ABSTRACT
If bacteria come in contact with a nitrified element in the absence of oxygen, the nitrates are reduced to nitrogen gas (denitrification) and escape the wastewater.
The denitrification process can be done in either an anoxic activated sludge system (suspended growth) or in a column system (fixed film).
After effective biological treatment there is little oxygen demanding material left in the wastewater when it reaches the denitrification process. The denitrification reaction will only occur if there is an oxygen demand source when no dissolved oxygen is present in the wastewater.
An oxygen demand source is usually added to reduce the nitrates quickly. The most common demand source added is soluble BOD or methanol. Approximately 3 mg/L of methanol is added for every 1 mg/L of nitrate-nitrogen.
Suspended growth denitrification reactors are mixed mechanically, but only enough to keep the biomass from settling without adding unwanted oxygen.
Submerged filters of different types of media may also be used to provide denitrification. A fine media, downflow filter is sometimes used to provide both denitrification and effluent filtration.
A fluidized sand bed where wastewater flows upward through a media of sand or activated carbon at a rate to fluidize the bed may also be used. The denitrifying bacteria grow on the media.
