In a wastewater treatment plant with four aerobic digesters in series, which tank will exert the highest oxygen demand?

In a system with four aerobic digesters operating in series, the first tank will exert the highest oxygen demand. This is because the initial tank is where the majority of the organic material is present and is actively undergoing decomposition by microorganisms. During the aerobic digestion process, microorganisms break down organic matter, which consumes oxygen.

As the wastewater flows through each successive tank, the concentration of organic material generally decreases due to the breakdown by bacteria. By the time the water reaches the second, third, or fourth tanks, a significant portion of the organic load has already been removed. Consequently, these subsequent tanks will typically have lower oxygen demand as there is less organic material remaining to decompose.

This principle of compound degradation through the sequence of digesters demonstrates how biological processes in the first tank generate the most significant oxygen consumption, leading to the highest oxygen demand occurring at that stage.