Question

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?

Answer 1

• Biochemical oxygen demand or biological oxygen demand (BOD) is the amount of dissolved oxygen needed by aerobic biological organisms to break down organic materials present in a given water sample at certain temperature over a specific time period. When BOD levels are high, dissolved oxygen levels decrease because the oxygen that is available in the water is consumed by the bacteria. Hence, the higher the BOD higher the water is polluted.
  
• The BOD value of sample A is 20 mg/L, which is the secondary effluent discharged from a sewage treatment plant.
• The BOD value of sample B is 8 mg/L, which is the river water and it is least polluted.
• The BOD value of sample C is 400 mg/L, which is untreated sewage water and it is highly polluted water.