Methods of air pollution control - zoning, source correction

METHODS OF AIR POLLUTION CONTROL – SOURCE CORRECTION

To effectively tackle the problem of air pollution, it is essential to prevent or minimize the formation of pollutants at the source.In case of industrial pollution, this can be achieved by analysing the process design amd selecting those methods that do not contribute to air pollution or have minimum impact due to air pollution. This technique is known as 'source correction methods'. The application of these methods is difficult, however some of these methods can be applied without having a major impact on economy of operation.
Below described are a few methods for control of pollution at source.

1. Raw material change – When raw material causes air pollution, a purer grade of raw material may reduce generation of undesirable substances. 

• An example in this regard is the use of low sulphur diesel in place of regular diesel which contains a higher sulphur content leading to effluents with a high concentration of sulphur particulates. 
• Another example would be usage of natural gas in place of coal to reduce the generation of particulates (both suspended and respirable).
• Desulphurization of fuel is an alternative, however it is expensive and poses technical problems. Another problem is lack of availability of better alternatives and the cost involved. 
• Coal combustion can be carried out with least air pollution by coal gasification. Coal gasification can be carried out by destructive distillation of coal or gasification of coke residues of carbonization with steam. 

2.  Operational change

• By causing all dust creating activities that are generated in a process to be effectively controlled and separated by effecting an operational change in the manufacturing industry
• Moistening the dust thereby binding the dust is a time old method to prevent dust from spreading.

3.  Modification or replacement of process equipment – This involves use  of new or modified techniques to lower emission of atmospheric pollutants.

An effective method to control dust in industries is by casing all dust creating activities               and the dust generated can be effectively controlled and separated.

Moistening the powder in order to bind the dust is an old method prevent dust from                    spreading

     Examples are listed below:

• Unburnt carbon monoxide (CO) and hydrocarbons (HCs) from cylinders of an automobile engine can be burnt by injecting air into the hot exhaust manifold of the engine.
• Hydrocarbons (HCs) released into the atmosphere from petroleum storage tanks due to temperature changes, direct vapourization and displacement due to filling can be reduced by designing tanks with floating roof covers or pressurising the tanks.
• Replacing the open hearth furnace by oxygen furnace in steel industries helps in reducing air pollution
• Alternate power for automobiles (Ex: Hydrogen power, Solar power) in place of internal combustion engines that use fossil fuels will help in significant reduction of air pollution.
• Air pollution due to industries can be reduced by proper maintenance of equipment, housekeeping and cleanliness of facilities helps reduce air pollution.
• Ore handling operations result in emission of large quantities of dust. In steel industries, raw ore is replaced with sintered pelletized ore to reduce dust emissions and blast furnace “slips”

4.   Effective operation of existing equipment

• Preventing leakage around ducts, piping and valves by checking seals and gaskets regularly air pollution from industries can be minimized.

                                  METHODS OF AIR POLLUTION CONTROL – ZONING

Air pollution control by zoning:

The CPCB (Central Pollution Control Board) has developed a tool for environmental planning for proper siting of industries thereby reducing the risks due to pollution and protect the environment. The CPCB in consultation with the SPCBs (State Pollution Control Boards) has developed a zoning atlas for siting of industries based on environmental considerations, district-wise, through-out the country. The zoning atlas for siting of industrial zones, classifies the environment in a region and presents the pollution receiving potential of various locations along with possible alternate sites through easy-to-read maps. The objectives for preparing the zoning atlas are:
1. To zone and classify regions
2. To identify locations for siting of industries and
3. To identify industries suitable for identified sites

The zoning atlas considers only environmental aspects. The zoning atlas helps in stream-lining the decision-making process along with the following benefits:

• It provides a ready-reckoner for best suitable site and relevant environmental information
• It helps to make decisions that are simple, faster, realistic, transparent and reliable
• It provides a basis for incorporating environmental aspects into land use planning
• It helps to plan for cost effective pollution control measures and programs
• It helps an entrepreneur save money, time, efforts and risk
• Helps develop infrastructural facilities
• It helps check additional pollution in areas already stressed with pollution
• It ensures that pollution potential of an industry is compatible with local conditions
• It ensures that industries with high pollution potential that are desirous to locate an industry in a high risk area adopts clean technologies so that generation of wastes is prevented or made compatible with the receiving environment.
• Helps in creating awareness among people regarding type of industries and nature of pollution anticipated in their neighbourhood
• In view of all the above mentioned issues, an EIA helps achieve sustainable development.

Selection of proper air pollution control equipment

Selection of a particular air pollution control device greatly depends on:
1. Particle size
2. Concentration
3. Desired efficiency of collection
4. Cost involved
5. Availability of space
6. Maintenance factors

Stack height calculation

Wind rose diagrams

WIND ROSE
Wind rose is defined as "any one class of diagrams designed to show the distribution of wind direction experienced at a given location over a considerable period."

• Wind rose shows the prevailing direction of wind. 
• It consists of a circle from which eight or sixteen lines emerge
• Each line signifies a specific direction
• Length of a line is proportional to frequency of wind from that direction
• Frequency of calm conditions is entered in the center
• Wind roses are constructed from data obtained over a particular month, season or year
• Wind direction refers to the direction FROM which wind is blowing
• Sometimes, instead of wind speed, the parameters of precipitation, smoke, sulphur dioxide, smok, hydrocarbons, etc are attached to wid direction. These diagrams are known as POLLUTION ROSES.

Diagram of a typical wind rose is shown below:

Air quality standards

Air Quality Standards

• The current National Ambient Air Quality Standards were notified on 18 November 2009 by the Central Pollution Control Board.
• The National Ambient Air Quality Standards specified by the CPCB are mentioned in the table below for various pollutants

Lapse rates, inversion, types of inversion, atmospheric stability and dispersion of air pollutants

LAPSE RATE

In well mixed dry air, temperature falls by 3.3F for every increase in 100 ft altitude. This vertical temperature gradient is called 'lapse rate'.

The value mentioned above is called NORMAL LAPSE RATE or ENVIRONMENTAL LAPSE RATE.

INVERSION
If the cold layer of air at ground level is covered by warmer air at a higher level, the phenomenon is called inversion. During inversion, the vertical air movement is stopped and pollutants are concentrated in the inversion layer below. In this state, the atmosphere is stable and very less turbulence or mixing takes place. As a result, the pollutants in the air do not disperse. Inversion occurs typically in the months of October to February. The accumulation of smoke and other pollutants aggravates the problem of pollution by preventing the sun's rays from heating the ground and adjacent air. Fog is generally associated with inversions. Narrow valleys are favorable to inversions as horizontal air movement is restricted. During inversions visibility is greatly reduced and contaminant concentration is maximum.

TYPES OF INVERSION

1. Radiation inversion
• This type of inversion occurs at night
• It occurs when the vertical movement of air is stopped
• Fog forms in this type of inversion if air is moist and temperature is below the dew point
• It is common in winter due to longer nights
• It frequently occurs in valley areas
2. Subsidence inversion
• This type of inversion occurs at modest altitudes and remains for several days
• It is caused due to sinking of air in high pressure areas surrounded by low pressure areas
• As air sinks, it is compressed and gets heated to form a warm dense layer that prevents upward movement of contaminants
• Inversion height varies from ground layer to a height of 1600 m.
• At inversion height of 200 m, extreme pollution occurs.

If radiation and subsidence inversion occur simultaneously, the phenomenon is called "double inversion".

ATMOSPHERIC STABILITY
Stability is an important characteristic of the atmosphere. It is simply the ability to resist vertical motion. Stability affects the ability to disperse pollutants.
Lapse rate is the rate at which atmospheric temperature decreases with an increase in altitude.
Lapse rate is the negative of the rate of temperature change with altitude change.
Three different lapse rates are:

1. Dry adiabatic lapse rate
2. Moist adiabatic lapse rate and
3. Environmental lapse rate
   

The actual temperature profile of ambient air is the environmental lapse rate or atmospheric lapse rate or prevailing lapse rate. The environmental lapse rate varies from time to time and from place to place. A radiosonde is an instrument to measure environmental lapse rate.
Negative lapse rate is an inversion.
Static stability of the atmosphere can be determined by comparing the dry, moist and environmental lapse rates.
IF:

• ELR < MALR then the atmosphere is absolutely stable
• ELR > DALR then the atmosphere is absolutely unstable
• MALR < ELR < DALR then the atmosphere is conditionally unstable

ELR     - Environmental Lapse Rate
MALR   - Moist Adiabatic Lapse Rate
DALR   - Dry Adiabatic Lapse Rate

During the day, the surface of the Earth get heated more due to insolation while at night, a terrestrial radiation loss causes a temperature inversion.
The range of stability indices are:

1. Very stable
2. Stable
3. Marginally unstable
4. Moderately unstable
5. Very unstable and
6. Extremely unstable

DISPERSION OF AIR POLLUTANTS

The degree to which air pollutants are discharged from various sources and their subsequent concentration in a particular area depends on meteorological conditions. Hence, the application of dispersal theory and knowledge of local weather conditions are essential for:

1. determination of required stack height and
2. evaluation of intensity of air pollution

Degree of air pollution varies spatially and temporally due to difference of meteorological conditions.

Adverse weather can trigger an air pollution episode like the "london killer smog". The important meteorological parameters that influence air pollution are:

1. Primary parameters
1. Wind speed and direction
2. Temperature
3. Atmospheric stability and
4. Mixing height
2. Secondary parameters
1. Precipitation
2. Humidity
3. Solar radiation and
4. Visibility

The above parameters vary as a function of

• Latitude
• Season and
• Topography

The following are the effects of air pollution on weather:

1. Reduced visibility
2. Frequent episodes of fog
3. Reduction in incoming solar radiation

Wind direction and speed influence the movement and diffusion of air pollutants discharged at ground level. high wind speeds diffuse pollutants from the source. Dispersed pollutants get rapidly diluted with increasing volume of air.

Gustiness is an important characteristic of surface winds is proportional to speed and determines the extent to which pollutants are mixed and diluted with surrounding air.

Concentration of a pollutant downwind is inversely proportional to wind speed.

Ambient air quality monitoring

Ambient air quality refers to the condition or quality of air surrounding us in the outdoors. National Ambient Air Quality Standards are the standards for ambient air quality set by the Central Pollution Control Board (CPCB) and is applicable nationwide.
The CPCB has been conferred this power by the Air (Prevention and Control of Pollution) Act, 1981.

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According to the National Ambient Air Quality Monitoring Program:

• The following factors influence site selection:

1. The site should be away from source and other interference (Inlet should be at least 15 m away from source or traffic artery)
2. Height of the inlet should be more than 3 m (preferably between 3 - 10 m) or double the height of nearby wall or obstruction.
3. The air at site should be free flowing and well mixed
4. The elevation angle should be less than 30 from inlet to top of building
5. Collocated samplers should be at least 2 m apart

• The following factors influence parameter selection

1. Sensitive location
2. Health impact stations (All pollutants)
3. Population and exposure in case of priority pollutants
4. Criteria pollutants and CO are typically sampled at traffic intersections
5. Concentration of criteria pollutants and ozone are accumulative and are sampled at 50 m from traffic intersections core commercial areas in the city

In case of manual monitoring, 

• Gravimetric methods are used to estimate the concentration of PM10 and PM2.5 
• Wet chemical methods are used to estimate the concentration OF SO2, NO2, NH3 and benzene
• Gravimetric method of sample processing and chemical analysis is done to estimate the concentration of Benzene, B(a)P, Ni, As and Pb in PM10

In case of automatic monitoring, 

• Sophisticated analysers are used for instant data generation, online data dissemination, air quality index, early warning system, forecasting, modeling of pollutants such as Sulphur dioxide, Nitrogen dioxide, particulate matter smaller than ten microns, particulate matter smaller than 2.5 microns, Ozone, Carbon monoxide, Ammonia and benzene.