Denitrification is the process by which microorganisms reduce nitrate to nitrogen gas. A number of species that occur in wastewater are capable of accomplishing denitrification. These are sometimes referred to as facultative organisms. All of the organisms that can accomplish denitrification are Heterotrophic, because they can metabolize complex organic substances. Normally, Heterotrophic organism will metabolize waste using dissolved oxygen whenever it is available (> 0.1 mg/L). When placed in anoxic conditions, (an environment having < 0.1 mg/L dissolved oxygen), the facultative organisms can turn to the oxygen bound in nitrate as a means of metabolizing waste. Utilizing the oxygen contained within the nitrate molecule results in the release of nitrogen gas. Given time or agitation, the nitrogen gas will escape from solution and exit into the atmosphere. This phenomenon is commonly observed in secondary clarifiers, where the rising gas bubbles float particles of sludge to the surface.
Denitrification occurs in two steps. First, nitrate is reduced to nitrite. Next, nitrite is reduced by the microorganism dissimulation process to gaseous forms of nitrogen (primarily N2).
Denitrification can be accomplished using fixed growth reactors, such as trickling filters, RBCs and constructed wetlands system. More commonly, denitrification is accomplished in modified activated sludge systems that incorporate anoxic zones.
The rate at which denitrification can occur is limited by the presence of dissolved oxygen. If more than 0.1 mg/L of D.O. is present, the facultative organisms will use the D.O. for respiration, instead of turning to nitrate. For this reason, it is critical that dissolved oxygen be eliminated. This typically requires that a large source of carbon be provided for the microorganisms to metabolize. Influent is commonly fed into anoxic zones to provide a carbon source, however, in some cases, alternative sources of carbon must be used. Methanol has been successfully employed as a source of carbon to drive denitrification, although the complication and expense often outweigh the benefits.
Temperature has a profound effect upon denitrification. At liquid temperatures of 10º C (50º F), denitrification tapers off dramatically. In fact, 10º C can be considered a lower limitation for denitrification. Below this temperature, biological denitrification cannot be used to effectively remove nitrate. This fact causes many problems with wastewater treatment facilities in cold climates that must remove nitrogen for permit compliance. The impracticality of warming large amounts of wastewater for the purposes of achieving denitrification makes the 10º C temperature wall a very hard barrier to contend with.
Land Application of Effluent
Dischargers who utilize land application of effluent as a means of removing residual nitrogen from treated effluent under permits issued. In this method, nitrogen in the effluent is applied at the agronomic uptake rate of a crop that is grown with the effluent, such as turf, landscaping or feed crops. As the nitrogen is applied to the crops, the growing plants take it up as fertilizer. When the green plants are cut and removed, the nitrogen bound in them is removed, and thus prevented from contaminating the underlying ground water.