Which of the following has the highest Biological Oxygen Demand (BOD)?

ADrinking water

BUnpolluted surface water

CContaminated surface water

DMunicipal sewage effluent ✓ Correct

Correct answer: (D) Municipal sewage effluent

Explanation

Municipal sewage effluent has the highest BOD as it is loaded with organic waste that microbes consume.

A high BOD reflects the large amount of oxygen microbes need to decompose that waste.

Drinking water has almost no BOD because it is treated and nearly free of organic matter.

Unpolluted surface water carries very little organic load and so a low BOD.

Contaminated surface water has a raised BOD but far below that of raw sewage.

High BOD depletes dissolved oxygen and can suffocate aquatic life.

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