Water Pollution And Biochemical Oxygen Demand

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Water Pollution and Biochemical Oxygen Demand The solubility of oxygen in water is low and so any factor that affects concentration of dissolved oxygen has a major impact on aquatic life because water pollutants reduce the amount of dissolved oxygen in the water. The extent to which these pollutants reduce oxygen is measured using a Biochemical Oxygen Demand (B.O.D.) test. Biochemical Oxygen Demand Biochemical Oxygen Demand is defined as the amount of dissolved oxygen consumed by biological action when a sample of water is kept in the dark at 20°C for five days. The more polluted the water, the more oxygen used up by micro-organisms. The rate at which micro-organisms use oxygen depends on temperature. A standard temperature is needed for valid comparisons between samples of water, so 20°C is used. The samples must be kept in the dark to prevent photosynthesis from replacing the oxygen used by the micro-organisms. The samples are kept in the dark for five days because a standard length of time is needed for comparison. Five days allows the micro-organisms to adjust to their environment. Any longer would cause all the oxygen to be used up and measurements of the amount of dissolved oxygen would be impossible. B.O.D. Test When collecting samples for the B.O.D test, the bottle must be filled under the surface of the water to prevent oxygen from the air entering the bottle, and should be filled to the brim to prevent air from being trapped under the lid. If the water is very polluted it must first be diluted because otherwise all the oxygen would be used up before the five day period was over. Distilled water should be used for dilution and it should be well oxygenated as this ensures that there will be enough oxygen at the end of the five day period to carry out the test properly. The experimental result might need to be multiplied by the dilution factor.

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