a small amount of nitrification may occur. However, it is not likely to be significant, so to be conservative fNl, was taken as zero. Examination of the SOL into stages 4 and 5 suggests that organic substrate removal will not restrict nitrification in them, making fsn equal to 1.0. Only in stage 3 will significant competition occur between heterotrophs and autotrophs, giving an fNM value of 0.77. The fNM values are summarized in Table E20.2.

e. What is the ammonia-N profile through a train?

As long as the ammonia-N concentration is above 5 mg/L, calculation of the ammonia-N profile is straightforward because the nitrification rate is not influenced by the ammonia-N concentration. However, once the concentration drops below 5 mg/L. the effect of the concentration must be considered, which requires an iterative procedure, as illustrated in Example

As indicated in Table E20.2, no nitrification will occur in stages 1 and 2. and thus the ammonia-N concentration in them is the same as the influent concentration, 25 mg/L. It is likely that the ammonia-N concentration will be high enough in stage 3 to allow nitrification to occur at a rate that is unaffected by the concentration. From Figure 20.5 the maximum rate in the absence of carbon oxidation is found to be 1.5 g NH,-N/(m day). However, from Table E20.2 the value of fNM is seen to be 0.77. Therefore, the ammonia-N removal rate is 1.16 g NH ,-N/(m: ■ day). Using this, the effluent ammonia-N concentration can be calculated from a mass balance on the stage.

Table E20.2 Ammonia-N Concentrations Through the RBC Stages in Example

Influent soluble

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