Environmental Pollution

1. Environmental Pollution Environmental pollution can be defined as any undesirable change in the physical, chemical or biological characteristics of any component of the environment ( air, water, soil), which can cause harmful effects on various form of life or property. Pollutant are defined as matter or energy which leads to undesirable changes in the environment. Pollutants include solid, liquid or gaseous substances present in greater than natural abundance produced due to human activity, which have a detrimental effect on our environment. From an ecological perspective pollutants can be classified as follows: • Degradable or non-persistent pollutants: These can be rapidly broken down by natural processes. Eg: domestic sewage, discarded vegetables etc. • Slowly degradable or persistent pollutants: Pollutants that remain in the environment for many years in an unchanged condition and take decades or longer to degrade. Eg: DDT and most plastics. • Non-degradable pollutants: These cannot be degraded by natural processes. Once they are released into the environment they are difficult to eradicate and continue to accumulate. Eg: toxic elements like lead or mercury.

2. Environmental Pollution could be of various types: • Air Pollution: Air pollution occurs due to the presence of certain substances (including the normal constituent in excess) in concentration which can cause undesirable effects on human health, property and structure. These substance include gases, particulate matter, radioactive substances etc. Classification of Air Pollutants: Based upon their origin air pollutant are classified into two type: • Primary Pollutants: Pollutants that are emitted directly from identifiable sources are produced both by natural events (for example, dust storms and volcanic eruptions) and human activities (emission from vehicles, industries, etc.). There are five primary pollutants that together contribute about 90 percent of the global air pollution. These are carbon oxides (CO and CO2), nitrogen oxides, sulfur oxides, volatile organic compounds (mostly hydrocarbons) and suspended particulate matter.

3. Major primary pollutant: • Particulate matter: Particulates are small pieces of solid material (for example, smoke particles from fires, bits of asbestos, dust particles and ash from industries) dispersed into the atmosphere. The effects of particulates range from soot to the carcinogenic (cancer causing) effects of asbestos, dust particles and ash from industrial plants that are dispersed into the atmosphere. Some definition: • Aerosol : General term for particles suspended in air. Example: Sprays from pressurized cans • Mist : Aerosol consisting of liquid droplets. Example: Sulfuric acid mist • Dust : Aerosol consisting of solid particles that are blown into the air or are produced from larger particles by grinding them down. Example :Dust storm

4. d ) Smoke: Aerosol consisting of solid particles or a mixture of solid and liquid particles produced by chemical reaction such as fires. Example: Cigarette smoke, smoke from burning garbage. e) Fume : It applies specifically to aerosols produced by condensation of hot vapors of metals. Example: Zinc/lead fumes f) Plume :Geometrical shape or form of the smoke coming out of a chimney. g) Fog : Aerosol consisting of water droplets. h) Smog : Term used to describe a mixture of smoke and fog. 2) Hydrocarbon: Hydrocarbons are a group of compounds consisting of carbon and hydrogen atoms. They either evaporate from fuel supplies or are remnants of fuel that did not burn completely. They act as precursor for the production of secondary pollutant. 3) Nitrogen oxides: Nitrogen oxides are found in vehicular exhausts. Nitrogen oxides are significant, as they are involved in the production of secondary air pollutants such as ozone. They are also important component of acid rain.

5. 4) Sulfur oxides: are produced when sulfur containing fossil fuels are burnt. They are major source of increase acidity in the atmospheric rain, a phenomenon known as acid rain. 5) Carbon monoxide: is a colorless, odorless and toxic gas produced when organic materials such as natural gas, coal or wood are incompletely burnt. Vehicular exhausts are the single largest source of carbon monoxide. Carbon monoxide is however not a persistent pollutant. Natural processes can convert carbon monoxide to other compounds that are not harmful. 6) Carbon dioxide: It is a component of atmospheric system and currently make 0.038 per cent of the atmosphere. It is important for maintaining the biotic system as source of carbon. However during past decade its concentration is increasing in atmosphere due to burning of fossils fuel and land use changes. It shows property of green house gas, thereby results into a phenomenon known as global warming. Beside these primary pollutant there are certain pollutant which are not a natural component of atmosphere, however it is introduced by human activities, such as CFC, MIC (methyl isocyanate).

6. b) Secondary Pollutant: Pollutants that are produced in the atmosphere when certain chemical reactions take place among the primary pollutants are called secondary pollutants. Eg: sulfuric acid, nitric acid, carbonic acid, etc. Some more example of Secondary pollutant: a) Tropospheric Ozone: The majority of tropospheric ozone formation occurs when nitrogen oxides (NOx), carbon monoxide (CO) and volatile organic compounds (VOCs), such as xylene, react in the atmosphere in the presence of sunlight. NOx, CO, and VOCs are called ozone precursors.

7. b) Peroxyacetyl nitrate (PAN): Acyl peroxy nitrates, or Peroxyacyl nitrates (also known as APNs, or PANs), are powerful respiratory and eye irritants present in photochemical smog. CH3C(O)OO· + NO2 → CH3C(O)OONO2 General equation: Hydrocarbons + O2 + light → CxHyC(O)OO· CxHyC(O)OO· + NO2 → CxHyC(O)OONO2 Sources of Air Pollutants: It is mainly divided into two types: • Natural sources: The natural sources of air pollution are volcanic eruption, forest fire, sea salt sprays, biological decay, pollen grains of flowers, spore etc. Radioactive material present in earth crust are the sources of radioactivity in the atmosphere. • Man-made (Anthropogenic sources): it include thermal power plant, industrial units, vehicular emissions, fossil fuel burning, agricultural activities etc.

8. Human can be affected by both indoor or outdoor air pollution. The most important indoor air pollutant is radon gas ( causing lung cancer) a radioactive atomic gas that results from the radioactive decay of radium, which may be found in rock formations beneath buildings or in certain building materials (bricks, concrete, tiles etc) themselves. Beside radon carbon monoxide, sulphur dioxide and hydrocarbon released due to incomplete combustion of fossil fuel, wood for cooking purpose are other major air pollutant. Fate of pollutant in the atmosphere: • Once pollutants enter the troposphere they are transported downwind, diluted by the large volume of air, transformed through either physical or chemical changes or are removed from the atmosphere by rain during which they are attached to water vapor that subsequently forms rain or snow that falls to the earth’s surface. • However this transportation and dilution of air pollutant depend strongly upon the metrological condition prevailing in that area. In an unstable atmosphere with high wind speed dispersion and dilution rate of pollutant are much higher compare to a stable atmosphere.

9. Stable atmosphere: If we (somehow) lift the parcel: It will cool at the dry adiabatic lapse rate. The parcel will find itself cooler than the environmental (sounding) temperature. At the same pressure, a cooler parcel will be more dense than the environment. Being denser, the parcel will descend back to where it came from. STABLE!

10. Unstable atmosphere: If we (somehow) lift the parcel:  It will cool at the dry adiabatic lapse rate. The parcel will find itself warmer than the environmental (sounding) temperature. At the same pressure, a warmer parcel will be less dense than the environment. Being less dense, the parcel will ascend and move farther from where it came from. UNSTABLE!

11. Temperature Inversion: • Although the temperature of air relatively near the earth’s surface normally decreases with increasing altitude, certain atmospheric conditions can result in the opposite condition—increasing temperature with increasing altitude. Such conditions are characterized by high atmospheric stability and are known as temperature inversions. Because they limit the vertical circulation of air, temperature inversions result in air stagnation and the trapping of air pollutants in localized areas. Frontal inversion: An inversion can form from the collision of a warm air mass (warm front) with a cold air mass (cold front). The warm air mass overrides the cold air mass in the frontal area, producing the inversion. Radiation inversions: Itis the most common form of surface inversion and occurs when the earth's surface cools rapidly. As the earth cools, so does the layer of air close to the surface. If this air cools to a temperature below that of the air above, it becomes very stable, and the layer of warmer air impedes any vertical motion. Radiation inversions usually occur in the late evening through the early morning under clear skies with calm winds, when the cooling effect is greatest.

12. Subsidence inversions: It takes place in hill area, where cooler surface air tends to flow into valleys at night, where it is overlain by warmer, less dense air., often accompanied by radiation inversions, can become very widespread. marine inversion is produced during the summer months when cool air laden with moisture from the ocean blows onshore and under warm, dry inland air. Illustration of pollutants trapped in a temperature inversion.

13. Impact of Temperature Inversion: Due to temperature inversion condition, environment become more stable thereby prevent dispersal and dilution of air pollutant from localized areas. One major air pollution scenario due to temperature inversion condition is London Smog (winter of 1952). The weather in Greater London had been unusually cold for several weeks leading up to the event. Because of the cold weather, households were burning more coal than usual to keep warm. The smoke from approximately one million coal-fired stoves, in addition to the emissions from local industry, was released into the atmosphere. Thousands of tons of black soot, tar particles, and sulfur dioxide had accumulated in the air from the heavy coal combustion. Estimates of PM10 concentrations during December, 1952, range between 3,000 and 14,000 μg/m³ with the high range being approximately 50 times higher than normal levels at the time. PM10 is particulate matter less than 10 micrometers in diameter. Due to temperature inversion condition the thick, smoke-polluted air to be trapped under the inversion. The following 114 hours in London experienced visibility less than 500 meters with 48 hours below 50 meters visibility. The smog-related deaths were primarily attributed to pneumonia, bronchitis, tuberculosis, and heart failure.

14. Effect of air pollution: Air pollution has adverse effects on living organisms and materials. • Effects on human health: • Air pollutants especially particulate matter( PM) is related to respiratory disease in human being. Particles come in a wide range of sizes. Those less than 10 micrometers in diameter (PM10) are so small that they can get into the lungs, potentially causing serious health problems. Suspended particulates can cause damage to lung tissues and diseases like asthma, chronic bronchitis and emphysema( damage of air sacs leading to loss of lung elasticity and acute shortness of breath. Metal in form of PM like lead, Asbestos etc also cause cancer or affect vital organs like kidney, liver, spleen etc. • Carbon monoxides combine with hemoglobin of blood to form carboxyhaemoglobin, due to which oxygen carrying capacity to various part of the body decrease which may results into suffocation, dizziness, unconsciousness and even death. • Hydrocarbon emission from vehicle or industrial units ( benzene, formaldehyde etc.) may results into mutation, reproductive problem or even cancer.

15. 2) Effects on Plant: Air pollution affects plant by entering through stomata (leaf pores through which gases diffuse), destroy chlorophyll and affect photosynthesis. Gaseous pollutant like SO2, ozone etc can leads to necrosis( dead areas of leaf), chlorosis( yellowing of leaf due to reduction of chlorophyll), abscission ( dropping of leaves). 3) Effects on aquatic life: Air pollutants (SOx and NOx) when mix up with rain can results into acid rain which reduces the pH of fresh water lakes especially in the higher latitude. This reduction in pH has adverse effect on aquatic biotic life. 4) Effects on materials: Metal parts of building, vehicle, bridges, wires and railway tracks are affected due to corrosion by particulate matter which further get accerlated in presence of SO2 and moisture. Acid rain also affect the structure made up of marble and limestone. Example: Yellowing of Tajmahal in Agra. Ozone in the atmosphere can cause cracking of rubber tyres. Beside these effect, air pollutants also leads to stratospheric Ozone depletion which results into increase UV-rays exposure on earth surface and Global warming due to green house gases are major environmental problem .

16. Control of Air Pollution: Air pollution can be controlled by applying various measures such as: 1) Zonation in landuse pattern: Industries should be set up far away from the residential areas and Industries should be located in places so as to minimize the effects of pollution after considering the topography and the wind directions. 2) Dilution of emission: It can be done by increasing stack height, beyond inversion layer. Wind current will disperse the pollutant. 3) Low Sulphur coal in industries to reduce emission of SOx from coal burning. 4) Installment of pollutant control equipment such as cyclones, bag house filters etc to reduce pollutant emission from industry. 5) Establish vehicular emission norm to reduce emission from vehicles, regular tuning of engines, installing catalytic converters, engine modification to have fuel efficient (lean) mixture to reduce CO and hydrocarbon emission, slow and cooler burning of fuels to reduce NOx emission( Honda Technology). 6) Using mass transport system, bicycles etc.

17. Bharat stage emission standards are emission standards instituted by Government of India to regulate the output of air pollutants from internal combustion engine equipments, including motor vehicles. The standards and the timeline for implementation are set by the Central Pollution Control Board under the Ministry of Environment & Forests.

18. Air Pollution control through pollution control equipment: • Particulate matter: Many devices are available for removal of particulate matter and choice of which depend upon characteristics of particulate matter, flow rate, collection efficiency, costs etc. a) Cyclone: These are employed for large size particle. The gas with particle in it enters tangentially at the top of cylinder and spin forming a vortex. Due to centrifugal force, particle strike the wall of cylinder.

19. b) Bag house filter: It contain large number of filter bags made of fabric. Dirty gas is passed through the filter bags which leaves the bags through their pores. The dust particle get deposited on the inner surface of the bag filters and may form a cake which can be removed by shaking. c) Wet scrubber: Dirty gas is passed through water in the chamber or water is sprayed on the gas. Particles are made wet and are removed from the gas stream which leaves from the top of scrubber.

20. d) Electrostatic precipitator: It can be plate type or cylinder type. Vertical wires are placed between the parallel plates or wires is hung along the axis of the cylinder. Higher negative voltage is applied to the wire. Dust particle while passing from the lower end get negatively charged and are collected on the positively charges surface. Electrostatic precipitators utilize electric energy and can efficiently remove even submicroscopic particles. 2) Gaseous pollutant: Gaseous pollutant can be reduced by physical absorption on porous solid materials like activated charcoal, silica gel etc. Effluent gases can be absorbed in liquid absorbent, example: SO2 absorbed in ammonia solution.

21. Air quality monitoring: The Central Pollution Control Board (CPCB) initiated its own national Ambient Air Quality Monitoring (NAAQM) program in 1985. Ambient air quality standards in India developed by the Central Pollution Control Board

22. Legal aspects of air pollution control in India: The Air (Prevention and Control of Pollution) Act was legislated in 1981. The Act provided for prevention, control and abatement of air pollution. In areas notified under this Act no industrial pollution causing activity could come up without the permission of the concerned State Pollution Control Board. To regulate vehicular pollution the Central Motor Vehicles Act of 1939 was amended in 1989. Following this amendment the exhaust emission rules for vehicle owners were notified in 1990 and the mass emission standards for vehicle manufacturers were enforced in 1991 for the first time. The mass emission norms have been further revised for 2000.

23. Water Pollution When the quality or composition of water changes directly or indirectly as a result of man’s activities such that it becomes unfit for any purpose it is said to be polluted. Water pollution can be defined as alteration in physical, chemical or biological characteristics of water making it unsuitable for the designated use in its natural state. Source of water pollution: Water pollution can be divided into two broad category depending upon source nature: Point sources: When a source of pollution can be readily identified because it has a definite source and place where it enters the water it is said to come from a point source. Eg. Municipal and Industrial Discharge Pipes. 2) Non-point sources: When a source of pollution cannot be readily identified, such as agricultural runoff, atmospheric deposition , etc, they are said to be non-point sources of pollution.

24. Source of water pollution can be different for both ground water and surface water. Ground water Pollution: While surface water pollution are highly visible and often get a lot of media attention, a much greater threat to human life comes from our groundwater being polluted which is used for drinking and irrigation. Major source of ground water pollution are: a) Urban run-off of untreated or poorly treated waste water and garbage. Industrial waste storage located above or near aquifers. Agricultural practices such as the application of large amounts of fertilizers and pesticides, animal feeding operations, etc. in the rural sector. Leakage from underground storage tanks containing gasoline and other hazardous substances. e) Leachate from landfills. f) Poorly designed and inadequately maintained septic tanks.

25. Beside these sources of ground water pollution, high metal concentration in ground water in some part of country is also very serious environmental issue. The high Arsenic concentration in ground water in West Bengal is known today as the worst case of groundwater pollution. The arsenic poisoning was first noticed by K C Saha, former professor of dermatology at the School of Tropical Medicine, Kolkata when he began to receive patients with skin lesions that resembled the symptoms of leprosy which was in reality not leprosy. Since all the patients were from the district of 24-Parganas, Saha along with others began to look for the cause and found it to be arsenic toxicity. There are two theories that have been put forth to explain this unusually high content of arsenic in groundwater. a) According to the first hypothesis, arsenic probably originates in the Himalayan headwaters of the Ganga and the Brahmaputra rivers and has been lying undisturbed beneath the surface of the region’s deltas for thousands of years in the thick layers of fine alluvial mud across the banks of these rivers.

26. The mud in these areas is thicker, wider and flatter than almost anywhere else on earth. It can thus take hundreds or thousands of years for underground water to percolate through the mud before reaching the sea and thus it absorbs arsenic for a long period. b) Other researchers feel that the excess amount of arsenic in groundwater can be contributed to by the high rate of groundwater extraction. Their hypothesis called the pyrite oxidation thesis describes how arsenic can get mobilized in the groundwater. In this hypothesis arsenic is assumed to be present in certain minerals (pyrites) that are deposited within the aquifer sediments. Due to the lowering of the water table below the deposits, arseno-pyrite which is oxidized in a zone of the aquifer called the Vadose zone releases arsenic as arsenic adsorbed on iron hydroxide. During the subsequent recharge period, iron hydroxide releases arsenic into groundwater. Arsenicosis or arsenic toxicity develops after two to five years of exposure to arsenic contaminated drinking water depending on the amount of water consumption and the arsenic concentration in water. Initially the skin begins to darken (called diffuse melanosis) which later leads to spotted melanosis when darkened sports begin to appear on the chest, back and limbs. At a later stage leucomelanosis sets in and the body begins to show black and white spots.

27. Surface water pollution: The major source of surface water pollution are: Sewage: Domestic waste mostly dumped into the nearby surface water bodies through drain and sewers are major source of pollution especially near the big cities. Industrial effluents: waste emerging out of industrial units are major source of toxic chemicals, acids, alkalis, metallic salts, organic material to the surface water bodies. Synthetic detergents: They are used in washing and cleaning processes in domestic and industrial unit and are major source of phosphate to surface water bodies. Agrochemicals: Chemicals like fertilizers, pesticides and insecticides applied in the agricultural land or urban garden areas washed away with rainwater and surface runoff from these area are major pollution source. Oil: Oil spillage into sea-water during drilling and shipment transport are major source of water pollution.

28. 6) Waste heat: Thermal pollution occurs when industry returns the heated water to a water source. The warm water not only decreases the solubility of oxygen but changes the breeding cycles of various aquatic organisms. The state of India’s rivers Urbanization, industrialization, excess withdrawal of water, agricultural run-off, improper agricultural practices and various religious and social practices all contribute to river pollution in India. Waters from the Ganga and the Yamuna are drawn for irrigation through the network of canals as soon as these rivers reach the plains reducing the amount of water that flows downstream. What flows in the river is water from small nalas, and streams that carry with them sewage and industrial effluents. Sewage and municipal effluents account for 75% of the pollution load in rivers while the remaining 25% is from industrial effluents and non-point pollution sources. In 1985, India launched the Ganga Action plan (GAP) the largest ever river clean-up operation in the country. The GAP Phase II in 1991 included cleaning operations for the tributaries of the Ganga, ie; the Yamuna, Gomti and the Damodar. Thus the Yamuna Action Plan (YAP), Gomti Action Plan and the Damodar Action plan were added.

29. In 1995 the National River Conservation plan( NRCP) was launched. Under this all the rivers in India were taken up for clean-up operations. In most of these plans, attempts have been made to tap drains, divert sewage to sewage treatment plants before letting out the sewage into the rivers. NRCP is scheduled to be completed by March 2005. The approved cost for the plan is Rs. 772.08 crores covering 18 rivers in 10 states including 46 towns. The cost is borne entirely by the Central Government and the Ministry of Environment and Forests is the nodal agency that co-ordinates and monitors the plan. Under this plan the major activities include treating the pollution load from sewer systems of towns and cities, setting up of Sewage treatment plants, electric crematoria, low cost sanitation facilities, riverfront development, afforestation and solid waste management. Water pollutants are classified into different class depending upon their nature: 1) Oxygen demanding waste: These are organic wastes that can be decomposed by aerobic (oxygen requiring) bacteria. Large populations of bacteria use up the oxygen present in water to degrade these wastes.

30. The amount of oxygen required to break down a certain amount of organic matter is called the biological oxygen demand (BOD). The amount of BOD in the water is an indicator of the level of pollution. If too much organic matter is added to the water all the available oxygen is used up. This causes fish and other forms of oxygen dependent aquatic life to die. 2) Disease-causing agents: It include bacteria, viruses, protozoa and parasitic worms that enter water from domestic sewage and untreated human and animal wastes. Human wastes contain concentrated populations of coliform bacteria such as Escherichia coli and Streptococcus faecalis. These bacteria are not harmful in low numbers. Large amounts of human waste in water, increases the number of these bacteria which cause gastrointestinal diseases. 3) Inorganic plant nutrients: These are water soluble nitrates and phosphates that cause excessive growth of algae and other aquatic plants. The excessive growth of algae and aquatic plants due to added nutrients is called eutrophication. While excess fertilizers cause eutrophication, pesticides cause bioaccumulation and biomagnification. At each link in the food chain these chemicals which do not pass out of the body are accumulated and increasingly concentrated resulting in biomagnification of these harmful substances.

32. 4) Water soluble inorganic chemicals: It includes acids, salts and compounds of toxic metals such as mercury and lead. High levels of these chemicals can make the water unfit to drink, harm fish and other aquatic life, reduce crop yields and accelerate corrosion of equipment that use this water. The disease called Minamata disease occurred due to consumption of methyl mercury contaminated fish caught from Minamata bay in Japan. The disease claimed 50 lives and permanently paralysed over 700 persons. Pollution from hevy metal cadmium had caused the disease Itai-Itai in the people of Japan. This disease was caused by cadmium contaminated rice. In this disease bones, liver, kidney, lungs, pancreas and thyroid are affected. 5) organic chemicals: It include oil, gasoline, plastics, pesticides, cleaning solvents, detergent and many other chemicals. These are harmful to aquatic life and human health. 6) Sediment of suspended matter: These are insoluble particles of soil and other solids that become suspended in water. This occurs when soil is eroded from the land. High levels of soil particles suspended in water, interferes with the penetration of sunlight. This reduces the photosynthetic activity of aquatic plants and algae disrupting the ecological balance of the aquatic bodies.

33. 7) Water soluble radioactive isotopes: These can be concentrated in various tissues and organs as they pass through food chains and food webs. Ionizing radiation emitted by such isotopes can cause birth defects, cancer and genetic damage. 8) Hot water: Water is used for cooling purpose in Thermal power plant and other industries. Thermal pollution occurs when industry returns the heated water to a water source. This heated water, which is at least 15oC higher than the normal is discharged back into the water body. The warm water not only decreases the solubility of oxygen but changes the breeding cycles of various aquatic organisms. Control of water pollution: Non-point sources are difficult to control in respect to point sources, discharge from point sources can be controlled by using different treatment process. Non-point pollution can be reduced by reducing use of chemical fertilizer, pesticides and insecticides applied to agricultural field. By reducing the amount of surface runoff by lining the street with percolating materials and prevent the mixing of storm water with the sewer water to avoid overflowing of sewer line.

34. Municipal waste water treatment Plant: The municipal waste water treatment plant are deigned to reduced the BOD and suspended solid load from the domestic sewage. Some of the treatment plant also use advanced treatment (tertiary) for removal of nutrients such as nitrogen and phosphate.

35. Primary treatment: These treatment plants use physical processes such as screening and sedimentation to remove pollutants that will settle, float or, that are too large to pass through simple screening devices. This includes, stones, sticks, rags, and all such material that can clog pipes. A screen consists of parallel bars spaced 2 to 7cms apart followed by a wire mesh with smaller openings. After screening the wastewater passes into a grit chamber. The detention time is chosen to be long enough to allow lighter, organic material to settle. From the grit chamber the sewage passes into a primary settling tank (also called as sedimentation tank) where the flow speed is reduced sufficiently to allow most of the suspended solids to settle out by gravity. Primary treatment normally removes about 35 percent of the BOD and 60 percent of the suspended solids. Secondary treatment: There are three commonly used approaches: trickling filters, activated sludge process and oxidation ponds. Secondary treatment can remove at least 85 percent of the BOD.

36. Trickling filter: It consists of a rotating distribution arm that sprays liquid wastewater over a circular bed of ‘fist size’ rocks or other coarse materials. The spaces between the rocks allow air to circulate easily so that aerobic conditions can be maintained. The individual rocks in the bed are covered with a layer of slime, which consists of bacteria, fungi, algae, etc. which degrade the waste trickling through the bed. Trickling filter

37. Activated sludge process: The sewage is pumped into a large tank and mixed for several hours with bacteria rich sludge and air bubbles to facilitate degradation by micro-organisms. The water then goes into a sedimentation tank where most of the microorganisms settle out as sludge. This sludge is then broken down in an anaerobic digester where methane-forming bacteria slowly convert the organic matter into carbon dioxide, methane and other stable end products.

38. Oxidation ponds These are large shallow ponds approximately 1 to 2 meters deep where raw or partially treated sewage is decomposed by microorganisms. They are easy to build and manage and accommodate large fluctuations in flow and can provide treatment at a much lower cost. They however require a large amount of land and hence can be used where land is not a limitation. Advanced sewage treatment: This involves a series of chemical and physical process that removes specific pollutants left in the water after primary and secondary treatment. Sewage treatment plant effluents contain nitrates and phosphates in large amounts. These contribute to eutrophication. Thus advanced treatment plants are designed to specifically remove these contaminants. Chlorination of water is generally done to kill harmful bacteria and some viruses. Advanced treatment plants are very expensive to build and operate and hence are rarely used.

39. Thermal Pollution It is defined as presence of waste heat in the water which can cause undesirable changes in the natural environment. Causes of thermal pollution: Heat producing industries i.e., thermal power plant, nuclear power plants, refineries, steel mills, etc are the major sources of thermal pollution. Power plants heat water to convert it into steam, to drive the turbines that generate electricity. For efficient functioning of the steam turbines, the steam is condensed into water after it leaves the turbines. This condensation is done by taking water from a water body to absorb the heat. This heated water, which is at least 15oC higher than the normal is discharged back into the water body. Effect of Thermal Pollution: 1) The dissolved oxygen concentration (DO) of water is decreased as the solubility of oxygen in water is decreased at high temperature.

40. 2) Toxicity of pesticides, detergents and chemicals in the effluent increases with increase in temperature. 3) The composition of flora and fauna changes because the species sensitive to increased temperature due to thermal shock will be replaced by temperature tolerant species. 4) Metabolic activities of aquatic organisms increase at high temperature and require more oxygen, whereas oxygen level falls under thermal pollution. 5) Discharge of heated water near the shores can disturb spawning and can even kill young fishes. 6) Fish migration is affected due to formation of various thermal zones. Control of Thermal Pollution: Thermal pollution are control by using following majors: 1) Cooling ponds: Water from condenser is stored in ponds where natural evaporation cools the water which can then be recirculated or discharged in nearby water body.

41. Cooling ponds 2) Spray Ponds: The water from condenser is received in spray ponds. Here the water is sprayed through nozzles where fine droplets are formed. Heat from these fine droplet is dissipated to the atmosphere.

42. 3) Cooling towers: It can be of two types: Wet cooling tower: Hot water is sprayed over baffles. Cool air entering from sides take away the heat and cools the water. This cool water can be recycled or discharged. Dry cooling tower: The heated water flow through pipes. Air is passed over these hot pipes with fans. It is costlier than wet cooling tower.

43. Wet Cooling Tower

45. Marine Pollution Marine pollution can be defined as the introduction of substances to the marine environment directly or indirectly by man resulting in adverse effects such as hazards to marine biota, obstruction of marine activities and lowering the quality of sea water. Sources of Marine Pollution: The municipal waste and sewage from residences and hotels in coastal towns are directly discharged into the sea. Pesticides and fertilizers from agriculture which are washed off the land by rain, enter water courses and eventually reach the sea. 3) Petroleum and oils washed off from the roads normally enter the sewage system but storm water overflows carry these materials into rivers and eventually into the seas. 4) Oil release due to Ship accidents and accidental spillages at sea can be very damaging to the marine environment.

46. 5) Offshore oil exploration and extraction also pollute the seawater to a large extent. Pollution due to oil: Oil pollution of the sea normally attracts the greatest attention because of its visibility. There are several sources though which the oil can reach the sea. Tanker operations: After a tanker has unloaded its cargo of oil it has to take on seawater as ballast for the return journey. This ballast water is stored in the cargo compartments that previously contained the oil. load-on-top system and ‘crude oil washing’ are used to reduced the amount of oil in the ballast water. 2) Dry docking: All ships need periodic dry docking for servicing, repairs, cleaning the hull, etc. During this period when the cargo compartments are to completely emptied, residual oil finds its way into the sea. 3) Bilge and fuel oils : This generate in the process of emptying fuel tanker. 4) Tanker accidents: Tanker accidents are major source of oil-spillage in the sea.

47. 5) Offshore oil production: Oil that is extracted from the seabed contains some water. Even after it is passed through oil separators the water that is discharged contains some oil, which adds to marine pollution. Uncontrolled release of oil from the wells can be catastrophic events resulting in oil pollution. Effects of marine pollution: When liquid oil is spilled on the sea it spreads over the surface of the water to form a thin film called an oil slick. The rate of spreading and the thickness of the film depends on the sea temperature and the nature of the oil. Oil slicks damage marine life to a large extent. Salt marshes, mangrove swamps are likely to trap oil and the plants, which form the basis for these ecosystems thus suffer. If liquid oil contaminates a bird’s plumage its water repellent properties are lost. Water thus penetrates the plumage and displaces the air trapped between the feathers and the skin. This air layer is necessary as it provides buoyancy and thermal insulation. Birds often clean their plumage by preening and in the process consume oil which depending on its toxicity can lead to intestinal, renal or liver failure.

48. Control measures for oil pollution: The natural process of emulsification of oil in the water can be accelerated through the use of chemical dispersants which can be sprayed on the oil. The oil pollution control has physical strategy i.e. mechanical booms or barriers are spread around an oil slick to check it progress. Natural material like peat moss, straw, sawdust, and pine bark can be used as dispersants. Bioremediation is fast emerging technology for cleaning the clean oil spill by using strain of microbes which decomposes oil. Control of marine pollution: Toxic pollutant from industries and sewage treatment plant should not be discharged in coastal area. Run off from non-point sources should be prevented to reach coastal area. Ecological sensitive coastal area should be protected from oil-drilling activiteis. Oil Ballast should not be dumped to the sea.

49. Soil Pollution Soil is a thin covering over the land consisting of a mixture of minerals, organic material, living organisms, air and water that together support the growth of plant life. Sources of soil pollution: Dumping of domestic and industrial waste on soil surface results into soil pollution. Domestic waste include garbage, rubbish material like glass, plastic, metallic can, paper, fibers etc. Industrial wastes are effluent discharged from industries and type of contaminant are dependent upon the type of Industry present. For example: paper and pulp industry mostly generate organic waste, Thermal power plant generate fly ash, or precipitate forms during treatment of pollutant gas produced due to combustion processes. • Application of fertilizers and Insecticides into agricultural land are other important category of soil pollution. Persistent pesticides once applied are effective for a long time. However as they do not break down easily they tend to accumulate in the soil and in the bodies of animals in the food chain.

50. Soil also receive excreta from animals and humans. The sewage sludge contain many pathogenic organism, bacteria, viruses and intestinal worm which cause pollution in the soil. • The source of radioactive substances in soil are explosion of radioactive devices, radioactive waste discharged from industries and laboratories, aerial fall out etc. The main isotopes are radium, uranium, thorium, strontium etc. Effects of soil pollution: • Sewage and industrial effluents which pollute the soil ultimately affect human health. One example is Itai-Itai disease which occurs due to consumption of Cd containing rice. The chemical discharge on soil such as acid and alkali also affects the soil fertility. • Some of the persistent toxic chemicals target the non-target organisms, soil flora and fauna and reduced soil fertility. These chemical accumulates in food chain and ultimately affect human health. • Radioactive fallout on soil are source of radio-isotopes which enter the food chain. Some of these replace essential elements in the body and cause abnormalities. Example: strontium-90 instead of calcium gets deposited in the bones and tissue.