[Sno 1 f rs1

where 6cdn = MCRT for the denitrifiers, kddn = kd for denitrifiers, and [ SNO lim ] = limiting value of [SNO ]. Values for kddn range from 0.04-0.08 per day, typical = 0.04/d.

15.10.5 Carbon Kinetics

As mentioned, three types of kinetics are involved in the removal of nitrogen: nitrification kinetics, denitrification kinetics, and carbon kinetics. Carbon kinetics refer to the kinetics of the heterotrophic aerobic reactions. By analogy with the nitrification or denitrification kinetics,

Pcm Uc+c[sj kd

Pcm IKsc + [Scl) - kdc where ¡c = specific growth rate of heterotrophs, kdc = kd of heterotrophs, 6cc = 6c of heterotrophs, ¡¡cm = maximum ¡¡c, Ksc = Ks of heterotrophs, [Sc] = concentration of carbonaceous limiting nutrient, and [Sclim] = limiting value of [Sc]. Values for kdc range from 0.04-0.075 per day, typical = 0.06/d. Values for ¡cm range from 0.8-8 per day, typical = 2/d.

Example 15.6 An activated sludge process is used to nitrify an influent with the following characteristics to 2.0 mg/L ammonia: O0 = 20,000 m /d, BOD5 = 200 mg/L, TKN = 60 mg/L, and average operating temperature = 22°C. The effluent is to have a BOD5 of 5.0 mg/L. The DO is to be maintained at 2.0 mg/L. Assume KsO = 1.3 mg/L. Calculate the limiting concentration of ammonia and the MCRT needed to nitrify to this limiting concentration.


Let kdn = 0.05 per day r„ J = _(0.05 >10 0 051(22>-1158_

0.47e0 098(22-15> (^)[ 1 - 0.833(7.2 - 7.2>J - 0.05

At the limiting concentration, MCRT — 6cn — ^ Ans 15.10.6 Reactor Sizing

Two possible calculations can be used to determine the volume of the denitrification reactor: calculation in which denitrification is allowed to occur in the same tank where nitrification is also occurring and calculation in which denitrification is done in a separate tank or in a separate portion of the same tank as in the oxidation ditch reactor. Where denitrification and nitrification occurs in the same tank, the process is a batch process, an example of which is the sequencing batch reactor (SBR).

In the SBR process, the timing of the nitrification and denitrification processes are properly sequenced at appropriate time intervals in a fill-and-draw mode consisting of four phases: fill, react, settle, and draw. In the fill phase, influent is introduced to fill the reactor vessel. In the react phase, the nitrification reaction is first allowed to take place by aerating the tank. Aeration is then stopped to allow denitrification to set in. In the settling phase, the microorganisms are allowed to settle, followed by the draw phase, where the supernatant formed during the settling phase is decanted. After decanting, the cycle starts all over again. Each of these phases has a predetermined period of time.

In the operation of the nitrification unit, the nitrifiers and the heterotrophs are coexisting. This means that there are two mean cell retention times that must be satisfied. The nitrifiers are slow growers compared to the heterotrophs, therefore, their mean cell retention time, 6cn, is greater. Therefore, in the design of the reactor, 6cc must be made equal to 6cn. In this case, the retention time for the heterotrophs is satisfied as well as the retention time for the nitrifiers. If the reverse is made equal, that is, if 6cn is made equal to 6cc, the time will be very short and the nitrifiers will not have sufficient time to grow and get established and will simply be washed out into the effluent. In addition, a safety factor, SF, should be applied to obtain the design 0c(Qicc — Qdm). A value for the safety factor of 2.0 may be used. In equation form, this is written as

cn cn

Once the MCRT has been established, the volume of the reactor may be determined. Substituting j — V/Qo into Equation (15.81), the reactor volume required is

[X] is the combined concentration of the heterotrophs and nitrifiers.

As noted in Equation (15.100), the design is based on the heterotrophs with the mean cell retention of the nitrifiers being the one used. Thus, the process is considered as simply the normal activated sludge process, with nitrification considered incidental to the scheme. Also, note that in the equation, concentrations are found both in the numerator and the denominator. This means that any unit of measurement for concentrations can be used.

The proportion of the respective population of heterotrophs and nitrifiers varies with the ratio of the influent BOD5 to influent TKN as shown in Table 15.4. Based on this table, the concentrations of the respective species may be calculated. Also, the design total volatile solids (MLVSS, for mixed liquor volatile suspended solids) vary depending upon the type of activated sludge process being used. Table 15.5 shows some representative values. Note that the air supplied is measured at normal air temperatures.

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