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Environmental Engineering

Dr. Ramakrishna Bag

Dept of Civil Engineering

NIT Rourkela

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Air pollution

Sampling techniques

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Sampling and measurement of air pollutants known as air qualitymonitoring, is an integral component of any air pollution controlprogramme.

Through monitoring, the current trends in air quality can be evaluatby comparing the data with regulated standards.

The information obtained based on regulations is helpful inimplementing control measures for reducing pollutant concentrationto acceptable limits.

Air quality is measured either a) at ambient condition or b) stacksampling.

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Collecting genuine and representative sample is the most importanttask.

Sampling should be carried out for long enough time and at a ratethat allows collection of an analytically measurable sample.

The amount of pollutant present is very small. Sometimes air pollutants may be reactive and changes their

properties after collection. So special care must be taken in that case

SO2 usually oxidise to SO 3, therefore analysis carried out for SObe misleading.

Ambient air pollution sampling

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Typical air-sampling set up

(a)

Sample collector Flow meter Pump

Flow meter Sample collector Pump

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Components of air sampling system

Sample collector To collect the sample and retain the desire pollutant ealone or as a mixture.

Flow meter - to measure airflow through the collector Pump To draw the air sample through the system

The choice of sample collector to be used is governed by the natu

of the pollutant and the method of analysis to be employed.

Samples collectors can collect either (a) gaseous pollutant or particulate pollutants.

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Collection of gaseous air pollutants Grab sampling : Sample is collected by filling an evacuated flask or inflat

Plastics bags are frequently used for grab sampling method. Possibility of losing sample by moisture condensation or by diffusion throu

wall. Losses can be minimised by performing the analysis immediately after colleti

sample. Rigid wall containers made of stainless steel or glass also used for grab samplin

These containers are evacuated and then collected sample by allowing air to en Alternatively, the containers are filled with water, then air sample is collect

draining the water. In order to reduced the condensation, containers are heated by wrapping them w

heating tapes or nichrome wires.

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Absorption of liquids : Absorption of gaseous pollutants into a liquid medmost commonly employed method of collecting air sample.

Absorption separates desired pollutant from air either by solubility in absorbing medium or by chemical reactions.

Fritted glass absorber is commonly used air sampler of this type; where the stream is broken into extremely small bubbles and promote intimate conta

between liquid and gas.

Another type of collector is known as Impinger. In the impinger the gas streamimpinged at high velocity onto a flat surface thus provide good contact betwegas and the liquid.

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Adsorption of liquids : This method is based on tendency of gases to be on the surface of solid materials.

The air sample is passed through a packed column containing a finely adsorbent on whose surface pollutants are retained and concentrated.

Most commonly used solid adsorbents are granular porous solids such as actcharcoal and silica gel with very large surface area.

After adsorption, sample gases are desorbed by heating or by washing in oranalyse the sample.

Analysis is carried out by gas chromatographic technique.

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Freeze-out sampling: In this method a series of cold traps are used .

Progressively low temperatures are maintained at the traps.

The sample is collected whereby pollutants are condensed.

The traps are brought to the laboratory and sample is analysed by gas

chromatography.

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Collection of particulate pollutants Particulates are those which settle out from air by gravity and those

remain suspended are known as aerosol.

Sedimentation technique is used to collect particulates whose diametermore than 10 m.

For collecting aerosol following methods are used High-volume filtration Tape sampler Impingement Electrostatic precipitation Thermal precipitation

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Sedimentation : Simplest device useful for collecting dustfall. Typical collector consists of a plastic jar of about 20 to 35 cm height and 10

to 15 cm dia. at the base.

A slight tapered of the walls from top to bottom. A handle is provided to ensure safe and upright positioning of the

collector. Sample is deposited for a month. Material is dried, weighed and reported the amount of dustfall in mg/cm

or tons/km 2. Dust particles larger than 10 m are seldom carried for more than a

kilometre distance, therefore dustfall stations must be installed closely inorder to collect meaningful data.

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High-voltage filtration: High voltage filtration is used for measurement of mconcentration of suspended particles smaller than 10 m.

A known volume of air is drawn by a high speed blower through a fine filter anincrease in weight due to the trapped particles is measured.

Filter is usually made of fibrous or granular material. The sampling time is generally 24 hours and during this time over 2000 m

passed through filter.

Tape sampler: This method is known as soil-haze particle sampler. A known volume of air is passed through a paper tape. Sampling time varied from 10 mins to 2 hrs. The air is passed through a nozzle and the particulates are collected on the pape

tape forming a spot.

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Spots so obtained are evaluated by analyzing the amount of light thcan be transmitted through the spot.

The measurement is reported as COH/1000 ft where COH is knoas coefficient of optical haze, calculated as:

where T 0 = Transmittance of clean tape (100%), T = percentage transmitted through the spot, A = area of the spot in ft 2, V =of sample in ft 3.

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Impingement: In an impinger the separation of particulates from air streamtakes place by collision against a flat surface.

Wet impingement is used to collect the particulates in a liquid phase.

Air stream containing particulates impinged at a flat surface immersed in aliquid.

Dry impingers are not very efficient for collecting smaller particles.

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Electrostatic precipitation : Air sampling is also carried out by elprecipitation.

A negative charge is imparted to a wire placed axially inside a cylinder wpositively charged.

Once airstream is passes though the cylinder, the particles acquire a negcharge from a corona discharge occurring on the central wire.

The particles migrate toward inner surface of the cylinder and are removesubsequent chemical or microscopic analysis.

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Thermal precipitation : Thermal precipitators works on the principle thatparticles, under the influence of a strong temperature gradient between two

surfaces, have a tendency to move towards the lower temperature and getdeposited on the colder of two surfaces.

The temperature gradient are normally 3000 oC/cm. Velocity of air is maintained very low. The efficiency of thermal precipitators are very high.

The particles collected on a grid or a thin microscope cover glass for analys

Stack

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Stacksampling

Stack sampling is carried out when a stack of air-pollutant (e.g. smois coming out from the source.

Stack sampling provides information on the nature and quantities various pollutants that are emitted to the atmosphere.

Major difficulty in this process is to obtain representative sample. Selecting proper site for sampling is the most important factor. The gas stream in a stack is normally under turbulent flow conditio

and any flow disturbance would cause non-uniform and unstable gflow profile.

To avoid this problem, sampling should be done at a proper distance.

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Multiple samples are collected where uniform sample pattern is not fo In stack sampling, a part of the gas stream is diverted through a sampl

train and the representative sample is collected for analysis. Sample train consists of a nozzle placed in the gas stream, a sampling

probe through which the sample is drawn, particulate/gas collectiondevice, flow meter and a pump.

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Air pollution Control methods

The most effective way of dealing with air pollution is to prevent theformation of the pollutants or minimize their emission at the source itself.

Control of the pollutants at the source can be accomplished by rawmaterial changes, operational changes, modification or replacement of

equipment or by more effective operation of existing equipment.

In case source correction method can not be achieved, effluent gascleaning technique is used.

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Source correction method Raw material changes : Sometimes raw materials is responsible for c

pollution. Use of purer quality raw material would reduce the air pollution. Burnnatural gas produces less pollution than that of coal.

Process change : This involves use of new or modified techniques for atmospheric pollutant emission. For example, paper industries produces objectionable sulphurous material. This emission can be prevented by major pchanges such as hydro-metallurgical separations of ores.

Equipment modification/ replacement : Air pollution emissions can be msuitable modification or replacement of process equipment. In petroleum refinhydrocarbon vapours are released into the atmosphere from storage tank dutemperature changes, direct evaporation and displacement during filling. This lossebe reduced by designing the tanks with floating roof covers.

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Cleaning of Gaseous effluents : Where emission of pollutant cprevented, cleaning techniques are being applied.

The equipment's are installed where the pollutants are present in smallest pvolume with higher concentrations.

Gas cleaning technology is used alone or in conjunction with source correction me Gaseous pollutants are cleaned by dry absorption, wet absorption or com

methods. Particulate emission control : Particulate matter emitted in gaseous stream

of discrete and minute suspended particles of size varies from 100 m to 0.1 m. Chparticulate collection device depends on

Physical and chemical properties of particulates Particulate size and concentration in the gas Volume of particulates to be handled Temperature and humidity of the gaseous medium.

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Collection efficiency : Efficiency of the collection device expressed a

T = 100* (Wt. of material collected/ Total amount of material enteringcollector)

The efficiency of collection system varies with particle size.

The variation of efficiency can be expressed as fractional efficiency

i = 100* (fraction in range i collected / m i )

where, m i = amount of pollutant entering the collector in range .

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Particulate control equipment

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Particulate control equipment Basic mechanisms of removing particulate matter from gas stream are

Gravitational settling Centrifugal impaction Inertial impaction Direct interception Diffusion Electrostatic precipitation

The various equipment's used for removing particulate matter are Gravitational settling chambers Cyclone separators Fabric filters Electrostatic precipitator Wet collectors