Previous Year Question

Biochemical Oxygen Demand (BOD) is an important standard for assessing Water Quality and Organic Pollution. It represents the amount of Dissolved Oxygen (DO) consumed by Aerobic Microorganisms during the decomposition of Biodegradable Organic Matter present in water. Organic substances originating from Domestic Sewage, animal waste, and food-processing industries can increase BOD. Therefore, it is used to assess organic pollution in rivers, lakes, and other aquatic ecosystems.

BOD is generally measured over a period of five days at a temperature of 20°C, known as BOD₅. The initial Dissolved Oxygen concentration of a water sample is measured, and the sample is then kept in the dark under specified conditions for five days. The remaining Dissolved Oxygen is subsequently measured, and the difference between the two readings determines oxygen consumption. BOD is generally expressed in milligrams of oxygen per litre of water (mg/L). This test estimates the oxygen demand resulting from biodegradable organic matter present in water.

A higher BOD value generally indicates greater organic pollution. When the concentration of organic matter increases, microorganisms consume more oxygen during decomposition, potentially reducing Dissolved Oxygen levels. Severe oxygen deficiency is called Hypoxia, which can threaten the survival of fish and other aquatic organisms. According to NCERT, clean water may generally have a BOD value below 5 ppm, whereas heavily polluted water may have a BOD value of 17 ppm or more. These are illustrative values; actual water-quality limits depend on the designated use of the water.

The distinction between BOD, DO, and COD is important. DO (Dissolved Oxygen) indicates the actual concentration of oxygen available in water, whereas BOD represents the oxygen demand arising from microbial processes. In contrast, COD (Chemical Oxygen Demand) measures the equivalent oxygen demand through chemical oxidation. BOD primarily indicates biodegradable organic pollution; it does not directly measure every pollutant, such as heavy metals. BOD is not used to determine blood oxygen levels, atmospheric oxygen in high-altitude regions, or oxygen levels in forest ecosystems.