Nitrification

"A biological process, during which nitrifying bacteria convert toxic ammonia to less harmful nitrate. It is commonly used to remove nitrogen substances from wastewater, but in lakes and ponds it occurs naturally." (Lenntech, 2019)

Sources

Lenntech. Water glossary: An explanation of  terminology concerning water related issues. Accessed March 4, 2019. Available at: https://www.lenntech.com/water-glossary.htm