The Wuhrman process was first suggested in 1964. This process depended upon post denitrification using a secondary anoxic reactor following the aerobic zone. Secondary anoxic reactors are inefficient and offer only very slow rate of denitrification due to the lack of RBCOD. The overall effect of such a zone would be a relatively small decrease in effluent nitrate compared to a fully aerobic system (the difference between say 28 mg/L and 25 mg/L nitrate-N in the effluent).
Numerous plants of this type supplement the nitrogen removal by the addition of an external carbon source such as methanol which can rapidly and effectively lower the effluent nitrate concentration in the process effluent.
Although the Wuhrman process is classed as a nitrifying/denitrifying (N/D) process, post denitrification in the secondary anoxic reactor allows only limited removal of nitrate. More complete nitrate removal is often achieved however through carbon dosing of the anoxic reactor.
The limiting factor for denitrification here is the scarcity of organic carbon in the anoxic zone. Denitrification requires organic carbon to fuel the process, and in this configuration the only organic carbon available comes from the slow breakdown of dead and dying bacteria which limits the rate of denitrification by a factor of 6-10 compared to high rate RBCOD driven denitrification.
This process also commonly uses a flash aeration reactor to clean up ammonia and condition (make aerobic) the mixed liquor by raising its redox potential prior to secondary clarification. Another function of flash aeration is to remove any bubbles of nitrogen associated with the floc – an issue often caused by the rapid denitrification arising with methanol dosing.