Final answer:
The final stage of sewage treatment removing nitrogen and phosphorus is tertiary treatment, often involving denitrification and advanced filtration.
Step-by-step explanation:
The final stage of sewage treatment that removes nitrogen and phosphorus from treated water is known as tertiary treatment, which may involve different processes such as advanced filtration, denitrification, and enhanced biological phosphorus removal.
Tertiary treatment is crucial for the reduction of nutrients like nitrogen and phosphorus that can contribute to environmental problems such as eutrophication in water bodies when released in excessive amounts.