What is Denitrification?

Denitrification is the conversion of soluble nitrate to nitrogen gas in the nitrogen cycle. For denitrification to occur, three factors are needed:

  • Presence of Nitrate
  • Anoxic conditions (no free dissolved oxygen)
  • A carbon source (soluble BOD)

Floating Sludge

Denitrification is useful and desired in certain anoxic zones designed to remove nitrogen. However, the process can cause problems in the final clarifiers by floating sludge and hindering settling. Often a denitrification scum is associated with bubbling as it is caused by nitrogen gas escaping the flocs and entering the atmosphere. It is common to see floc size in the scum of denitrification foam twice the size of the flocs in the mixed liquor due to nitrogen gas entrapment. When the sludge floats in the clarifier, it can go over the effluent weirs and cause suspended solids violations. To solve denitrification problems, one of the three above variables must be eliminated.

Common operational suggestions include:

  • Increase dissolved oxygen prior to the clarifier
  • Improve treatment to reduce residual soluble BOD
  • Reduce sludge retention time in the clarifier
  • Optimizing denitrification in its desired location
  • Limit nitrification which forms nitrate (only possible if there is no ammonia limit on the effluent).


Denitrification and rising sludge is a very frequent problem facing wastewater operational staff.  By knowing what causes denitrification to occur, steps can be taken to discourage it from happening in the clarifier. It is always recommended to perform a microscopic evaluation of the mixed liquor as well as the scum/ foam to help diagnose the problem. Once the problem has been diagnosed, changes can be made to address it.