Pollution of Air and Water

Overview & Fundamental Concept

In-depth AISSEE Class 9 General Science guide for Pollution of Air and Water (Topic 12). Contains pointwise environmental facts, Taj Mahal corrosion chemistry, eutrophication flowcharts, water purification tables, solved models, and 10 practice drills.

• Normal Composition of Clean Air: Nitrogen (approx. 78%), Oxygen (approx. 21%), Carbon Dioxide (0.03%), Argon (0.9%), along with minute traces of methane, ozone, water vapor, and dust particles.
• Air Pollution Definition: When air is contaminated by unwanted, harmful substances (both gases and particulate matter) that have a damaging effect on both living organisms (humans, animals, plants) and non-living structures, it is called air pollution.
• Major Air Pollutants & Sources:
  • 1. Carbon Monoxide (CO): Produced by the incomplete combustion of fuels such as petrol and diesel in automobiles. It is an extremely poisonous, colorless, odorless gas. It binds with blood hemoglobin to form carboxyhemoglobin, reducing the oxygen-carrying capacity of blood and causing suffocation.
  • 2. Smog: A dense, toxic atmospheric layer formed by the combination of Smoke + Fog. Contains oxides of nitrogen that react with other pollutants. Causes severe respiratory ailments like asthma, bronchitis, and wheezing in children.
  • 3. Sulphur Dioxide ($SO_2$): Produced by burning coal in thermal power plants and petroleum refining (e.g. Mathura Oil Refinery). Causes severe lung damage and is a primary precursor to Acid Rain.
  • 4. Nitrogen Oxides ($NO_x$): Emitted by vehicle exhausts at high combustion temperatures.
  • 5. Chlorofluorocarbons (CFCs): Used as refrigerants in refrigerators, air conditioners, and aerosol propellant sprays. CFCs rise to the stratosphere and deplete the protective Ozone layer ($O_3$), allowing harmful cancer-causing Ultraviolet (UV) rays to penetrate to Earth.
  • 6. Suspended Particulate Matter (SPM / $PM_{2.5}$): Tiny unburnt solid particles emitted by steel mills, cement factories, and diesel engines that remain suspended in air for weeks, causing visibility reduction and silicosis.

• Acid Rain Formation: Industries in Agra and neighboring Mathura (especially the Mathura Oil Refinery, rubber processing units, and glass factories) release high volumes of Sulphur Dioxide ($SO_2$) and Nitrogen Dioxide ($NO_2$).
• These pollutant gases react chemically with water vapor present in the atmosphere to form Sulphuric Acid ($H_2SO_4$) and Nitric Acid ($HNO_3$):
  $$SO_2 + \text{Water Vapor} + O_2 \longrightarrow H_2SO_4 \quad (\text{Sulphuric Acid})$$
  $$NO_2 + \text{Water Vapor} + O_2 \longrightarrow HNO_3 \quad (\text{Nitric Acid})$$
• The acids drop down with rainwater, making the rain acidic (pH < 5.6). This is called Acid Rain.
• Marble Cancer Phenomenon: The Taj Mahal in Agra is constructed from pure white Makrana marble (Calcium Carbonate, $CaCO_3$). Acid rain reacts chemically with the marble:
  $$CaCO_3 + H_2SO_4 \longrightarrow CaSO_4 \text{ (Calcium Sulphate)} + H_2O + CO_2 \uparrow$$
• This chemical corrosion slowly dissolves and pits the white marble, turning the glistening facade dull and yellow. This degradation is termed 'Marble Cancer'.
• Additionally, unburnt carbon soot particles emitted by the Mathura refinery have deposited on the stone, accelerating yellowing.
• Supreme Court Intervention (Taj Trapezium Zone - TTZ):
  • Ordered all industries in the TTZ to switch exclusively to clean fuels: CNG (Compressed Natural Gas) and LPG.
  • Mandated that all vehicular traffic passing through the zone must run on unleaded petrol and CNG.

• Water Pollution Definition: When toxic chemical substances, untreated sewage, or agricultural run-offs mix with water bodies (rivers, lakes, groundwater), changing its physical, chemical, and biological properties so that it becomes harmful to humans, animals, and aquatic life.
• Major Sources of Water Pollution:
  • 1. Untreated Municipal Sewage: Carries disease-causing microbial pathogens (bacteria, viruses, protozoa, helminths) causing water-borne epidemics like cholera, typhoid, jaundice (hepatitis), and dysentery.
  • 2. Industrial Chemical Effluents: Chemical factories, paper mills, textile dye units, and tanneries discharge toxic untreated wastes containing Arsenic, Lead, Mercury, and Fluorides directly into rivers. These heavy metals are non-biodegradable and bioaccumulate in the food chain.
  • 3. Hot Water from Power Plants: Thermal power stations discharge superheated coolant water into rivers. Hot water decreases the solubility of dissolved oxygen, killing cold-adapted fish (Thermal pollution).
• Eutrophication (Nutrient Enrichment Cascade):
  • Farmers apply excessive chemical fertilisers (NPK — Nitrogen, Phosphorus, Potassium) to crop fields.
  • Rains wash these excess nitrates and phosphates into nearby ponds and lakes (Agricultural Runoff).
  • These nutrients act as high-potency food for green aquatic algae, triggering explosive proliferation called an Algal Bloom.
  • The dense algal bloom covers the entire surface of the water body, blocking sunlight from reaching submerged aquatic plants.
  • When the massive mass of algae dies, aerobic decomposer bacteria multiply rapidly and consume virtually all dissolved oxygen to break down the dead matter.
  • Catastrophe: Dissolved Oxygen (DO) levels drop to near zero (Biochemical Oxygen Demand / BOD surges), and fish and other aquatic fauna suffocate and die en masse.

• The Ganga River Pollution Crisis: The sacred River Ganga sustains over 40% of India's population. A 2007 WWF study listed the Ganga as one of the ten most endangered rivers in the world due to the discharge of millions of litres of untreated industrial effluents, domestic sewage, and immersed idols.
• 1. Ganga Action Plan (GAP): Launched by Prime Minister Rajiv Gandhi in 1985 to reduce pollution levels in the river by installing sewage treatment plants (STPs).
• 2. National Mission for Clean Ganga (NMCG / Namami Gange): Launched in 2014 as an integrated flagship conservation mission with a budget of ₹20,000 crore to achieve effective abatement of pollution, conservation, and rejuvenation of the National River Ganga.
• Potable Water: Water that is chemically pure, free of microbial pathogens, clear, odorless, and safe for human drinking.
• Methods of Making Water Potable:
  • 1. Physical Filtration: Using candle filters, activated charcoal, or modern RO (Reverse Osmosis) semi-permeable membranes to remove suspended particles.
  • 2. Boiling: The simplest and most dependable domestic method. Boiling water vigorously for 10–15 minutes kills virtually all disease-causing bacteria, viruses, and parasites.
  • 3. Chlorination: A commonly used chemical method where calculated quantities of Chlorine tablets or Bleaching powder [$CaOCl_2$] are added to water. Chlorine is a powerful oxidizing biocide that destroys water-borne pathogens without harming humans (excess chlorine must be avoided).
  • 4. Ultraviolet (UV) Radiation: UV purifiers use germicidal UV rays to damage the DNA of microbes, inactivating them without adding chemicals.