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From Equation (15.31), let Yc be the cell yield in moles per unit mole of sewage. Then, Yc = ((q/20)/(r + q/50)) = 5q/2(r + q). And,

The reaction involving sewage is called a carbonaceous reaction, because it is the reaction where organisms utilize the carbon of sewage for synthesis of the cells. Thus, Yc is also called a carbonaceous cell yield.

15.6.5 Denitrification: Normal Anoxic Stage

Let s, based on the synthesis reaction, be the equivalents of cells produced for the regular anoxic denitrification reaction. The organisms in denitrification are het-erotrophic, which must use sewage because, as mentioned, removal of nitrogen is done in conjunction with the treatment of sewage. Again, the ammonium ion is produced in the process, making it the source of nitrogen in the synthesis reaction. Thus, modifying the ammonium synthesis reaction, using the s equivalents of cells, produces the following reaction:

5CO2 + ¿HCO- + + sH+ + se~ — 2OC5H7NO2 + —O^O (15.33)

Denitrification is an anaerobic process; thus, it will not be using oxygen as the electron acceptor. From Table 15.2, two possibilities exist for the electron acceptor: nitrite or nitrate. In nitrate, the oxidation state of nitrogen is 5+; in nitrite, the oxidation state of nitrogen is 3+. The nitrate ion is at the higher oxidation state, so it is easier for it to be reduced than the nitrite ion. Thus, nitrate is the electron acceptor.

Let p, based on the NO- acceptor reaction, be the equivalents of NO3-N utilized. The revised acceptor reaction is

From Eqs. (15.33) and (15.34), the total e- needed from the donor reaction is s + p electron moles. Thus, the donor reaction using sewage is s-50PC1oH19NO3 + 99(s+P-H2O — 9-s0P-CO2 + ~+0pNH+

Adding Eqs. (15.33), (15.34), and (15.35), the overall reaction for denitrification is produced,