I would say that denitrification is not biochemical, it is biological. Nitrification have to be done before any denitrification can happen. Also the sludge blanket should not have any dissolved oxygen,the denitrifying bacteria should be present as well as soluble carbon. If one of those one is not there you cannot get a denitrification. If you have denitrification of your sludge blanket, you will get flaoting sludge for sure. In the sludge blanket it is usually caused by septic conditions.