Ravi:

N₂ gas is a very stable compound due to its triple bonds.  There are few ways of converting N₂ gas into a form that can be used by organisms.  Nitrogen fixation (process of converting inert gaseous N₂ into a form that can be used by plants and bugs) can be done by lightening bolts (sufficient energy to destabilize the N bonds) and fixation by specific bacteria (diazothropes)--some of these form a symbiotic relationship with certain plants, legumes primarily.  Cyanobacteria are also capable of doing this but more than likely you will not find these in wastewater treatment unless you have lagoons.  These are slow growing organisms so for this to occur at any given appreciable concentrations in your treatment scheme is negligeable--not saying it does not happen, but not to the point where you will have to worry.

If you want you can try to get a copy of the following EPA manual:

Nitrogen Control Manual, EPA/625/R-93/010, September 1993.