The activated sludge systems have been designed so that, in addition to decomposition of organic material, partial removal of nitrogen compounds can also take place.
The nitrogen compound removal process is a biological process known as nitrification-denitrification (see the equations below).
The nitrification process, in which ammonia in the wastewater is oxidized into nitrate, occurs all the time using particular microbes during aeration due to the permanent presence of oxygen.
In contrast, in the denitrification process in which the nitrates are converted into gaseous nitrogen, an anoxic zone (without oxygen) must be maintained in the forward part of the basins where this process takes place with the use of denitrification microbes.
The carbon source that is essential in the denitrification process is the organic material in the wastewater, while the oxygenated nitrogen compounds that supply the oxygen required for this process arrive in an internal circulation process performed using surface aerators.
The aerobic zone where removal of most of the organic material as well as the nitrification process takes place is located after the anoxic zone and constitutes the principal volume in the basins.
Slowly rotating mixers are stationed in the anoxic zone of the basins that have the function of maintaining the mix of wastewater and biomass in suspension but without injecting oxygen that may harm the denitrification process that requires oxygen-free conditions.
Removal of nitrogen in nitrification and denitrification processes