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

1. Polluted water has BOD value higher than 10 ppm
2. Aerobic bacteria decreases the BOD value
3. Anaerobic bacteria increases the BOD value
4. Clean water has BOD value higher than 10 ppm

Correct answer: Polluted water has BOD value higher than 10 ppm

Solution

BOD measures oxygen demand from organic pollution. Clean water has BOD <= 5 ppm, while polluted water has BOD higher than 10 ppm (often 17 ppm or more). Aerobic bacteria do not decrease BOD, and anaerobic bacteria do not increase it. So the correct statement is that polluted water has BOD value higher than 10 ppm.

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