Environmental Pollution

L1S1- Process Engineering

ENVIRONMENTAL POLLUTION Pollution can be described as “the deliberate or accidental contamination of the environment with waste that is created by human action”. A pollutant is a substance or effect which adversely alters the environment by changing the growth rate of species, interferes with the food chains, is toxic or interferes with health, comfort, amenities or property values of people. Pollutants are introduced into the environment in significant amounts in the form of sewage, waste, accidental discharge, or as a by-product of a manufacturing process or other human activity. The damage caused to the biosphere may be briefly summarized as follows: 1. Damage to human health caused by specific chemical substances present in the air, food, water and radioactivity materials 2. Damage to the natural environment which affects vegetation, animals, crops, soil and water 3. Damage to the aesthetic quality of the environment caused by smoke, chemical fumes, dust, noise, the dumping of solid waste and waste water 4. Damage caused by long term pollution effects which are not immediately apparent. The dangerous effects are caused by low level pollution absorbed into the body over long periods of time, for example carcinogenic substances, radioactivity and excessive noise Types of Pollution (a) (b) (c) (d) (e) (f)

Water Pollution Air Pollution Land Pollution Noise Pollution Thermal Pollution Solid Wastes

(a) Water Pollution Water pollution occurs due to the presence of dissolved inorganic materials, organic materials such as proteins, fats, carbohydrates and other substances found in domestic and industrial waste waters. It also gets polluted due to physical factors such as turbidity, colour, temperature of effluent, associated radioactivity etc. Organic pollutants Organic pollution is due to the presence of high molecular weight compounds such as sugars, oils and fats, and proteins obtained from distillery, canning, sugar and other food processing industries. They impart a high Biological Oxygen Demand (BOD) load to the liquid waste. These organic compounds are readily degraded in aqueous medium by soil and microorganisms present in the sewage. During this process, dissolved oxygen (DO) in the stream is used up. When the DO is reduced below a certain limit, aquatic life is affected adversely. Oil spillage from tankers and ships leads to the pollution of beaches. Some wastes from pharmaceutical and petrochemical industries and coke-ovens contain phenols which are toxic to fish, microorganisms and other aquatic life. Liquid effluents from industries manufacturing drugs, dyestuffs, pesticides and detergents can also be toxic. Usage of anionic detergents which are not biodegradable leads to a lot of foaming and frothing. 

Dept. of Chemical and Process Eng.

Inorganic pollutants and heavy metal pollution Alkalis, acids, inorganic salts and other chemicals formed during processing lead to inorganic pollution. Besides being the cause of corrosion of metals, these chemicals are toxic to aquatic life. Industries like paper and pulp, tanneries, textiles and coke-ovens among many others, discharge these chemicals. Inorganic chemicals such as free chlorine, ammonia, hydrogen sulfide and other sulfides, salts of metals like Cr, Ni, Zn, Cd, Cu, Ag etc. are usually found in metal plating liquid wastes, alkali producing units, polyvinyl chloride, coke-oven and fertilizer industries. Pharmaceutical industries also produce large quantities of free acids and neutralized chemicals during different unit processes. Chromates, phosphates, ammonia and urea are typical chemicals found in effluents from fertilizer industries. Pollution due to mercury in chloro-alkali industries has forced to switch from mercury to diaphragm cells although there are many operating difficulties in the latter. 

Eutrophication is a phenomenon that occurs due to the presence of inorganic matter in the water. Though it is a natural process in lakes and rivers, human activities can accelerate the rate at which nutrients enter these water bodies. Inorganic pollutants containing nitrates and phosphates are essential for plants. When these compounds are present in excess in a water body there is an over growth of plant matter creating an imbalance in the aquatic environment. Apart from these factors, effluent waters can have pathogenic micro organisms and harmful substances like organochlorides, phenolic and benzenic compounds that cause various types of acute and chronic diseases among living beings. Therefore, it is most desirable that industrial liquid effluents are pretreated or reduced with undesirable chemicals before being thrown on the land and in rivers, seas or public sewers. If discharged into public sewers they can corrode the pipe lines and treatment equipment as well as reduce biological activity due to the presence of toxic materials.

(b) Air Pollution Apart from natural causes, pollution of air occurs due to increased industrialization and urbanization. In nature, thousands of tonnes of dust, sand and obnoxious gases are carried from one place to another by wind, cyclones, volcanic eruptions and forest fires. A healthy man inhales about 16.5 kg air/day. If this air is polluted, intake of oxygen is reduced. Corrosion of materials by acid mist and acid gases like SO2, CO2 and oxides of nitrogen can be very significant. Pollution of air occurs from process industries such as sulfuric acid plants, power station boilers, nitric acid plants, cement plants, foundries, chlor-alkali industries and plastics industries among many others. Technically solutions exist for all pollution problems, but economically they may not be favourable. Pollution of air is broadly due to particulate matter dispersed in it or gaseous pollutants completely miscible with it in all proportions. Dusts refer to coarser sizes of solids (1-100 microns) that are carried away due to the turbulent forces of flow. Aerosols refer to particles smaller than 1 micron. Smoke is obtained during incomplete combustion of organic matter. The sizes of particles vary between 0.01-1 micron. Fumes, mists and fog are other forms of Dept. of Chemical and Process Eng. 2

pollution which could occur due to various operations in chemical industry. Gaseous pollutants are gaseous solutes such as SO2, Cl2, NOx, CO2, CO, mercury or organic vapour dispersed in air. Another source of ambient air pollution is the emission from automobiles. The pollutants in this case are CO, NOx, hydrocarbons, particulate matter and trace quantities of SO2, formaldehyde and lead.

Effects of air pollution  Global warming (Green house effect) - This is the observed increase in the average temperature of the earth's atmosphere and oceans in recent decades. The increased amounts of carbon dioxide (CO2) and other greenhouse gases (CH4, O3) are the primary causes. They are released by the burning of fossil fuels, land clearing and agriculture, etc. and lead to an increase in the greenhouse effect. 

Acid rain (SOx, NOx) - Acid rain occurs when sulfur dioxide and nitrogen oxides are emitted into the atmosphere. These substances undergo chemical transformations and are absorbed by water droplets in clouds. The droplets then fall to earth as rain or snow. This can increase the acidity of the soil, and affect the chemical balance of lakes and streams.

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Depletion of the ozone layer (CFCs, HCFCs etc.) - Ozone layer prevents most harmful UV wavelengths (270- 315 nm) of ultraviolet light from passing through the earth's atmosphere. The decrease in ozone is due to the production of CFCs and ozone depleting chemicals such as carbon tetrachloride and trichloroethane It is suspected that a variety of biological consequences, including, for example, increases in skin cancer, damage to plants, and reduction of plankton in the ocean result from the increased UV exposure due to ozone depletion.

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Photochemical smog & air borne toxic substances - Photochemical smog is due to the chemical reaction of sunlight, nitrogen oxides (NOx) and volatile organic compounds (VOC's) in the atmosphere, which leaves a yellow brown haze that reduces visibility. Nitrogen oxides are released in the exhaust of fossil fuel-burning engines in cars, trucks, coal power plants, and industrial manufacturing factories. VOC's are vapors released from gasoline, paints, solvents, pesticides, and other chemicals.

(c) Land Pollution The greatest land pollution by far occurs due to urbanization and concentration of population. Garbage, waste paper, packing materials and rejects from households and industries form solid wastes. In Sri Lanka, the per capita production of solid waste is 0.89 kg/day. Improper disposal by burning in open dumps adds to air pollution. Proper management of solid wastes is required to prevent environmental pollution, nuisance and deterioration of the quality of life. Land pollution also occurs when deforestation is carried out on a large scale, agricultural operations are extended to forest and mountainous areas, large areas covered with green vegetation are submerged in water by the construction of dams. It can also get polluted when coal, oil, ores, stones, sand and other construction materials are mined and transported. All these operations adversely affect ecological aspects of the environment. Ever increasing Dept. of Chemical and Process Eng. 3

demands for paper, fuel, furniture and packing materials due to exploding populations combined with increasing urbanization have aggravate the unhappy situation. The result is that we now live in a rapidly changing environment, the effects of which are not easily predictable.

(d) Noise Pollution Of all the pollutants it is the most insidious and a menace in thickly populated areas. There is ample medical evidence that it affects speech, hearing and the general health and behaviour of people exposed to it over extended periods of time. The measurements designed to determine noise levels include the intensity and frequency of noise periods of exposure and duration of noise. The community noise levels are expressed as a weighted sound-pressure level in decibles dB. The sources of noise in environs of industries include metal fabrication processes, high pressure burners in furnaces, rotary equipment like turbines, compressors, pumps etc. pipelines carrying high velocity fluids and solids and vibrating and grinding equipment among many others.

POLLUTION CONTROL ASPECTS Pollution can be controlled by proper choice of preventive and remedial measures. Good housekeeping and maintenance of process equipment go a long way towards reducing pollution of water and air and increasing the life of the plant. Wastewater Treatment Preventive Measures 1. Volume Reduction of wastewater 2. Strength reduction Curative Measures 1. Physical 2. Biological 3. Chemical 4. Combination of above Different standards are laid down for the discharge of effluents into natural water bodies, municipal sewers and into the land. 

Physical Methods

Physical methods aim at removing solid or liquid pollutants based on their density difference from water. They are essentially wastewater clarification methods and remove suspended or floating solids or liquids. Physical methods of treatment are reverse osmosis, electrodialysis, filtration, foam separation, porous-bed filtration, adsorption etc. They help remove fine particles, and organic and inorganic dissolved materials, resulting in better water quality for re-use or disposal. Dept. of Chemical and Process Eng. 4

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Biological Wastewater Treatment

In this method colloidal and dissolved solids are converted into settle-able solids by microorganisms under favourable environmental conditions. Anaerobic treatment takes place in the total absence of oxygen and is a rather slow process. Aerobic biological treatment methods include the activated sludge process, trickling filter process and stabilization ponds. 

Chemical Wastewater Treatment

Industrial effluents usually contain acids, alkalis, undesirable chlorides, phenols, sulfates, chromates, phosphates and salts of mercury, lead, calcium, barium, zinc etc. The best way to remove these is to incorporate inplant changes in process design and operation. Acids are neutralized with caustic soda or lime. Neutralization of alkalis is done with calculated quantity of sulfuric or hydrochloric acid. Flocculation and precipitation can help reduce the organic and inorganic load by settling the sludges or precipitates formed. More than 90% of the dissolved phosphates and 55-70% of the BOD load of domestic sewage can be removed if domestic wastewater clarification is combined with iron and aluminum salts. Oxidation with chlorine and sodium hypochlorite is used for killing pathogenic organisms in the treated liquid waste as well as for treating industrial wastes. Air Pollution Control One method of disposing of air pollutants is to make use of meteorological parameters to dilute and dispose them, to get ground level concentrations within acceptable limits by using tall stacks. This is a rather costly method and no recovery of useful chemical is possible. A better way to handle the pollution control problem is to reduce the concentration of pollutants, at the source, by incorporating process changes and making careful choices of raw materials. Particulate matter can be separated from gases by making use of gravity, centrifugal force, inertia and diffusion, adhesion and cohesion, thermal and electrostatic forces acting on the particles. In many cases one has to use a combination of equipment for separation if the size distribution of the particles is wide.

Solid Waste Management The first objective of solid waste management is to remove discarded materials from inhabited places in a timely manner to prevent the spread of disease, to minimize the likelihood of fires, and to reduce aesthetic insults arising from putrifying organic matter. The second objective which is equally important is to disposal of the discarded material in a manner that is environmentally acceptable. The following should be addressed in an effective solid waste management program. 1. Storage 2. Collection Dept. of Chemical and Process Eng. 5

3. Transfer 4. Treatment 5. Disposal The prevention of waste generation and the productive use of waste material are ways of alleviating some of the problems of solid waste management. Some of the materials that can be recovered are copper, lead, aluminum, paper, plastics, glass etc. Segregation is quite important in implementing solid waste management – and hope that all of you will learn this by participating in the University of Moratuwa scheme.

OCCUPATIONAL HEALTH AND SAFETY Any organization has a legal and moral obligation to safeguard the health and welfare of its employees and the general public. Safety is also good business; the good management practices needed to ensure safe operation will also ensure efficient operation. All manufacturing processes are to some extent hazardous, but in chemical processes there are additional, special hazards associated with the chemicals used and the process conditions. The designer must be aware of these hazards, and ensure systematic application of sound engineering practice, so that the risks are reduced to acceptable levels. Safety in process design can be considered under the following broad headings. 1. Identification and assessment of the hazard Dept. of Chemical and Process Eng. 6

2. Control of hazards: e.g. containment of flammable and toxic materials 3. Control of the process: Prevention of hazardous deviations in process variables (pressure, temperature, flow), by provision of automatic control systems, interlocks, alarms, trips; together with good operating practices and management 4. Limitation of the loss. The damage and injury caused if an incident occurs: pressure relief, plant layout, provision for fire fighting equipment.

Dept. of Chemical and Process Eng. 7