To understand the concept of Biochemical Oxygen Demand (BOD), it is essential to grasp its significance in evaluating water pollution:
Biochemical Oxygen Demand (BOD): BOD is a measure of the amount of oxygen required by aerobic microorganisms to decompose organic matter in water. It is expressed in parts per million (ppm) or milligrams per liter (mg/L).
High BOD: Indicates a high level of organic pollution, meaning more oxygen is required for microbial activity, which is typical for polluted water.
Low BOD: Signifies clean water with less organic matter, resulting in less oxygen consumption by bacteria.
Now, let's evaluate the given statements:
Aerobic bacteria decrease the BOD value: This statement is incorrect. Aerobic bacteria consume oxygen to break down organic matter, which increases the BOD. They do not decrease the BOD value.
Anaerobic bacteria increase the BOD value: This statement is misleading. Anaerobic bacteria do not require oxygen for their processes and are associated with the production of by-products like methane, but they don't increase BOD since BOD measures aerobic processes.
Clean water has BOD value higher than 10 ppm: This statement is incorrect. Clean water has a low BOD value, typically less than 5 ppm, as it contains minimal organic pollutants.
Polluted water has BOD value higher than 10 ppm: This statement is correct. Polluted water usually has high levels of organic waste and, consequently, a BOD value greater than 10 ppm.
Therefore, the correct answer is: Polluted water has BOD value higher than 10 ppm.
