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?
(labels) Sample A (20mg/L) = Secondary effluent; Sample B (8mg/L) = River water; Sample C (400mg/L) = Untreated sewage water.
(most_polluted) Sample C (BOD 400mg/L) is the most polluted water sample.
Explanation
According to the context, BOD (biochemical oxygen demand) is a measure of the organic matter present in water. The greater the BOD of waste water, more is its polluting potential. The context also states that secondary treatment significantly reduces the BOD of the effluent. Since river water is relatively clean, it would have the lowest BOD. Untreated sewage contains large amounts of organic matter, so it has the highest BOD. Secondary effluent has reduced BOD due to treatment but still has some organic matter remaining.
Solution Steps
Step 1: Understand that higher BOD means more pollution (greater polluting potential).
Step 2: Compare BOD values: C (400mg/L) > A (20mg/L) > B (8mg/L).
Step 3: Identify that Sample C with highest BOD (400mg/L) is untreated sewage water (most polluted).
Step 4: Identify that Sample B with lowest BOD (8mg/L) is river water (relatively clean).
Step 5: Identify that Sample A with intermediate BOD (20mg/L) is secondary effluent (BOD reduced after treatment).