GAS AND LECHATE MOVEMENT AND CONTROL IN LANDFILL

GAS MOVEMENT

Mostly 90 % of gas produced from decomposition of solid waste includes methane and carbon dioxide. The methane in air in the concentration of 5 to 15 % is explosive. In case of high concentration of methane in landfill, there is no oxygen present so there is no danger of explosion. Most of the methane releases into the atmosphere. Both methane and carbon dioxide are found in concentrations of 40 % at lateral distances of 400 ft from the edges of landfill. 

If proper venting is applied than methane does not create any problem. Carbon dioxide is problematic because of its density. Carbon dioxide is 1.5 times denser than air and 2.8 times denser than methane. It tends to move at the bottom of landfill. The concentrations of carbon dioxide in the bottom is thus high.

CONTROL OF GAS MOVEMENT BY PERMEABLE METHODS

The lateral movement of gas could be controlled by installing the vents that are made of materials that are more permeable than surrounding soil. Normally gas vents are made of gravel. The spacing of vents depends upon width of waste cells, mostly it varied between 60 to 200 ft. The thickness of gravel layer should be continuous and recommended between 12 to 18 in.  Barrier vents and well vents are also use to control lateral movement of gases. 

When well vents are used, waste gas burners are also installed often. It is recommended well penetrates in the upper waste cell. The height of waste burner varied between 10 to 20 ft. The burner can be burned by hand or by pilot flame. 

The downward movement of gases can be controlled by perforated pipes in gravel layer at the bottom of landfill. 

LECHATE MOVEMENT

Lechate is generally found in the bottom of landfills. It movement is by underlying strata. Lateral movement can also occur depending on properties of surrounding materials. 

Darcy's Law: 

The rate of seepage of lechate from the bottom of landfill can be estimated by this law. It is written as:

Q= -KA dh/dL

Q= lechate discharge per unit time

K= coefficient of permeability

A= cross sectional area by which lechate flow

dh/dL= hydraulic gradient

The - sign comes from the fact that head loss dh is always negative. Coefficient of permeability is also termed as hydraulic conductivity, effective permeability or seepage coefficient. 

Read More

REACTIONS OCCURRING IN COMPLETED LANDFILL

Solid waste in sanitary landfill is undergone by many chemical, biological and physical changes. The most important changes are as follows:

• Biological decay of organic material either aerobically or anaerobically with the emission of gases and liquids
• Chemical oxidation of materials
• Release of gases from the landfill and lateral diffusion of gases
• Movement of liquids by differential heads
• Leaching and dissolving of organic and inorganic materials by water and lechate movement through the fill
• Movement of dissolved material by concentration gradient
• Irregular settlement of materials by merging of materials in spaces

1) DECOMPOSITION IN LANDFILL

The waste that is placed in the landfill is undergone by bacterial decomposition. At first the decomposition occurs in aerobic condition because air is soon lost. Long term decomposition occurs in anaerobic conditions. The source of aerobic and anaerobic organisms that deals with decomposition is soil that is used as final cover material daily.

The rate of decomposition of organic material depends upon their properties and to much extent on the moisture conditions. The organic material in solid waste is classified into 3 categories:

• those containing cellulose and derivatives of cellulose
•  those not containing cellulose and their derivatives
• plastics, rubber and leather

Cellulose is major part of organic waste e.g. paper, straw, string and plant tissues. Non cellulose organics are proteins, carbohydrates and fats. Mineral salts in less amount and moisture are attached with these materials. 

The end result products from anaerobic decomposition are intermediate volatile organic acids, partially stable organic materials and many gases. 

2) GASES IN LANDFILL

Gases in landfill consist of air, ammonia, carbon dioxide, carbon monoxide, hydrogen, hydrogen sulphide, nitrogen, methane and oxygen. The main gases evolved from anaerobic decomposition of organic material are carbon dioxide and methane. The high carbon dioxide concentration is due to aerobic decomposition. It occurs until the oxygen in air is depleted. After this anaerobic decomposition occurs. After 18 months the composition of gas varies. If the landfill is not vented the percentage of methane increases. 

The total volume of gas released during anaerobic decomposition is estimated by various methods. One is for example if all the organic materials have the formula CaHbOcNd then total volume could be estimated with the assumption of its complete conversion into carbon dioxide and methane.   

3) LECHATE IN LANDFILLS 

Lechate is defined as liquid that percolates through solid waste and has extracted dissolved or suspended materials from it. In landfills the liquid is produced from decomposition of waste and from external sources such as groundwater, rainfall, surface drainage and water from underground springs. 

When lechate percolates through solid waste that undergoes decomposition, both biological and chemical materials are picked up.

Quantity of lechate is direct function of the amount external water entering the landfill.  If landfill is constructed properly then production of lechate could be eliminated. 

COMPLETE LANDFILL

Read More

LANDFILLING METHODS AND OPERATIONS

The methods used for landfilling dry areas are classified into area, trench and depression.

1) AREA METHOD

Area method is used when the land is not suitable for the excavation of trench where solid waste is placed. In this method the waste is unloaded and spread in the long narrow strips in layers that vary in depth from 16 to 30 in. Each layer is compacted in day's operation until its height reaches the 6 to 10 ft. At the end of each day's operation 6 to 12 in layer of cover material is placed over the completed fill. The cover material is hauled from the nearest borrow pit or adjacent land from trucks or earth moving equipment. 

The filling operation is started by building an earthen lavee against which wastes are hauled and compacted. The length of unloading area is based on site conditions and size of operation. The width of compacted waste varies from 8 to 20 ft depending on the terrain. A complete lift having cover material is called a cell. Successive lifts are placed on top of one another until the final grade is reached that is called ultimate development plan. The length of unloading area used every day should be such that final height of fill is achieved at the end of days operation.

2)TRENCH METHOD

The trench method is used when there is enough amount of cover material available at the site and the water table is near the surface. The solid waste is placed in trenches that vary in length from 100 to 400 ft, depth from 3 to 6 ft and width from 15 to 25 ft. In the beginning of the process some part of trench is dug and waste is piled to form an embankment behind the first trench. Wastes are placed there and thin layers of 18 to 24 in are made and compacted. This process continued until the required height is achieved. The length of trench should be such that at the end of days operation final height is achieved. The cover material is gained from the excavation of nearest trench or continuing the trench that is being filling.

3) DEPRESSION METHOD

In the areas where artificial or natural depressions are present, they can be used for landfilling operations. Canyons, Ravines, quarries and dry borrow pits all can be used for this. The technique of placing and compacting the waste depends upon the geometry of the site, properties of cover material, hydrology and geology of site and access to the site. 

In canyon the floor is flat and the first filling is done here by the method of trench. After this the filling is done at the head end of canyon than at the mouth of canyon. This avoids the water accumulation behind the landfill. Wastes are deposited at canyon floor and pushed against canyon face at the slope of 2 to 1 . High compaction is achieved. The compacted density achieved as high as 1,200 Ib/yd3 was seen. 

4) CONVENTIONAL METHODS FOR WET AREAS

Swamps, marshes, tidal areas, ponds and quarries are wet areaas that could be used for landfilling. Due to the problems of odor and ground water contamination the design of landfill in wet areas is a serious concern. 

To bear mudwaves and to enhance structure stability dikes are used to divide the cells or lagoons by riprap trees, lumber demolition waste,, tree limbs and other relevant materials with clean fill material. Sometimes to avoid the malodorous lechate movement and gases in lagoons, the clay and wood sheet piling or leightweight interlocking steel are used. 

The problems could also be ressolved by first draining the site and then lining the bottom with clay liner and other sealants. 

Read More

SITE SELECTION PARAMETERS FOR A SANITARY LANDFILL SITE

The following considerations are taken while designing a sanitary landfill site:

SITE SELECTION

Factors considered while evaluating the potential of solid waste disposal sites are:

• availability of land area
• Processing and resource recovery impacts
• Haul distance
• soil conditions and topography
• climatic conditions
• surface water hydrology
• geologic and hydrologic conditions
• local environmental conditions
• Final use of complete site

1) LAND AREA AVAILABILITY

In order to select a suitable land disposal site sufficient land area should be ensured. There are no fixed rules prescribed for area but its is needed to have the area operated for atleast 1 year at a site. For shorter times the operations become more expensive because of site preparation, provision of auxiliary facilities and final cover completion. 

2) RESOURCE RECOVERY

During the assessment of potential disposal sites the extent of resource recovery processing activities occurring in future should be predicted. These activities have impact on quantity and condition of residual materials to be disposed off. As an example for the recycling of 50 % of paper  the weight of disposed materials and requirement of landfill area must be reduced. The recovery facility presence should also be in consideration at the disposal sites.

3) HAUL DISTANCE

For selecting a disposal site the haul distance is an important parameter. The length of haul affect the design and waste management operation. It is suggested to use minimum haul distance but along with this other factors should also be considered such as: collection route location, local traffic patterns, condition of routes, traffic patterns and access conditions.

4) SOIL CONDITIONS AND TOPOGRAPHY

The characteristics of soils in the area must be provided. If the soil of proposed landfill area is used for cover material than its geologic and hydrlogic investigations must be there. If the cover material is taken from borrow pit than test borings are needed to characterize the material. The local topography must be considered.

5) CLIMATIC CONDITIONS

Climatic conditions must be considered to evaluate the potential sites. Many locations access is affected by winter season. When freezing is severe landfill cover material must be available in stockpiles. Wind and its patterns must be considered carefully. Windbreaks must be used to avoid the blowing or flying papers. Specific type of wind break depends upon local conditions.

6) LOCAL ENVIRONMENTAL CONDITIONS

To build landfill sites close to residential or industrial areas care must be taken while it is environmentally acceptable or not in terms of noise, odor, dust and vector control. Flying papers and plastic films must be controlled.

Read More

INTRODUCTION AND ADVANTAGES, DISADVANTAGES OF SANITARY LANDFILL SITES

INTRODUCTION

For the waste that has no use after its collection and after its processing and recovery of conversion products, something must be done. There are two alternatives for the long term handling of solid waste: One is disposal on or in the earths mantle and second is disposal at the bottom of the ocean. Disposal on land is most commonly used method today. Disposal in the atmosphere provides a third alternative. It is not a good method because the discharge in atmosphere ultimately deposits the waste in earth or in the ocean by natural phenomenon (such as rainfall).    

Ocean dumping of solid waste was practiced at the end of 20 th century and it continued till 1933. Than it was prohibited by US Supreme Court. Industrial waste are also discharged at sea. Recently the ocean floors are used as waste storage sites. 

Land disposal in the form of sanitary landfill has been proved as more economical and accepted method for the disposal of solid wastes.

SANITARY LANDFILL SITES

The sanitary landfill means an operation in which waste to be disposed is compacted and covered with layer of soil at the end of each day operation. When disposal site has reached its ultimate capacity than a final layer of 2 ft or more of cover material is applied. 

ADVANTAGES

• Where land is available sanitary landfill is a good method for waste disposal.
• The initial investment is low as compared to the other disposal methods.
• It is final or complete disposal method as compared to composting and incineration that require additional treatments.
• Sanitary landfill can receive all types of waste. It avoids the need of separate collection of waste.
• A sanitary landfill is flexible, high quantity of waste can be disposed without additional equipment requirement.
• Some marginal land can be reclaimed for use such as: playgrounds, golf courses and airports.

DISADVANTAGES

• In high populated areas land might not be available within economical hauling distance.
• Proper sanitary landfill standards must be adhered to daily operation may result in an open dump.
•  A sanitary landfill in residential areas is highly opposed by people.
• A complete landfill requires the periodic maintenance.
• Special design and construction must be used for the buildings that are constructed on landfill due to settlement factor.
• Methane that is an explosive gas and other gases produced from decomposition of waste produce the hazard and interfere with the use of complete landfill.

Read More

SECONDARY TREATMENT OF WATER BY FILTERATION

FILTRATION

The most important stage in water treatment is filtration. In filtration the water is passed through he thick layer of sand. Through filtration following effects are seen:

• Chemical properties of water are changed.
• Suspended and colloidal impurities in finely divided state are removed. 
• The number of bacteria are also reduced.

The filtration is based on four actions:

1) Mechanical Staining

The suspended particles that can not pass through the pores of sand are arrested and removed through the  action of mechanical straining.

2) Sedimentation

The spaces in the sand particles act as the small sedimentation tanks. The impurities are arrested in voids of sand and these adhere to the particles of sand due to 2 reasons:

• due to physical attraction between two particles of matter
• due to presence of gelatinous film developed on sand grains by previous bacteria.

3) Biological Metabolism

The growth and life processes in living cells is called as biological metabolism. The action of filter is based on biological metabolism process. When bacteria are held in voids of sand, a zoological jelly is formed around. The film consist of large colonies of bacteria. Bacteria feed on organic impurities in water. They convert impurities to harmless compounds by biochemical reactions.

4) Electrolytic Changes

The action of filter can also be explained by ionic theory. It describes that when two opposite charges come close in contact to each other the charges are neutralized and new chemical substances are made. It is seen that some of the sand grain filters are charged with the electricity of some polarity. When particles of suspended and dissolved matter containing electricity of opposite polarity come in contact with sand grains, they neutralize each other and results in alteration of chemical properties of water. 

FILTER SAND

The sand used in filter is free from clay, loam, vegetable matter and organic impurities. It should be uniform in nature and size. The classification of filter sand depends upon effective size and uniformity coefficient. The effective size of sand depicts the size of sieve in mm through which 10 % of sample by weight is passed. The uniformity coefficient is the ratio of sieve size in mm through which 60 % of the sample of sand by weight will pass to the effective size of sand.

CLASSIFICATION OF FILTERS

The filters are classified into two categories:

• Slow Sand Filters
• Rapid Sand Filter

Rapid sand filters are further categorized into two sub classes:

• Gravity type rapid filters
• Pressure type rapid filters

SLOW SAND FILTERS

The purpose of slow sand filters is to pass the water slowly through the layer of sand that is placed above the base material and thus purification process is aimed to improve the biological, chemical and physical properties of water. These are best suitable for rural areas in developing countries. because of its simple and maintenance procedures. It pure water at low cost. 

ESSENTIAL PARTS 

A slow sand filter consist of following parts:

• Enclosure tank
• Under drainage system
• Base material
• Filter media of sand
• Appurtenances

The rate of filtration from slow sand filters varies from 100 to 200 liters / hour/ meter square of filter area. 

EFFICIENCY OF SLOW SAND FILTERS

• They remove about 98 to 99 % of bacterial load from raw water.
• They may remove 20 to 25 % color of raw water.
• They can remove the turbidity to an extent of 50 ppm. 

RAPID SAND FILTERS

The main disadvantage of slow sand filters is that it needs considerable space for its installation. This makes it uneconomical for the places where the land values are high. This led the engineers the need to increase the rate of filtration that could be increased by two ways:

• by increasing the size of sand so that the friction of water passing through the filter media is minimized.
• by allowing the water to pass under pressure though the filter media

The first one is achieved by gravity type rapid sand filter and second is achieved by pressure rapid filters. 

The parts of rapid sand filters are same as the slow sand filters.

Read More

COAGULATION AS PRIMARY TREATMENT OF WATER

Coagulation is the process used to make bigger sized particles by adding certain chemicals called as coagulants. These coagulants attach and react with the impurities in water and convert them into settled sizes. 

GENERAL COAGULANTS

Some coagulants that are used for coagulation are as follows:

• Aluminium sulphate
•  Chlorinated copperas
• Ferrous sulphate and lime
• Magnesium Carbonate
• Polyelectrolytes
• Sodium aluminate

1) Aluminium Sulphate

It is also known as filter alum or alum. Its chemical formula is Al₂ (SO4)₃, 18 H₂O. Alum is an effective coagulant and its use in water treatment is universal. Alum in water treatment is supplied and used in the form of flakes, solid lumps and in solution form.

The benefits from alum are as follows:

• It reduce the taste and odor.
• It reduces the turbidity of water.
• It is cheap.
• It is simple in working and not require skilled supervision.
• It produce crystal clear water.
• The floc formed by this coagulant is better.
• The floc formed is tough and can not be broken easily.

Normally bicabonate alklanity is present in water. The chemical reaction involved in this is:

Al₂ (SO4)₃. 18 H₂O + 3Ca(HCO₃)₂   ----->       2Al (OH)₃ + 3CaSO₄ + 18 H₂O + 6CO₂

_{The aluminium hydrooxide formed is not soluble in water. It acts as floc. Some of permanent hardness is caused due to calcium sulphate and carbon dioxide cause hardness.}

Read More

PRE TREATMENT OF WATER

If the raw water has good quality than it can be directly goes in secondary treatment processes of flocculation/ coagulation and sedimentation. Prior to the secondary treatment some steps are followed. They are as follows:

• Screening
• Storage
• Chemical pre-treatment

SCREENING

Coarse screens with inclined bars of 25 mm in diameter and 100 mm spacing avoid the large floating materials from entering the treatment plant. Raking is done with the inclination of bars. The velocities set for screens is 0.5 m/s that may be automatically or manually raked down. If there is no facilitation of storage than fine screens are fitted after the coarse screens. If here is storage than fine screens are placed at the outlet of storage tanks. Fine screens have the openings with 6 mm diameter or square. Exclusive forms of screens are generally used and they are normally automatically cleaned. 

The screens can be of circular drum type or travelling belt type as in vertical escalator. Screens poses the head loss that accounts for hydraulic calculations. Another type of screening is micro screening with mesh opening of 20 micrometer to 40 micrometer. Such screens are used to treat the uncontaminated waters and moderately colored waters. 

STORAGE

Storage is required for municipal water supply systems to meet variable water demand, fire protection, and for emergency needs. The reservoirs used are of three types:

• Surface Reservoir
• Standpipes
• Elevated Tanks

Surface Reservoirs

Surface reservoirs are located at the location where sufficient water pressure is provided. They are normally covered to prevent the contamination. 

Standpipes

These are tall cylindrical tanks. Its upper portion is used for storage and its lower portion supports the structure. The standpipes with the height of 15 m are not economical but above this height the storage tanks become the choice. Water demands of residential areas changes over day. 

Elevated Tanks

In recent years elevated tanks have becoming less famous due to their high cost and due to availability of invariable speed pumps and controls that make it possible to adjust pumping rates with varying demand. The location, type of storage and storage size must be determined. This depend on population and purpose of storage.

Read More

INTRODUCTION OF WATER TREATMENT PROCESSES

Modern technologies have greatly reduced the spread of waterborne diseases such as cholera and typhoid fever. These diseases are not much a concern now for public health as they were before. The main key for this advancement is the recognition that human wastes are the source of contamination of public water and it could be eliminated by effective water treatment strategies and waste disposal systems. 

In 1802 the filtration of drinking water was practiced in Paisly, Scotland. It was used by water vendors in London, England in 1828. In U.S the filtration of drinking water was first used in 1872 by the city of Poughkeepsie, New York. In this century the technology to make water safe for drinking has becoming widespread in Europe and North America. 

There are four classes of water treatment:

Class A  

In this class no treatment is required for some borehole water Occasional upland water

Class B

In this class borehole water or occasional water is used in public water supply. Disinfection is practiced to maintain the purity along the water pipelines. Chlorination with chlorine has been very debated. The alternative of chlorine has been searching out. Sometimes in this class aeration is used to remove the hydrogen sulphide odors and taste and to increase the level of oxygen in water. 

Class C

It is known as standard water treatment and it is used for lowland rivers and reservoirs.

Class D

Class D is special water treatment. It is used when the source is downstream of urban developments or when high quality water is required by the industries. Th other processes include: membrane technology, iron and manganese removal, chemical oxidation and carbon adsorption. 

Water treatment is designed to provide the standard quality water at taps. Four considerations are used in this:

• Source Selection
• Water Quality Protection
• Treatment method to be used
• prevention of re contamination

Precautions used to avoid groundwater and surface water pollution are:

• Prohibition of discharge of sanitary and storm sewers close to water reservoir
• installing fences to avoid pollution from recreational use of water
• restriction on application of fertilizers and pesticides in areas that drain to reservoirs

Screening, coagulation, flocculation, filtration and disinfection are used to surface water treatment.

These plants have the task to remove:

• Particulate substances such as sand and clay, organic matter, bacteria and algae
• dissolved substances causing color and hardness
• Pathogenic bacteria and viruses

Read More

TREATMENT OF AIR PARTICLE EMISSION BY BAGHOUSE COLLECTORS AND ELECTROPRECIPITATORS

BAGHOUSE COLLECTORS

Baghouse or fabric collectors are same like vacuum cleaner on large scale. Through these collectors dry particles are removed from dry and low temperature gas stream (0 to 275 degree centigrade). Cloth sock of about 15 cm in diameter or up to 10 m long is suspended in the chamber. The air is forced to pass through the sock and is discharged by the fabric. The fabric may be woven that is more common. The other fabric materials used are cotton, synthetics, fiberglass. Each material has its own adaptance to the gas, particle temperature and physical and chemical characteristics. 

The cloth from which bag or sock are made may consist of holes that exceed 100 µm. If these are correctly performed then greater than 99 % efficiency could be achieved for particles of diameter above than 1 µm. For smaller particles collection the filter cake is used on the cloth as filtration medium. As the filter cake thickens, the pressure loss and power cost increases. If the porous filter cake thickens too much, then the pressure loss increases that cake may collapse into more compact masses. If these pores are filled by liquid then same problems occurs. So, it is said that these collectors are used for dry particles collection and special care must be taken to avoid excessive condensation from the gas stream.

The filter cake is removed from small bag by simply shaking the bag so that the cake falls by itself. For large industrial collectors bag is cleaned by passing the ring jel of air alongside the bag and after some moments the flow is reversed. Some particles reentrainment occurs while cleaning. These particles are again removed in second cycle of cleaning. To avoid the need of bag shaking, to maintain the thickness of filter cake, and to avoid excessive pressure loss the volume flow rate by collectors is maintained at 0.5 to 2 m³ per meter square of cloth. 

The pressure drop across baghouse range from 5 to 40 cm of water for shaking periods ranged between 4 to 5 times per hour to once in many hours. A typical life of bag is 2 to 3 years. 

Fiber mat particle collectors are performed at low pressure drop and are disposable. They can be washed and reused many times. They are used in air conditioning system and hot air domestic systems.  

ELECTROSTATIC PRECIPITATORS

The voltage difference is maintained at as high level as possible. Electrons are released at the electrode in the cornea discharge and attach to particles and charge the particles. The charged particles or the molecules having same polarity as electrodes move toward ground surface due to electrostatic forces. 

Electrostatic Precipitators

Migrating ions liquid or solid and particles in gas stream and thus giving the particles charge that result in particle motion towards collector plates. When particles attach to plates they stick there and form an insulating blanket.   

For precipitator design gas and particle resistivity are important parameters. Resistivity varies with temperature and chemical composition. Precipitator efficiency is as high as 99 % for particles above than 2 µm at pressure loss of 5 cm of water or less. 

  

Read More

AIR PARTICLE EMISSION BY SCRUBBERS

Scrubbers or wet collectors are designed to increase the particle size by using water or slurry droplets because it is easy to collect the larger particles. 

Wet Scrubber

There are various types of scrubbers but two types are discussed here:

• Conventional Scrubbers
• Venturi Scrubbers

There are several modes of particle collection in scrubbers. In the upper part of scrubber the falling water droplets collide and collect the particles from the upward moving dity gas. In packed section special shapes are used to increase the area of contact between liquid and aerosols. The plugging is a problem faced in packed section despite the specific shapes used. Below the packed section there exists a flooded perforated disc. that supports many centimeters of water and allowed the contact between liquid and water bubbles containing particles. The liquid passes through perforations and fall in another falling drop collection section. 

The particles collection is not achieved in all the droplets and particles collision due to surface tension of droplets and particles wettability properties. Chemicals are added to reduce the surface tension of droplets and to improve their abilities to absorb the gases and particles. 

Venturi Scrubber

The liquid containing particles are moved to the bottom of tower and shifted to settling basin or filtering device for the removal of particles. The water is recirculated with or without treatment to result zero discharge system. 

The demistor the exit point of scrubber is a particles collector. It is designed to remove the drops of liquid from gas stream. 

The size of water drops is critical in determining the performance of scrubber. If the water drops are large than particles then drag forces displace the particles out of the path of falling drops and collision number decreases. 

The pressure loss from conventional scrubbers is 15 to 40 cm of water. Collector efficiency increase with the pressure loss and it may be as high as 95 %.

Scrubbers are designed to operate at high temperature and avoid corrosion. Operating cost is high for pressure loss scrubbers but the capital cost is comparatively low from other collectors. 

Read More

USE OF GRAVITATIONAL SETTLING CHAMBERS AND CYCLONES COLLECTORS FOR AIR EMISSION CONTROL

The air particles emission must control the particles emissions ranging in size from 1 µm to more than 100 µm in diameter. Collectors are designed according to the physics of collecting mechanisms.

GRAVITATIONAL SETTLING CHAMBERS

These are simple chambers and expensive. In this gravitational forces dominate vertical particle motion. These are simple expansions in duct where horizontal velocity of particles is reduced to give time to settle the particles by gravity. On particle viscous force and gravitational forces are equal but in opposite direction and the particle is expected to fall at terminal velocity. The horizontal component of viscous force is negligible because the particle moves with the velocity of gas stream.

The expressions used for the efficiency of the collector are:

η_g= 1- exp {-µ_tL/µH}

η_g= 1- exp {-gd2p _PpL/18µuH}

exp is used to represent exponential

η_{g = efficiency of removal as a fraction}

_{L=Length of collector in meter}

_{H= Depth of collector in meter}

_{u= horizontal velocity of gas and particles through collector}

d_{p=diameter of particle}

_{Pp= density of particle}

_{The performance of the collector varied from design prediction due to turbulence and variation in flow in collector.}

CYCLONES/ INTERNAL COLLECTORS

These collectors depends upon centrifugal forces to separate the heavier particles from lighter gas molecules. Particles laden gases enter in cyclone at top and spiral downward along casing in solid body rotation at entrance velocity u. Particles move outside of spiral. The only exit for particles is upward from central pipe. 

The magnitude of centrifugal force is 

F_c= m_p u²_T/r

UT= tragential velocity of particle

r= radius of curvature of particle trajectory

The pressure drop through conventional cyclone is 5 to 15 cm of 

water by high efficiency cyclone is 10 to 30 cm. of water.

Gravitational settling chambers and simple inertial separators have 

no moving parts. They may be fabricated by metals that can 

withstand the high temperature and resist corrosion. They are 

effective for solid and liquid particles. 

Read More

INTRODUCTION AND OBJECTIVES OF AIR QUALITY CONTROL

INTRODUCTION OF AIR POLLUTION CONTROL 

Air pollution control s defined as the measures taken to achieve the certain standards of emissions.

When the standards are met, it is said that pollution is in control. The measures taken are as follows:

• change in processes
• change in raw materials
• modification of equipment
• installation of treatment device at the end of processing equipment

The devices that control the air pollution are called as air pollution control devices. 

The control is related with another term known as abatement. 

While the control is just referred to the meeting of certain standards of emissions but control is defined as:

The measures taken to reduce the amount of emissions regardless the standards. 

When the measures of abatement meets the standards of emissions than abatement becomes control.

There are two types of air pollution control depending upon the types of air pollution.

• Particulate Control
• Control of gaseous pollutants

AIR QUALITY CONTROL

The main objective of air pollution control is to maintain such an atmosphere that has no negative influence on the human activities. The best solution to control the pollution is simply by not producing the pollutants. For Example by burning non leaded fuels instead of leaded fuels eliminates the lead emissions. Similarly nitrogen oxide emissions are reduced by redesigning the engines.

A substitute of nitrogen oxides pollution is to shift the location of nitrogen oxides sources from automobiles tailpipe to stack of electric generating station by utilizing hydrogen fuel cars. 

By legislating the amount of of ash and sulphur in fuels emissions can also be reduced of materials and its end products. 

Pollution can also be reduced by using add on devices. In vehicles carbon canisters are used to absorb the hydrocarbons vapors from carburetor and gas tanks. In automobile exhaust catalytic converters are utilized. for reducing the hydrocarbons emissions.the energy in hydrocarbons is released as heat. 

In industrial applications scrubbers are used to to remove pollutants from stack or gas streams. The scrubbers enforce the pollutants to chemically react with the materials to form stable compounds for collection and storage.

Planned dispersion may also provide a solution for the local air quality where other techniques cannot control the pollution. Tall stacks require the long time fort he pollutants to reach on the earth surface to make harm to the humans. materials and vegetation. For Example electric heating shifts the emissions from short chimneys in houses to tall stacks in remote areas. 

Read More