In biochemical nitrogen removal, BNR, two steps are required: oxidation of nitrogen to nitrate and subsequent reduction of the nitrate to gaseous nitrogen, N2. The oxidation steps are mediated by Nitrosomonas and by Nitrobacter, as mentioned previously. The reduction step is mediated by the normal heterotrophic bacteria. We will now discuss the chemical reactions involved in these oxidations and reduction.
15.6.1 Nitrification: Nitrosomonas Stage
From Table 15.2, the generalized donor reaction, acceptor reaction, and synthesis reaction mediated by Nitrosomonas are, respectively, shown by the following half-cell reactions:
1 NH+ + 1 H2O — 1 NO- + 4 H+ + e~ donor reaction (15.13)
— 20 C5H7NO2 + — H2O synthesis reaction (15.15)
The table shows three possibilities for a donor reaction: domestic wastewater, nitrite, and ammonia.
Note: Ammonia and ammonium are interchangeable, since one converts to the other.
We are nitrifying ammonia, so the ammonium is the donor. The table also shows other possibilities for the electron acceptor. Because nitrification is aerobic, however, oxygen is the acceptor. Lastly, the synthesis reaction has two possibilities: one using ammonia and the other using nitrate. Of these two possible species, organisms prefer the ammonium ion to the nitrate ion. Only when the ammonium ions are consumed will the nitrates be used in synthesis.
If the previous equations are simply added without modifications, electrons will remain free to roam in solution; this is not possible. In actual reactions, the previous equations must be modified to make the electrons given up by Equation (15.13) be balanced by the electrons accepted by Equations (15.14) and (15.15). Starting with one gram-equivalent of NH4-N, based on Equation (15.15), assume m equivalents are incorporated into the cell of Nitrosomonas, C5H7NO2. Thus, the NH4-N equivalent remaining for the donor reaction of Equation (15.13) is 1 - m, equals (1 - m) [((1/20)N)/1] (1/N) = -0(1 - m) moles. [The 1/20 came from Equation (15.15) used to compute the equivalent mass of NH4-N.] Thus, 2o(1 - m) moles of NH4-N is available for the donor reaction to donate electrons. By Equation (15.13), the modified donor reaction is then
From Equation (15.15), the m equivalents of NH4-N is m[((1/20)N)/1] (1/N) = -0 moles. Thus, the synthesis reaction becomes
1/20I20J CO' + 1720 I20J HC° + H/-2) y nh4 + ^ (20 J H + 1720(2oJe
From Eqs. (15.16) and (15.17), the electron-moles left for the acceptor reaction is
JL(J_J(i _ ) - JL/m-J = 3-13m 1/6U-J( 1 m) 1/20U-J 10
Hence, the acceptor reaction, Equation (15.14), modifies to
1 (3-13mJ„ 3-13mTT+ 3-13m - 1 (3-13mJTT „ /^-mx
Adding Eqs. (15.16), (15.17), and (15.18) produces the overall reaction for the Nitrosomonas reaction shown next. Note that, after addition, the electrons e~ are gone. The overall reaction should indicate no electrons in the equation, since electrons cannot just roam around in the solution. They must be taken up by some atom. The overall reaction is
15.6.2 Nitrification: Nitrobacter Stage
Nitrobacter utilizes the nitrites produced by Nitrosomonas for energy. The unmodified donor, acceptor, and synthesis reactions are written below, as taken from Table 15.2:
2n°-+ 2h2° ^ 2n°- + H+ + e- donor reaction (15.20)
In the BNR process, the Nitrobacter reaction follows the Nitrosomonas reaction. From Equation (15.19), 1/20(1 - m) moles of NO2-N have been produced from the original one equivalent of ammonia nitrogen based on Equation (15.15). These nitrites serve as the elector donor for Nitrobacter. Thus, the donor reaction of Equation (15.20) becomes
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