Question 97 from UPSC CSE 2017 - Environment

Question

Biological Oxygen Demand (BOD) is a standard criterion for

Accepted Answer

Correct Option: C. Pollution assay in aquatic ecosystems

Strategy: The term **'Biological Oxygen Demand'** itself is a strong clue. 'Biological' refers to microorganisms, and 'Oxygen Demand' refers to the amount of oxygen they consume to break down organic waste. This process is central to water pollution. A higher BOD means more organic waste is present, indicating greater pollution. Therefore, BOD is a direct measure, or 'assay', of pollution. Looking at the options, only (c) **'Pollution assay in aquatic ecosystems'** correctly places this concept in the context of water pollution. The other options - blood, forests, and high altitudes - are clearly incorrect environments for applying the BOD test.

Key Info: ## Biological Oxygen Demand (BOD)
Biological Oxygen Demand (BOD) is the amount of dissolved oxygen (DO) needed by aerobic biological organisms to break down organic material present in a given water sample at a specific temperature over a designated period. It is a widely used indicator of the organic quality of water.

- **Indicator of Pollution:**
  - BOD is a measure of the degree of organic pollution in water. A higher BOD value indicates a greater amount of organic waste (e.g., from sewage or industrial effluents), which means more oxygen will be consumed by microorganisms, leaving less for other aquatic life.
  - Clean water typically has a BOD of less than 1 mg/L. Water with a BOD of 3-5 mg/L is considered moderately polluted, while a value above 5 mg/L indicates significant pollution.
- **Measurement:**
  - It is commonly expressed in milligrams of oxygen consumed per liter of sample (mg/L).
  - The standard test involves incubating a sealed water sample for 5 days at 20°C, and the result is referred to as BOD5.

## Dissolved Oxygen (DO)
Dissolved Oxygen refers to the level of free, non-compound oxygen present in water or other liquids. It is a critical parameter for assessing water quality as it is essential for the survival of most aquatic organisms.

- **Inverse Relationship with BOD:**
  - BOD and DO are inversely related. As microorganisms consume organic matter (a process measured by BOD), they use up the dissolved oxygen in the water, causing DO levels to drop.
  - Low DO levels, a condition known as hypoxia, can lead to the death of fish and other aquatic life, creating 'dead zones'.

## Chemical Oxygen Demand (COD)
Chemical Oxygen Demand (COD) is another measure of water quality that determines the quantity of oxygen required to oxidize both organic and inorganic compounds in water through a chemical reaction. Unlike BOD, COD measures both biodegradable and non-biodegradable pollutants.

- **Comparison with BOD:**
  - The COD value for a water sample is always higher than its BOD value because it accounts for a wider range of oxidizable substances.
  - The COD test is much faster to perform (a few hours) compared to the BOD test (5 days), making it useful for rapid monitoring, especially of industrial wastewater.

Answer

Option A: Measuring oxygen levels in blood

Answer

Option B: Computing oxygen levels in forest ecosystems

Answer

Option D: Assessing oxygen levels in high altitude regions