Environmental Engineering (Air Pollution)

7/27/2019 Environmental Engineering (Air Pollution)

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[Air Pollution]

Er. Harsh Mahajan BE civil ME structure

1#Environmental Engineering#

Part III: Engineering of Air Pollution

Air pollution means the presence in the outdoor atmosphere of one or more contaminants, such as

dust, fumes, gas, mist, order, smoke or vapour in quantities with characteristics and of duration such

as to be injurious to human, plant or to property or which unreasonably interfere with the comfortable

enjoyment of life and property

# Air pollutant:

The atmosphere contains hundreds of air pollutants from natural or from anthropogenic sources. All

such pollutant are called as Primary pollutants.

Important primary pollutant:

1. Sulphur oxides2. Carbon monoxide

3. Nitrogen oxides

4. Lead

5. Hydrocarbon

6. Allergic agent like pollen and spores

7. Radioactive substances

8. H2S, fluorides

The primary pollutant often reacts with each other or with water vapours in presence of sunlight, to

form an entirely new set of pollutant called as Secondary Pollutant.

Important secondary pollutant :

1. Sulphuric acid

2. Ozone

3. Formaldehydes

4. Peroxy-acyl-nitrate (PAN)

Sulphur dioxide :

Irritant gas, when inhaled, affects ourMUCOUS MEMBRANEs

Increase breathing rate and causes oxygen deficits in the body, leading to bronchial spasms.

SO2 may oxidise into sulphur trioxide (SO3) which may dissolve in body fluid and forms

sulphuric acid. SO3 thus cases high and worse irritation, leading to Severe Bronchospasm.

SO2 is also responsible for causing acidity in fog, smokes and rain (acid rain)

Major source of corrosion of building and metal objects

Originated from: refineries and chemical plants, smelting operation and burning fuels, Open

burning of garbage and municipal incineration plants.


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2 IS standard limit in ambient air : 80mg/m

3or 0.03ppm

*[The concentration of suspended particles in air are generally expressed as microgram per

cubic meter.

1g/m3=

1 10^3

]

Carbon monoxide:

CO possesses about 200 times affinity for blood haemoglobin than oxygen. So when inhaled,

CO replaces oxygen from haemoglobin and form Carboxy-haemoglobin.

When about half of the haemoglobin of the blood is used up in forming carboxy-

haemoglobin, leads to death.

Dying person exhibits pink colour carboxy- haemoglobin in their blood.

CO also affects the Central Nervous system, also responsible for heart attack and high

mortality rates.

Originated from: biggest source - Automobile exhaust 80%(due to incomplete combustion of

fuel), incineration, degradation, volcano eruption

Limit Concentration in air: 4mg/m3

Oxides of nitrogen:

Nitric oxides (NO) and nitrogen oxides (NO2) are injurious to human.

Eye and nasal irritation are common problem caused by about 28mg/m3

of NO2,

Respiratory discomfortif concentration rises to 47mg/m3.

Sources: natural- lighting and activity of soil bacteria.

Anthropogenic: Automobile exhaust, incineration plants, furnace smoke . caused by

combustion of organic matter.

Limit Concentration in air: 80mg/m3

Hydrogen sulphide: Foul smelling gas with typical odour of rotten gas.

Exposure may lead to loss of smell sense, headaches, conjunctivitis, sleeplessness and pain

in eye.

At higher concentration it may block oxygen transfer and damage the nerve tissue.

As it is not exhausted by automobile and it burnt to SO2, H2S gas in not included in ambient

air quality.

Origin: Oil refinery, rubber, tanneries plant and plant producing sulphur dye.

Also produce from sewage treatment and open burning of municipality and industrial waste.

Methyl and ethyl mercaptans: Other compound of sulphur having strong odour. This compound generally not found in

atmosphere and not so much harmful to us.

Hydrogen fluoride:

High concentration extremely irritant and corrosive

Low concentration- Causes fluorosis in cattle and plants less harmful to human.


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3 Origin- emitted fromAluminium plant, phosphate fertilizer plants, burning of coal,

refineries and chemical industries

Found in lower concentration hence not included in ambient air quality standard

Lead:

May cause irritation ofMucous membrane of nose, throat and lungs.

Lead poisoning leads to damage to gastrointestinal tract, liver and kidney. Causes

abnormalities in pregnancy and fertility.

Responsible for retarding mental growth in children.

Limit: 1.5mg/m3

Origin: Exhaust from automobile (patrol vehicle)

Particulates:

Particle pollution (also known as "particulate matter") in the air includes a mixture of

solids and liquid droplets. Some particles are emitted directly; others are formed in the

atmosphere when other pollutants react. 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. Ten micrometers is smaller than the width of a

single human hair.

Particles less than 2.5 micrometers in diameter are called "fine" particles. These particles are

so small they can be detected only with an electron microscope. Sources of fine particles

include all types of combustion, including motor vehicles, power plants, residential wood

burning, forest fires, agricultural burning, and some industrial processes.

Particles between 2.5 and 10 micrometers in diameter are referred to as "coarse." Sources of

coarse particles include crushing or grinding operations, and dust stirred up by vehicles

travelling on roads.

# Dispersion of air pollution into atmosphere:

When the pocket of air pollutant is released into atmosphere from the source then it will

dispersed into the atmosphere in various direction these direction and rate of dispersion depends

on the prevailing wind and temperature condition.

Temperature conditions of the environment are defined by a technical term called as LAPSE

RATE.

Lapse rate:

In the troposphere, the temperature of the ambient air normally decreases with increase inaltitude.

The rate of change of temperature is called as LAPSE RATE.

Rate will differ from place to place and time to time.

The lapse rate at a particular time and at a particular place can be determined by sending balloon

equipped with thermometer and a self recording mechanism, is known as Prevailing lapse rate or

Ambient lapse rate or the environmental lapse rate.(ELR)


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4

Adiabatic lapse

rate

Dry adiabaticlapse rate (9.80C

per Km )

Wet adiabatic

lapse rate (60Cper Km )

0

500

1000

1500

2000

2500

3000

0 5 10 15 20 25 30

ELR

ALR

Ht.inmeter

500

1000

1500

2000

2500

3000

ALR

When a parcel of air, which is hotter and lighter than surrounding air, is released it rises up until

it reaches the level at which its own temperature and density becomes equal to the surrounding.

Hence when the artificially heated air (i.e. automobile exhaust or stack gas) is emitted into

environment, it rises up, expands become lighter and get cooled.

The rate at which the temperature decreases with increase in height, internal temperature change

may be differing from ELR of the air though which the smoke parcel moves.

Decrease in temperature of parcel

theoretically assumed to be ADIABATIC (a

process occurring without the addition of

outside heat or loss of its own internal heat)

Means the temperature decrease of air

parcel is due to increase in volume.

Rate of decrease of temperature of hot air

parcel is known as ADIABATIC LAPSE

RATE.

Normally, emitted smokes are neither dry

nor purely saturated, hence the actual adiabatic lapse rate (ALR) will be somewhere between

9.80C and 6

0C.

Comparison of ELR and ALR line on graph sheet give the stability of environment.

Three major relative position of ELR with respect to ALR:

When the ELR is more than ALR then the environment is said to be UNSTABLE:

Rising parcel will always remains warmer than the surrounding because environment is getting

cooler more quickly as compared to air parcel.

In such condition, the rising parcel of warmer lighter air will contunue to lift up; whereas heavier

cooler will continue to come down.

The parcel of air will continue to

accelerate in the direction of

displacement.

In such circumstances, theenvironment is unstable, and the

dispersion of pollutant will be rapid

due to marker vertical mixing of the

air.

The prevailing environmental lapse

rate is known as Super adiabatic lapse

rate.

Reverse condition: environmental is

said to be stable ELR is less than

ALR. The prevailing ELR is saidto be Sub-adiabatic lapse rate.

Neutral Condition : ELR=ALR.

# Negative Lapse rate and

inversion:


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When the temperature of the

inverted or negative.

Negative lapse rate occurs und

cooler air below. Such tempera

There are two types of inversio

1. Radiation inversio

2. Subsidence invers

The radiation inversion

phenomenon occurring fro

unequal cooling rate for the e

the earth atmosphere. In oth

when the earth cools rapidly a

quickly than the air above

naturally the temperature

environment will be less at t

and will increases above it. (

possible at night)

Radiation inversion helps in foalso cause increase in concentr

Normally happens in winter m

Subsidence inversion is caused

pressure area surrounded by lo

A subsidence inversion deve

compressed and heated by the

lapse rate of temperature is red

becomes warmer than at lo

inversions are common over t

these regions generally have s

centers.

#Lapse rate and dispersion of air

By comparing the two lapse

a source.

The emitted gases being kno

The manner in which emitte

a) Looping plume:

Plume has wavy character

adiabatic process.

Produces an unstable atmo

rapid mixing.

High concentration near groto turbulence before the d

completed.

Hence areas where super ad

is found there great height

throw gases away from grou

Er. Harsh Mahajan BE civil ME s

nvironment increases with altitude, then the

r conditions (Inversion), a state in which the w

ure inversion represents a highly stable environ

:

n

on

is a

m the

rth and

r word

d more

it, then

in the

e earth

ight be

mation of fog if air is wet and catches gases ation of pollutants in close environment.

re and also in valley region.

by the characteristic sinking or subsiding moti

pressure area.

lops when a widespread layer of air desce

resulting increase in atmospheric pressure, a

uced. If the air mass sinks low enough, the air

er altitudes, producing a temperature inve

e northern continents in winter and over the s

ubsiding air because they are located under l

ollutant:

rates, we can predict that what will happen to

wn as PLUME and their source of origin is call

plume behaves is explained below

nd occurs in super

sphere, because of

und may occur dueispersion is finally

abatic environment

of stack required to

nd.

[Air Pollution]

ructure

5apse rate become

rmer air lies over

ment.

d particulates and

on of air is a high

ds. The layer is

d as a result the

at higher altitudes

sion. Subsidence

btropical oceans;

rge high-pressure

ases emitted from

ed as STACK.


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b) Neutral plume:

Upward vertical rise of plu

Occurs when ELR is very n

Upward lifting of plume wil

c) Coning Plume

The neutral plume is tends

wind velocity is greater t

when cloud over blocks the

day and terrestrial radiation

Occurs under sub-adi

Environment is slightly st

limited vertical mixing, ther

probability of air pollution i

d) Fanning Plume: Under extreme inversio

caused by negative EL

ground to a height greater th

of stack then emission will

horizontally as it cannot

extremely stable environme

In such a case there will b

mixing, and the plume

horizontally to a great distan

In area where radiation

present, there high stack (

inversion it may uneconomi Good condition

e) Lofting plume:

Exist in a strong super adia

above the surface inversion.

Plume has minimum down

its downward mixing is

inversion.

The upward mixing will be

and rapid.

No concentration will touch

f) Fumigating plume:

Occurs when the inversion

short distance above the to

and super adiabatic cond

below the stack.


e from stack.

ar to the ALR.

be continued till air density becomes equal to

to cone when the

an 32km/hr and

solar radiation by

t night.

batic condition

ble and there is

eby increasing the

the area.

condition,

, from the

an the height

spread only

lift due to

t.

e no vertical

will extend

ce.

inversion is

ht. higher than inversion height), But in ca

al.

batic lapse rate

ard mixing, as

prevented by

quite turbulent

the ground, most ideal condition of dispersion

layer is at a

of the stake

tion prevails

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ructure

6

lume density.

se of subsidence

f emission.


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Pollutant cannot escape ab

turbulence.

Bad condition

g) Trapping plume:

Inversion layer exist abo

source as well as below the

adiabatic condition in betwe

Naturally plume cannot be g

will confined in between the

Bad condition

# Impact of atmospheri

dispersion: High pressure system (antic

atmospheric stability, may p

Low pressure system (Cyc

conditions, generally lead to

# Effective height of stack: H= h+h

h= vs D [1.5 + 2.68e-3 PD

u

Where h = ris

Vs = Stack gas velocity m/

D = Inside exit diameter o

u = Wind speed m/se

P = atmospheric pres

Ts= Stack gas tempe

Ta= Air temperature For unstable co

For stable cond

# Air pollution Control: Air possesses self cleansin

from the atmosphere.

When the discharge of

environment become pollu

Natural self cleansing pr1. Dispersion:

Dispersion of pollutant repollutant not removing it.

Long chimney helps us in

2. Gravitational settling:

Large particle from the am

Particle size greater than 2

Flocculated particles are al


ve the top of stack and will be brought down

e the emission

source and super

en them

o up and down it

inversion layer.

pressure on

yclone), which are accompanied by clear skie

ove to be bad for dispersion of pollutants.

lone), which are associated with highly uns

good mixing and rapid dispersion of pollutant.

Ts-Ta)]

Taof plume above the stack in m

sec

stack in m

c

sure in millibar

ature inoK

ino

K.ndition h= 1.1 to 1.2 h

calculated

ition h = 0.8 to 0.9 hcalculated

properties, which continuously clean and re

ollutant exceed the natural capcity of atm

ed.

perties of environment:

uces the concentration of pollutants at one pl

iluting the air pollutant near the source.

ient air settle down on building, trees other obj

m

so form by combining particle of lower size.

[Air Pollution]

ructure

7to ground due to

s, light winds and

able atmospheric

ove the pollutants

osphere then our

ace, it only dilute

ect.


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83. Absorption:

In natural absorption, gases and particulates pollutant get collected in rain or mist and settle

down.

Absorption takes place below cloud level.

Does not help in removing particles smaller than 1mm in size.

4. Rainout:

Rainout is the process involving precipitation above the cloud level, where submicron

particles present in the atmosphere in the clouds, serves as condensation nuclei, may fall out

as rain.

This phenomenon helps in increased rainfall and fog formation.

5. Adsorption:

In adsorption, the gaseous, liquid or solid pollutants present in ambient air are kept attracted

generally electro-statically by the surface.

Building, trees grass may attract and retain pollutant on its surface.

# Control of Particulate Pollutant in industries:1. Gravitational settling chamber

2. Centrifugal collectors3. Wet scrubber

4. Electrostatic precipitation

5. Fabric filter

1. Gravitational Filter: Minimum particle size that can be

removed >50mm

Efficiency = 50%

Simple to design and low pressure

loss

Require larger space and

collection efficiency is low

Working on the principle ofstocks law.

2. Centrifugal collector: Minimum particle size that can be removed

>5-25mm

Efficiency = 50%-90%

Relatively inexpensive, require less floor area,

easy to design and maintain, Low to moderate

pressure loss (2.5 to 20 cm), can handle large volume

of gases

Require much head room, collection efficiency

is low for smaller particles

A cyclone collection is a closed chamber in

which velocity of inlet gas is transferred into spinning

vortex and due to centrifugal force all particulates

sticks to wall and thus collected into hopper bottom

and cleaned.

Efficiency depends on the Mass Of particulate,


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9inlet gas velocity and radius of cyclone.

A Dynamic precipitator imparts centrifugal force to gas with the help of rotating vanes.

7times more effective than cyclone collection.

Cyclone collection is used in Cement factories, fertilizer plants, Grain mill, cotton mill.

Dynamic precipitator is used in ceramic, food and pharmaceutical and wood working

industries.

3. Wet scrubber:

Name Minimum particle size Efficiency

Spray tower >10 m 2.5mm 0.5mm 1mm

Efficiency = 95%-99%

Merit: Particle collected may be wet or dry;small particles can be removed; maintenance

is nominal; Contains few moving part;

Operation temperature is high (300 to

4500C)

Demerit: High initial cost; sensitive to

variable dust load; Uses high voltage hence


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10operational cost is too high; not very much safe.

Fine gas is made to pass through a highly ionized zone created by plates of high voltage, thus

particle having charges on their surface experience powerful electrostatic force which pull it

out from the flue gases.

Used in thermal power plant, pulp and paper plants, mining and metallurgical industries,

chemical industries.

5. Fabric filters:

Minimum particle size that can be removed >1mm

Efficiency = >99%

Merit: highly efficient;

performance decrease give visible pre-

warning.

Demerit: High temperature gases

need to be cooled down; flue gas must be

dry; fabric is liable to chemical action.

Flue gas is allowed to pass through

a woven or felted fabric, which filters our

particulates and allows gas to pass.

# Control of Gaseous Pollutant in

industries:1. Absorption unit

2. Adsorption unit

3. Incineration equipment

1. Absorption Unit:

spray tower, venture-scrubber, plate tower

Plate tower and packed tower are also most

effective devices.

Absorption unit works on the principle of transferof the pollutant from gas phase to liquid phase by

absorbing it into solvent like water.

Efficiency of this unit depends on the capability of

absorbing solvent towards main emission gases like

H2S, SO2, NO etc..

2. Adsorption Unit:

Using adsorption like activated carbon, molecular

sieve (dehydrated zeolite or silica gel)

Most of adsorbents are likely to catch water vapor

first so effluent should be free from water.

All the absorbents are not suitable at high

temperature, they get destruction (e.g. Activatedcarbon 150

oC, Silica gel-400

oC)

3. Incineration equipment:

When the pollutants are oxidized into inert gases

pollutant like hydrocarbons and CO are removed

from gases.


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11# Acid rain:

"Acid rain" is a broad term referring to a mixture of wet and dry deposition (deposited material)

from the atmosphere containing higher than normal amounts of nitric and sulphuric acids. The

precursors, or chemical forerunners, of acid rain formation result from both natural sources, such

as volcanoes and decaying vegetation, and man-made sources, primarily emissions of sulphur

dioxide (SO2) and nitrogen oxides (NOx)resulting from fossil fuel combustion. In the United

States, roughly 2/3 of all SO2 and 1/4 of all NOx come from electric power generation that relies

on burning fossil fuels, like coal.

Acid rain occurs when these gases

react in the atmosphere with

water, oxygen, and other

chemicals to form various acidic

compounds. The result is a mild

solution of sulphuric acid and

nitric acid. When sulphur dioxide

and nitrogen oxides are released

from power plants and othersources, prevailing winds blow

these compounds across state and

national borders, sometimes over

hundreds of miles.

Wet DepositionWet deposition refers to acidic

rain, fog, and snow. If the acid chemicals in the air are blown into areas where the weather is

wet, the acids can fall to the ground in the form of rain, snow, fog, or mist. As this acidic water

flows over and through the ground, it affects a variety of plants and animals. The strength of the

effects depends on several factors, including how acidic the water is; the chemistry and buffering

capacity of the soils involved; and the types of fish, trees, and other living things that rely on the

water.Dry DepositionIn areas where the weather is dry, the acid chemicals may become incorporated into dust or

smoke and fall to the ground through dry deposition, sticking to the ground, buildings, homes,

cars, and trees. Dry deposited gases and particles can be washed from these surfaces by

rainstorms, leading to increased runoff. This runoff water makes the resulting mixture more

acidic. About half of the acidity in the atmosphere falls back to earth through dry deposition.

#Photochemical fog:

Photochemical smog is a unique type of air pollution which is caused by reactions between

sunlight and pollutants like hydrocarbons and nitrogen dioxide. Although photochemical smog is

often invisible, it can be extremely harmful, leading to irritations of the respiratory tract and

eyes. In regions of the world with high concentrations of photochemical smog, elevated rates ofdeath and respiratory illnesses have been observed.

Smog itself is simply airborne pollution which may obscure vision and cause various health

conditions. It is caused by small particles of material which become concentrated in the air for a

variety of reasons. Commonly, smog is caused by an inversion, in which cool air presses down

on a column of warm air, forcing the air to remain stationary. Inversions are notorious in


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12Southern California, where smog can sometimes get so severe that people are warned to stay

indoors.

Some of the particulate matter in the air can oxidize very readily when exposed to the UV

spectrum. Nitrogen dioxide and various hydrocarbons produced through combustion will interact

with sunlight to break down into hazardous chemicals. It doesn't have to be sunny for

photochemical smog to form; UV light can also penetrate clouds. The pollutants released

through human activity in this situation are known as primary pollutants, and they include

sulphur dioxide, carbon monoxide, and other volatile organic compounds. When these

compounds interact with the sun, they form secondary pollutants like ozone and additional

hydrocarbons.

# Environmental Impact Assessment:Environmental Impact Assessment (EIA) is a process of evaluating the likely environmental

impacts of a proposed project or development, taking into account inter-related socio-economic,

cultural and human-health impacts, both beneficial and adverse.

UNEP defines Environmental Impact Assessment (EIA) as a tool used to identify the

environmental, social and economic impacts of a project prior to decision-making. It aims to

predict environmental impacts at an early stage in project planning and design, find ways and

means to reduce adverse impacts, shape projects to suit the local environment and present thepredictions and options to decision-makers. By using EIA both environmental and economic

benefits can be achieved, such as reduced cost and time of project implementation and design,

avoided treatment/clean-up costs and impacts of laws and regulations.

Although legislation and practice vary around the world, the fundamental components of an EIA

would necessarily involve the following stages:

a. Screening to determine which projects or developments require a full or partial impact

assessment study;

b. Scoping to identify which potential impacts are relevant to assess (based on legislative

requirements, international conventions, expert knowledge and public involvement), to

identify alternative solutions that avoid, mitigate or compensate adverse impacts on

biodiversity (including the option of not proceeding with the development, finding alternative

designs or sites which avoid the impacts, incorporating safeguards in the design of the project,or providing compensation for adverse impacts), and finally to derive terms of reference for

the impact assessment;

c. Assessment and evaluation of impacts and development of alternatives , to predict and

identify the likely environmental impacts of a proposed project or development, including the

detailed elaboration of alternatives;

d. Reporting the Environmental Impact Statement (EIS) or EIA report, including an

environmental management plan (EMP), and a non-technical summary for the general

audience.

e. Review of the Environmental Impact Statement (EIS), based on the terms of reference

(scoping) and public (including authority) participation.

f. Decision-making on whether to approve the project or not, and under what conditions; and

g. Monitoring, compliance, enforcement and environmental auditing. Monitor whether thepredicted impacts and proposed mitigation measures occur as defined in the EMP. Verify the

compliance of proponent with the EMP, to ensure that unpredicted impacts or failed

mitigation measures are identified and addressed in a timely fashion.


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