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Appropriate Technology: Water


Denitrification is the biological conversion of nitrate to nitrogen gas, nitric oxide or nitrous oxide.  These compounds are gaseous compounds and are not readily available for microbial growth; therefore they are typically released to the atmosphere.  Nitrogen gas makes up over 70% of atmospheric gases, thus the release of N2 to the atmosphere is benign. 

Biological denitrification is an anaerobic respiration reaction in which nitrate (NO3) is reduced. Denitrifying bacteria are aerobic autotrophs or heterotrophs that can switch to anaerobic growth when nitrate is used as an electron acceptor (Bitton 1994).  Denitrification can occur by two pathways.  The dissimilative nitrate reduction pathway requires anoxic conditions and results in the liberation of nitrogen gas from the water column (Reed et al. 1988; Madigan et al. 1997).  Under aerobic conditions denitrification results in the assimilative pathway or accumulation of nitrogen into biomass (Bitton 1994; Madigan et al. 1997).  It is desirable to encourage the dissimilative pathway of denitrification so that nitrogen may be completely removed from the system in gaseous form rather than simply recycled through the system in biomass.  In order for this to occur, there must be insufficient molecular or dissolved oxygen present so that the bacteria use the nitrate rather than the oxygen.  The rate of the denitrification reaction is relatively fast when there is no free oxygen present (< 0.5 mg/l is ideal).  The denitrification rate drops to zero when the dissolved oxygen level reaches 2.0 mg/l.

The denitrification process partially reverses the effects of the nitrification process in regards to alkalinity concentration.  For every milligram of nitrate reduced to nitrogen gas, around 3.57 mg of alkalinity, in the form of CaCO3, are created.

Denitrifiers also require the presence of organic matter (carbon source) to act as an electron donor (see carbon and nitrogen cycle diagrams below).  The presence of a carbon source is the primary determinant of denitrification rates in water (Weier et al. 1994).  Sources of such electron donors may be raw wastewater, methanol, and decomposing organic matter (Bowmer 1987; Bitton 1994; Weier et al. 1994).  Alternatively, organic molecules resulting from solubilisation can be used, but are less efficient.  If the main engine of dentrification is a subsurface flow constructed wetland, raw wastewater and decomposing plant matter provide the necessary carbon source for denitrification.

Denitrifiers belong to several genera including Pseudomonas, Bacillus, Spirillum, Hyphomicrobium, Agrobacterium, Acinetobacter, Propionobacterium, Rhizobium, Cornebacterium, Cytophata, Thiobacillus, and Alcaligenes.  However the most wide spread in water and wastewater are Pseudomonas fluorescens, P. Aeruginosa, P. denitrificans and Alcaligenes sp. (Smith et al. 1994; Bitton 1994).  These organisms are ubiquitous and commonly found in natural soils and wetland environments.

Nitrogen Cycle

Nitrogen Cycle in Wetlands
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Carbon Cycle in Wetlands
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