Water Pollution
What is Water Pollution?
Water pollution simply means:
Any undesirable substance enters water → water quality deteriorates → water becomes unfit for use.
These undesirable substances are called POLLUTANTS, and they may be:
- Organic pollutants – e.g., sewage, plant and animal waste
- Inorganic pollutants – e.g., heavy metals, salts, acids
- Biological pollutants – e.g., pathogens, microbes
- Radiological pollutants – radioactive substances
- Heat (thermal pollution) – hot effluents from industries or power plants
Even nature contributes to pollution. For example:
- Soil erosion increases silt load
- Minerals leach from rocks naturally, or due to acid rain
- Decaying organic matter releases nutrients and reduces water quality
So, pollution is not always man-made, but human activities greatly magnify its scale and impact.
Types of Pollution Sources: Point vs. Non-Point
UPSC often asks this distinction because it forms the foundation of pollution management.
(A) Point Source Pollution
Think of a single identifiable pipe releasing pollutants directly into a water body.
Examples:
- A drainpipe releasing industrial effluents
- A sewage outlet discharging into a river
Here, the source is fixed and traceable. Regulation becomes easier.
(B) Non-Point Source Pollution
Here, pollutants come from wide, diffused areas, not one identifiable point.
Examples:
- Runoff from agricultural fields (fertilizers, pesticides)
- Grazing lands
- Construction sites
- Abandoned mines
This type is difficult to monitor and control because the pollution is spread over a large area.
Measuring Water Pollution
To understand the health of water, we measure how much oxygen is available and how much oxygen is being consumed.
Dissolved Oxygen (DO)
Dissolved Oxygen is the oxygen present in water, essential for fish and other aquatic life—just like the air we breathe.
Why does DO decrease?
When water has many organic and inorganic wastes:
- Decomposition increases
- Microorganisms consume more oxygen
→ DO levels fall
Factors influencing DO:
- Surface turbulence – more turbulence = more oxygen absorbed
- Photosynthesis – aquatic plants release oxygen
- Respiration and decomposition – consume oxygen
Interpreting DO values:
- Below 8.0 mg/L → contaminated water
- Below 4.0 mg/L → severely polluted (aquatic life struggles to survive)
Lower DO is a classic indicator of poor water quality.
Biological Oxygen Demand (BOD)
This is a crucial term for UPSC.
What is BOD?
BOD measures:
How much oxygen bacteria need to decompose organic wastes in water.
Higher organic waste → more bacterial activity → higher BOD.
Interpretation:
- High BOD = high pollution due to biodegradable organic matter
- High BOD = low DO (because bacteria consume oxygen)
Limitation:
BOD measures only the biodegradable portion of wastes.
So it cannot assess pollution from non-biodegradable substances, chemicals, or toxins.
Therefore, BOD alone is not a complete indicator.
BOD Level and Water Quality
| BOD Level (mg/L) | Water Quality | Description |
|---|---|---|
| 1 – 2 | Very Good | Very little organic matter present in water. |
| 3 – 5 | Fair | Moderately clean water. |
| 6 – 9 | Poor | Somewhat polluted; indicates presence of organic matter and active microbial decomposition. |
| 100 or more | Very Poor | Highly polluted; contains large amounts of organic matter. |
Chemical Oxygen Demand (COD)
Think of COD as a more comprehensive measure than BOD.
What does COD measure?
COD tells us the oxygen required to oxidise both:
➡️Biodegradable organic matter, and
➡️non-biodegradable organic matter, and
➡️Oxidisable inorganic compounds
It is expressed in parts per million (ppm).
Why is COD considered better?
Because it captures total pollution load, not just the biodegradable component.
In simple terms:
- BOD = partial picture
- COD = full picture
This is why agencies often use COD for regulatory monitoring.