An anaerobic denitrification question

[skyshrimp]

https://www.mankysanke.co.uk/html/anoxic_filtration.html

This paragraph in the article confuses me,

'Beneficial filter bacteria will be as busy as ever converting ammonia to nitrate in the aerobic areas of the biological filter, but bugs that can live in anaerobic conditions will rapidly colonise the anaerobic areas and will become equally busy taking the oxygen that they need from the nitrate that has just been produced by their cousins. This will result in that nitrate being converted back to ammonia again before it leaves.

This is a completely pointless exercise, yet it is exactly what is happening in many filter systems where the biological media is not clean. At least part of the good work being done by the aerobic bugs in the oxygen-rich areas, is being undone by anaerobic bugs in areas that are deprived of oxygen.'

I thought anaerobic areas have denitrifiying bacteria that convert NO3 to NO2 to N2O and N2. The author is stating nitrate gets converted to ammonia in anaerobic areas of a filter if the media is not clean. Is the author reffering to ammonifying bacteria that converts NO3 to NH4? A dirty filter will produce ammonia from the detritus breaking down within it. What am I missing here?

 

[BillTre]

I think your interpretation of denitrifiying filter function is correct.
NO3 to NO2 to N2O and N2 is the most common form of denitrifying filtration.
There are several ways to run this kind of filtration.