ANS:B - Dissolved Oxygen (D.O.)

1. Bio-Chemical Oxygen Demand (B.O.D.): B.O.D. is a measure of the amount of dissolved oxygen consumed by aerobic microorganisms while they decompose organic matter in water. It is expressed in milligrams of oxygen per liter of water (mg/L). B.O.D. is typically measured over a specific period, often five days at a specified temperature (usually 20°C). Higher B.O.D. values indicate higher levels of organic pollution in the water. B.O.D. testing is widely used to assess the quality of water bodies, the efficiency of wastewater treatment processes, and the level of pollution in aquatic environments.
2. Dissolved Oxygen (D.O.): Dissolved oxygen refers to the amount of oxygen dissolved in water. It is essential for the survival of aquatic organisms, such as fish and other aquatic life, as they rely on dissolved oxygen for respiration. D.O. levels in water bodies can fluctuate due to various factors, including temperature, pressure, and the presence of photosynthetic organisms. Low levels of dissolved oxygen can lead to hypoxia (oxygen deficiency) in aquatic environments, which can harm aquatic organisms and disrupt ecosystems.
3. Chemical Oxygen Demand (C.O.D.): C.O.D. is a measure of the amount of oxygen required to chemically oxidize both organic and inorganic compounds in water. It represents the total oxygen demand in a water sample, including oxygen consumed by aerobic and anaerobic processes. C.O.D. is often used as a measure of the overall pollution or organic strength of wastewater and industrial effluents. Unlike B.O.D., which relies on biological processes, C.O.D. measurements are based on chemical oxidation reactions and provide a more rapid assessment of water quality.