question_answer 11)

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:

BOD values indicate the amounts of organic wastes present in the matter. The greater the BOD of waste water, more is its polluting potential. On the basis of this criterion, we can conclude: (i) The sample labelled as C is of untreated sewage water because such sewage water has highest quantities of organic matter and so has highest BOD value (i.e., 400 mg/L). (ii) The sample labelled as A is of water of secondary effluent chamber of STP because it comes after the primary treatment and is with less organic matter than untreated sewage water so has less BOD (i.e., 20 mg/L). (iii) The sample labelled as B has minimum BOD (i.e., 8 mg/L) and it has been assumed that river water is relatively clean so B sample is of river water.