Air & Noise Pollution | Environmental Engineering - Civil Engineering (CE) PDF Download

Introduction

Developmental activities such as construction, transportation and manufacturing consume natural resources and generate large quantities of wastes. These wastes lead to pollution of air, water, soil and oceans, and contribute to problems such as global warming and acid rain.

Pollution & Pollutants

Automobiles emit from their tailpipes oxides of nitrogen, sulfur dioxide, carbon dioxide, carbon monoxide and a complex mixture of unburnt hydrocarbons and black soot which pollute the atmosphere. Domestic sewage and run-off from agricultural fields, laden with pesticides and fertilisers, pollute water bodies. Effluents from tanneries contain many harmful chemicals and often produce foul odours.

• Pollution is the addition of undesirable material into the environment as a result of human activities.
• Pollutants are physical, chemical or biological substances unintentionally released into the environment that are directly or indirectly harmful to humans and other living organisms.

Types of Pollution

Pollution may be of the following types:

• Air pollution
• Noise pollution
• Water pollution
• Soil pollution
• Thermal pollution
• Radiation pollution

Air Pollution

Air pollution is the presence of any solid, liquid or gaseous substance (including noise and radioactive radiation as pollutants in the atmosphere) in concentrations that are injurious to humans, other living organisms, plants or property, or that interfere with normal environmental processes.

Air pollutants are generally of two broad types:

• Suspended particulate matter (SPM)
• Gaseous pollutants such as carbon dioxide (CO₂), oxides of nitrogen (NOx), sulfur dioxide (SO₂), carbon monoxide (CO) and unburnt hydrocarbons.

Some of the major air pollutants, their sources and effects are given in the table:

Particulate Pollutants

Particulate matter suspended in air includes dust and soot released from industrial chimneys and other combustion sources. Particle diameters range from about 0.001 µm to 500 µm.

• Particles smaller than 10 µm (PM10) remain suspended and are inhalable; those larger than 10 µm tend to settle.
• Particles smaller than 0.02 µm form persistent aerosols and can remain airborne for long periods.
• Major sources of SPM include vehicles, thermal power plants, construction activities, oil refineries, railway yards, marketplaces and various industries.

Fly Ash

Fly ash is a particulate by-product from coal-burning thermal power plants.

• Fly ash can pollute air and water and may introduce heavy metals into water bodies.
• Vegetation is affected by direct deposition on leaves and indirectly through changes to soil properties.
• Beneficial uses include manufacturing fly ash bricks and use as landfill or fill material when managed appropriately.

Lead and Other Metal Particles

• Tetraethyl lead (TEL) has been used as an anti-knock agent in petrol; lead emitted from exhausts mixes with ambient air.
• Lead entering food and water causes cumulative poisoning and long-term neurodevelopmental effects in children, including reduced intelligence.
• Oxides of iron, aluminium, manganese, magnesium, zinc and other metals are produced during mining and metallurgical operations and cause adverse effects due to dust deposition on plants and soil.

Gaseous Pollutants

Power plants, industries and vehicles burn fossil fuels (petrol, diesel, coal) and release gaseous pollutants such as CO₂, NOx and SO₂ together with particulate matter (smoke). These gases have harmful effects on plants, animals and humans. Control of gaseous pollutants requires different treatment techniques than particulate matter.

Air Pollution Control & Principles

Air pollution control applies engineering and management measures to reduce emissions, limit exposure and remediate polluted air. Control strategies include source control (reducing emissions at their origin), end-of-pipe devices (removing pollutants from exhaust streams) and regulatory or operational measures (fuel switching, process modification, maintenance and work-practice changes).

Source Control Technology

• Air quality management provides tools to control pollutant emissions through technology, process changes and regulations.
• Control measures include equipment, process changes or actions used to reduce air pollution at the source.
• The effectiveness of a control measure depends on the pollutant type, engineering feasibility, environmental benefits and economic considerations.

Settling Chambers

• Settling chambers remove larger solid particles by gravity.
• The gas stream is slowed inside a chamber so large particles fall out and are collected in hoppers.
• Settling chambers are simple and inexpensive but remove only larger particles; they are often used before more efficient devices.

Cyclones

• Cyclones separate particles by inertia: the particulate-laden gas is forced to change direction and heavier particles continue in the original direction and collect at the walls.
• Walls narrow toward the bottom so collected particles fall into a hopper.
• Cleaner gas exits through the top in a spiral vortex; cyclones are efficient for large particles but less so for small particles, so they are often followed by other control devices.

Venturi Scrubbers

• Venturi scrubbers use a liquid stream (usually water) to capture particulate matter and soluble gases.
• The gas passes through a constricted throat where velocity and turbulence increase, promoting particle-liquid contact and collection.
• At the downstream expansion zone velocity drops and droplets containing particles separate from the gas and are removed.
• Venturi scrubbers can achieve removal efficiencies up to about 99% for small particles, but they generate wastewater that requires treatment.

Electrostatic Precipitators (ESPs)

• An ESP charges particles in a gas stream and collects them on oppositely charged metal plates using electrostatic forces.
• Collected particles are dislodged by mechanical rapping and fall into hoppers for disposal.
• ESPs can achieve high removal efficiencies (around 99% for many particle sizes) and are widely used in power plants, cement, pulp and paper, petrochemical and steel industries.

Absorption (Scrubbing)

• Absorption removes gaseous pollutants by dissolving them in a liquid solvent; equipment used for this is commonly called a scrubber.
• Gas-liquid contact devices include spray towers, packed columns and spray chambers.
• Absorbers can achieve removal efficiencies greater than 95% for many soluble gases.
• A principal drawback is the generation of contaminated wastewater that must be treated, converting an air pollution issue into a water pollution management task.

Adsorption

• Adsorption is the capture of gas or vapour molecules on the surface of a solid adsorbent rather than their dissolution into a liquid.
• Common industrial adsorbents include activated carbon, silica gel and alumina, which have very large surface areas per unit weight.
• Activated carbon is a standard medium for removing trace organics and odorous compounds from vapour or liquid streams.
• Adsorption systems can be regenerative (carbon beds are cyclically desorbed and reused) or non-regenerative (spent carbon is replaced and disposed).

Condensation

• Condensation converts vapour to liquid by reducing temperature and/or increasing pressure; it is used to reduce gas volumes before further treatment.
• Contact condensers mix gas with a cold liquid; surface condensers cool the gas on cold surfaces such as tube walls.
• Condensers are often used as pretreatment and can achieve removal efficiencies from about 50% to over 95%, depending on the vapour and design.

Incineration (Combustion)

• Incineration oxidises organic compounds in waste gases to carbon dioxide and water vapour when combustion is complete.
• It is widely used to control emissions of volatile organic compounds (VOCs) and odorous organics from process industries.
• Types of combustion control equipment include direct flares, thermal incinerators and catalytic incinerators.

Noise Pollution

• Noise is commonly defined as unwanted sound or "sound without value" to the recipient.
• Noise level is measured in decibels (dB). The World Health Organization (WHO) recommends an optimum noise level of about 45 dB by day and 35 dB by night.
• Continuous exposure to noise levels above about 80 dB is hazardous and can cause temporary or permanent hearing loss, increased blood pressure, reduced work efficiency and other adverse health effects.
• Everyday sources of hazardous noise include stone cutting and crushing, heavy machinery, loud music, continuous traffic, railways and aircraft operations.

Sources of some noises and their intensity

Sources of Noise Pollution

• All human activities contribute to noise pollution to varying degrees; sources may be indoor or outdoor.
• Indoor sources include radios, televisions, generators, electric fans, air coolers, air conditioners and various household appliances.
• Outdoor sources include loudspeakers, industrial activities, road and rail traffic, aeroplanes and activities in marketplaces, religious and social functions, sports and political rallies.

Prevention and Control of Noise Pollution

The following measures can reduce or control noise pollution:

• Reduce road traffic noise by better vehicle design, proper maintenance and traffic management to avoid congestion.
• Construct noise mounds and attenuation walls and maintain smooth, well-surfaced roads to reduce tyre/road noise.
• Reduce railway noise by retrofitting, using continuously welded rails, electric locomotives and quieter rolling stock.
• Mitigate air traffic noise by insulating buildings near airports and enforcing noise regulations for aircraft take-off and landing patterns.
• Soundproof noisy industrial equipment and enclose high-noise areas; use silencing and vibration-damping measures on machinery.
• Restrict use of loud public address systems, very loud music and noisy public functions at night; regulate horns, alarms and other nuisance devices.
• Promote green belts and tree planting as natural noise absorbers and visual barriers.
• Limit use of noisy fireworks and heavy machinery at night and in residential areas.