Why is copper sulfate-sulfamic acid solution added to a sample of mixed liquor during a D.O. test?

The inclusion of copper sulfate-sulfamic acid solution in a sample of mixed liquor during a dissolved oxygen (D.O.) test serves to kill bacteria that utilize oxygen. This is crucial because the objective of the test is to accurately measure the amount of dissolved oxygen in the sample, which can be affected by the biological activity of microbes that consume oxygen. By eliminating these bacteria, the test can provide a more accurate representation of the dissolved oxygen levels, reflecting only the physical and chemical aspects of the water sample without the interference from biological processes.

The other options might seem relevant in different contexts of wastewater treatment or monitoring, but they do not align with the primary purpose of the copper sulfate-sulfamic acid addition in this specific D.O. test scenario. For example, preventing sludge settling, maintaining the organic process, or enhancing oxygen absorption are considerations related to overall treatment processes but are not the direct reasons for the use of this specific solution in a D.O. test context.