Nitrate denitrification

Bacterial remediation of selenium oxyanions in San Joaquin, California, drainage water is under active investigation (96,97), but has not yet been commercialized. Agricultural drainage rich in selenium is also typically rich in nitrates, so bioremediation must also include conditions that stimulate denitrification (98). Phytoextraction of selenium is also being tested, but is not yet being used on a large scale. 

Denitrification is a process in which facultative organisms will reduce nitrate to nitrogen gas in the absence of molecular oxygen. This consequendy results in the removal of BOD. The denitrification process also generates one hydroxyl ion so that alkalinity requirements are reduced to half when both nitrification and denitrification are practiced. 

Nitrogen oxides are formed at various stages of the biological denitrification process. This process starts with nitrate as the nitrate is reduced through various steps, NO2, NO, N2O, and N2 can be formed and, depending on the conditions, released to the atmosphere. 

Denitrification The reduetion of nitrates to nitrites and finally to nitrous oxide or even to moleeular nitrogen eatalyzed by faeultative aerobie soil baeteria working under anaerobie eonditions. 

Fig. 2 Longitudinal changes in nutrient concentrations below the effluent input of a WWTP without tertiary treatment in La Tordera Stream. Values are the average ( SEM) of monthly measurements done over a year (see more details in [47]). In the left panel, note the net decline of ammonium concentration with concomitant net increases in nitrate concentration, suggesting a potential hot spot for nitriflcation. However, in the latest meters downstream, dissolved inorganic nitrogen (DIN) tends to decrease, which indicates net lost of DIN possibly due to denitrification. The right panel shows net changes in phosphate and dissolved organic carbon (DOC) concentrations. While phosphate does not exhibit any clear trend on an annual basis, DOC seems to decline similarly to DIN, which supports the relative dominance of denitrification...

Hydrogenation reactions, particularly for the manufacture of fine chemicals, prevail in the research of three-phase processes. Examples are hydrogenation of citral (selectivity > 80% [86-88]) and 2-butyne-l,4-diol (conversion > 80% and selectivity > 97% [89]). Eor Pt/ACE the yield to n-sorbitol in hydrogenation of D-glucose exceeded 99.5% [90]. Water denitrification via hydrogenation of nitrites and nitrates was extensively studied using fiber-based catalysts [91-95]. An attempt to use fiber-structured catalysts for wet air oxidation of organics (4-nitrophenol as a model compound) in water was successful. TOC removal up to 90% was achieved [96]. 

The adverse properties of red beet preparations are without doubt, their high nitrate content, their unpleasant smell due to pyrazine derivatives and geosmin with a low odor threshold, and finally the risk of carry-over of earth-bound germs. - " Since nitrate has been associated with nitrite and nitrosamine formation, the latter exhibiting cancer-inducing properties, procedures have been developed to reduce the nitrate content of red beet preparations. This has been achieved by microbial denitrification. - ... 

Moir JWB, DJ Richardson, SJ Ferguson (1995) The expression of redox proteins of denitrification in Thiosphaerapantropha grown with oxygen, nitrate, and nitrous oxide. Arch Microbiol 164 43-49. 

The theoretical formaldehyde requirements for denitrifying nitrite or nitrate, if biomass production is not considered, are 0.64 and 1.07 kg C/kg N-NO , respectively. Garrido and colleagues3 found C/N ratios of 0.8 and 1.3 kg C/kg N-NO for denitrification of nitrite and nitrate, respectively, these values being 20% higher than the theoretical ones. 

Denitrification can be affected by free ammonia, but this inhibition does not appear up to 300 to 400 mg/L NH3.46 This high concentration can justify that no inhibition of the denitrification process has been reported for this kind of wastewater.3-4 Eiroa and colleagues37 observed that nitrate was eliminated much faster at higher initial urea concentrations. However, they also found an increase of nitrite accumulation, which was later removed, due to high urea concentrations. 

In systems treating formaldehyde, the loading rates of removed nitrate ranged from 0.44 kg/m3 d to 0.94 kg/m3 d N-NO,. 33-49 These values are in the range of denitrifying loading rates obtained for other kinds of wastewaters (1.1 kg/m3 d or 1.5kg/m3 d N-NO,),50-51 which means formaldehyde can be used efficiently as an electron donor for denitrification. 

When denitrification via nitrate is not possible (COD/N ratios lower than 3.5) there are two possible options to remove nitrogen ... 

Anoxic. Anoxic reactions also function without the presence of free oxygen. However, the principal biochemical pathways are not the same as in anaerobic reactions, but are a modification of aerobic pathways and hence termed anoxic. Anoxic reactions are used for denitrification to convert nitrate to nitrogen ... 

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