Oxygen Sag Curve in Polluted Rivers

Steps:

• Sewage introduces organic matter, increasing biochemical oxygen demand (BOD) as bacteria decompose it, consuming dissolved oxygen (DO).
• DO levels drop sharply downstream from the release point due to high BOD.
• Further downstream, oxygen re-enters via reaeration and reduced organic load, causing DO to recover.
• The curve shows an initial decline followed by gradual recovery, peaking near normal levels.

Why A is correct:

• Curve A depicts the classic oxygen sag: rapid DO decrease then upstream recovery, matching the BOD dilution and reaeration law in polluted water bodies.

Why the others are wrong:

• B shows steady DO decline without recovery, ignoring reaeration processes.
• C indicates constant DO, failing to account for bacterial oxygen consumption.
• D displays increasing DO, contradicting the pollution-induced depletion.

Final answer: A