The first stage is for BOD removal. The second stage is for nitrification. The BOD removal efficiency of a plant is always an important factor in nitrification efficiency. Compared to heterotrophic bacteria, which consume organic compounds, the growth and reproduction rates of nitrifying bacteria are quite low. Heterotrophic bacteria take as little as 30 minutes to reproduce.
Nitrifying bacteria take several hours to reproduce.
For every gram of organic substrate consumed, roughly 0.4 grams of new heterotrophic bacteria are produced. However, for every gram of ammonia converted to nitrate, less than 0.2 grams of nitrifying bacteria are produced. There are only two main species of nitrifying bacteria. Nitrosomonas and Nitrobacter. There are variety of chemicals that are known to inhibit these bacteria.