Question

A laboratory assistant collected three water samples, namely river water, untreated sewage water and secondary effluent discharged from a sewage treatment plant. But the laboratory attendant did not note which was which. The samples were labelled A, B and C and subjected to a BOD test. The BOD values of the three samples A, B and C were recorded as 30 mg/L, 8 mg/L and 400 mg/L respectively.

(a) Which sample of the water is most polluted ?

(b) Can you assign the correct label to each, assuming the river water is relatively clean ?

Hint:

Higher BOD means more organic waste: sewage highest, treated effluent in the middle, river lowest.

Answer 1

Step 1: Meaning of BOD:
Biochemical Oxygen Demand (BOD) is the amount of oxygen that microbes use to break down the organic matter in one litre of water. More organic matter means more microbes and a higher BOD.

Step 2: Answer (a):
The BOD values are A = 30 mg/L, B = 8 mg/L and C = 400 mg/L. The highest value is 400 mg/L. So sample C is the most polluted.

Step 3: Answer (b):
River water is relatively clean, so it has the lowest BOD. That is B with 8 mg/L. Untreated sewage is full of organic waste, so it has the highest BOD. That is C with 400 mg/L. Secondary effluent has been treated and has a value in between. That is A with 30 mg/L.

Final Answer:
(a) Sample C. (b) A is secondary effluent, B is river water and C is untreated sewage.
\[ \boxed{\text{A = effluent, B = river, C = sewage}} \]