Biochemical oxygen demand (BOD) value can be a measure of water pollution caused by the organic matter. Which of the following statements is correct?

Biochemical Oxygen Demand (BOD) is defined as the amount of dissolved oxygen (in parts per million, ppm) required by aerobic microorganisms to oxidise the biodegradable organic matter present in a given volume of water at a specified temperature and for a specific time (usually 5 days at 20 °C).

Interpretation of BOD:

• High concentration of organic matter → more food for aerobic microbes → they consume more $$O_2$$ while oxidising this matter → BOD value becomes high.
• Low concentration of organic matter (clean water) → less microbial oxidation → BOD value remains low (typically $$\lt 5$$ ppm).

Now analyse each statement:

Option A: “Polluted water has BOD value higher than 10 ppm.”
Water is considered significantly polluted if the BOD exceeds about 10 ppm. Hence this statement is correct.

Option B: “Aerobic bacteria decreases the BOD value.”
Aerobic bacteria actually consume dissolved $$O_2$$ to degrade organic matter, thereby reflecting (and often increasing) the BOD measurement. They do not decrease the BOD; they are the reason the demand exists. Hence this statement is incorrect.

Option C: “Anaerobic bacteria increases the BOD value.”
BOD is defined for aerobic oxidation only; anaerobic bacteria operate in absence of oxygen and therefore are not considered in BOD estimation. Thus this statement is incorrect.

Option D: “Clean water has BOD value higher than 10 ppm.”
Clean (unpolluted) water should have a BOD well below 5 ppm; therefore this statement is incorrect.

Therefore, the correct choice is:

Option A which is: Polluted water has BOD value higher than 10 ppm