Why would any filtration be a nitrate factory?
One explanation is that if nitrification is the major means of creating anaerobic condition for denitrification, then nitrification has to take place not far from denitrification.
In the denitrification coil setup, it is suggested that nitrification is the major means of creating anaeobic condition for denitrification further down the coil.
If bulk consideration pervails, then there cannot be a nitrate factory, as denitrification can take place anywhere.