Q.Three water samples namely river water, untreated sewage water and secondary effluent discharged from a sewage treatment plant were subjected to BOD test. The samples were labelled A, B and C; but the laboratory attendant did not note which was which. The BOD values of the three samples A, B and C were recorded as 20mg/L, 8mg/L and 400mg/L, respectively. Which sample of the water is most polluted? Can you assign the correct label to each assuming the river water is relatively clean?

Biochemical Oxygen Demand (BOD) measures the amount of oxygen consumed by microorganisms to decompose organic matter in a water sample. A higher BOD value indicates a greater presence of biodegradable organic pollutants, signifying a higher level of water pollution.

Considering the given BOD values:

• Sample C has the highest BOD of 400 mg/L, indicating it is the most polluted. This corresponds to untreated sewage water, which is rich in organic matter.
• Sample A has a BOD of 20 mg/L. This value is significantly lower than untreated sewage but higher than clean river water, making it the secondary effluent discharged from a sewage treatment plant. …

Water pollution is a critical environmental concern, and understanding its extent is crucial for effective management. One of the most significant measures used to gauge the organic pollution load in a water body is the Biochemical Oxygen Demand, or BOD. This concept is fundamental to environmental science and plays a key role in assessing water quality.

Biochemical Oxygen Demand refers to the amount of dissolved oxygen required by aerobic biological microorganisms to break down organic material present in a given water sample at a certain temperature over a specific period, typically five days at 20°C. Essentially, it quantifies the oxygen consumed by bacteria and other microbes as they decompose organic waste.

When organic matter, such as that found in sewage, is discharged into a water body, decomposer microorganisms proliferate rapidly. These microbes consume dissolved oxygen from the water to break down the organic waste. If the organic load is very high, the demand for oxygen by these microbes can deplete the dissolved oxygen levels in the water, leading to conditions that are detrimental or even fatal for aquatic life like fish and insects, which rely on dissolved oxygen for survival. This is why BOD is such a crucial indicator of water quality and the impact of pollutants.

Let's analyze the given samples:

• Sample A: BOD = 20 mg/L
• Sample B: BOD = 8 mg/L
• Sample C: BOD = 400 mg/L

Comparing these values, Sample C has the highest BOD at 400 mg/L. This immediately tells us that Sample C contains the largest amount of biodegradable organic matter and therefore represents the most polluted water sample among the three.

Now, let's assign the correct labels to each sample, assuming the river water is relatively clean:

• Untreated sewage water: This type of water is rich in organic matter from human and animal waste. It has a very high concentration of biodegradable pollutants, leading to an extremely high BOD value. Among the given samples, Sample C (400 mg/L) perfectly fits this description, indicating severe organic pollution. …

Instead of matching numbers to labels by elimination, reason from the one physical principle that links them -- more untreated organic matter means more oxygen demand -- and rank the three BOD values on that single scale (400 > 20 > 8) before mapping the ranked order onto the known pollution ranking ( …