Which process is defined as the biological oxidation of ammonia to nitrite and then to nitrate?

The biological oxidation of ammonia to nitrite and then to nitrate is known as nitrification. This process is crucial in the nitrogen cycle, particularly within wastewater treatment. During nitrification, specific bacteria, such as ammonia-oxidizing bacteria and nitrite-oxidizing bacteria, facilitate the conversion of toxic ammonia into nitrite, and then further convert nitrite into the less harmful nitrate.

This two-step process helps reduce the levels of ammonia in wastewater, making it safer for discharge into natural waters or further processing. Understanding nitrification is vital for managing the nitrogen levels in wastewater treatment systems to prevent environmental issues such as eutrophication, which is caused by excess nutrients in water bodies.

Other processes mentioned in the options, such as denitrification, are focused on the conversion of nitrate into nitrogen gas, which is part of a different step in the nitrogen cycle. The activated sludge process refers to a method of degrading organic matter, while RAS flow control pertains to the management of return activated sludge in treatment systems, neither of which define the specific oxidation process of ammonia described in the question.