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Same reactor. After the nitrifiers and the heterotrophs, the denitrifiers will now also be considered. Considering the denitrification is to occur in the same vessel as for the nitrification-heterotrophic reaction step, a second volume will be compared with the first volume computed previously; the larger of the two controls the design. The applicable equation for the second case (denitrification) is

[Xdn] is the concentration of the denitrifiers. Since the denitrifiers are basically heterotrophs, [Xdn] is equal to [Xc]. As in previous equations, concentrations are found both in the numerator and the denominator. Again, this means that any unit of measurement for concentrations can be used.

Separate reactors. Where denitrification and nitrification are done in separate tanks or in a separate portion of the same tank, as in the oxidation ditch process, each will require its own reactor volume. The volume of the reactor for denitrification will be given by Equation (15.101) and that for nitrification will be given by Equation (15.100).

Incorporating the SF, the modified forms of Eqs. (15.100) and (15.101) are, respectively, as follows:

Vcarbnit is the volume of the activated sludge reactor, where the carbonaceous and the nitrification reactions occur. Vdenit is the volume for denitrification.

Example 15.7 A domestic wastewater with a flow of 4000 m /d is nitrified. The influent to the reactor tank contains 60 mg/L of TKN, 25 mg/L of ammonia nitrogen, and 200 mg/L of BOD5. The effluent from the tank contains 2.0 mg/L of dissolved oxygen, 0.5 mg/L of NO2-N, 2.0 mg/L of ammonia nitrogen, and 10 mg/L of BOD5. (a) Calculate the volume of the reactor. (b) How long does it take to attain the limiting nitrate ion concentration?

Solution:

0 47'10 's- + [Snh--)(™>1 "»-833<7 2 -pH)] - kd

Let "cm — 2 per day Ksc — 50 mg/L kdc — 0.05 per day

Let T — 10°C pH — 7.2 kdn — 0.06 per day KsO — 1.3 mg/L Therefore,

0.47e0098(1°-15>( + J^)[ 1 -0.833(7.2- 7.2)] -0.06

Let SF — 2 Yc — 0.35 mg VSS/mg BOD5. Assume MLVSS — 3000 mg/L composed of hetrotrophs and nitrifiers.

From Table 15.4, nitrifier population:

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