How does denitrification work and simultaneous nitrification/denitrification

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In Wastewater treatment denitrification describes the final step of the conversion from ammonium NH4+ to Nitrogen gas N2. After ammonium is oxidized by microorganisms in a 2-step process ammonium is at first converted to Nitrate NO3-. Nitrate NO3- is still in the wastewater and although is less harmful than ammonia it can still cause diseases.
Microorganisms for denitrification are more difficult to cultivate compared to nitrifying microorganisms such as Nitrosomonas or Nitrobacter. Denitrifying microorganisms are also called heterotrophic bacteria and they require organic carbon as food as well as oxygen. Therefore, in wastewater treatment applications a certain BOD amount has to be maintained in the anoxic basin. This is achieved by recirculating sludge from the secondary clarifier and aerobic basin back to the anoxic basin.
The heterotrophic bacteria then try to oxidize the available BOD under the consumption of oxygen. However, as no dissolved oxygen is present in an anoxic basin, they have to look for alternative sources. Heterotrophic bacteria has the ability to strip the oxygen from Nitrate NO3- and to convert Nitrate to Nitrogen gas N2. The nitrogen gas floats then as bubbles to the water surface and into the atmosphere.
The activated sludge process usually separates the anoxic basin from the aerobic basin either by building two different tanks or by creating different zones within one basin with controlled aeration or by on/off operation of aerators.
It was found that under certain conditions micro anoxic zones can be formed inside bacteria layers. This effect which is also known as simultaneous nitrification/denitrification (SNDN) is especially benefited in submerged fixed bed reactors. Here, microorganisms are attached to a fill media surface and form a biofilm layer. While the top layers of microorganisms consume the available oxygen to oxidize ammonia, deeper layers are forced to strip oxygen from Nitrate NO3- to produce nitrogen gas N2. Within a fixed bed reactor the SNDN process can also be combined with different aeration zones or on/off aeration for improved denitrification results.