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 20mg/L, 8mg/L and 400mg/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?

Explanation

BOD (Biological Oxygen Demand) is the method of determining the amount of oxygen required by microorganisms to decompose the waste present in water supply. If the quantity of organic waste in the water supply is high, then the number of decomposing bacteria present in the water will also be high. as a result, the BOD value also will incrase. therefore it can be concluded that if the water supply is more polluted since it has the maximum BOD value of 400 mg/L. after this, secondary effluent discharge from a sewage treatment plant is most polluted. thus sample A has the BOD value of 20 mg/L while sample B is river water and has the BOD value of 8 mg/L. hence correct label for each sample is.
A-20mg/L-Secondary effluent discharge from a sewage treatment plant
B-8 mg/L -River water
C-400 mg/L-Untreated sewage water.