Step 1: Understanding the Problem:
Biochemical Oxygen Demand (BOD) is a standard measure of water pollution, specifically relating to the concentration of organic matter present in a water body.
Step 2: Approach and Formula:
Define BOD technically: It relies on measuring the activity of aerobic microbes decomposing organic matter. Determine what is physically measured during this process.
Step 3: Detailed Explanation:
- BOD is defined as the amount of oxygen that would be consumed if all the organic matter in one liter of water were oxidized by bacteria.
- While BOD gives an indirect estimate of the biodegradable organic matter present in the water, the actual physical quantity that is estimated/measured in the laboratory test is the amount of oxygen consumption over a period of 5 days at $20^\circ$C.
- A higher rate of oxygen consumption signifies a higher BOD, which in turn implies that the water is highly polluted with organic waste.
Step 4: Final Answer:
BOD is estimated by measuring oxygen consumption.