Biological Oxygen Demand (BOD) is a measure of how much oxygen microorganisms need to break down organic matter in water. High BOD indicates significant organic pollution—when bacteria decompose sewage, food waste, or other organic materials in water, they consume oxygen that fish and other aquatic life need to survive. Municipal wastewater treatment plants are designed to reduce BOD before releasing effluent into waterways. Water quality regulations specify maximum acceptable BOD levels. Monitoring BOD helps environmental agencies assess water quality and identify pollution sources threatening aquatic ecosystems.