Figure 19.10 Effect of the SRT in a suspended growth bioreactor and the nitrification efficiency in an upstream trickling filter on the effluent ammonia-N concentration from a coupled TF/AS system. (From G. T. Daigger, L. E. Norton, R. S. Watson, D. Crawford, and R. B. Sieger, Process and kinetic analysis of nitrification in coupled trickling filter/activated sludge processes. Water Environment Research 65:750- 758, f993. Copyright © Water Environment Federation; reprinted with permission.)

ling filter allows the suspended growth bioreactor to maintain nitrification even when operating at a nitrification design factor that would otherwise cause washout of the nitrifiers. Furthermore, the greater the seeding affect, i.e., the greater the ammonia-N conversion in the trickling filter, the lower the effluent ammonia-N concentration. The potential adverse affects of sloughing from the upstream trickling filter on the validity of this approach have been discussed in the literature.040 Nevertheless, the results suggest that more fundamental procedures may be developed in the future for the design of suspended growth bioreactors in coupled TF/AS processes.

If the characteristics of the trickling filter effluent are described sufficiently, the procedures of Section 5.2.3 can also be used to calculate the oxygen requirement in the suspended growth bioreactor of a coupled TF/AS process. Just as with any sus pended growth process, the oxygen requirements must be compared with the energy input required for mixing and the larger of the two selected. The relationship between oxygen and mixing requirements differs among the various coupled TF/AS processes. The primary function of the suspended growth bioreactor in a TF/SC process is flocculation. Consequently, the TF/SC suspended growth bioreactor will generally be mixing limited. In contrast, substantial stabilization of biodegradable organic matter occurs in the suspended growth bioreactor of BF/AS or RF/AS systems, so that the energy input may be determined either by oxygen or mixing requirements. The difficulty, in any case, is in determining the degree of stabilization of biodegradable organic matter in the upstream trickling filter.

If the trickling filter effluent cannot be characterized sufficiently well to allow the relationships of Section 5.2.3 to be used to calculate the suspended growth bioreactor oxygen requirements, empirical correlations can be used. It will be recalled from Section 9.4.1 that the process oxygen stoichiometric coefficient, Yn:, is often used with Eq. 9.4 to estimate the oxygen requirement for the activated sludge process in the absence of other data. Figure 9.8 shows how the value of that parameter varies with the SRT. A similar approach can be used to estimate the oxygen requirement in coupled TF/AS systems, except that an equation expresses the effect of SRT (or F/M ratio, U) on the process oxygen stoichiometric coefficient, rather than a figure. Based on studies of several full-scale coupled TF/AS processes, Harrison " used the following equation to estimate the overall oxygen stoichiometric coefficient that would occur in the suspended growth bioreactor in the absence of the trickling filter:

where Y(1-, is the oxygen stoichiometric coefficient for synthesis, taken equal to 0.6 mg OVmg BOD,; Y0, d is the oxygen stoichiometric coefficient for decay, taken equal to 1.2 mg 0:/mg VSS; b„ is the decay coefficient, taken equal to (0.115)(1.025)' 2" day ', where T is the temperature of the mixed liquor in the suspended growth bioreactor in °C; and U is the F/M ratio, based on the process influent BOD, loading and the MLVSS inventory in the suspended growth bioreactor. In addition, Harrison2" developed the following equation relating the oxygen stoichiometric coefficient for the trickling filter, Yn:to its TOL, As, expressed in units of kg BOD,/(m'-day):

The oxygen stoichiometric coefficient for the suspended growth bioreactor, Y0;S{;, in a coupled TF/AS system can be estimated as the difference between Y,):, as calculated with Eq. 19.12, and YOJ.,,., as calculated with Eq. 19.13:

The oxygen requirement in the suspended growth bioreactor can then be estimated by multiplying Yo:s<1 by the mass of BOD, entering the coupled TF/AS system per unit time, as indicated by Eq. 9.4. The following example illustrates the technique.

What is the oxygen requirement for the suspended growth bioreactor in the RF/ AS system sized in Example 19.3.6.2, as estimated using the procedure of Harrison? Assume that the temperature is 25°C and that the MLSS is 75% volatile

What is the F/M ratio for the process based on the organic matter entering the system and the mass of MLSS in the suspended growth bioreactor? The F/M ratio can be calculated with Eq. 5.37 by extending it to account for the particulate contribution to the total BOD,. From Example 19.3.6.2, the MLSS concentration 2,500 mg/L and the bioreactor volume is 480 m\ Because the MLSS is 75% volatile, the MLVSS concentration is (0.75)(2,500) = 1,875 mg/L = 1,875 g/m\ The wastewater flow rate is 5,000 m'/day and the influent BOD, concentration is 150 mg/L = 150 g/m\ Therefore,

= (5,000)050) = (1,875)(480) 6 fe b. What would the process oxygen stoichiometric coefficient be if no trickling filter was present?

This can be calculated with Eq. 19.12 after substituting the appropriate values for the oxygen stoichiometric coefficients for synthesis and decay:

c. What is the oxygen stoichiometric coefficient for the trickling filter? This can be calculated with Eq. 19.13. The roughing filter TOL is 2.5 kg BOD,/(m'day). Therefore:

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