What happens to the Biochemical Oxygen Demand (BOD) as sewage strength increases?

Biochemical Oxygen Demand (BOD) is an important indicator of the organic pollution present in water, particularly in wastewater such as sewage. As the strength of sewage increases—meaning that there is a higher concentration of organic matter present—the demand for oxygen by microorganisms that decompose that organic material also increases. This is because more organic material requires more oxygen to be broken down by aerobic bacteria and other microorganisms during the decomposition process.

Therefore, an increase in BOD signifies that there is a greater amount of organic pollutants in the water, which in turn means that more oxygen will be needed for microbial activity to effectively process this organic waste. This relationship is critical for assessing the impact of sewage and wastewater on aquatic ecosystems, as high BOD levels can deplete oxygen in receiving waters, leading to adverse effects on fish and other aquatic life.

The other options do not accurately represent the relationship between sewage strength and BOD. For example, if BOD were to decrease or remain constant with increased sewage strength, it would suggest that the water is cleaner or that organic matter has little to no impact on oxygen demand, which contradicts the fundamental principles of wastewater management and aquatic health.