April 2013, Vol. 25, No.4

Operator essentials

What every operator should know about denitrification

Woodie Mark Muirhead

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A practical consideration   




Denitrification is the biological reduction of nitrate (and nitrite) to nitrogen gas.  


Nitrification oxidizes ammonia to nitrate but does not remove nitrogen from wastewater; nitrogen merely is converted from one form to another. Denitrification is the final stage of the nitrogen cycle and removes nitrogen from wastewater when it is released as nitrogen gas to the air, which already is approximately 78% nitrogen gas.  


Environmental need  


Nitrite and nitrate in drinking water can affect human health, particularly in infants.  


All forms of inorganic nitrogen can serve as nutrients for undesirable biological growth in receiving water.  


Excess nutrients cause eutrophication, which can result in algae blooms. In fresh water, phosphorus often is the limiting factor for nuisance algae growth. In marine waters, nitrogen typically is the limiting factor, often in the form of nitrate.  




Denitrifying organisms use nitrate (and nitrite) as a terminal electron acceptor, much the same way aerobic organisms use oxygen. Most denitrifying organisms are facultative and will use oxygen first, even if nitrate (and nitrite) are present.  


The term “anoxic” is used to describe the denitrification respiratory process. This term is unique to the wastewater profession to differentiate denitrification from aerobic and other types of anaerobic respiration. In biological and medical textbooks, the term anoxic means “without oxygen.” For the wastewater profession it also means the presence of nitrate or nitrite. The relationship between nitrate (NO3) or nitrite (NO2) and denitrification in anoxic zones can be remembered when the term is written as  




The expression NOx refers to the NO3 or NO2 


Energy source  


Denitrifying organisms are heterotrophic. They use organic carbon compounds as an energy source.   


Approximately 2.8 mg of chemical oxygen demand (COD) are oxidized per milligram of nitrate–nitrogen reduced to nitrogen gas.   


Because a significant amount of energy is invested in nitrification to oxidize ammonia to nitrate, the use of the nitrate to help oxidize COD recovers some of the investment.  


Anoxic zones  


Anoxic zones often are used in suspended growth processes to support denitrification and nitrogen removal.   


Preanoxic zones are located at the beginning of activated sludge basins and take advantage of the high organic carbon and low dissolved oxygen in influent wastewater or primary effluent to support denitrification. Since there typically is little or no nitrate in influent wastewater or primary effluent, mixed liquor is recycled to the anoxic zone from the end of the aerobic zone of nitrifying processes to provide the nitrate needed for denitrification.  


Because denitrifying organisms are facultative, the residual dissolved oxygen in the mixed-liquor recycle from the aerobic zone can hinder the denitrification process. Control of the dissolved oxygen and/or the size of the anoxic zone is important to minimize this effect. In practice, more COD will be needed if oxygen also is present.  


Post-anoxic zones (or post-denitrification facilities, such as filters) can achieve lower effluent nitrogen concentrations, but supplemental carbon might be required to optimize denitrification.  


Promoting denitrification in anoxic zones can minimize its occurrence in the secondary clarifier sludge blanket. Denitrification in the clarifier is undesirable, because the nitrogen gas bubbles can cause floc particles to disperse and solids to float, increasing effluent total suspended solids and turbidity.  




The reduction of 1 mg of nitrate–nitrogen to nitrogen gas produces 3.6 mg of alkalinity.   


Some facilities that are required to nitrify but not remove nitrogen employ anoxic zones in their activated sludge systems solely to recover and supplement alkalinity lost during nitrification.   




Anoxic zones have been demonstrated to control many types of nuisance filamentous organisms.  


Many filamentous organisms are aerobic and cannot adsorb organic compounds — meaning carbonaceous biochemical oxygen demand — for later use. Under anoxic conditions, denitrifying organisms use the readily biodegradable carbon that filamentous organisms need and remove it under anoxic conditions, making it unavailable to the filamentous organisms in subsequent aerobic conditions.   




Woodie Mark Muirhead is a vice president and operations specialist in the Honolulu office of Brown and Caldwell (Walnut Creek, Calif.).