Question

BOD values of four samples \(A, B, C\) and \(D\) of water are \(60,80,100\) and \(150 \mathrm{mg}\) /itre, respectively. Which is more polluted? (a) \(\mathrm{A}\) (b) B (c) \(\mathrm{C}\) (d) D

Short answer

The most polluted sample is D.

Step-by-step solution

Understand BOD (Biochemical Oxygen Demand)

BOD indicates the amount of dissolved oxygen needed by microorganisms to decompose organic matter in the water. Higher BOD values mean more organic pollution.

Compare BOD values

Given BOD values for samples are: Sample A: 60 mg/L Sample B: 80 mg/L Sample C: 100 mg/L Sample D: 150 mg/L. Identify which value is the highest.

Determine the most polluted sample

The sample with the highest BOD value is the most polluted. Compare the values and find the highest one.

Identify the Sample

Sample D has the highest BOD value of 150 mg/L, indicating that it is the most polluted among the given samples.

Key concepts

Water Pollution

Water pollution is the contamination of water bodies, such as lakes, rivers, oceans, and groundwater. This typically occurs due to human activities. When harmful substances like chemicals, waste products, or microorganisms enter the water, they degrade its quality.

• Contaminants can be physical, chemical, or biological.
• Both visible and invisible pollutants affect water.

Common sources include industrial discharge, agricultural runoff, and sewage leaks. This pollution not only harms aquatic life but can also make water unsafe for human consumption and use.

Dissolved Oxygen

Dissolved oxygen (DO) refers to the amount of oxygen present in water. It's a critical component for the survival of fish, invertebrates, and microorganisms. Without it, aquatic life cannot sustain itself.
The level of dissolved oxygen can indicate the health of a water body.

• High DO levels mean a healthier ecosystem.
• Low DO levels can indicate pollution or an imbalance.

Decomposition of organic matter and respiration from aquatic organisms can deplete DO levels, leading to hypoxic conditions.

Organic Matter Decomposition

Organic matter decomposition is the process where dead plants, animals, and waste materials break down. Microorganisms, such as bacteria and fungi, play a critical role in this process.
As these microorganisms consume organic matter, they use dissolved oxygen. This is why higher levels of organic matter lead to higher BOD values, indicating more pollution.

• Decomposition releases nutrients back into the ecosystem.
• It is a natural recycling process but can be harmful in excessive amounts.

Proper waste management can help control the amount of organic matter entering water bodies.

Microorganisms

Microorganisms are tiny living organisms found everywhere, including in water. They are essential for breaking down organic pollutants and recycling nutrients.
In the context of BOD, these microorganisms use oxygen to decompose organic matter.

• Bacteria are the primary microorganisms involved in BOD.
• Their activity can deplete oxygen levels in water, affecting the quality.

While microorganisms help clean up pollutants, an overload can lead to oxygen depletion, causing harm to aquatic life.