Three Water Samples Namely River Water, Untreated Sewage Water and Secondary Effluent Discharged from a Sewage Treatment Plant Were Subjected To BOD Test. The Samples Were Labelled A, B and C; but the Laboratory Attendant Did Not Note Which Was Which. The BOD Values of the Three Samples A, B and C Were Recorded as 20 mg/L, 8 mg/L and 400 mg/L, Respectively. Which Sample of the Water Is Most Polluted? Can You Assign the Correct Label to Each Assuming the River Water is Relatively Clean?
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Ans.
The amount of oxygen required by micro-organisms to decompose the waste present in the water supply is determined using the Biological Oxygen Demand (BOD) method. When the water supply contains high levels of organic wastes, the number of decomposing bacteria also increases, leading to a higher BOD value. Consequently, water supplies that are more polluted tend to have higher BOD values. Sample C, with a BOD value of 400 mg/L, is the most polluted among the three samples. The discharge of secondary effluent from a sewage treatment plant is more polluted than river water, which explains why sample A has a higher BOD value of 20 mg/L compared to sample B, which has a BOD value of 8 mg/L and is sourced from a river.
Hence, the correct label for each samples are:
Label | BOD Value | Sample |
Sample A | 20 mg/L | Secondary effluent discharge from a sewage treatment plant |
Sample B | 8 mg/L | River water |
Sample C | 400 mg/L | Untreated sewage water |