Biological removal of phosphorus and nitrogen from wastewater using a stressed contact zone and a luxury contact zone

DETAILED DESCRIPTION OF THE INVENTION As used herein, the term "anaerobic" refers to the unavailability of free oxygen.
Referring to the Figure, the process for biological nutrient removal includes providing wastewater influent and activated sludge in a stressed contact zone 12 to form a mixed liquor.
"Mixed liquor" means a mixture of raw or settled wastewater and activated sludge.
The method further includes mixing and aerating the mixed liquor in a luxury contact zone 14, and separating the activated sludge from the mixed liquor in a separation zone 16.
The process further includes the step of returning at least a portion of the activated sludge from the separation zone 16 to the stressed contact zone 12, using a return activated sludge (RAS) pipe 18 or other RAS conduit as is known in the art.
In addition, if desired, the process also may include the step of passing the wastewater through a primary treatment zone 20 such as a Parshall flume, bar screen, grit chamber, primary clarifier and the like, or combination thereof, or other means of primary treatment as is well known in the art.
Wastewater influent and RAS are provided in the stressed zone to form a mixed liquor.
Preferably, the influent and RAS are mixed within this zone to minimize settling, although mixing is not essential.
If suspended solids fall out of the mixed liquor, they will migrate toward the bottom of the zone, where they will become highly stressed.
At that point, the suspended solids will form gases which will float the solids toward the top of the mixed liquor, thereby creating a natural mixing effect.
The stressed contact zone is characterized by having low levels of dissolved oxygen, and the DO concentration should be maintained at a level of about 0.
5 ppm or less.
"Stressed contact zone" means a contact zone having a DO concentration of about 0.
5 ppm or less, without placing the organisms in an anaerobic state.
This relatively low level of DO is needed to stress the organisms and achieve biological denitrification