Read the following statements and answer the questions that follow:-
The primary effluent is passed into large aeration tanks where it is constantly agitated mechanically and air is pump into it. This allows vigorous growth of useful aerobic microbes into flocs.

What happens when these microbes grow in the aeration tanks?

Biochemical Oxygen Demand (BOD) decreases.

When microbes grow in the aeration tanks, the Biochemical Oxygen Demand (BOD) decreases. The aeration tanks provide an oxygen-rich environment where aerobic microbes can thrive and consume the organic matter present in the wastewater. As these microbes metabolize and break down the organic compounds, they utilize oxygen from the water, reducing the amount of dissolved oxygen available. This decrease in BOD is an indication that the organic matter is being consumed and the wastewater is being treated. Therefore, the correct statement is: "Biochemical Oxygen Demand (BOD) decreases."