Final answer:
Nutrient removal from wastewater is primarily achieved through tertiary (chemical or biological) treatment, which is the advanced phase of sewage treatment following primary and secondary treatments. Option d.
Step-by-step explanation:
The removal of nutrients, such as nitrogen and phosphorus, from wastewater is critical to prevent the disruption of aquatic ecosystems. These nutrients can cause problems such as algae blooms and excessive weed growth. The treatment process which is used for nutrient removal is the tertiary (chemical or biological) treatment.
In this phase, advanced methods including sand filtration and enhanced biological phosphorus removal are employed. Specific bacteria known as polyphosphate accumulate organisms play a crucial role in the biological removal of phosphorus.
Different steps in a conventional sewage treatment plant include primary treatment, where solids are settled or floated away, secondary treatment, where aerobic bacterial decomposition of organic solids occurs, and then tertiary treatment, which includes bacterial decomposition of nutrients and further filtration.
After the secondary treatment, sewage still contains dissolved and suspended biological matter. Thus, tertiary treatment is necessary not only to reduce these nutrients but also to polish the effluent if the water is discharged into a sensitive ecosystem.
This advanced level of treatment ensures the water's safety for the environment and public health when it is reintroduced into natural water sources or reused. Option d.