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

Biological oxygen demand(BOD)is the method of determining the amount of oxygen required by micro-organisms to decompose the waste present in the water supply. If the quantity of organic wastes in the 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 will increase. 
Therefore it can be concluded that if the water supply is more polluted then it will have a higher BOD value. Out of the above three samples sample C is the most polluted since it has the maximum BOD value of 400mg/L. After untreated sewage water secondary effluent discharge from a sewage treatment plant is most polluted. Thus sample A is secondary effluent discharge from a sewage treatment plant and has the BOD value of 20mg/L while sample B is river water and has the BOD value of 8mg/L. 
Hence the correct label for each sample is:

Label	BOD value	Sample
A	20 mg/L	Secondary effluent discharge from a sewage treatment plant
B	8mg/L	River water
C	400mg/L	Untreated sewage water