Ab F

<0.6

<1.0

system. Less information is available about net process yields in coupled TF/AS systems than is available for activated sludge systems. However, because the retention of biomass in a trickling filter increases as the TOL is decreased, the value of Y„ typically is influenced more by the TOL on the trickling filter than by the SRT of the suspended growth bioreactor and decreases as the TOL is decreased. Furthermore, values are typically on the order of 0.7 to 0.9 mg TSS/mg BODs. The following example illustrates this approach for the design of a TF/SC system.

A high-quality effluent in terms of effluent BOD, and suspended solids is desired for the trickling filter sized in Example 19.3.2.2. Size a solids contact unit to achieve this goal, thereby converting the system into a coupled TF/SC system. Assume that the net process yield, Y,„ has a value of 0.70 mg TSS/mg BOD. and that the MLSS concentration in the suspended growth bioreactor is 2500 mg/L.

a. What SRT value would be appropriate for this application?

The TOL used for the trickling filter in Example 9.3.2.2 is 0.75 kg BOD,, (m' day). Thus, from Table 19.6, a suspended growth bioreactor with an SRT of 1.0 day would be appropriate.

b. What should the volume of the suspended growth bioreactor be?

From Example 19.3.2.2, the tlow rate is 5,000 m'/day and the total BOD, concentration is 150 mg/L (150 g/m'). The required reactor volume can be calculated with Eq. 9.2 by making use of the fact that the desired MLSS concentration is 2,500 mg/L (2,500 g/m'):

2,500

This gives an HRT of 1.0 hr. The acceptability of this size from a mixing energy and oxygen transfer perspective would have to be verified using the procedures discussed in Section 10.2.5.

c. What is the excess solids production rate? This can be calculated with Eq. 9.3:

WM = (5,000)(0.70)(150) = 525,000 g/day = 525 kg/day

A similar approach can be used to design a coupled RF/AS system, as illustrated in the following example.

Example 19.3.6.2

The roughing filter sized in Example 19.3.2.1 is to be used in a coupled RF/AS process. What size suspended growth bioreactor is required. Assume that the net process yield, Y„, has a value of 0.80 mg TSS/mg BOD, and that the MLSS concentration in the suspended growth bioreactor is 2500 mg/L. The net process yield is higher than in the preceding example because the TOL on the trickling filter is higher. Also assume that because the subject wastewater is readily bio degradable, flocculation will control the design.

a. What SRT value would be appropriate for this application?

From Example 19.3.2.1, the TOL is 2.5 kg BODJ(m' • day). Consequently, from Table 19.6, an SRT of 2 days is necessary to obtain good flocculation.

b. What should the volume of the suspended growth bioreactor be?

From Example 19.3.2.1, the flow rate is 5,000 mVday and the total BOD. concentration is 150 mg/L (150 g/m1). The required reactor volume can be calculated with Eq. 9.2 by making use of the fact that the desired Ml.SS concentration is 2,500 mg/L (2,500 g/m'):

2,500

This gives an HRT of 2.3 hr. The acceptability of this size from a mixing energy and oxygen transfer perspective would have to be verified using the procedures discussed in Section 10.2.5.

c. What is the excess solids production rate? This can be calculated with Eq. 9.3:

WM = (5,000)(0.80)(150) = 600,000 g/day = 600 kg/day

More solids are produced than in the TF/SC system because the TOL on the roughing filter is much higher, thereby making Y„ higher and the SRT of the suspended growth bioreactor is not large enough to reduce it.

While both process kinetics and flocculation must be considered when sizing a coupled TF/AS process, the criteria presented in Table 19.6 will typically control the process size since flocculation is generally the governing event. The concentration of soluble, biodegradable organic matter in the coupled TF/AS process effluent can be calculated by using the specific growth rate of the biomass in the suspended growth bioreactor, just as with other suspended growth bioreactors. However, for coupled TF/AS processes a significant fraction of the biodegradable organic matter contained in the process influent wastewater will be removed in the trickling filter, and this will result in a significant input of microorganisms into the suspended growth bioreactor. This must be considered in the calculation of the specific growth rate if accurate predictions of the effluent soluble substrate concentration are to be made. The effect of influent biomass on the specific growth rate in a suspended growth bioreactor is discussed in Section 5.2.3, and the results are presented as Eq 5.50. It is repeated here, with the only modification being that the source of the heterotrophic biomass in the suspended growth bioreactor influent is identified:

where XH H ,H is the heterotrophic biomass concentration in the trickling filter effluent resulting from a single pass of the wastewater over the trickling filter. In other words, it is the concentration that would result if the trickling filter were the only biochemical operation being used. All other symbols refer to the suspended growth bioreactor. As illustrated, the feed of microorganisms into the suspended growth bioreactor reduces the specific growth rate, thereby reducing the effluent substrate concentration below the value that would be obtained in a system with the same SRT. but with no biomass input.

If the characteristics of the trickling filter effluent, i.e., SSl. and X„ M rH , could be defined, the relationships presented in Section 5.2.3 could be used to size the suspended growth bioreactor of a coupled TF/AS system. Unfortunately this cannot be done easily. First, we saw in Section 19.3.1 that trickling filter design procedures focus on the removal of substrate rather than on the growth of biomass. Thus, while Ss, may be well defined, X,, ,, U1. is not. If an attempt were made to estimate biomass growth by using the true growth yield alone and neglecting cell decay, X,,,,,,, would be overestimated, thereby causing the substrate removal capability of the suspended growth bioreactor to be overestimated. Second, the suspended solids in the effluent from the trickling filter consist of heterotrophic biomass, cell debris, inert influent suspended solids, and unmetabolized substrate, with their relative quantities depending on the characteristics of the influent wastewater, as well as on the TOL and THL of the trickling filter. Precise prediction of the concentrations of these constituents, which are required for use of the equations in Section 5.2.3, is not currently possible. Third, both aerobic and anaerobic metabolism can occur within the biofilm of a trickling filter. Although the outer portion of the biofilm is aerobic, the inner portion may be anaerobic. Moreover, the relative importance of aerobic and anaerobic metabolism will vary depending on the nature and concentration of the biodegradable organic matter in the influent wastewater, the availability of oxygen, and hydraulic conditions affecting biofilm thickness. Since yields are quite different under aerobic and anaerobic conditions, biomass production can vary significantly. This further complicates prediction of the concentrations of various types of suspended solids in the trickling filter effluent. Finally, biomass can accumulate within a trickling filter and be sloughed periodically, as discussed in Section 19.4. This results in time-variant concentrations of biomass and other particulate constituents in the trickling filter effluent. In many instances, sloughing cycles occur over the course of several days, or even several weeks, a time interval that can significantly exceed the suspended growth bioreactor SRT. In fact, significant variations in suspended growth bioreactor MLSS concentrations have been observed as a result of trickling filter sloughing cycles.'"" These variations can affect the performance of the suspended growth bioreactor in a significant manner. Consequently, the suspended growth bioreactor is typically sized by using the net process yield approach as illustrated in Examples 19.3.6.1 and 19.3.6.2.

Some of the difficulties discussed above can be avoided when the focus is on nitrification because then only the autotrophic biomass concentration need be known. As a result, the relationships presented in Section 5.2.3 have been used successfully to characterize the removal of ammonia-N in a coupled TF/AS process accomplishing combined carbon oxidation and nitrification.'" The concentration of nitrifiers in the trickling filter effluent was estimated as the concentration of ammonia-N nitrified in the trickling filter multiplied by the nitrifier true growth yield. This was permissible because there is little difference between the true growth yield and the observed yield for autotrophic bacteria. This concentration was used, along with Eq. 19.11, to predict the nitrifier specific growth rate in the suspended growth bioreactor allowing estimation of the effluent ammonia-N concentration. The performance relationship developed is presented in Figure 19.10 where the effluent ammonia-N concentration is plotted as a function of the suspended growth bioreactor SRT divided by the nitrifier minimum SRT. Several curves are presented corresponding to different trickling filter nitrification efficiencies. As seen, the sloughing of nitrifiers from the trick-

Influent Ammonia - N Cone. = 20 mg N/L Temp. = 20°C

Trickling Filter

Nitrification

Efficiency

Trickling Filter

Nitrification

Efficiency

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