In a wastewater treatment system, what is the sequence that biologically removes nitrogen?

• A. Sedimentation followed by filtration
• B. Denitrification followed by nitrification
• C. Chlorination followed by aeration
• D. Nitrification followed by denitrification

Answer: (D)

Biological nitrogen removal relies on two distinct processes that must happen in sequence. First, nitrification occurs in the presence of oxygen, where ammonium is oxidized first to nitrite and then to nitrate. This step converts nitrogen into a form (nitrate) that can be removed in the next stage. Then denitrification takes place under low-oxygen (anoxic) conditions, where bacteria use nitrate as an electron acceptor and convert it to nitrogen gas, which is released to the atmosphere. This two-step order is essential because denitrification needs nitrate to proceed, so you must form nitrate first through nitrification.

The other options don’t accomplish biological nitrogen removal in the required way: sedimentation and filtration are physical separation steps, chlorination and aeration relate to disinfection and gas exchange rather than nitrogen conversion, and doing denitrification before nitrification wouldn’t have nitrate to reduce and wouldn’t effectively remove nitrogen.