Nitrification

Nitrification is carried out by unique specialized bacteria in a two-step reaction. The first step, oxidation of ammonium (NH4) to nitrite (NOp, is catalyzed by a few species of bacteria that have names beginning with nitroso-, for example, nitrosomonas and nitrosospira. The reaction is as follows  

The second step, nitrite (NO ) to nitrate (N03), is carried out by a different bacteria— nitrobacter  

Based on Reactions 8.14 and 8.15, nitrification is energetically favorable (overall AG° = -84 kcal). The intermediate nitrogen form, NO , rarely accumulates in significant concentrations because nictrobacter normally acts as fast or faster than the N02-producing bacteria. However, nitrobacter is more sensitive to ammonia than nitrosomonas, and for this reason nitrite may accumulate under high concentrations of NH (Fig. 8.5) but not under low concentrations of NH4 (Fig. 8.6) 

The rate of bacterial growth and hence the rate of nitrification is both temperature and pH dependent. Maximum bacterial activity occurs at about 28°C and a pH of about 8, Below a temperature of about 2°C, the reaction is very slow (Fig. 8.7). Below pH 5.5, the nitrifying bacteria decrease their activity, and below pH 4.5 the nitrification process is severely restricted lack of oxygen also inhibits nitrification. As noted above, oxidation of NH to NO is an enzyme-driven reaction and commonly the Km (see Chapter 7) under optimum conditions is observed to be somewhere around 2.5 mM. Some Km values below 2.5 mM are observed under high pH values when a large fraction of the ammonium is in the NH3 form. 

The overall nitrification rate which describes conversion of NH to NO can be expressed by 

Nitrification in wetlands is restricted to aerobic zones of soil and water column or under drained soils conditions, where ammonium is oxidized to nitrate. Nitrification reaction supports denitrification by supplying heterotrophs with nitrate as their electron acceptor. In a broader sense, nitrification is defined as the conversion of organic or inorganic compounds from reduced state to a more oxidized state. Three groups of microorganisms are capable of oxidizing ammonium under aerobic conditions  

Nitrification is a microbial process by which ammonia is sequentially oxidized to nitrite and then to nitrate. This 

In nitrification, ammonia or ammonium ions are oxidized to nitrite ions and then to nitrate ions  

Nitrification seems limited to a number of autotrophic bacteria. The dominant genus that is capable of oxidizing ammonia to nitrite in soils is Nitmsomonas, and the dominant genus capable of oxidizing nitrite to nitrate is Nitrobacter. Normally, the two processes are closely connected and nitrite accumulation does not occur. Nitrifying bacteria are chemolithotrophs that utilize the energy derived from nitrification to assimilate C02. 

In recent literature some authors have thrown doubt on the value of nitrification. Alexander (1965) states that it is a mixed blessing and, possibly, a frequent evil. He bases his conclusion chiefly on the fact that nitrates are easily lost through leaching or as gases by denitrification and nitrites are chemically unstable. In addition, the acids formed during nitrification may, if not neutralized, form soluble K, Ca, Mg, Mn, P and Al. In extreme cases aluminum toxicity may be encountered. Presumably since most plants can utihze ammonia nitrogen, and some prefer it, nitrate formation is not considered especially essential. 

The importance, or lack of importance, that one attaches to nitrification is largely a matter of emphasis that he places on the various observed facts. Although the author admits that there are a few mixed blessings, he considers that on the whole nitrification is a very essential and valuable process. If nitrification in soil could be prevented completely it is doubtful if we could have an agriculture such as we have today, at least not without supplying much of our fertilizer nitrogen in the nitrate form. Some of the reasons for this viewpoint are discussed at the end of this chapter. 

Both Nitrosomonas and Nitrobacter, being obligate autotrophs, require no organic source of energy. Nitrosomonas obtains its energy by oxidizing ammonia to nitrite, and Nitrobacter by oxidizing the nitrite to nitrate. The reactions involved are as follows  

They obtain their carbon dioxide for cell synthesis from the air or from any available carbonate sources present. Our knowledge of the biochemistry of the oxidation processes and intermediates has been reviewed by Alexander (1965). Suffice it to say that such information is very limited. 

Nitrobacter is sensitive to the presence of ammonia even at comparatively low concentrations (Aleem et al., 1957 Broadbent et al., 1957 Tyler and Broadbent, 1960). Where such inhibition occurs considerable nitrite but little nitrate may be formed. In acid soils, where nitrites are unstable, some of the nitrogen may be lost as oxides of nitrogen or as nitrogen gas, as is pointed out in Chapters 11 and 13. Over the pH range of 3 to 7 the nitrite may also react with soil organic matter (Fiihr and Bremner, 1964a, b Stevenson and Swaby, 1964 Stevenson et al., 1970) to form stable compounds. Nitrites are known to be inhibitory to the respiration of Nitrobacter, as well as to most other bacteria, under acid conditions but only very mildly so in calcareous soils (Tyler and Broadbent, 1960). 

QXJANTITILb 01 SUBSTANCLS WHICH RLIARD NITROUS J LRML NTATION 

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For most crops, other than rice, urea in the soil must first undergo hydrolysis to ammonia and then nitrification to nitrate before it can be absorbed by plant roots. One problem is that in relatively cool climates these processes are slow thus plants may be slow to respond to urea fertilization. Another problem, more likely in warmer climates, is that ammonia formed in the soil hydrolysis step may be lost as vapor. This problem is particularly likely when surface appHcation is used, but can be avoided by incorporation of the urea under the soil surface. Another problem that has been encountered with urea is phytotoxicity, the poisoning of seed by contact with the ammonia released during urea hydrolysis in the soil. Placement of urea away from the seed is a solution to this problem. In view of the growing popularity of urea, it appears that its favorable characteristics outweigh the extra care requited in its use. 

U.S. EPA, Geosafe Corporation In Situ Nitrification Innovative Technology Evaluation Report, EPA/540/R-94/520, Washington, D.C., 1995. 

As in all biological reactions, nitrification is a function of temperature. For municipal wastewaters, a minimum sludge age of 3.5 d is required at 20°C while the minimum sludge age must be increased to 12 d at 10°C in order to achieve nitrification. 

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. 

The process of nitrification—denitrification can be practiced in one of two ways. In the oxidation ditch, nitrification occurs in the vicinity of the aerators. When the dissolved oxygen is depleted as the sludge—wastewater mixture passes from the aerator, denitrification occurs. 

In the two-stage process, nitrification occurs under aerobic conditions in the second stage. The nitrified mixed Hquor from the second stage is internally recycled to the anoxic first stage, where denitrification occurs. 

Note that the maintenance of water quaUty and hence stream standards are not static, but subject to change with the municipal and industrial environment. For example, as the carbonaceous organic load is removed by treatment, the detrimental effect of nitrification in the receiving water increases. Eutrophication may also become a serious problem in some cases. These considerations require an upgrading of the required degree of treatment. 

S. P. Landels, M. M. Smart,. Bakker, and. Shimosato, "Controlled Release Fertilisers and Nitrification Inhibitors," Chemical Economics Handbook, SRI International, Menlo Park, Calif., 1989. 

Nitrification—fish hatchery—Idaho Aerohic 820 Waste Mode mg/L IVft -d time, hr g/L... 

The interest in gaseous losses of nitrogen from soil is now extensive and includes the well established community of soil scientists concerned with losses of fertilizer-applied nitrogen by nitrification and denitrification. More recently, interest in ammonia losses from plants and soil has been stimulated by the very large emissions from intensive cattle production in the Netherlands and their... 

Figure 6 The production and emission of NO and N,0 during nitrification and denitrification. NO / ...

In soil, microbial nitrification and denitrification are the predominant sources of NO and NjO and the emission fiiixes may be regarded as leakage during the transformation processes shown in Figure 6. Nitrifiers can produce NO and NjO during the oxidation of NH4 to NO3". Both gases are by-products of the nitrification pathway and the typical yield of NO in well-aerated soil is 1-4% of the NH4 oxidized and for NjO is less than... 

W. J. Payne, in Denitrification, Nitrification and Atmospheric Nitrons Oxide, ed. C. C. Delwiche,... 

Figure 7 The production and emission of NO during denitrification in agricultural soil treated with NO3 fertilizer (KNO3) and the nitrification inhibitor Dyciandiamide (10%) under aerobic (air) and anerobic conditions (N,). Fluxes are means from three soil columns, error bars represent standard deviations from the mean. V = vertical flow through the column H = Horizontal flow over the soil surface.

Attached growth processes Wastewater treatment processes in which the microorganisms and bacteria treating the wastes are attached to the media in the reactor. The wastes being treated flow over the media. Trickling filters, bio-towers, and RBCs are attached growth reactors. These reactors can be used for removal of BOD, nitrification, and denitrification. 

Autotroph Organism which uses carbon dioxide as the sole carbon source. Autotrophic nitrification Oxidation of ammonium to nitrate through the combined... 

Nitrierungs-grad, m. degree of nitration or nitrification or nitridation. -stufe, /. stage or degree of nitration or nitridation. 

Salpeter-ather, m. nitric ether (ethyl nitrate), -bakterien, n.pl. nitrifying bacteria, -bil-dung, /. nitrification, -blumen, f.pl. niter efflorescence, -damp/, -dupst, m. nitrous fumes, -erde, /. nitrous earth, -erzeugung, /. niter production nitrification. 

Salpeter-schwefels ure, /. nitrosulfuric acid (a mixture) nitrosylsulfuric acid, -siederei, /. saltpeter works, -stkrkemehl, n. nitrated starch, -stoff, m. nitrogen, -strauch, m. niter bush Nitraria). -ung,/. nitrification, -verbindung, /. nitrate. -waC4)ge, /. nitrometer. 

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