Water Pollution
Understand the causes of water contamination, Biochemical Oxygen Demand (BOD), Eutrophication, and crucial Drinking Water Standards.
Water is essential for the survival of all living organisms. However, human activities often compromise its quality. Water pollution occurs when undesirable substances—such as organic, inorganic, biological, or radioactive pollutants—are introduced into water bodies, altering its natural properties and making it unfit for use.
1. Sources of Water Pollution
Pollution sources are generally categorized into two broad types based on how they enter the water supply:
Pollutants that are discharged from a single, identifiable location or source.
Examples: Discharge pipes from a specific factory, a municipal sewage treatment plant, or an underground storage tank leak.
Pollutants that come from diffuse, broad areas, making them difficult to track and control.
Examples: Agricultural runoff (fertilizers and pesticides from fields), acid rain, and storm water runoff from city streets.
Major Types of Pollutants
| Pollutant Type | Source | Examples / Effects |
|---|---|---|
| Pathogens | Domestic sewage, animal excreta | Bacteria (E. coli, Streptococcus), viruses causing gastrointestinal diseases. |
| Organic Wastes | Sewage, paper mills, food processing | Leaves, grass, trash. Causes severe depletion of dissolved oxygen when degraded by bacteria. |
| Chemical Pollutants | Industrial discharge, agriculture | Heavy metals ($Cd, Pb, Hg$), synthetic organic compounds (PCBs, pesticides), fertilizers. |
2. Dissolved Oxygen (DO) & Biochemical Oxygen Demand (BOD)
The health of an aquatic ecosystem is heavily dependent on the presence of oxygen dissolved in the water.
A. Dissolved Oxygen (DO)
In cold water, dissolved oxygen (DO) can reach a maximum concentration of about 10 ppm (parts per million). Unlike air, which is ~21% oxygen, aquatic life survives on very little.
Critical Limit: If the concentration of dissolved oxygen in water falls below 6 ppm, the growth of fish gets severely inhibited.
B. Biochemical Oxygen Demand (BOD)
When large amounts of organic waste are dumped into water, aerobic bacteria consume oxygen to break it down. BOD is the measure of the amount of oxygen required by bacteria to break down the organic matter present in a certain volume of a sample of water.
Important Note: A high BOD means there is a high level of organic pollution, which consequently leaves very little DO for fish and other aquatic life.
3. Eutrophication
Chemical fertilizers containing phosphates and nitrates are washed into lakes and water bodies via agricultural runoff. These nutrients act as food for algae, causing an explosion in their population, known as an Algal Bloom.
As this massive amount of algae eventually dies, aerobic bacteria decompose it, consuming vast quantities of Dissolved Oxygen (DO). The water becomes hypoxic (oxygen-starved), suffocating fish and destroying the aquatic ecosystem. This process of nutrient enrichment and subsequent loss of biodiversity is called Eutrophication.
4. International Standards for Drinking Water
Certain elements and ions are beneficial in small quantities but highly toxic at higher concentrations. The following limits are crucial for competitive exams:
- Standard: ~1 ppm is desirable (prevents tooth decay).
- > 2 ppm: Causes brown mottling of teeth.
- > 10 ppm: Extremely harmful to bones and teeth (skeletal fluorosis).
- Standard: Maximum limit is 50 ppb (parts per billion).
- Effect: Can damage the kidney, liver, and reproductive system. Plumbosolvency (lead pipes dissolving in water) is a major source.
- Standard: Maximum limit is 50 ppm.
- Effect: Excess nitrate in drinking water causes Methemoglobinemia, also commonly known as Blue Baby Syndrome.
- Standard: Harmless at moderate levels, but limit is >500 ppm.
- Effect: Above 500 ppm, it causes a laxative effect.
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