Civil Engineering - Waste Water Engineering - Discussion

"In treated sewage, 4 ppm of D. O is essential:

This is correct. The oxygen level in treated sewage is essential for maintaining the aerobic biological processes involved in the treatment. A minimum of 4 ppm of dissolved oxygen (D.O) is typically required to support these processes.

"Only very fresh sewage contains some dissolved oxygen":

This is also correct. Fresh sewage, being less decomposed, contains some dissolved oxygen. However, as the sewage ages and the organic matter breaks down, the oxygen is consumed by microorganisms, leading to lower D.O. Levels.

"The solubility of oxygen in sewage is 95% that is in distilled water": This statement is also correct.

While sewage typically has a lower oxygen solubility than distilled water due to impurities and organic matter, the solubility of oxygen in sewage is still comparable to distilled water, and this value is often cited as around 95%.

Please explain.