Removal of nitrogen compounds
- The activated sludge systems have been designed so that, in addition to decomposing organic material, nitrogen compounds can also be partially removed.
- 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 continuously using certain microbes during aeration due to the constant 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 tanks 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 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 takes up most of the volume of the tanks.
- Slowly rotating mixers are stationed in the anoxic zone of the tanks that work to maintain the mix of wastewater and biomass in suspension but without injecting oxygen that may harm the denitrification process that requires oxygen-free conditions.