This gives an idea about the water purity and quality. It is a measure of the amount of oxygen needed by bacteria and other microorganisms to oxidize the organic material present in a water sample over a period of 5 days. The biological oxygen demand (BOD) of drinking water should be less than 1. Nitrates and phosphates in water contribute to BOD. These are plant nutrients and facilitate plant and algae growth in water. When these plants die, it becomes an organic waste which gets decomposed by bacteria resulting in a high BOD level. When water temperature is increased, rate of photosynthesis increases. This enhances plant and algae growth. On death, they are decomposed and BOD level gets increased. Biological oxygen demand (BOD) can be determined experimentally by a method known as Winkler’s method. The basic idea employed here is that in alkaline medium, dissolved oxygen(DO) oxidizes manganese-2+ ions to manganese-4+ ions and in acidic medium, DO oxidizes iodine ions to free iodine.
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