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COD, or Chemical Oxygen Demand, measures the total amount of oxygen that organic compounds in water can consume during decomposition. It's a key indicator of water pollution, reflecting the presence of pollutants like sewage, food waste, or industrial chemicals. High COD levels suggest the water is heavily contaminated and may harm aquatic life by depleting oxygen needed for fish and other organisms. Testing COD helps assess water quality, guide treatment processes, and ensure environmental safety.