect and “global warming” that is currently under way. • Deforestation also contributes to global warming. Trees use carbon dioxide and give off oxygen in its place, which helps to create the optimal balance of gases in the atmosphere. As more forests are logged for timber or cut down to make way for farming, however, there are fewer trees to perform this critical function. • Population growth is another factor in global warming, because as more people use fossil fuels for heat, transportation and manufacturing the level of greenhouse gases continues to increase. As more farming occurs to feed millions of new people, more greenhouse gases enter the atmosphere. Therefore, more greenhouse gases mean more infrared radiation trapped and held by the green house gas, which gradually increases the temperature of the Earth’s surface and the air in the lower atmosphere. We should know that since the industrial revolution, there is an increased in the amount of greenhouse gases in the atmosphere like CO2, methane, tropospheric ozone, CFCs and nitrous oxide. Consider the number of cars in your city compared to ten years ago. Compare number of industries in your country compared to those we were having ten years ago. Figure out the increase in the concentration of gases like CO2. The concentrations of CO2 and methane have increased by 36% and 148% respectively since the mid-1700s. These levels are considerably higher than at any 51 time during the last 650,000 years, the period for which reliable data has been extracted from ice cores. Less direct geological evidence indicates that CO2 values this high were last seen approximately 20 million years ago. Fossil fuel burning has produced approximately three-quarters of the increase in CO2 from human activity over the past 20 years. Most of the rest is due to landuse change, in particular deforestation. The CO2 concentrations are continuing to rise due to burning of fossil fuels and land-use change. The future rate of rise will depend on uncertain economic, sociological, technological, and natural developments. The IPCC Special Report on Emissions Scenarios gives a wide range of future CO2 scenarios, ranging from 541 to 970 ppm by the year 2100. Global warming is the increase in the average temperature of the Earth's near-surface air and oceans since the mid-twentieth century and its projected continuation. Global surface temperature increased 0.74 ± 0.18 °C during the last century. Great scientists believe that greenhouse gases are responsible for most of the observed temperature increase since the middle of the twentieth century, and that natural phenomena such as solar variation and volcanoes probably had a small warming effect from pre-industrial times to 1950 and a small cooling effect afterward. These basic conclusions have been endorsed by more than 40 scientific societies and academies of science, including all of the national academies of science of the major industrialized countries. Climate model projections summarized in the latest IPCC report indicate that global surface temperature will probably rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F) during the twenty-first century. The uncertainty in this estimate arises from the use of models with differing climate sensitivity, and the use of differing estimates of future greenhouse gas emissions. Some other uncertainties include how warming and related changes will vary from region to region around the globe. Most studies focus on the period up to 2100. However, warming is expected to continue beyond 2100, even if emissions stop, because of the large heat capacity of the oceans and the long lifetime of carbon dioxide in the atmosphere. Increasing global temperature will cause sea levels to rise and will change the amount and pattern of precipitation, probably including expansion of subtropical deserts. The continuing retreat of glaciers consider mountain Kilimanjaro, permafrost and sea ice is expected, with the Arctic region being particularly affected. Other likely effects include shrinkage of the Amazon rainforest and Boreal 52 forests, increases in the intensity of extreme weather events, species extinctions and changes in agricultural yields. Study Questions 1. If the temperature of the earth continue to increase what will happen to island of Zanzibar and Mafia? 2. Discuss in detail the effect of global warming on agricultural sector. 3. Does global warming has anything to do with increase in population in the city like Dar es Salaam? Discuss. References 1. http://environment.about.com/od/globalwarming/a/greenhouse.htm 2. The American Heritage dictionary of the English language, 4th ed, Houghton Mifflin Company. 3. http://en.wikipedia.org/wiki/Joseph_Fourier 53 10: LECTURE TEN 10.1: How Does the Earth Balance the Radiations Introduction The radiation equilibrium of the Earth system is an accounting of the received and leaving components of radiation. These components are balanced over long time periods and over the Earth as whole. If they weren't the Earth would be continually cooling or warming. However, over a short period of time, radiant energy is unequally distributed over the Earth. Sunlight, in the broad sense, is the total spectrum of the electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the atmosphere, and the solar radiation is clear as daylight when the Sun is above the horizon. When the direct radiation is not blocked by clouds, it is experienced as sunshine, a mixture of bright light and heat. Radiant heat directly produced by the radiation of the sun is different from the increase in atmospheric temperature due to the radioactive heating of the atmosphere by the sun's radiation. The World Meteorological Organization defines sunshine as direct irradiance from the Sun measured on the ground of at least 120 W·m−2. 10.2: Radiation Balance Direct sunlight has a luminous efficiency of about 93 lumens per watt of radiant flux, which includes infrared, visible, and ultra-violet light. Sunlight is a key factor in the process of photosynthesis, crucially important for life on Earth. The Earth receives energy from the Sun in the form of radiation. Most of the energy is in visible wavelengths and in infrared wavelengths that are near the visible range (often called "near infrared"). The Earth reflects about 30% of the incoming solar radiation. The remaining 70% is absorbed, warming the land, atmosphere and ocean. For the Earth's temperature to be in steady state so that the Earth does not rapidly heat or cool, this absorbed solar radiation must be very closely balanced by energy radiated back to space in the infrared wavelengths. Since the intensity of infrared radiation increases with increasing temperature, 54 one can think of the Earth's temperature as being determined by the infrared flux needed to balance the absorbed solar flux. The visible solar radiation mostly heats the surface, not the atmosphere, whereas most of the infrared radiation escaping to space is emitted from the upper atmosphere, not the surface. The infrared photons emitted by the surface are mostly absorbed in the atmosphere by greenhouse gases and clouds and do not escape directly to space. The reason this warms the surface is most easily understood by starting with a simplified model of a purely radiative greenhouse effect that ignores energy transfer in the atmosphere by convection (sensible heat transport, Sensible heat flux) and by the evaporation and condensation of water vapor (latent heat transport, Latent heat flux). In this purely radiative case, one can think of the atmosphere as emitting infrared radiation both upwards and downwards. The upward infrared flux emitted by the surface must balance not only the absorbed solar flux but also this downward infrared flux emitted by the atmosphere. The surface temperature will rise until it generates thermal radiation equivalent to the sum of the incoming solar and infrared radiation. A more realistic picture taking into account the convective and latent heat fluxes is somewhat more complex. But the following simple model captures the essence. The starting point is to note that the opacity of the atmosphere to infrared radiation determines the height in the atmosphere from which most of the photons are emitted into space. If the atmosphere is more opaque, the typical photon escaping to space will be emitted from higher in the atmosphere, because one then has to go to higher altitudes to see out to space in the infrared. Since the emission of infrared radiation is a function of temperature, it is the temperature of the atmosphere at this emission level that is effectively determined by the requirement that the emitted flux balance the absorbed solar flux. But the temperature of the atmosphere generally decreases with height above the surface, at a rate of roughly 6.5 °C per kilometer on average, until one reaches the stratosphere 10–15 km above the surface. (Most infrared photons escaping to space are emitted by the troposphere, the region bounded by the surface and the stratosphere, so we can ignore the stratosphere in this simple picture.). A very simple model, but one that proves to be remarkably useful, involves the assumption that this temperature profile is simply fixed, by the non-radiative energy fluxes. Given the temperature at the emission level of the infrared flux escaping to space, one then computes the surface temperature by increasing temperature at the rate of 6.5 °C per kilometer, the environmental lapse rate, until one reaches the surface. The more opaque the atmosphere, and the higher the emission level of the escaping infrared radiation, the warmer the surface, since one then needs to 55 follow this lapse rate over a larger distance in the vertical. While less intuitive than the purely radiative greenhouse effect, this less familiar radiative-convective picture is the starting point for most discussions of the greenhouse effect in the climate modeling literature. In order to have a picture of what happen in the atmosphere and earth surface we can summarize the process using the following picture. SUN 1 9 5 4 8 7 2 3 10 6 EARTH Fig. 8: Radiation Balance Let us first have a look what happens to the sunlight. (1) The sun is the source of all radiation and energy coming to the Earth from the space. (2) A part of the sunlight reaches the Earth surface and all its different landscapes: forests, oceans, deserts, savannah, cities, ice and snow (3) The Earth's surface does not take up all the sunlight, but sends a certain part of it directly back (reflection). In particular very bright surfaces like ice and snow are excellent reflectors. (4) Reflection does not only occur at the Earth surface. Some light is already sent back from the top side of the clouds and smaller amounts by aerosols. (5) Also the uptake of the light (we call it absorption) does not only take place at the Earth surface. Also gas molecules and particles in the air absorb some sunlight. The portion of the 56 sunlight reaching the Earth warms up its surface. The Earth sends this warmth back as infrared radiation. (6) The Earth's surface warmed up by the sun is a source of heat radiation (long wave infrared radiation) (7) A bit of the energy is needed to evaporate water. As you know from kettles, you need energy to bring water from the liquid phase (e.g. oceans) to the vapour phase (water vapour in the air). (8) Some infrared radiation goes directly back to the space. (9) Clouds do not only reflect sunlight, they also absorb and re-emitted infrared radiation back to the Earth. A cloudy sky keeps the Earth warmer, like a blanket. (10) Finally there are particles and gases in the air absorbing the infrared radiation. These gases are called greenhouse gases. They keep the energy of this heat radiation near the ground. All this interaction of light with the atmosphere and the surface happens in our climate system and we have to take them into account, in order to understand the climate. But why do we call it a greenhouse effect? Study Questions 1. How do greenhouse gases influence earth's surface temperature? 2. The heat-trapping ability of a greenhouse is influenced by a number of factors including the transparency of the greenhouse cover, color of the surfaces inside the greenhouse, and type of surfaces inside. Discuss in detail with examples. Refer ref. 2 below. 3. What Factors Impact a greenhouse? Refer ref. 3 below. References 1. http://oceanworld.tamu.edu/resources/oceanography-book/radiationbalance.htm 2. http://www.ucar.edu/learn/1_3_1.htm 3. http://www.ucar.edu/learn/1_3_2_13t.htm 57 11: LECTURE ELEVEN 11.1: Components of the Atmosphere: Clouds and Aerosols Introduction Let us start defining these two terms. The word cloud according to American heritage dictionary is a visible body of very fine water droplets or ice particles suspended in the atmosphere at attitude ranging up to several miles above sea level. Therefore a cloud is a visible mass of droplets or frozen crystals suspended in the atmosphere above the surface of the Earth. A cloud is also a visible mass attracted by gravity, such as masses of material in space called interstellar clouds and nebulae. Clouds are studied in the nephology or cloud physics branch of meteorology. On Earth the condensing substance is normally water vapor, which forms small droplets or ice crystals, typically 0.01 mm in diameter. When surrounded by billions of other droplets or crystals they become visible as clouds. Dense deep clouds display a high reflectance (70% to 95%) throughout the visible range of wavelengths. They thus appear white, at least from the top. Cloud droplets tend to scatter light efficiently, so that the intensity of the solar radiation decreases with depth into the gases, hence the gray or even sometimes dark appearance at the base. Thin clouds may appear to have acquired the color of their environment or background and clouds illuminated by non-white light, such as during sunrise or sunset, may appear colored accordingly. In the nearinfrared range, clouds look darker because the water that constitutes the cloud droplets strongly absorbs solar radiation at those wavelengths. Fig. 9: Picture of the Clouds 58 On the other hand aerosol is different from the clouds. According to American heritage dictionary aerosol is a gaseous suspension of fine solid or liquid particles. Technically, an aerosol is a suspension of fine solid particles or liquid droplets in a gas. Examples are smoke, oceanic haze, air pollution, smog and CS gas. In general discussion, "aerosol" usually refers to an aerosol spray can or the output of such a can. The word aerosol derives from the fact that matter "floating" in air is a suspension (a mixture in which solid or liquid or combined solid-liquid particles are suspended in a fluid). To differentiate suspensions from true solutions, the term sol evolved—originally meant to cover dispersions of tiny (sub-microscopic) particles in a liquid. With studies of dispersions in air, the term aerosol evolved and now embraces both liquid droplets, solid particles, and combinations of these. Anthropogenic aerosols, particularly sulfate aerosols from fossil fuel combustion, exert a cooling influence on the climate which partly counteracts the warming induced by greenhouse gases such as carbon dioxide. This effect is accounted for in many climate models. Recent research, as yet unconfirmed, suggests that aerosol diffusion of light may have increased the carbon sink in the earth's ecosystem. The question which environmentalists ask several times is what is the effect of these natural things to our surroundings? We will go though on this so that we can get picture if they can affect our environment. 11.2: Aerosols and Clouds 11.2.1: Clouds We have learned that cloud is a visible mass of droplets or frozen crystals floating in the atmosphere above the surface of the Earth or another planetary body. A cloud is also a visible mass attracted by gravity, such as masses of material in space called interstellar clouds and nebulae. On Earth the condensing substance is typically water vapor, which forms small droplets or ice crystals, typically 0.01 mm in diameter. When surrounded by billions of other droplets or crystals they become visible as clouds. Dense deep clouds exhibit a high reflectance (70% to 95%) throughout the visible range of wavelengths. They thus appear white, at least from the top. Cloud droplets tend to scatter light efficiently, so that the intensity of the solar radiation decreases with depth into the gases, hence the gray or even sometimes dark appearance at the base. Thin clouds may appear to have acquired the colour of their environment or background and clouds illuminated by non-white light, such as during sunrise or sunset, may appear coloured accordingly. In the near-infrared range, clouds look 59 darker because the water that constitutes the cloud droplets strongly absorbs solar radiation at those wavelengths. The colour of a cloud, as seen from the Earth, tells much about what is going on inside the cloud. Clouds form when water vapor is light enough to rise due to becoming warmer than its surrounding. As it rises it cools and the vapor condenses out of the air as micro-droplets. These tiny particles of water are densely packed and sunlight cannot penetrate far into the cloud before it is reflected out, giving a cloud its characteristic white colour. As a cloud matures, the droplets may combine to produce larger droplets, which may combine to form droplets large enough to fall as rain. By this process of accumulation, the space between droplets becomes increasingly larger, permitting light to penetrate farther into the cloud. If the cloud is sufficiently large and the droplets within are spaced far enough apart, it may be that a percentage of the light which enters the cloud is not reflected back out before it is absorbed. A simple example of this is being able to see farther in heavy rain than in heavy fog. This process of reflection/absorption is what causes the range of cloud colour from white to black. For the same reason, the undersides of large clouds and heavy overcasts can appear as various degrees of grey shades, depending on how much light is being reflected or transmitted back to the observer. Other colours occur naturally in clouds. Bluish-grey is the result of light scattering within the cloud. In the visible spectrum, blue and green are at the short end of light's visible wavelengths, while red and yellow are at the long end. The short rays are more easily scattered by water droplets, and the long rays are more likely to be absorbed. The bluish colour is evidence that such scattering is being produced by rain-sized droplets in the cloud. A greenish tinge to a cloud is produced when sunlight is scattered by ice. A cumulonimbus cloud emitting green is an imminent sign of heavy rain, hail, strong winds and possible tornadoes. Yellowish clouds are rare but may occur in the late spring through early fall months during forest fire season. The yellow colour is due to the presence of smoke. Red, orange and pink clouds occur almost entirely at sunrise/sunset and are the result of the scattering of sunlight by the atmosphere. The clouds do not become that colour; they are reflecting long and unscattered rays of sunlight, which are predominant at those hours. The effect is much like if one were to shine a red spotlight on a white sheet. In combination with large, mature thunderheads this can produce blood-red clouds. 60 11.2.2: Aerosols We can still define aerosols as particulates, alternatively referred to as particulate matter or fine particles, and are tiny particles of solid or liquid suspended in a gas or liquid. That means from this definition aerosol refers to particles and the gas together. Sources of particulate matter can be man made or natural. In water pollution, particulates can either be in a solid or dissolved state. Solid particulates can be removed by filters or settle from the water, and is referred to as insoluble particulate matter. Whereas, dissolved particulate matter in water is collected by allowing the water to evaporate, leaving behind the dissolved particulate matter. Salt is an example of dissolved particulate matter. Some particulates occur naturally, originating from volcanoes, dust storms, forest and grassland fires, living vegetation, and sea spray. Human activities, such as the burning of fossil fuels in vehicles, power plants and various industrial processes also generate considerable amounts of aerosols. The term anthropogenic aerosols refer to those made by human activities and currently account for about 10 percent of the total amount of aerosols in our atmosphere. 11.3: The importance of Clouds and Aerosols to Climate Change Everything, from an individual person to Earth as a whole, emits energy. Scientists refer to this energy as radiation. As Earth absorbs incoming sunlight, it warms up. The planet must emit some of this warmth into space or increase in temperature. Two components make up the Earth's outgoing energy: heat (or thermal radiation) that the Earth's surface and atmosphere emit; and sunlight (or solar radiation) that the land, ocean, clouds and aerosols reflect back to space. The balance between incoming sunlight and outgoing energy determines the planet's temperature and, ultimately, climate. Both natural and human-induced changes affect this balance, called the Earth's radiation budget. 61 Fig. 10: Earth's Radiation Balance Between Incoming and Outgoing Radiation (Adopted from http://earthobservatory.nasa.gov/Features/CALIPSO/CALIPSO2.php) Clouds affect the radiation budget directly by reflecting sunlight into space (cooling the Earth) or absorbing sunlight and heat emitted by the Earth. When clouds absorb sunlight and heat, less energy escapes to space and the planet warms. To understand how clouds impact the energy budget, environmentalists need to know the composition of cloud particles, the altitude of clouds and the extent to which clouds at different altitudes overlap each other. Both natural processes and human activities produce aerosols. They either reflect or absorb energy, depending on their size, chemical composition and altitude. The haze layer that is commonly seen in the summertime is one example of an aerosol that primarily reflects sunlight. Soot emitted by diesel engines is an example of an aerosol that absorbs sunlight. The reflection and absorption of energy by aerosols act in a direct way to change the balance between incoming and outgoing energy. These effects are called direct aerosol radiative forcing. Aerosols also can affect the Earth's radiation budget indirectly by modifying the characteristics of clouds. Cloud particles almost always form around aerosols such as natural sea salt particles or human-made sulfate particles. The presence of additional aerosols can change the way clouds 62 radiate energy and the length of time they stay intact. A good example is the way that exhaust particles emitted into the atmosphere by ships can increase the brightness of clouds along their course. These effects are called indirect aerosol radiative forcing. 11.4: Healthy Hazards of the Aerosols The health effect of the particles is mainly depending on the size of the particle. The range is the one determine the effect. The size of the particle is a main determinant of where in the respiratory tract the particle will come to rest when inhaled. Common particles are airborne pollutants that fall between 2.5 and 10 micrometers in diameter. The effects of inhaling particulate matter have been widely studied in humans and animals and include asthma, lung cancer, cardiovascular issues, and premature death. Because of the size of the particle, they can penetrate the deepest part of the lungs. Larger particles are generally filtered in the nose and throat and do not cause problems, but particulate matter smaller than about 10 micrometers, referred to as PM10, can settle in the bronchi and lungs and cause health problems. The 10 micrometer size does not represent a strict boundary between respirable and non-respirable particles, but has been agreed upon for monitoring of airborne particulate matter by most regulatory agencies. Similarly, particles smaller than 2.5 micrometers, PM2.5, tend to penetrate into the gas-exchange regions of the lung, and very small particles (< 100 nanometers) may pass through the lungs to affect other organs. In particular, a study published in the Journal of the American Medical Association indicates that PM2.5 leads to high plaque deposits in arteries, causing vascular inflammation and atherosclerosis a hardening of the arteries that reduces elasticity, which can lead to heart attacks and other cardiovascular problems. Researchers suggest that even short-term exposure at elevated concentrations could significantly contribute to heart disease. 63 Study Question and Model Answer: What is the origin and effect of fossil fuel in the environment Answer: Fossil fuels are formed by the anaerobic decomposition of remains of organisms including phytoplankton and zooplankton that settled to the sea (or lake) bottom in large quantities under anoxic conditions, millions of years ago. Over geological time, this organic matter, mixed with mud, got buried under heavy layers of sediment. The resulting high levels of heat and pressure caused the organic matter to chemically alter, first into a waxy material known as kerogen which is found in oil shales, and then with more heat into liquid and gaseous hydrocarbons in a process known as catagenesis. Terrestrial plants, on the other hand, tend to form coal. Many of the coal fields date to the Carboniferous period of Earth's history. According to data, the United States, more than 90% of greenhouse gas emissions come from the combustion of fossil fuels. Combustion of fossil fuels also produces other air pollutants, such as nitrogen oxides, sulfur dioxide, volatile organic compounds and heavy metals like Pb, Hg, Cd, Cu just to mention few . Nitrogen oxides and sulphur dioxide emissions, which contribute to smog and acid rain and the formation of fine particulate matter. It is the largest uncontrolled industrial source of mercury emissions in parts of developed countries like Canada and USA. Fossil fuel-fired electric power plants also emit carbon dioxide, which may contribute to climate change. In addition, the sector has significant impacts on water and habitat and species. In particular, hydro-dams and transmission lines have significant effects on water and biodiversity. Fossil fuels also contain radioactive materials, mainly uranium and thorium, which are released into the atmosphere. In 2000, about 12,000 metric tons of thorium and 5,000 metric tons of uranium were released worldwide from burning coal. Study Question When the water evaporates into the sky the water particles form the cloud. There are 10 types of clouds in total however, there are only three main types, and they are cirrus, stratus, and cumulus. Discuss. References 1. Hinds, W. C., Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles, Wiley-Interscience 2. Lave, L. B.; Eugene P. S. (1973), An Analysis of the Association Between U.S. Mortality and Air Pollution, J. Amer. Statistical Association 68: 342. 3. Pope, C. A. (2002), Cancer, cardiopulmonary mortality, and long-term exposure to fine particulate air pollution, J. Amer. Med. Assoc. 287: 1132–1141. 64 12: LECTURE TWELVE 12.1: Air Pollution and Management Introduction We have just learned from the last lecture that particulate matter has a problem that researches shows that about 1.9% increased risk of dying from cardiovascular disease. Taking example, what will happen to those people living closer to the rough roads? In Tanzania what can you say about those people live closer to cement industry like Twiga cement industry or Wazo hill? All these contribute to Air pollution. Air pollution is the introduction of chemicals, particulate matter, or biological materials that cause harm or discomfort to humans or other living organisms, or damages the natural environment, into the atmosphere. Let us have a look on the picture of cement industry in Texas USA producing dust storm. The same can be observed in our cement industries. Fig. 11: Air Pollution (Adopted from http://en.wikipedia.org/wiki/File 28/04/2009) Apart from industries, there are other sources of air pollution. We will discuss few of air pollutants them and highlight the environmental effect to this pollutants. 12.2: Air Pollutants An air pollutant is known as a substance in the air that can cause harm to humans and the environment. Pollutants can be in the form of solid particles, liquid droplets, or gases. In addition, they may be natural or man-made. Pollutants can be classified as either primary or secondary. Usually, primary pollutants are substances directly emitted from a process, such as ash from a 65 volcanic eruption, the carbon monoxide gas from a motor vehicle exhaust or sulfur dioxide released from factories. Secondary pollutants are not emitted directly. Rather, they form in the air when primary pollutants react or interact. An important example of a secondary pollutant is ground level ozone - one of the many secondary pollutants that make up photochemical smog. Note that some pollutants may be both primary and secondary: that is, they are both emitted directly and formed from other primary pollutants. 12.2.1: Air Major Primary Pollutants (a) Sulfur oxides (SOx) - Especially sulfur dioxide, a chemical compound with the formula SO2. Sulfur dioxide is produced by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide. Further oxidation of SO2, usually in the presence of a catalyst such as NO2, forms H2SO4, and thus acid rain. This is one of the causes for concern over the environmental impact of the use of these fuels as power sources. Sulfur dioxide is a colorless, non-flammable gas that is heavier than air. It has a strong, suffocating odor that most people can readily identify as one of rotting eggs. It is a liquid when under pressure, and dissolves in water very easily. Over 65% of this gas is released in the air, more than 13 million tons per year, comes from electric utilities, especially those that burn coal. On a global basis, fossil fuel combustion accounts for 75 to 85% of manmade of sulfur dioxide emissions, and industrial processes such as refining and smelting account for the remainder. As a gas, sulfur dioxide major route of toxicity is inhalation. However, it is also a strong irritant to the eyes and moist skin where the gas combines with water to form both sulfuric and sulfurous acids. Victims with chronic pulmonary disease, particularly asthma, have been shown to be much more sensitive to the gas at lower concentrations. Children, too, may be at higher risk because of their greater lung surface area to body weight ratio, increased minute volume to weight ratio, and shorter structure. (b) Nitrogen oxides (NOx) - Especially nitrogen dioxide are emitted from high temperature combustion. Nitrogen dioxide is the chemical compound with the formula NO2. It is one of the 66 several nitrogen oxides. This reddish-brown toxic gas has a characteristic sharp, biting odor. NO2 is one of the most prominent air pollutants. The toxicity of nitrous oxide (N2O) or laughing gas, which is used as an anesthetic, is different from that of the other nitrogen oxides. Children exposed to the same levels of nitrogen oxides as adults may receive larger doses because they have greater lung surface area:body weight ratios and increased minute volumes: weight ratios. In addition, they may be exposed to higher levels of nitrogen dioxide than adults in the same location because of their short figure and the higher levels of nitrogen dioxide found nearer to the ground. Exposure to relatively high air concentrations can produce eye irritation and inflammation. (c) Carbon monoxide - is a colourless, odourless, non-irritating but very poisonous gas. It is a product by incomplete combustion of fuel such as natural gas, coal or wood. Vehicular exhaust is a major source of carbon monoxide. (d) Carbon dioxide (CO2) - a greenhouse gas emitted from combustion but is also a gas very important to living organisms. It is a natural gas in the atmosphere. (e) Volatile organic compounds - VOCs are an important outdoor air pollutant. In this field they are often divided into the separate categories of methane (CH4) and non-methane (NMVOCs). Methane is an extremely efficient greenhouse gas which contributes to enhance global warming. Other hydrocarbon VOCs are also significant greenhouse gases via their role in creating ozone and in prolonging the life of methane in the atmosphere, although the effect varies depending on local air quality. Within the NMVOCs, the aromatic compounds benzene, toluene and xylene are suspected carcinogens and may lead to leukemia through prolonged exposure. 1,3-butadiene is another dangerous compound which is often associated with industrial uses. (f) Particulate matter - Particulates, alternatively referred to as particulate matter (PM) or fine particles, are tiny particles of solid or liquid suspended in a gas. In contrast, aerosol refers to particles and the gas together. Sources of particulate matter can be man made or natural. Some particulates occur naturally, originating from volcanoes, dust storms, forest and grassland fires, living vegetation, and sea spray. Human activities, such as the burning of fossil fuels in vehicles, power plants and various industrial processes also generate significant amounts of aerosols. Averaged over the globe, anthropogenic aerosols—those made by human activities—currently account for about 10 percent of the total amount of aerosols in our atmosphere. Increased levels 67 of fine particles in the air are linked to health hazards such as heart disease, altered lung function and lung cancer. (f) Toxic metals, such as lead, cadmium and copper. (g) Chlorofluorocarbons (CFCs) - harmful to the ozone layer emitted from products currently banned from use. (h) Ammonia (NH3) - emitted from agricultural processes. Ammonia is a compound with the formula NH3. It is normally encountered as a gas with a characteristic pungent odor. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to foodstuffs and fertilizers. Ammonia, either directly or indirectly, is also a building block for the synthesis of many pharmaceuticals. Although in wide use, ammonia is both caustic and hazardous. (i) Odors - such as from garbage, sewage, and industrial processes (j) Radioactive pollutants - produced by nuclear explosions, war explosives, and natural processes such as the radioactive decay of radon. 12.2.2: Air Major Secondary Pollutants (a) Particulate matter formed from gaseous primary pollutants and compounds in photochemical smog. Smog is a kind of air pollution; the word "smog" is a portmanteau of smoke and fog. Classic smog results from large amounts of coal burning in an area caused by a mixture of smoke and sulfur dioxide. Modern smog does not usually come from coal but from vehicular and industrial emissions that are acted on in the atmosphere by sunlight to form secondary pollutants that also combine with the primary emissions to form photochemical smog. A good example is fossil fuels or mineral fuels. These are fuels formed by the anaerobic decomposition of buried dead organisms that lived up to 300 million years ago. These fuels contain high percentage of carbon and hydrocarbons. Fossil fuels range from volatile materials with low carbon:hydrogen ratios like methane, to liquid petroleum to nonvolatile materials composed of almost pure carbon, like anthracite coal. Methane can be found in hydrocarbon fields, alone, associated with oil, or in the form of methane clathrates. It is generally accepted that they formed from the fossilized remains of dead plants and animals by exposure to heat and pressure in the Earth's crust over hundreds of millions of years. 68 Ground level ozone (O3) formed from NOx and VOCs. Ozone (O3) is a key constituent of the troposphere (it is also an important constituent of certain regions of the stratosphere commonly known as the Ozone layer). Photochemical and chemical reactions involving it drive many of the chemical processes that occur in the atmosphere by day and by night. At abnormally high concentrations brought about by human activities (largely the combustion of fossil fuel), it is a pollutant, and a constituent of smog. Peroxyacetyl nitrate (PAN) - similarly formed from NOx and VOCs. Peroxyacyl nitrates, or PANs, are powerful respiratory and eye irritants present in photochemical smog. They are formed from a peroxyacyl radical and nitrogen dioxide, for example peroxyacetyl nitrate, CH3COOONO2: Hydrocarbons + O2 + NO2 + light → CH3COOONO2 The general equation is; CxHyO3 + NO2 → CxHyO3NO2 PANs are both toxic and irritating, as they dissolve more readily in water than ozone. They are lachrymators, causing eye irritation at concentrations of only a few parts per billion. At higher concentrations they cause extensive damage to vegetation. Both PANs and their chlorinated derivates are said to be mutagenic, as they can be a factor causing skin cancer. PANs are secondary pollutants, which mean they are not directly emitted as exhaust from power plants or internal combustion engines, but they are formed from other pollutants by chemical reactions in the atmosphere. Free radical reactions catalyzed by ultraviolet light from the sun oxidize unburned hydrocarbons to aldehydes, ketones, and dicarbonyl compounds, whose secondary reactions create peroxyacyl radicals, which combine with nitrogen dioxide to form peroxyacyl nitrates. The most common peroxyacyl radical is peroxyacetyl, which can be formed from the free radical oxidation of acetaldehyde, various ketones, or the photolysis of dicarbonyl compounds such as methylglyoxal or diacetyl. Since they dissociate quite slowly in the atmosphere into radicals and NO2, PANs are able to transport these unstable compounds far away from the urban and industrial origin. This is important for tropospheric ozone production as PANs transport NOx to regions where it can more efficiently produce ozone. 69 12.2.3: (a) Air Minor pollutants A large number of minor hazardous air pollutants. Some of these are regulated in USA under the Clean Air Act and in Europe under the Air Framework Directive. (b) A variety of persistent organic pollutants, which can attach to particulate matter. Persistent organic pollutants (POPs) are organic compounds that are resistant to environmental degradation through chemical, biological, and photolytic processes. Because of this, they have been observed to persist in the environment, to be capable of long-range transport, bioaccumulate in human and animal tissue, biomagnify in food chains, and to have potential significant impacts on human health and the environment. Example of compounds that make up POPs are also classed as PBTs (Persistent, Bioaccumulative and Toxic) or TOMPs (Toxic Organic Micro Pollutants). Many POPs are currently or were in the past used as pesticides. Others are used in industrial processes and in the production of a range of goods such as solvents, polyvinyl chloride, and pharmaceuticals. Though there are a few natural sources of POPs, most POPs are created by humans in industrial processes, either intentionally or as byproducts. Exposure to POPs can take place through diet, environmental exposure, or accidents. POP exposure can cause death and illnesses including disruption of the endocrine, reproductive, and immune systems; neurobehavioral disorders; and cancers possibly including breast cancer. The lipid solubility of POPs allows them to bioaccumulate in fatty tissues of animals. Many of the first generation organochlorine insecticides such as DDT were particularly noted for this characteristic. Study Question (i) Carbon monoxide (CO) is one of the known highly toxic gas. Discuss the sources and the biological toxicity of this gas. (ii) Draw the general formula of peroxyacetyl nitrate. References: 1 http://www.atsdr.cdc.gov/mhmi/mmg175.html 2 http://209.85.229.132/custom?q=cache:t5NPaOyWlMQJ:www.bioterrorism.slu.edu/bt/products/ahec_chem/scripts/ SulfurDioxide.pdf+toxicity+of+sulfur+dioxide&cd=1&hl=en&ct=clnk&client=pub 70 13: LECTURE THIRTEEN 13.1: Water Pollution and Management Introduction Water pollution is a major problem in the global context. It has been suggested that it is the leading worldwide cause of deaths and diseases, and that it accounts for the deaths of more than 14,000 people daily. In addition to the acute problems of water pollution in developing countries like Tanzania or industrialized countries continue to struggle with pollution problems as well. In the most recent national report on water quality in the Tanzania shows that the country is blessed by having big fresh water lakes and rivers although these water sources are not evenly distributed. There are areas therefore where water is scarce while in other areas there is enough water. Environmental degradation due to water pollution and poor land management affects water source availability, suitability and sustainability. Water is typically referred to as polluted when it is impaired by anthropogenic contaminants and either does not support a human use, like serving as drinking water, and/or undergoes a marked shift in its ability to support its constituent biotic communities, such as fish. Natural phenomena such as volcanoes, algae blooms, storms, and earthquakes also cause major changes in water quality and the ecological status of water. Water pollution has many causes and characteristics. 13.2: Water in the Atmosphere Water vapour also called aqueous vapour is the gas phase of water. Water vapor is one state of the water cycle within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under normal atmospheric conditions, water vapor is continuously generated by evaporation and removed by condensation. Whenever a water molecule leaves a surface, it is said to have evaporated. Each individual water molecule which transitions between a more associated (liquid) and a less associated (vapor/gas) state does so through the absorption or release of kinetic energy. The aggregate measurement of this kinetic energy transfer is defined as thermal energy and occurs only when there is differential in the temperature of the water molecules. Liquid water that becomes water vapor takes a parcel of heat 71 with it, in a process called evaporative cooling. The amount of water vapor in the air determines how fast each molecule will return back to the surface. When a net evaporation occurs, the body of water will under go a net cooling directly related to the loss of water. Evaporative cooling is restricted by atmospheric conditions. Humidity is the amount of water vapor in the air. The vapor content of air is measured with devices known as hygrometers. The measurements are usually expressed as specific humidity or percent relative humidity. The temperatures of the atmosphere and the water surface determine the equilibrium vapor pressure; 100% relative humidity occurs when the partial pressure of water vapor is equal to the equilibrium vapor pressure. This condition is often referred to as complete saturation. Humidity ranges from 0 gram per cubic metre in dry air to 30 grams per cubic metre when the vapor is saturated at 30 °C. Another form of evaporation is sublimation, by which water molecules become gaseous directly from ice without first becoming liquid water. Sublimation accounts for the slow mid-winter disappearance of ice and snow at temperatures too low to cause melting. Water vapor will only condense onto another surface when that surface is cooler than the temperature of the water vapor, or when the water vapor equilibrium in air has been exceeded. When water vapor condenses onto a surface, a net warming occurs on that surface. The water molecule brings a parcel of heat with it. In turn, the temperature of the atmosphere drops slightly. In the atmosphere, condensation produces clouds, fog and precipitation (usually only when facilitated by cloud condensation nuclei). The dew point of an air parcel is the temperature to which it must cool before water vapor in the air begins to condense. Also, a net condensation of water vapor occurs on surfaces when the temperature of the surface is at or below the dew point temperature of the atmosphere. Deposition, the direct formation of ice from water vapor, is a type of condensation. Frost and snow are examples of deposition. 13.2: Physical Properties of Water in the Atmosphere The molecular mass or weight of water is 18.02g/mol, as calculated from the sum of the atomic masses of its constituent atoms. The average molecular mass of air (Approx. 79% nitrogen, N2; 21% Oxygen, 02) is 28.57g/mol at standard temperature and pressure (STP). Using Avogadro's Law and the ideal gas law, water vapor and air will have a molar volume of 22.414 litre/mol at STP. A molar 72 mass of air and water vapor occupy the same volume of 22.414 litres. The density (mass/volume) of water vapor is 0.804g/litre, which is significantly less than that of dry air at 1.27g/litre at STP. Note that STP conditions include a temperature of 0°C, at which the ability of water to become vapor is very restricted. Its concentration in air is very low at 0°C. The red line on the chart to the right is the maximum concentration of water vapor expected for a given temperature. The water vapor concentration increases significantly as the temperature rises, approaching 100% (steam, pure water vapor) at 100°C. However the difference in densities between air and water vapor would still exist. 13.3: Chemical Properties of Water in the Atmosphere A chemical property is any of a material's properties that becomes evident during a chemical reaction; that is, any quality that can be established only by changing a substance's chemical identity. Simply speaking, chemical properties cannot be determined just by viewing or touching the substance; the substance's internal structure must be affected for its chemical properties to be investigated. Water is a single chemical compound whose molecules consist of two hydrogen atoms attached to one oxygen atom. The chemical formula of this compound is H2O. Considering that a hydrogen atom weighs only about one-sixteenth as much as an oxygen atom, most of the weight in water is due to oxygen: 88.8 percent of the weight is oxygen and 11.2 percent is hydrogen. This percentage remains the same from a single water molecule to a lake full of water molecules. Water can be made (synthesized) from hydrogen and oxygen, both of which are gases. When these two gases are mixed, however, they do not react unless the reaction is started with a flame or spark. Then they react with explosive violence. The tremendous energy that is released is a signal that water is an extremely stable compound. It is hard to break a water molecule apart into its components. The normal boiling point of water is 100°C and its freezing point is 0°C. As water is cooled to make ice, it becomes slightly denser, like all liquids. But at 4°C, it reaches its maximum density. When cooled below that temperature, it becomes less dense. At 0°C, water freezes and expands. Since ice is less dense than water, ice floats on it. 73 In pure water, 1 out of every 555 million molecules is broken down into a hydrogen ion and a hydroxide ion (an ion is an electrically charged atom or group of atoms). These ions are enough to make water a slight conductor of electricity. That is why water is dangerous when there is electricity around. Because water dissolves so many substances (it is called the universal solvent), all of the water on Earth is in the form of solutions. Water can react with many things in the atmosphere to produce different compounds. Some of the compounds formed and there effect in the environment is acid rain. In the following lecture we will discuss the formation and effect of acid rain to our environment. Study Question Discuss in detail the difference between smog and water vapor References: 1. Chang, R. (1991), Chemistry, 4th ed McGraw-Hill, Inc 2. Manahan S. E. (2000), Environmental Chemistry, 7th ed Lewis Publisher 3. Wright, J. (2005), Environmental Chemistry, Taylor & Francis e-Library. NY. 4. Petrucci, H. and Harwood, W. S. (1997), General Chemistry, Principles and Modern Application, 7th ed, Prentice hall Upper Saddle River, New Jersey. 74 14: LECTURE FOURTEEN 14.1: Acid rain and the Effect to the Environment Introduction An acid a word from the Latin word acidus meaning sour is traditionally considered as any chemical compound that, when dissolved in water, gives a solution with a hydrogen ion with a pH less than 7.0. other scientists like Johannes Nicolaus Brønsted and Martin Lowry, defined an acid as a compound which donates a hydrogen ion (H+) to another compound (called a base). The following is the reaction to describe what we just say: O O C R H O + H O H C R O + OH3 O R C O H O + NH3 R C + O NH4 You can just see how acid donate the Hydrogen ion (H+). Another scientist is A Arrhenius who defines an acid as a molecule which increases the concentration of the hydronium (H3O+) ion when dissolved in water. On the other hand an Arrhenius base is a molecule which increases the concentration of the hydroxide ion when dissolved in water. A third concept was proposed by Gilbert N. Lewis which includes reactions with acid-base characteristics that do not involve a proton transfer. A Lewis acid is a species that accepts a pair of electrons from another species; in other words, it is an electron pair acceptor. Brønsted acid-base reactions are proton transfer reactions while Lewis acid-base reactions are electron pair transfers. Example of reaction showing Lewis acid is showing in the reaction below: 75 In the first reaction a fluoride ion, F-, donate an electron pair to boron trifluoride to form the product tetrafluoroborate. Fluoride "loses" a pair of valence electrons because the electrons shared in the B—F bond are located in the region of space between the two atomic nuclei and are therefore more distant from the fluoride nucleus than they are in the lone fluoride ion. BF3 is a Lewis acid because it accepts the electron pair from fluoride. Common examples of an acid include acetic acid (in vinegar) and sulfuric acid (used in car batteries). We have learned earlier that water moisture is present in the atmosphere. Therefore, it is wise to say acid is possible to be formed in the atmosphere. That’s where the concept of acid rain comes. Let us discuss in detail about this acid rain and the consequences it has on our environment. 14.2: Acid Rain and its Source "Acid rain" is a popular term referring to the deposition of wet (rain, snow, sleet, fog and cloudwater, dew) and dry (acidifying particles and gases) acidic components. A more accurate term is “acid deposition”. Distilled water, which contains no carbon dioxide, has a neutral pH of 7. Liquids with a pH less than 7 are acidic, and those with a pH greater than 7 are bases. “Clean” or unpolluted rain has a slightly acidic pH of about 5.2, because carbon dioxide and water in the air react together to form carbonic acid, a weak acid (pH 5.6 in distilled water), but unpolluted rain also contains other chemicals. H2O (l) + CO2 (g) → H2CO3 (aq) Carbonic acid then can ionize in water forming low concentrations of hydronium ions: 2H2O (l) + H2CO3 (aq) CO32- (aq) + 2H3O+(aq) 76 The most important gas which leads to acidification is sulfur dioxide. Emissions of nitrogen oxides which are oxidized to form nitric acid are of increasing importance due to stricter controls on emissions of sulfur containing compounds. The form of SO2 comes from fossil fuel combustion and industry, or from wildfires and also from volcanoes. There are several sources of the acidity rain. These are: 14.2.1: Human Activity The principal cause of acid rain is sulfur and nitrogen compounds from human sources, such as electricity generation, factories, and motor vehicles. Coal power plants are one of the most polluting. The gases can be carried hundreds of km in the atmosphere before they are converted to acids and deposited. In the past, factories had short funnels to let out smoke, but this caused many problems locally; thus, factories now have taller smoke funnels. However, dispersal from these taller stacks causes pollutants to be carried farther, causing widespread ecological damage. 14.2.2: Chemical Processes In the gas phase sulfur dioxide is oxidized by reaction with the hydroxyl radical via an intermolecular reaction: SO2 + OH· → HOSO2· which is followed by: HOSO2· + O2 → HO2· + SO3 In the presence of water, sulfur trioxide (SO3) is converted rapidly to sulfuric acid: SO3(g) + H2O(l) → H2SO4(l) Nitric acid is formed by the reaction of OH with nitrogen dioxide: NO2 + OH· → HNO3 14.3: The Effect of Acid Rain in the Environment Acid rain has been shown to have adverse impacts on forests, freshwaters and soils, killing insect and aquatic life-forms as well as causing damage to buildings and having impacts on human health. 77 14.3.1: Effect on Surface Waters and Aquatic Animals Both the lower pH and higher aluminum concentrations in surface water that occur as a result of acid rain can cause damage to fish and other aquatic animals. At pHs lower than 5 most fish eggs will not hatch and lower pHs can kill adult fish. As lakes and rivers become more acidic biodiversity is reduced. Acid rain has eliminated insect life and some fish species, including the brook trout in some lakes, streams, and creeks in geographically sensitive areas, such as the Adirondack Mountains of the United States. However, the extent to which acid rain contributes directly or indirectly via runoff from the catchments to lake and river acidity (i.e., depending on characteristics of the surrounding watershed) is variable. 14.3.2: Effect on Soils Soil biology and chemistry can be seriously damaged by acid rain. Some microbes are unable to tolerate changes to low pHs and are killed. The enzymes of these microbes are denatured (changed in shape so they no longer function) by the acid. The hydronium ions of acid rain also mobilize toxins, e.g. aluminium, and leach away essential nutrients and minerals. 2H+ (aq)+ Mg2+ (clay) 2H+ (clay)+ Mg2+(aq) Soil chemistry can be dramatically changed when base cations, such as calcium and magnesium, are leached by acid rain thereby affecting sensitive species, such as sugar maple (Acer saccharum). 14.3.3: Effect on Forests and other Vegetation Fig. 12: Effect of acid rain on a forest, Jizera Mountains, Czech Republic 78 Adverse effects may be indirectly related to acid rain, like the acid's effects on soil (see above) or high concentration of gaseous precursors to acid rain. High altitude forests are especially vulnerable as they are often surrounded by clouds and fog which are more acidic than rain. Other plants can also be damaged by acid rain but the effect on food crops is minimized by the application of lime and fertilizers to replace lost nutrients. In cultivated areas, limestone may also be added to increase the ability of the soil to keep the pH stable, but this tactic is largely unusable in the case of wilderness lands. When calcium is leached from the needles of red spruce, these trees become less cold tolerant and exhibit winter injury and even death. 14.3.4: Effect on Human Health Fine particles, a large fraction of which are formed from the same gases as acid rain (sulfur dioxide and nitrogen dioxide), have been shown to cause illness and premature deaths such as cancer and other diseases. For more information on the health effects of aerosols see particulate health effects. 14.3.5: Other Adverse Effects Acid rain can also cause damage to certain building materials and historical monuments. This results when the sulfuric acid in the rain chemically reacts with the calcium compounds in the stones (limestone, sandstone, marble and granite) to create gypsum, which then flakes off. CaCO3 (s) + H2SO4 (aq) CaSO4 (aq) + CO2 (g) + H2O (l) This result is also commonly seen on old gravestones where the acid rain can cause the inscription to become completely illegible. Acid rain also causes an increased rate of oxidation for iron. Visibility is also reduced by sulfate and nitrate aerosols and particles in the atmosphere. 79 Study Questions: 1. With vivid equations differentiate between Arrhenius acid, Lewis acid and Brønsted acid. 2. In terms of electron losing and gaining define the terms oxidation and reduction process. Using equations of definition of acid, show which specie is oxidizing agent and which one is reducing agent. References: 1. Chang, R. (1991), Chemistry, 4th ed McGraw-Hill, Inc 2. Manahan S. E. (2000), Environmental Chemistry, 7th ed Lewis Publisher 3. Wright, J. (2005), Environmental Chemistry, Taylor & Francis e-Library. NY. 4. Petrucci, H. Harwood, W. S. (1997), General Chemistry, Principles and Modern Application, 7th ed, Prentice hall Upper Saddle River, New jersey. 5. http://en.wikipedia.org/wiki/File:Acid-base.png 6. Ebbing, D. D., & Gammon, S. D. (2005). General chemistry, 8th ed, Boston, MA: Houghton Mifflin 80 15: 15.1: LECTURE FIFTEEN Soil and Ground Water Pollution Management Introduction Until the late 1960s, soil chemistry focused primarily on chemical reactions in the soil that contribute to affect of plant growth. Since then concerns have grown about environmental pollution, organic and inorganic soil contamination and potential ecological health and environmental health risks. Consequently, the emphasis in soil chemistry has shifted from pedology and agricultural soil science to an emphasis on environmental soil science. Knowledge of environmental soil chemistry is paramount to predicting the fate, mobility and potential toxicity of contaminants in the environment. The vast majority of environmental contaminants are initially released to the soil. Once a chemical is exposed to the soil environment a myriad of chemical reactions can occur that may increase/decrease contaminant toxicity. These reactions include adsorption/desorption, precipitation, polymerization, dissolution, complexation, and oxidation/reduction. These reactions are often disregarded by scientists and engineers involved with environmental remediation. Understanding these processes enable us to better predict the fate and toxicity of contaminants and provide the knowledge to develop scientifically correct and costeffective remediation strategies. Contamination of water and soil is one of the critical issue in our country. There are several laws and bylaws to protect these systems. Despite the questions that have ensued concerning the strictness and perhaps the inappropriateness of some of the regulations contained in environmental laws, the fact remains that the public is very concerned about the quality of the environment. They have expressed an overwhelming willingness to spend substantial tax shillings to ensure a clean and safe environment. 15.2: Soil and Groundwater This is water located beneath the ground surface in soil pore spaces and in the fractures of lithologic formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock become 81 completely saturated with water is called the water table. Groundwater is recharged from, and eventually flows to, the surface naturally; natural discharge often occurs at springs and seeps, and can form oases or wetlands. Groundwater is also often withdrawn for agricultural, municipal and industrial use by constructing and operating extraction wells. The study of the distribution and movement of groundwater is hydrogeology, also called groundwater hydrology. Typically, groundwater is thought of as liquid water flowing through shallow aquifers, but technically it can also include soil moisture, permafrost (frozen soil), immobile water in very low permeability bedrock, and deep geothermal or oil formation water. Groundwater is hypothesized to provide lubrication that can possibly influence the movement of faults. It is likely that much of the Earth's subsurface contain some water, which may be mixed with other fluids in some instances. Groundwater may not be confined only to the Earth. The formation of some of the landforms observed on Mars may have been influenced by groundwater. An aquifer is a layer of relatively porous substrate that contains and transmits groundwater. When water can flow directly between the surface and the saturated zone of an aquifer, the aquifer is unconfined. The deeper parts of unconfined aquifers are usually more saturated since gravity causes water to flow downward. The upper level of this saturated layer of an unconfined aquifer is called the water table or phreatic surface. Below the water table, where generally all pore spaces are saturated with water is the phreatic zone. Substrate with relatively low porosity that permits limited transmission of groundwater is known as an aquitard. An aquiclude is a substrate with porosity that is so low it is virtually impermeable to groundwater. A confined aquifer is an aquifer that is overlain by a relatively impermeable layer of rock or substrate such as an aquiclude or aquitard. If a confined aquifer follows a downward grade from its recharge zone, groundwater can become pressurized as it flows. This can create artesian wells that flow freely without the need of a pump and rise to a higher elevation than the static water table at the above, unconfined, aquifer. The characteristics of aquifers vary with the geology and structure of the substrate and topography in which they occur. Generally, the more productive aquifers occur in sedimentary geologic formations. By comparison, weathered and fractured crystalline rocks yield relatively smaller quantities of groundwater in many environments. Unconsolidated to poorly cemented alluvial 82 materials that have accumulated as valley-filling sediments in major river valleys and geologically subsiding structural basins are included among the most productive sources of groundwater. The high specific heat capacity of water and the insulating effect of soil and rock can mitigate the effects of climate and maintain groundwater at a relatively steady temperature. In some places where groundwater temperatures are maintained by this effect at about 10°C, groundwater can be used for controlling the temperature inside structures at the surface. For example, during hot weather relatively cool groundwater can be pumped through radiators in a home and then returned to the ground in another well. During cold seasons, because it is relatively warm, the water can be used in the same way as a source of heat for heat pumps that is much more efficient than using air. The relatively constant temperature of groundwater can also be used for heat pumps. Groundwater makes up about twenty percent of the world's fresh water supply, which is about 0.61% of the entire world's water, including oceans and permanent ice. Groundwater is naturally replenished by surface water from precipitation, streams, and rivers when this recharge reaches the water table. It is estimated that the volume of groundwater comprises 30.1% of all freshwater resource on earth compared to 0.3% in surface freshwater; the icecaps and glaciers are the only larger sources of fresh water on earth at 68.7%. 15.3: Contamination of Water and Soil There are a number of inorganic and organic contaminants that are important in water and soil. These include plant nutrients such as nitrate and phosphate; heavy metals such as cadmium, chromium, and lead; oxyanions such as arsenite, arsenate, and selenite; organic chemicals; inorganic acids; and radionuclides. The sources of these contaminants include fertilizers, pesticides, acidic deposition, agricultural and industrial waste materials, and radioactive fallout. We can just find out human activity play key role in the contamination of water and soil. Let us now start one by one looking on how different sources are contaminated. 15.3.1: Groundwater Pollution Interactions between groundwater and surface water are complex. Consequently, groundwater pollution, sometimes referred to as groundwater contamination, is not as easily classified as surface water pollution. By its very nature, groundwater aquifers are susceptible to contamination from 83 sources that may not directly affect surface water bodies, and the distinction of point vs. non-point source may be irrelevant. Ground water contamination is the result of polluted water infiltrating through the soil and rock and eventually reaching the ground water. This process might take many years and might take place at a distance from the well where the contamination is found. Once the ground water is contaminated, it is very difficult to remediate. No doubt that the new technologies will always reduce the pollution level. But the underground water quality in this basin based on various factors, influx of industrial effluent, influx of water through rainfall, soil, agriculture pattern etc., so we can say that by these factors, the underground water quality can be varied qualitatively and quantitatively. A spill of a chemical contaminant on soil, located away from a surface water body, may not necessarily create point source or non-point source pollution, but nonetheless may contaminate the aquifer below. Analysis of groundwater contamination may focus on soil characteristics and hydrology, as well as the nature of the contaminant itself. 15.3.2: Surface and Ground Water Quality Situation: Tanzania Case Natural mineralization as in the case of occurrence of high concentrations of fluoride in water in some parts of Arusha, Kilimanjaro, Singida, Shinyanga and other areas is an example of natural factors. An interpretation of the data obtained from the sampling of boreholes, protected springs open springs, streams, ponds around the country indicates that boreholes are the best water sources except for natural mineralization as in the case with many borehole with high fluoride concentration or salinity, or if the beneficiaries pollute during water drawing in the case of open wells. The quality of drinking water in the investigated sources in the country reveals how the sources are as indicated in the following table. Table 1: Types of Water Sources in Tanzania Water sources Description Boreholes Suitable for use without treatment provided pump base and sealants are firmly intact. Rain water and protected Suitable as well Springs and impoundments Moderate quality depending on the location Streams, rivers, lakes, open Worst in quality, thus need treatment wells 84 15.3.3: Pollution of Water Resources There are two basic types of pollution—point and nonpoint. Point pollution is contamination that can be traced to a particular source such as an industrial site, septic tank, or wastewater treatment plant. Nonpoint pollution results from large areas and not from any single source and includes both natural and human activities. Sources of nonpoint pollution include agricultural, human, forestry, urban, construction, and mining activities and atmospheric deposition. There are also naturally occurring nonpoint source pollutants that are important. These include geologic erosion, saline seeps, and breakdown of minerals and soils that may contain large quantities of nutrients. The specific contaminants leading to pollution in water include a wide spectrum of chemicals, pathogens, and physical or sensory changes such as elevated temperature and discoloration. While many of the chemicals and substances that are regulated may be naturally occurring (calcium, sodium, iron, manganese, etc.) the concentration is often the key in determining what is a natural component of water, and what is a contaminant. Oxygen-depleting substances may be natural materials, such as plant matter (e.g. leaves and grass) as well as man-made chemicals. Other natural and anthropogenic substances may cause turbidity (cloudiness) which blocks light and disrupts plant growth, and clogs the gills of some fish species. Many of the chemical substances are toxic. Pathogens can produce waterborne diseases in either human or animal hosts. Alteration of water’s physical chemistry includes acidity (change in pH), electrical conductivity, temperature, and eutrophication. Eutrophication is the fertilization of surface water by nutrients that were previously scarce. High levels of pathogens may result from inadequately treated sewage discharges. This can be caused by a sewage plant designed with less than secondary treatment (more typical in lessdeveloped countries). In developed countries, older cities with aging infrastructure may have leaky sewage collection systems (pipes, pumps, valves), which can cause sanitary sewer overflows. Some cities also have combined sewers, which may discharge untreated sewage during rain storms. Contaminants may include organic and inorganic substances. 85 15.3.4: Organic Water Pollutants: • Detergents • Disinfection by-products found in chemically disinfected drinking water, such as chloroform • Food processing waste, which can include oxygen-demanding substances, fats and grease • Insecticides and herbicides, a huge range of organohalides and other chemical compounds • Petroleum hydrocarbons, including fuels (gasoline, diesel fuel, jet fuels, and fuel oil) and lubricants (motor oil), and fuel combustion byproducts, from storm water runoff • Tree and brush debris from logging operations • Volatile organic compounds (VOCs), such as industrial solvents, from improper storage. Chlorinated solvents, which are dense non-aqueous phase liquids (DNAPLs), may fall to the bottom of reservoirs, since they don't mix well with water and are denser. • Various chemical compounds found in personal hygiene and cosmetic products 15.3.5: Inorganic Water Pollutants: • Acidity caused by industrial discharges (especially sulfur dioxide from power plants) • Ammonia from food processing waste • Chemical waste as industrial by-products • Fertilizers containing nutrients--nitrates and phosphates--which are found in storm-water runoff from agriculture, as well as commercial and residential use[12] • Heavy metals from motor vehicles (via urban storm-water runoff)[12] [13] and acid mine drainage • Silt (sediment) in runoff from construction sites, logging, slash and burn practices or land clearing sites 15.3.6: Macroscopic Pollution These are visible items polluting the water may be termed "floatables" in an urban storm-water context, or marine debris when found on the open seas, and can include such items as: Trash (e.g. paper, plastic, or food waste) discarded by people on the ground, and that are washed by rainfall into storm drains and eventually discharged into surface waters Nurdles, small ubiquitous waterborne plastic pellets, shipwrecks, large derelict ships 86 15.3.7: Thermal Pollution Thermal pollution is the rise or fall in the temperature of a natural body of water caused by human influence. A common cause of thermal pollution is the use of water as a coolant by power plants and industrial manufacturers. Elevated water temperatures decreases oxygen levels (which can kill fish) and affects ecosystem composition, such as invasion by new thermophilic species. Urban runoff may also elevate temperature in surface waters. Thermal pollution can also be caused by the release of very cold water from the base of reservoirs into cooler rivers. 15.7: Waste Management as Source of water Pollution Tanzania Case Waste management is the collection, transport, processing, recycling or disposal, and monitoring of waste materials. The term usually relates to materials produced by human activity, and is generally undertaken to reduce their effect on health, the environment or aesthetics. Waste management is also carried out to recover resources from it. Waste management can involve solid, liquid, gaseous or radioactive substances, with different methods and fields of expertise for each. Waste management practices differ for developed and developing nations like Tanzania, for urban and rural areas, and for residential and industrial producers. Management for non-hazardous residential and institutional waste in metropolitan areas is usually the responsibility of local government authorities, while management for non-hazardous commercial and industrial waste is usually the responsibility of the generator. Waste collection methods vary widely between different countries and regions. Domestic waste collection services are often provided by local government authorities, or by private industry. Some areas, especially those in less developed countries, do not have a formal waste-collection system. In Dar es salaam city we have management structure of the Dar es Salaam Health department under City council as shown here: 87 Fig. 13: Waste Management Structure of the Dar es Salaam Health Department Each of Dar es Salaam’s three districts (Kinondoni, Temeke, and Ilala) has its own day-to-day setup for solid-waste management, headed by the health officer, after who come the foreperson, the heads, and the cleaners. All daily operations are based at site offices in the three districts. The activities of city cleansing are supervised by the Health Standing Committee, which plans, evaluates, and advises on all matters concerning health, including waste removal and disposal. To see waste removal as a health issue is perhaps to take a narrow focus, as it subsumes other aspects of waste generation and management. The concentration on cleansing does not bring into the forefront important aspects, such as waste generation and recycling. Some times the house hold waste products are just thrown closer to the premises. Let us see the picture taken at to illustrate the environmental impact. 88 Fig. 14: Trash Closer to Living Premises with Colored Discharge Find its Way to Water/Soil System 15.4: Soil Pollution Soil pollution is caused by the presence of man-made chemicals or other alteration in the natural soil environment. This type of contamination typically arises from the rupture of underground storage tanks, application of pesticides, percolation of contaminated surface water to subsurface strata, oil and fuel dumping, leaching of wastes from landfills or direct discharge of industrial wastes to the soil. The most common chemicals involved are petroleum hydrocarbons, solvents, pesticides, lead and other heavy metals. This occurrence of this phenomenon is correlated with the degree of industrializations and intensities of chemical usage. Soil contaminants can have significant toxic consequences for ecosystems. These changes can be evident in the alteration of metabolism of common microorganisms and arthropods local in a given soil environment. The result can be virtual abolition of some of the primary food chain, which in turn have major consequences for predator or consumer species. Even if the chemical effect on lower life forms is small, the lower pyramid levels of the food chain may ingest alien chemicals, which normally become more concentrated for each consuming rung of the food chain. Many of 89 these effects are now well known, such as the concentration of persistent DDT materials for avian consumers, leading to weakening of egg shells, increased chick mortality and potentially species extinction. Effects occur to agricultural lands which have certain types of soil contamination. Contaminants typically alter plant metabolism, most commonly to reduce crop yields. This has a secondary effect upon soil conservation, since the languishing crops cannot shield the Earth's soil mantle from erosion phenomena. Some of these chemical contaminants have long half-lives and in other cases derivative chemicals are formed from decay of primary soil contaminants. Study Questions: 1. Consider Table 1 above. Discuss each category shown by identifying how they can be contaminated. 2. What can you tell about contamination of ground water using figure 1 above? Class Activity: Discuss in detail the role of the private formal sector in waste management in your city. What do you think the Government can do the rescue the life of the citizens due to waste around our houses? References: 1. http://ipsnews.net/news.asp?idnews=47932 2. Mwanthi, M. A., Nyabola, L. O., (1997), Solid Waste Management in Nairobi City: Knowledge and Attitudes, Environmental Health Dec. 1997(Dec. 1997): 23-29. 3. Teerlink, H. and Frank, E. D., (1993), The Brown Environment, Employment and Local Agenda 21, Third World Planning Review 15(2): 195-207. 4. Yhdego, M., (1995), Urban solid waste management in Tanzania Issues, concepts and challenges, Resource, Conservation and Research 14: 1-10. 90 16: Lecture 16 16.1: Health Problems of Water Pollution Introduction People have known for a long time that drinking water can spread disease; however, until it was understood that microbes in the water were responsible for disease, not much could be done to prevent waterborne illness. It is not true that the outbreak of diseases due to unsafe water is only in Africa. Example is in 1854, one of the first incidents occurred that indicated drinking water could be treated to prevent disease. An outbreak of cholera was raging through several London neighborhoods and no one understood the cause (cholera is caused by bacteria and one of the sources for the bacteria is feces, or stool, from infected people). In one of the worst infected neighborhoods, an individual named John Snow made an interesting observation. We will discuss these problems in detail in this lecture. 16.2: Unsafe Water a Source of Waterborne Diseases Several reasons are responsible for the possible contamination of the big water systems, the main ones being aging infrastructure and most likely poor water treatment practices, which have led to a dramatic emergence of cholera epidemics and high rate of diarrhea diseases in the country. These are normally spread when infected human faeces gain access to water. The disease causing organisms, which may be present in the excreta, are often passively carried into the water. Out breaks occur when the contaminated water is drunk untreated. To prevent the water borne diseases water for domestic use must always be free from faecal contamination. The situation calls for improvement of water quality and this can be achieved by disinfection. But generally the water being supplied should adhere to drinking water quality standards. Waterborne diseases are pathogenic microorganisms which are directly transmitted when contaminated fresh water is consumed. Contaminated fresh water, used in the preparation of food, can be the source of food-borne disease through consumption of the same microorganisms. According to the World Health Organization, diarrheal disease accounts for an estimated 4.1% of the total daily global burden of disease and is responsible for the deaths of 1.8 million people every year. It was estimated that 88% of that burden is attributable to unsafe water supply, sanitation and 91 hygiene, and is mostly concentrated in children in developing countries. Waterborne disease can be caused by protozoa, viruses, or bacteria, many of which are intestinal parasites. Table 2: Protozoal Infections Disease and Transmission Microbial Agent Sources of Agent in Water Supply General Symptoms Protozoan (Entamoeba Abdominal discomfort, fatigue, Amoebiasis (handSewage, non-treated drinking water, flies histolytica) (Cyst-like weight loss, diarrhea, bloating, to-mouth) in water supply appearance) fever, abdominal pain Cryptosporidiosis (oral) Protozoan (Cryptosporidium parvum) Flu-like symptoms, watery Collects on water filters and membranes diarrhea, loss of appetite, that cannot be disinfected, animal substantial loss of weight, manure, seasonal runoff of water. bloating, increased gas, nausea Cyclosporiasis Protozoan parasite (Cyclospora cayetanensis) Sewage, non-treated drinking water Giardiasis (oralfecal) (hand-tomouth) Protozoan (Giardia lamblia) Most common intestinal parasite Untreated water, poor disinfection, pipe breaks, leaks, groundwater Diarrhea, abdominal contamination, campgrounds where discomfort, bloating, and humans and wildlife use same source of flatulence water. Beavers and muskrats create ponds that act as reservoirs for Giardia. Microsporidiosis The genera of Encephalitozoon Protozoan phylum intestinalis has been detected in (Microsporidia), but groundwater, the origin of drinking closely related to fungi water cramps, nausea, vomiting, muscle aches, fever, and fatigue Diarrhea and wasting in immunocompromised individuals Table 3: Parasitic Infections (Kingdom Animalia) Disease and Transmission Microbial Agent Sources of Agent in Water Supply General Symptoms Schistosomiasis (immersion) Members of the genus Schistosoma Fresh water contaminated with certain types of snails that carry schistosomes Rash or itchy skin. Fever, chills, cough, and muscle aches Dracunculiasis (Guinea Worm Disease) Dracunculus medinensis Stagnant water containing larvae Allergic reaction, urticaria rash, nausea, vomiting, diarrhea, asthmatic attack. Taeniasis Tapeworms of Drinking water the genus Taenia contaminated with eggs Intestinal disturbances, neurologic manifestations, loss of weight, cysticercosis Fasciolopsiasis Fasciolopsis buski Drinking water contaminated with encysted metacercaria GIT disturbance, diarrhea, liver enlargement, cholangitis, cholecystitis, obstructive jaundice. Hymenolepiasis (Dwarf Tapeworm Infection) Hymenolepis nana Drinking water contaminated with eggs Abdominal pain, anorexia, itching around the anus, nervous manifestation Echinococcosis Echinococcus Drinking water Liver enlargement, hydatid cysts press on bile duct 92 (Hydatid disease) granulosus contaminated with feces and blood vessels; if cysts rupture they can cause (usually canid) anaphylactic shock containing eggs coenurosis multiceps multiceps contaminated drinking water with eggs increases intacranial tension Ascariasis Ascaris lumbricoides Drinking water contaminated with feces (usually canid) containing eggs Mostly, disease is asymptomatic or accompanied by inflammation, fever, and diarrhea. Severe cases involve Löffler's syndrome in lungs, nausea, vomiting, malnutrition, and underdevelopment. Enterobiasis Enterobius vermicularis Drinking water contaminated with eggs Peri-anal itch, nervous irritability, hyperactivity and insomnia Table 4: Bacterial Infections Disease and Transmission Microbial Agent Sources of Agent in Water Supply Bacteria can enter a wound from contaminated water sources. Can enter the gastrointestinal tract by consuming contaminated drinking water or (more commonly) food General Symptoms Dry mouth, blurred and/or double vision, difficulty swallowing, muscle weakness, difficulty breathing, slurred speech, vomiting and sometimes diarrhea. Death is usually caused by respiratory failure. Botulism Clostridium botulinum Campylobacteriosis Most commonly caused Drinking water contaminated by Campylobacter with feces jejuni Produces dysentery like symptoms along with a high fever. Usually lasts 2-10 days. Cholera Spread by the bacterium Vibrio cholerae Drinking water contaminated with the bacterium In severe forms it is known to be one of the most rapidly fatal illnesses known. Symptoms include very watery diarrhea, nausea, cramps, nosebleed, rapid pulse, vomiting, and hypovolemic shock (in severe cases), at which point death can occur in 1218 hours. E. coli Infection Certain strains of Escherichia coli (commonly E. coli) Water contaminated with the bacteria Mostly diarrhea. Can cause death in immunocompromised individuals, the very young, and the elderly due to dehydration from prolonged illness. M. marinum infection Mycobacterium marinum Naturally occurs in water, most cases from exposure in swimming pools or more frequently aquariums; rare infection since it mostly infects immunocompromised individuals Symptoms include lesions typically located on the elbows, knees, and feet (from swimming pools) or lesions on the hands (aquariums). Lesions may be painless or painful. Dysentery Caused by a number of species in the genera Water contaminated with the Shigella and bacterium Salmonella with the most common being Frequent passage of feces with blood and/or mucus and in some cases vomiting of blood. 93 Shigella dysenteriae Pontiac fever produces milder symptoms resembling acute influenza without pneumonia. Legionnaires’ disease has severe symptoms such as fever, chills, pneumonia (with cough that sometimes produces sputum), ataxia, anorexia, muscle aches, malaise and occasionally diarrhea and vomiting Legionellosis (two distinct forms: Legionnaires’ disease and Pontiac fever) Caused by bacteria belonging to genus Legionella (90% of cases caused by Legionella pneumophila) Leptospirosis Water contaminated by the Caused by bacterium of animal urine carrying the genus Leptospira bacteria Begins with flu-like symptoms then resolves. The second phase then occurs involving meningitis, liver damage (causes jaundice), and renal failure Otitis Externa (swimmer’s ear) Caused by a number of Swimming in water bacterial and fungal contaminated by the species. responsible pathogens Ear canal swells causing pain and tenderness to the touch Salmonellosis Caused by many bacteria of genus Salmonella Drinking water contaminated with the bacteria. More common as a food borne illness. Typhoid fever Salmonella typhi Characterized by sustained fever up to 40ºC (104ºF), profuse sweating, diarrhea, less commonly a rash may Ingestion of water contaminated with feces of an occur. Symptoms progress to infected person delerium and the spleen and liver enlarge if untreated. In this case it can last up to four weeks and cause death. Vibrio Illness Vibrio vulnificus, Vibrio alginolyticus, and Vibrio parahaemolyticus Can enter wounds from contaminated water. Also got by drinking contaminated water or eating undercooked oysters. Contaminated water: the organism thrives in warm aquatic environments. Symptoms include diarrhea, fever, vomiting, and abdominal cramps Symptoms include explosive, watery diarrhea, nausea, vomiting, abdominal cramps, and occasionally fever. Table 5: Viral Infections Disease and Transmission Microbial Agent Sources of Agent in Water Supply Manifests itself in improperly treated water Adenovirus infection Adenovirus Gastroenteritis Astrovirus, Manifests itself in Calicivirus, Enteric improperly treated Adenovirus, and water Parvovirus General Symptoms Symptoms include common cold symptoms, pneumonia, croup, and bronchitis Symptoms include diarrhea, nausea, vomiting, fever, malaise, and abdominal pain SARS (Severe Acute Respiratory Coronavirus Syndrome) Manifests itself in improperly treated water Symptoms include fever, myalgia, lethargy, gastrointestinal symptoms, cough, and sore throat Hepatitis A Can manifest itself in Symptoms are only acute (no chronic stage to the Hepatitis A virus 94 (HAV) water (and food) virus) and include Fatigue, fever, abdominal pain, nausea, diarrhea, weight loss, itching, jaundice and depression. Enters water through the feces of infected individuals Poliomyelitis (Polio) Poliovirus Polyomavirus infection Very widespread, can manifest itself in water, Two of Polyomavirus: JC ~80% of the population virus and BK virus has antibodies to Poltomavirus 90-95% of patients show no symptoms, 4-8% have minor symptoms (comparatively) with delirium, headache, fever, and occasional seizures, and spastic paralysis, 1% have symptoms of non-paralytic aseptic meningitis. The rest have serious symptoms resulting in paralysis or death BK virus produces a mild respiratory infection and can infect the kidneys of immunosuppressed transplant patients. JC virus infects the respiratory system, kidneys or can cause progressive multifocal leukoencephalopathy in the brain (which is fatal). Class Activity Discuss in detail common epidemic diseases caused by unhygienic water at your place. References: 1. Berman, E. (1969), Toxic Metals and their analysis 6th Ed. Heyden and Sons Ltd 2. Jocobs, A. and Worwood, M. (1974); In Blood and its Disorder, Hardesty, R. M. and Weatherall, D. J. , Oxford, Blackwell 3. Jones, A.; Johnston, D. O.; Netterville, J. T. and Wood, J. L. (1983), Chemistry, Man and Society, Saunder. 95 17: LECTURE SEVENTEEN 17.1: Water Treatment Introduction Water treatment describes those processes used to make water more acceptable for a desired enduse. These can include use as drinking water, industrial processes, medical and many other uses. The goal of all water treatment process is to remove existing contaminants in the water, or reduce the concentration of such contaminants so the water becomes fit for its desired end-use. One such use is returning water that has been used back into the natural environment without adverse ecological impact. The processes involved in treating water for drinking purpose may be solids separation using physical such as settling and filtration, chemical such as disinfection and coagulation. Biological processes are also employed in the treatment of wastewater and these processes may include, for example, aerated lagoons, activated sludge or slow sand filters. On the other hand water purification is the removal of contaminants from untreated water to produce drinking water that is pure enough for its intended use, most commonly human consumption. Substances that are removed during the process of drinking water treatment include bacteria, algae, viruses, fungi, minerals such as iron, manganese and sulphur, and man-made chemical pollutants including fertilizers. It is important to take measures to make available water of desirable quality at the consumer end. That leads to protection of the treated water during conveyance and distribution after treatment. It is common practice to have residual disinfectants in the treated water in order to kill any bacteriological contamination after water treatment. World Health Organization (WHO) guidelines are generally followed throughout the world for drinking water quality requirements. In addition of the WHO guidelines, each country or territory or water supply body can have their own guidelines in order for consumers to have access to safe drinking water. 17.2: Various Processes Used in Drinking Water Treatment The combination of following processes is used for municipal drinking water treatment worldwide Pre-chlorination - For algae control and arresting any biological growth Aeration - along with prechlorination for removal of dissolved iron and manganese. 96 Coagulation –This process is used for flocculation. Coagulant aids also known as polyelectrolytes to improve coagulation and for thicker floc formation. Sedimentation - For solids separation, that is, removal of suspended solids entrapped in the floc. Filtration - for removal of carried over floc. Disinfection - for killing the bacteria Various technologies & chemicals are developed and are being developed continuously by various organizations to deliver the above mentioned processes. There is no unique solution (selection of processes) for any type of water. Also, it is very difficult to standardise the solution in the form of processes for water from different sources. Treatability studies for each source of water in different seasons need to be carried out to arrive at most appropriate processes. Let us discuss one type: 17.3: Sewage Treatment Sewage treatment is the process that removes the majority of the contaminants from wastewater or sewage and produces both a liquid effluent suitable for disposal to the natural environment and a sludge. To be effective, sewage must be conveyed to a treatment plant by appropriate pipes and infrastructure and the process itself must be subject to regulation and controls. Some wastewaters require different and sometimes specialized treatment methods. At the simplest level, treatment of sewage and most wastewaters is carried out through separation of solids from liquids, usually by settlement. By progressively converting dissolved material into solids, usually a biological floc which is then settled out, an effluent stream of increasing purity is produced. Sewage is created by residences, institutions, hospitals and commercial and industrial establishments. Raw influent (sewage) includes household waste liquid from toilets, baths, showers, kitchens, sinks, and so forth that is disposed of via sewers. In many areas, sewage also includes liquid waste from industry and commerce. The separation and draining of household waste into grey-water and black-water is becoming more common in the developed world, with grey-water being permitted to be used for watering plants or recycled for flushing toilets. A lot of sewage also includes some surface water from roofs or hardstanding areas. Municipal wastewater therefore includes residential, commercial, and industrial liquid waste discharges, and may include storm-water runoff. Sewage systems capable of handling storm-water are known as combined systems or combined sewers. Such systems are usually avoided since they complicate and thereby reduce the efficiency of sewage treatment plants owing to their seasonality. The variability in flow also leads to often larger than necessary, and subsequently more 97 expensive, treatment facilities. In addition, heavy storms that contribute more flows than the treatment plant can handle may overwhelm the sewage treatment system, causing a spill or overflow (called a combined sewer overflow, or CSO, in the United States). It is preferable to have a separate storm drain system for storm-water in areas that are developed with sewer systems. As rainfall runs over the surface of roofs and the ground, it may pick up various contaminants including soil particles and other sediment, heavy metals, organic compounds, animal waste, and oil and grease. Some jurisdictions require storm-water to receive some level of treatment before being discharged directly into waterways. Examples of treatment processes used for storm-water include sedimentation basins, wetlands, buried concrete vaults with various kinds of filters, and vortex separators (to remove coarse solids). Sewage can be treated close to where it is created (in septic tanks, biofilters or aerobic treatment systems), or collected and transported via a network of pipes and pump stations to a municipal treatment plant (see sewerage and pipes and infrastructure). Sewage collection and treatment is typically subject to local, state and federal regulations and standards. Industrial sources of wastewater often require specialized treatment processes. Conventional sewage treatment involves three stages, called primary, secondary and tertiary treatment. First, the solids are separated from the wastewater stream. Then dissolved biological matter is progressively converted into a solid mass by using indigenous, water-borne microorganisms. Finally, the biological solids are neutralized then disposed of or re-used, and the treated water may be disinfected chemically or physically (for example by lagoons and microfiltration). The final effluent can be discharged into a stream, river, bay, lagoon or wetland, or it can be used for the irrigation of a golf course, green way or park. If it is sufficiently clean, it can also be used for groundwater recharge or agricultural purposes. We can summarize the whole process in the fig. 13. 98 Fig. 15: Process Flow Diagram for a Typical Large-Scale Treatment Plant 17.3.1: Pre-treatment The typical materials that are removed during pre treatment include fats, oils, and greases (also referred to as FOG), sand, gravels and rocks (also referred to as grit), larger settleable solids and floating materials (such as rags and flushed feminine hygiene products). In developed countries, sophisticated equipment with remote operation and control are employed. The developing countries still rely on low cost equipment like manually cleaned screen etc. 17.3.2: Screening The influent sewage water is worried to remove all large objects carried in the sewage stream, such as rags, sticks, tampons, cans, fruit, etc. This is most commonly done with a manual or automatic mechanically raked bar screen. The raking action of a mechanical bar screen is typically paced according to the accumulation on the bar screens and/or flow rate. The bar screen is used because large solids can damage or clog the equipment used later in the sewage treatment plant. The large solids can also hinder the biological process. The solids are collected and later disposed in a landfill or incineration. Pre treatment also typically includes a sand or grit channel or chamber where the velocity of the incoming wastewater is carefully controlled to allow sand grit and stones to settle, while keeping the majority of the suspended organic material in the water column. This equipment is called a de- 99 gritter or sand catcher. Sand, grit, and stones need to be removed early in the process to avoid damage to pumps and other equipment in the remaining treatment stages. Sometimes there is a sand washer (grit classifier) followed by a conveyor that transports the sand to a container for disposal. The contents from the sand catcher may be fed into the incinerator in a sludge processing plant, but in many cases, the sand and grit is sent to a landfill. 17.3.3: ` Primary Treatment - Sedimentation In the primary sedimentation stage, sewage flows through large tanks, commonly called "primary clarifiers" or "primary sedimentation tanks". The tanks are large enough that sludge can settle and floating material such as grease and oils can rise to the surface and be skimmed off. The main purpose of the primary sedimentation stage is to produce both a generally homogeneous liquid capable of being treated biologically and a sludge that can be separately treated or processed. Primary settling tanks are usually equipped with mechanically driven scrapers that continually drive the collected sludge towards a hopper in the base of the tank from where it can be pumped to further sludge treatment stages. 17.3.4: Secondary Treatment Secondary treatment is designed to substantially degrade the biological content of the sewage such as are derived from human waste, food waste, soaps and detergent. The majority of municipal plants treat the settled sewage liquor using aerobic biological processes. For this to be effective, the biota require both oxygen and a substrate on which to live. There are a number of ways in which this is done. In all these methods, the bacteria and protozoa consume biodegradable soluble organic contaminants (e.g. sugars, fats, organic short-chain carbon molecules, etc.) and bind much of the less soluble fractions into floc. Secondary treatment systems are classified as fixed film or suspended growth. Fixed-film treatment process including trickling filter and rotating biological contactors where the biomass grows on media and the sewage passes over its surface. In suspended growth systems, such as activated sludge, the biomass is well mixed with the sewage and can be operated in a smaller space than fixed-film systems that treat the same amount of water. However, fixed-film systems are more able to cope with drastic changes in the amount of biological material and can provide higher removal rates for organic material and suspended solids than suspended growth systems. 100 Roughing filters are intended to treat particularly strong or variable organic loads, typically industrial, to allow them to then be treated by conventional secondary treatment processes. Characteristics include typically tall, circular filters filled with open synthetic filter media to which wastewater is applied at a relatively high rate. They are designed to allow high hydraulic loading and a high flow-through of air. On larger installations, air is forced through the media using blowers. The resultant wastewater is usually within the normal range for conventional treatment processes. 17.3.5: Activated Sludge In general, activated sludge plants encompass a variety of mechanisms and processes that use dissolved oxygen to promote the growth of biological floc that substantially removes organic material. The process traps particulate material and can, under ideal conditions, convert ammonia to nitrite and nitrate and ultimately to nitrogen gas. Others are calling this denitrification. 17.3.6: Surface Aerated Basins Most biological oxidation processes for treating industrial wastewaters have in common the use of oxygen (or air) and microbial action. Surface-aerated basins achieve 80 to 90% removal of Biochemical Oxygen Demand with retention times of 1 to 10 days. The basins may range in depth from 1.5 to 5.0 metres and use motor-driven aerators floating on the surface of the wastewater. In an aerated basin system, the aerators provide two functions: they transfer air into the basins required by the biological oxidation reactions, and they provide the mixing required for dispersing the air and for contacting the reactants (that is, oxygen, wastewater and microbes). Typically, the floating surface aerators are rated to deliver the amount of air equivalent to 1.8 to 2.7 kg O2/kW·h. However, they do not provide as good mixing as is normally achieved in activated sludge systems and therefore aerated basins do not achieve the same performance level as activated sludge units.[1] Biological oxidation processes are sensitive to temperature and, between 0 °C and 40 °C, the rate of biological reactions increase with temperature. Most surface aerated vessels operate at between 4 °C and 32 °C. 17.3.7: Filter Beds (Oxidizing Beds) In older plants and plants receiving more variable loads, trickling filter beds are used where the settled sewage liquor is spread onto the surface of a deep bed made up of coke (carbonized coal), limestone chips or specially fabricated plastic media. Such media must have high surface areas to 101 support the biofilms that form. The liquor is distributed through perforated rotating arms radiating from a central pivot. The distributed liquor trickles through this bed and is collected in drains at the base. These drains also provide a source of air which percolates up through the bed, keeping it aerobic. Biological films of bacteria, protozoa and fungi form on the media’s surfaces and eat or otherwise reduce the organic content. This biofilm is grazed by insect larvae and worms which help maintain an optimal thickness. Overloading of beds increases the thickness of the film leading to clogging of the filter media and ponding on the surface. 17.3.8: Biological Aerated Filters Biological Aerated (or Anoxic) Filter (BAF) or Bio-filters combine filtration with biological carbon reduction, nitrification or de-nitrification. BAF usually includes a reactor filled with a filter media. The media is either in suspension or supported by a gravel layer at the foot of the filter. The dual purpose of this media is to support highly active biomass that is attached to it and to filter suspended solids. Carbon reduction and ammonia conversion occurs in aerobic mode and sometime achieved in a single reactor while nitrate conversion occurs in anoxic mode. BAF is operated either in up flow or down flow configuration depending on design specified by manufacturer. 17.3.9: Membrane Bioreactors Membrane bioreactors (MBR) combines activated sludge treatment with a membrane liquid-solid separation process. The membrane component uses low pressure microfiltration or ultra filtration membranes and eliminates the need for clarification and tertiary filtration. The membranes are typically immersed in the aeration tank (however, some applications utilize a separate membrane tank). One of the key benefits of a membrane bioreactor system is that it effectively overcomes the limitations associated with poor settling of sludge in conventional activated sludge (CAS) processes. The technology permits bioreactor operation with considerably higher mixed liquor suspended solids (MLSS) concentration than CAS systems, which are limited by sludge settling. The process is typically operated at MLSS in the range of 8,000–12,000 mg/L, while CAS is operated in the range of 2,000–3,000 mg/L. The elevated biomass concentration in the membrane bioreactor process allows for very effective removal of both soluble and particulate biodegradable materials at higher loading rates. Thus increased Sludge Retention Times (SRTs)—usually exceeding 15 days—ensure complete nitrification even in extremely cold weather. 102 The cost of building and operating a MBR is usually higher than conventional wastewater treatment, however, as the technology has become increasingly popular and has gained wider acceptance throughout the industry, the life-cycle costs have been steadily decreasing. The small footprint of MBR systems, and the high quality effluent produced, makes them particularly useful for water reuse applications. 17.4: Secondary Treatments 17.4:1: Secondary Sedimentation The final step in the secondary treatment stage is to settle out the biological floc or filter material and produce sewage water containing very low levels of organic material and suspended matter. 17.4.2: Rotating Biological Contactors Rotating biological contactors (RBCs) are mechanical secondary treatment systems, which are robust and capable of withstanding surges in organic load. RBCs were first installed in Germany in 1960 and have since been developed and refined into a reliable operating unit. The rotating disks support the growth of bacteria and micro-organisms present in the sewage, which breakdown and stabilise organic pollutants. To be successful, micro-organisms need both oxygen to live and food to grow. Oxygen is obtained from the atmosphere as the disks rotate. As the micro-organisms grow, they build up on the media until they are sloughed off due to shear forces provided by the rotating discs in the sewage. Effluent from the RBC is then passed through final clarifiers where the microorganisms in suspension settle as sludge. The sludge is withdrawn from the clarifier for further treatment. A functionally similar biological filtering system has become popular as part of home aquarium filtration and purification. The aquarium water is drawn up out of the tank and then cascaded over a freely spinning corrugated fiber-mesh wheel before passing through a media filter and back into the aquarium. The spinning mesh wheel develops a biofilm coating of microorganisms that feed on the suspended wastes in the aquarium water and are also exposed to the atmosphere as the wheel rotates. This is especially good at removing waste urea and ammonia urinated into the aquarium water by the fish and other animals. 103 17.4.3: Tertiary Treatment The purpose of tertiary treatment is to provide a final treatment stage to raise the effluent quality before it is discharged to the receiving environment (sea, river, lake, ground, etc.). More than one tertiary treatment process may be used at any treatment plant. If disinfection is practiced, it is always the final process. It is also called "effluent polishing". 17.4.4: Filtration Sand filtration removes much of the residual suspended matter. Filtration over activated carbon removes residual toxins. Lagooning provides settlement and further biological improvement through storage in large man-made ponds or lagoons. These lagoons are highly aerobic and colonization by native macrophytes, especially reeds, is often encouraged. Small filter feeding invertebrates such as Daphnia and species of Rotifera greatly assist in treatment by removing fine particulates. 17.4.5: Constructed Wetlands Constructed wetlands include engineered reedbeds and a range of similar methodologies, all of which provide a high degree of aerobic biological improvement and can often be used instead of secondary treatment for small communities. In Tanzania this is area of researching where most of Engineers has this project in Shinyanga. 17.4.6: Nutrient Removal Wastewater may contain high levels of the nutrients nitrogen and phosphorus. Excessive release to the environment can lead to a build up of nutrients, called eutrophication, which can in turn encourage the overgrowth of weeds, algae, and cyanobacteria (blue-green algae). This may cause an algal bloom, a rapid growth in the population of algae. The algae numbers are unsustainable and eventually most of them die. The decomposition of the algae by bacteria uses up so much of oxygen in the water that most or all of the animals die, which creates more organic matter for the bacteria to decompose. In addition to causing deoxygenation, some algal species produce toxins that contaminate drinking water supplies. Different treatment processes are required to remove nitrogen and phosphorus. 104 17.4.7: Nitrogen Removal The removal of nitrogen is effected through the biological oxidation of nitrogen from ammonia (nitrification) to nitrate, followed by denitrification, the reduction of nitrate to nitrogen gas. Nitrogen gas is released to the atmosphere and thus removed from the water. Nitrification itself is a two-step aerobic process, each step facilitated by a different type of bacteria. The oxidation of ammonia (NH3) to nitrite (NO2−) is most often facilitated by Nitrosomonas spp. (nitroso referring to the formation of a nitroso functional group). Nitrite oxidation to nitrate (NO3−), though traditionally believed to be facilitated by Nitrobacter spp. (nitro referring the formation of a nitro functional group), is now known to be facilitated in the environment almost exclusively by Nitrospira spp. Denitrification requires anoxic conditions to encourage the appropriate biological communities to form. It is facilitated by a wide diversity of bacteria. Sand filters, lagooning and reed beds can all be used to reduce nitrogen, but the activated sludge process (if designed well) can do the job the most easily. Since denitrification is the reduction of nitrate to dinitrogen gas, an electron donor is needed. This can be, depending on the wastewater, organic matter (from faeces), sulfide, or an added donor like methanol. Sometimes the conversion of toxic ammonia to nitrate alone is referred to as tertiary treatment. 17.4.8: Phosphorus Removal Phosphorus removal is important as it is a limiting nutrient for algae growth in many fresh water systems (for negative effects of algae see Nutrient removal). It is also particularly important for water reuse systems where high phosphorus concentrations may lead to fouling of downstream equipment such as reverse osmosis. Phosphorus can be removed biologically in a process called enhanced biological phosphorus removal. In this process, specific bacteria, called polyphosphate accumulating organisms (PAOs), are selectively enriched and accumulate large quantities of phosphorus within their cells (up to 20% of their mass). When the biomass enriched in these bacteria is separated from the treated water, these biosolids have a high fertilizer value. Phosphorus removal can also be achieved by chemical precipitation, usually with salts of iron (e.g. ferric chloride), aluminum (e.g. alum), or lime. This may lead to excessive sludge productions as hydroxides precipitates and the added chemicals can be expensive. Despite this, chemical phosphorus removal requires significantly smaller equipment footprint than biological removal, is 105 easier to operate and is often more reliable than biological phosphorus removal. Once removed, phosphorus, in the form of a phosphate rich sludge, may be land filled or, if in suitable condition, resold for use in fertilizer. 17.4.8: Disinfection The purpose of disinfection in the treatment of wastewater is to substantially reduce the number of microorganisms in the water to be discharged back into the environment. The effectiveness of disinfection depends on the quality of the water being treated (e.g., cloudiness, pH, etc.), the type of disinfection being used, the disinfectant dosage (concentration and time), and other environmental variables. Cloudy water will be treated less successfully since solid matter can shield organisms, especially from ultraviolet light or if contact times are low. Generally, short contact times, low doses and high flows all militate against effective disinfection. Common methods of disinfection include ozone, chlorine, or ultraviolet light. Chloramine, which is used for drinking water, is not used in wastewater treatment because of its persistence. Chlorination remains the most common form of wastewater disinfection in North America due to its low cost and long-term history of effectiveness. One disadvantage is that chlorination of residual organic material can generate chlorinated-organic compounds that may be carcinogenic or harmful to the environment. Residual chlorine or chloramines may also be capable of chlorinating organic material in the natural aquatic environment. Further, because residual chlorine is toxic to aquatic species, the treated effluent must also be chemically dechlorinated, adding to the complexity and cost of treatment. Ultraviolet (UV) light can be used instead of chlorine, iodine, or other chemicals. Because no chemicals are used, the treated water has no adverse effect on organisms that later consume it, as may be the case with other methods. UV radiation causes damage to the genetic structure of bacteria, viruses, and other pathogens, making them incapable of reproduction. The key disadvantages of UV disinfection are the need for frequent lamp maintenance and replacement and the need for a highly treated effluent to ensure that the target microorganisms are not shielded from the UV radiation (i.e., any solids present in the treated effluent may protect microorganisms from the UV light). In the developed coutries, light is becoming the most common means of disinfection because of the concerns about the impacts of chlorine in chlorinating residual organics in the wastewater and in chlorinating organics in the receiving water. Ozone O3 is generated by passing 106 oxygen O2 through a high voltage potential resulting in a third oxygen atom becoming attached and forming O3. Ozone is very unstable and reactive and oxidizes most organic material it comes in contact with, thereby destroying many pathogenic microorganisms. Ozone is considered to be safer than chlorine because, unlike chlorine which has to be stored on site (highly poisonous in the event of an accidental release), ozone is generated onsite as needed. Ozonation also produces fewer disinfection by-products than chlorination. A disadvantage of ozone disinfection is the high cost of the ozone generation equipment and the requirements for special operators. 17.4.9: Package Plants and Batch Reactors In order to use less space, treat difficult waste, deal with intermittent flow or achieve higher environmental standards, a number of designs of hybrid treatment plants have been produced. Such plants often combine all or at least two stages of the three main treatment stages into one combined stage. When a large number of sewage treatment plants serve small populations, package plants are a viable alternative to building discrete structures for each process stage. One type of system that combines secondary treatment and settlement is the sequencing batch reactor (SBR). Typically, activated sludge is mixed with raw incoming sewage and mixed and aerated. The resultant mixture is then allowed to settle producing a high quality effluent. The settled sludge is run off and re-aerated before a proportion is returned to the head of the works. The disadvantage of such processes is that precise control of timing, mixing and aeration is required. This precision is usually achieved by computer controls linked to many sensors in the plant. Such a complex, fragile system is unsuited to places where such controls may be unreliable, or poorly maintained, or where the power supply may be intermittent. Package plants may be referred to as high charged or low charged. This refers to the way the biological load is processed. In high charged systems, the biological stage is presented with a high organic load and the combined floc and organic material is then oxygenated for a few hours before being charged again with a new load. In the low charged system the biological stage contains a low organic load and is combined with flocculate for a relatively long time. 107 17.4.10: Electricity Use Wastewater treatment plants are, along with water treatment plants, often the largest users of energy in a community. In developed countries these processes account for 3% of electricity use. As an example, Nazareth Wastewater Treatment Plant in Nazareth, PA uses 1.6 million kWh per year to process 365 million gallons of wastewater. For each gallon of water treated, the electric power used is equivalent to leaving a 100-W equivalent (23W) Compact Fluorescent Light bulb on for 11.4 minutes, or a 60-W equivalent (14W) bulb for 18.8 minutes. 17.4.11: Sludge Treatment and Disposal The sludge’s accumulated in a wastewater treatment process must be treated and disposed of in a safe and effective manner. The purpose of digestion is to reduce the amount of organic matter and the number of disease-causing microorganisms present in the solids. The most common treatment options include anaerobic digestion, aerobic digestion, and composting. Choice of a wastewater solid treatment method depends on the amount of solids generated and other site-specific conditions. However, in general, composting is most often applied to smaller-scale applications followed by aerobic digestion and then lastly anaerobic digestion for the larger-scale municipal applications. 17.4.12: Anaerobic Digestion Anaerobic digestion is a bacterial process that is carried out in the absence of oxygen. The process can either be thermophilic digestion, in which sludge is fermented in tanks at a temperature of 55°C, or mesophilic, at a temperature of around 36°C. Though allowing shorter retention time (and thus smaller tanks), thermophilic digestion is more expensive in terms of energy consumption for heating the sludge. One major feature of anaerobic digestion is the production of biogas, which can be used in generators for electricity production and/or in boilers for heating purposes. 17.4.13: Aerobic Digestion Aerobic digestion is a bacterial process occurring in the presence of oxygen. Under aerobic conditions, bacteria rapidly consume organic matter and convert it into carbon dioxide. The 108 operating costs are characteristically much greater for aerobic digestion because of the energy costs needed to add oxygen to the process. 17.4.14: Composting Composting is also an aerobic process that involves mixing the sludge with sources of carbon such as sawdust, straw or wood chips. In the presence of oxygen, bacteria digest both the wastewater solids and the added carbon source and, in doing so, produce a large amount of heat. 17.4.15: Sludge Disposal When a liquid sludge is produced, further treatment may be required to make it suitable for final disposal. Typically, sludges are thickened (dewatered) to reduce the volumes transported off-site for disposal. There is no process which completely eliminates the need to dispose of biosolids. There is, however, an additional step some cities are taking to superheat the wastewater sludge and convert it into small pelletized granules that are high in nitrogen and other organic materials. In New York City, for example, several sewage treatment plants have dewatering facilities that use large centrifuges along with the addition of chemicals such as polymer to further remove liquid from the sludge. The removed fluid, called centrate, is typically reintroduced into the wastewater process. The product which is left is called "cake" and that is picked up by companies which turn it into fertilizer pellets. This product is then sold to local farmers and turf farms as a soil amendment or fertilizer, reducing the amount of space required to dispose of sludge in landfills. Many processes in a wastewater treatment plant are designed to mimic the natural treatment processes that occur in the environment, whether that environment is a natural water body or the ground. If not overloaded, bacteria in the environment will consume organic contaminants, although this will reduce the levels of oxygen in the water and may significantly change the overall ecology of the receiving water. Native bacterial populations feed on the organic contaminants, and the numbers of disease-causing microorganisms are reduced by natural environmental conditions such as predation or exposure to ultraviolet radiation. Consequently, in cases where the receiving environment provides a high level of dilution, a high degree of wastewater treatment may not be required. However, recent evidence has demonstrated that very low levels of certain contaminants in wastewater, including hormones (from animal husbandry and residue from human hormonal contraception methods) and synthetic materials such as phthalates that mimic hormones in their 109 action, can have an unpredictable adverse impact on the natural biota and potentially on humans if the water is re-used for drinking water. In the US and EU, uncontrolled discharges of wastewater to the environment are not permitted under law, and strict water quality requirements are to be met. A significant threat in the coming decades will be the increasing uncontrolled discharges of wastewater within rapidly developing countries. Class activity: Anaerobic digestion is one of famous technique used these days to treat sewage. Refer to website http://en.wikipedia.org/wiki/Anaerobic_digestion to discuss in detail the importance of this method over others. References 1. Seinfeld J. H. and Pandis, S.N. (1998), Atmospheric Chemistry and Physics, John Wiley & Sons, Inc. New York. 2. Hites, R. A. (2007), Elements of Environmental Chemistry, John Wiley & Sons, Inc. 3. Khopkar, S. M. (2004). Environmental Pollution Monitoring and Control. New Age International. 4. Beychok, M.R. (1971), Performance of surface-aerated basins. Chemical Engineering Progress Symposium Series 67 (107): 322–339. 110 18: Additional Practice Questions and Model Answers Questions on Primary Air Pollution Mention different known primary pollutants and their sources and also what are the health effect of each? (a) CO (carbon monoxide): The source of CO is natural-forest fires: Causes basically by human (anthropogenic) -transportation industry (automobile exhaust), incomplete combustion of fossil fuels: eg. 2CH4 + 3O2 → 2CO + 4H2O The health effect is acute exposure- headaches, dizziness, and decreased physical performance: chronic expo-stress on cardiovascular system, heart attack (b) NOx (nitrogen oxides): The source is natural-forest fires: Causes basically by human (anthropogenic) stationary combustion sources (factories and power plants), transportation: N2(g) + O2 (g) → 2NO (g) 2NO (g) + O2 (g) → 2NO2 (g) The health effect is acute-lung irritation: chronic-bronchitis (persistent inflammation of bronchial tubes) (c) SOx (sulfur oxides): The source is natural volcanoes Causes basically by human (anthropogenic)stationary combustion sources, industry, found in metal ores, coal: S + O2 → SO2 2S + O2 → 2SO3 The health effect is acute-inflammation of respiratory tract, asthma: chronic-emphysema (breakdown of alveoli in lungs), bronchitis 111 (d) Particulates: The source is natural forest fires, volcanoes, windstorms, pollen, sea salts: Causes basically by human (anthropogenic) industry (smoke, cement, ash), domestic-heating. The health effect is irritation of respiratory system and cancer. (e) HC (hydrocarbons): The source is from living and decaying plants: Causes basically by human (anthropogenic) transportation. What are the Possible Control and Prevention Measures of Primary air Pollution Catalytic converters- devices attached to a vehicle's exhaust system to convert CO and HC into water and carbon dioxide. (CO, HC) Scrubbers- pollutant-laden air is passed through a mixture of water and lime, trapping particulates and sulfur oxide gases Filters- cloth bags through which smoke is passed to stop particulates from flowing into the air. Question on Ozone Depletion What is the Evidence for Ozone Depletion? (a) A thin layer of ozone gas (O3) encircles the earth and prevents about 99% of the ultraviolet light (UV) from the sun from reaching the earth. (b) Measurements made by British scientists at Halley Bay, Antarctica demonstrated the thinning of the ozone layer. (c) Satellite measurements of the ozone layer over Antarctica revealed that there was a hole in the layer, the size of the United States. The ozone layer over Antarctica was depleted by more than 50% in 20 years (1965-1985) while other data from satellites showed that the ozone layer was depleted even over non-polar regions (about 3%) (d) A similar hole was discovered above the Arctic, but smaller in size. 112 How the ozone formed and the depletion of ozone: (the natural process) The UV light strikes O3 molecules, which split apart. The products reunite, giving off heat and reforming ozone. The ozone layer continues renewing itself while converting UV light into heat (infrared radiation). Balanced equations: O3(ozone) + UV light → O + O2 O + O2 → O3 + Infrared radiation (heat) What are the Pollutants of Ozone layer and their sources? (a) CFC's: Propellants-spray cans: insecticides, paint, aerosols: Coolants-refrigerators, air conditioners, freezers: Styrofoam (b) NOx: High-altitude jets: supersonic transport (SST), commercial jets: Detonation of nuclear weapons, fertilizers, volcanoes (minor) What are the Catalysts for ozone depletion? (a) CFC's: These react with stratospheric ozone. When CFC's are broken down, chlorine free radicals are produced. These can react with more than 100,000 molecules of ozone. (b) NOx: These nitric oxides react with ozone to form oxygen gas (O2), thus interfering with the natural process of formation of ozone. What is the Environmental Effects of Ozone Depletion? (a) Ozone depletion increases the amount of UV light that strikes the earth's surface. An excess of this type of light causes damage to living organisms, including: Eyes cataracts- blurred vision or blindness. Skin: severe burns/cancer. Immune system: weaker immune response therefore more susceptible to diseases. Crops: interference with photosynthesis and hence lower crop yields. On the other hand marine ecosystem: plankton near surface die therefore disrupts food chain 113 What is the evidence for global warming? The history shows the level of carbon dioxide in the Earth's atmosphere had remained relatively stable until about 100 years ago. This was the time that the human race began burning fossil fuels at a high level. In just over a century, industrial civilization has added 360 billion tons of carbon dioxide to our atmosphere. Carbon dioxide is very difficult to get rid of- the natural processes are very slow and we aren't intelligent enough to have invented our own way of doing it. It is well now documented that there is a correlation between the rise in CO2 levels and the rise in temperature over the years. What are the main gases involved in a global warming? The gases that trap the heat near the Earth's surface and radiate it back: CH4, H2O (water vapor), CO2 and chlorofluorocarbons (CFC's) and nitrous oxide. Their Sources: CH4: Methane is produced by the bacterial decomposition of vegetation under water that occurs in flooded rice fields. Cattle- "ruminants" (because they "ruminate") i.e. cows, etc. have four stomachs. In the complicated process of digesting the the food is fermented, producing a large quantity of methane gas as a by-product which the cow expels. H2O: Water vapor is always present in the atmosphere the water cycle. CO2 It is produced naturally by the decomposition of organic materials, growing plants, and weathering of rocks. (While they are burning millions of trees in the tropical rain forest, or using fire woods not only they suffocating us, and killing many species of animals, but they are also contributing to global warming. It is produced by the burning of fossil fuels- coal, oil and natural gas. i.e. use of furnaces and coal fires, aircrafts and cars, industrial plants, etc. 114 Influence of Gases on Global warming The greenhouse theory proposes that the greenhouse gases are increasing in the atmosphere to such an extent that less reradiated energy is able to escape from the atmosphere. The greater quantities of greenhouse gases absorb this long-wave radiation, effectively trapping it. The energy is then reradiated into the lower atmosphere. The gases may be said to be acting like a blanket, absorbing more and more radiation in the atmosphere. In theory, this could lead to a progressive increase in temperatures and eventually to climatic change. Methane: This gas is present in the atmosphere only in very small quantities, but it absorbs radiation more effectively than CO2. Carbon dioxide: Naturally occurring CO2 allows sunlight to pass through the atmosphere and heat the Earth, but also absorbs infrared radiation escaping from the Earth's surface and radiates it back to earth. This process helps maintain Earth's temperature. The increase of CO2 concentration in the air slows down the escape of heat global warming. Like almost anything in the world, little carbon dioxide is good; too much may be devastating. Effects of Global Warming Oceans will absorb more heat energy, making hurricanes and typhoons more common. There will be a change in ocean current patterns. The world's weather patterns will be altered significantly and flooding in some areas/droughts in others. Some areas like USA and Canada will become warmer and drier and the agricultural plains might become too dry to support dry-land farming. Melting of polar ice caps and glaciers: This would raise the sea level, flooding up to 20% of the world's land mass. (Sinking of islands like Zanzibar, Florida and especially Holland, which is already half-inside the sea-they just retrieved it by building canals. The construction of climate models is difficult because of feedback processes: The Earth warms then more water evaporates, more clouds form, clouds reduce the amount of heat reaching Earth 115 and the warming trend will slow down or polar ice caps melt, ice reflects back sunlight instead of absorbing, when ice decreases, warming speeds up. What is the Influence of Particulates on Temperature? Particulates consist of smoke, ash, soot, dust, lead, and other particles from burning fuel. They come from industrial processes and motor vehicles that burn fossil fuel, burning wood, and dust from construction and agriculture. They can form clouds that reduce visibility and cause a variety of respiratory problems. Particulates have also been linked with cancer. They also corrode metals, erode buildings and sculptures, and soil fabrics, and they can lower the temperature by reflecting sunlight. 116 19: Additional Reading List 1. Thomas, G. S. and William, M. S. (2003). Chemistry of the Environment, Prentice Hall. 2. Stanley, E. M. (2000). Environmental Chemistry, Lewis Publishers. 3. Miller, T. G . (1999). Living in the Environment. Brooks/Cole Publishing. 4. Baird, C. and Cann, M. Environmental Chemistry, (2004), 3rd edition, W.H. Freeman and Co, New York. 5. Jacob, D. J. (1999), Introduction to Atmospheric Chemistry, Princeton University Press. 6. Seinfeld J. H. and Pandis, S.N. (1998), Atmospheric Chemistry and Physics, John Wiley & Sons, Inc. New York. 7. Hites, R. A. (2007), Elements of Environmental Chemistry, John Wiley & Sons, Inc. 8. http://en.wikipedia.org/wiki/Ozone_depletion (15 April, 2009) 9. Chang, R. (1991), Chemistry, 4th Edn McGraw – Hill, Inc. 10. UNEP (1989) Environmental Effects Panel Report, UNEP, Nairobi, 64 pp. 11. http://en.wikipedia.org/wiki/Global_warming 12. Royal Society (2005). "Joint science academies' statement: Global response to climate change" Retrieved on 19 April 2009 13. http://en.wikipedia.org/wiki/Greenhouse_gas . Retrieved on 27/04/2009 14. Hites, R. A. (2007); Elements of Environmental Chemistry, John Wiley and Sons Inc. Publication. 15. McNeill, V. F.; et al (2006). "The effect of varying levels of surfactant on the reactive uptake of N2O5 to aqueous aerosol". Atmospheric Chemistry and Physics (Strasbourg, France: European Geosciences Union) 16. http://www.atmos-chem-phys-discuss.net/6/17/2006/acpd-6-17-2006-print.pdf. Retrieved on 2008-10-17. 17. Ritter L; Solomon KR, Forget J, Stemeroff M, O'Leary C.. "Persistent organic pollutants". United Nations Environment Programme. http://www.chem.unep.ch/pops/ritter/en/ritteren.pdf. Retrieved on 2007-09-16. 18. Berg, U. T.; Heilmann, S.; Hemmingsen, V.; Dahi, E. and Thogersen, J. water use Pattern and bacterial Growth During Water Storage by Consumers in Dar es Salaam,(1992) Workshop Research on Hygenic Water in Dare s Salaam. 117 19. Mihale, M. (2002); Chemometrics of Obsolete Pesticides at Vikuge Farm, Kibaha District, Tanzania , M. Sc. Thesis, University of Dar es Salaam. 20. United Republic of Tanzania, Ministry of Water and Irrigation, MAJI WEEK 16 – 22 March 2008. 21. Appropriate Technology for Sewage Pollution Control in the Wider Caribbean Region, Caribbean Environment Programme Technical Report #40 1998 22. Massoud, T. and Ahmad, A. (2005), Integrated Approach to Water and Wastewater Management for Tehran,Iran, Water Conservation, Reuse, and Recycling: Proceedings of the Iranian-American Workshop, National Academies Press . 23. Joshi, P.K., Tyagi, N.K. & Svendsen, M. 1995. Measuring crop damage due to soil salinity. In M.S.A. Gulati, ed. Strategic change in Indian irrigation. New Delhi, Macmillan India Limited. 24. Kahnert, F. & Levine, G. 1989. Key findings, recommendations, and summary. Groundwater irrigation and the rural poor: options for development in the Gangetic basin. Washington, DC, the World Bank. 25. Alexander, S. (1977), Soil Microbiology, 2nd Ed., Wiley Interscience 26. Bohn, M., and O'Connor, N. (1985), Soil Chemistry, 2nd Ed, Wiley Interscience 27. Seinfeld, J. H.; Pandis, S. N. (1998). Atmospheric Chemistry and Physics - From Air Pollution to Climate Change. John Wiley and Sons, Inc. 28. Schroeder, D. (2000), Thermal Physics, Addison Wesley Longman. 118