ENVIRONMENTAL POLLUTION Pollution can be described as “the

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ENVIRONMENTAL POLLUTION Pollution can be described as “the Powered By Docstoc
					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)   Water Pollution
                (b)   Air Pollution
                (c)   Land Pollution
                (d)   Noise Pollution
                (e)   Thermal Pollution
                (f)   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

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.

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.

   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.

   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

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.

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

(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 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.

   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

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.

        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.


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.

         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.