ANS:A - Liberation of ammonia-formation of nitrites-formation of nitrates-liberation of nitrogen

The nitrogen cycle in sewage involves a series of microbial processes that transform nitrogen compounds into different forms. Here's a detailed explanation of each step in the nitrogen cycle:

1. Liberation of Nitrogen: Nitrogen enters the sewage system in various forms, including organic nitrogen compounds and nitrogen gas (N2). These nitrogen compounds may originate from human waste, organic matter, or industrial sources. In the sewage system, organic nitrogen compounds are broken down, releasing nitrogen into the wastewater.
2. Liberation of Ammonia: Through a process known as ammonification, organic nitrogen compounds are decomposed by bacteria and other microorganisms, resulting in the release of ammonia (NH3) into the wastewater. Ammonia is a soluble form of nitrogen and serves as a primary nitrogen source for subsequent microbial processes.
3. Formation of Nitrites: Ammonia in the wastewater is oxidized by specific groups of bacteria into nitrites (NO2-). This process, called nitrification, occurs in two stages. Initially, ammonia-oxidizing bacteria (AOB) convert ammonia into nitrite ions, releasing energy in the process. Nitrite ions are the intermediate product formed during the nitrification process.
4. Formation of Nitrates: Nitrites are further oxidized by a different group of bacteria, known as nitrite-oxidizing bacteria (NOB), into nitrates (NO3-). This completes the nitrification process, where both ammonia and nitrite are converted into nitrate ions. Nitrates are relatively stable nitrogen compounds and can persist in the sewage system until further transformations occur.

The nitrogen cycle in sewage is crucial for the transformation and recycling of nitrogen compounds, which are essential nutrients for microbial growth and metabolism. Through the nitrogen cycle, nitrogen compounds undergo various transformations, converting between organic and inorganic forms, and contributing to the overall nutrient dynamics within the sewage system.