The literature reports cell yields (productions) for Nitrosomonas of 0.04 to 0.29 milligrams of the bacteria per milligram of NH4-N destroyed and cell yields for Nitrobacter of 0.02 to 0.084 milligrams of the bacteria per milligrams of NO2-N destroyed (Mandt and Bell, 1982). Yield or specific yield simply means the amount of organisms produced per unit amount of substrate consumed. Also, for nitrification to be the dominant reaction, the BOD5/TKN ratios should be less than 3. At these ratios, the nitrifier population is about 10% and higher. For BOD5/TKN ratios of greater than 5.0, the process may be considered combined carbonaceous-nitrification reaction. At these ratios, the nitrifier population is less than 4%. In addition, to ensure complete nitrification, the dissolved oxygen concentration should be, at least, about 2.0 mg/L.

15.6.4 Denitrification: Heterotrophic Side Reaction Stage

Aside from the normal anoxic reaction, two side reactions must be considered in denitrification: the continued oxidation using the leftover dissolved oxygen from the nitrification reaction stage, and nitrite reduction. Immediately after nitrification is stopped, a large concentration of dissolved oxygen still exists in the reactor—this would be around 2.0 mg/L. In nitrification, the nitrifiers are mixed with the heterotrophic bacteria. The heterotrophic bacteria are fast growers compared to Nitrosomonas and Nitrobacter, so they overwhelm the reaction and the overall chemical process is the normal heterotrophic carbonaceous reaction in the last stage of oxidation. By control of the process, the growth rates of the nitrifiers and the heterotrophs are balanced during nitrification and the two types of bacteria grow together. As soon as the oxygen supply is cut off, however, the nitrifiers cannot compete against the heterotrophs for the ever decreasing concentration of dissolved oxygen. Thus, the activities of the nitrifiers "fade away," and the heterotrophs predominate in the last stage of oxidation after aeration is cut off.

The other side reaction is the reduction of nitrite to the nitrogen gas. Although the process is aimed at oxidizing nitrogen to the nitrate stage, some nitrite can still be found. In the absence of oxygen, after the heterotrophs have consumed all the remaining oxygen, no other reaction can occur except for the reduction of nitrite. This is discussed further, after the discussion on the regular denitrification reaction.

Now, derive the chemical reaction for the heterotrophic stage. Let r be the equivalents of O2 (based on the oxygen acceptor reaction) used at this stage. The removal of nitrogen is done in conjunction with the treatment of sewage. Thus, sewage (C10H19NO3) must be the electron donor. Using sewage, the ammonium ion is produced, see Table 15.2. As mentioned before, given NO- and NH+ in solution, organisms prefer to use NH+ first, before NO- for synthesis. Thus, of the two competing synthesis reactions, the one using the ammonium is then the one favored by the bacteria. Letting q be the equivalents of cells, based on the synthesis reaction, produced during the last stage of the aerobic reaction, the synthesis reaction may be modified as follows:

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