This can be used in Eq. 16.8 to calculate the recirculation ratio, a:


Thus, the recirculation flow rate must be (3.84)(5,000) = 19,190 m'/day.

This example illustrates that a large media volume may be needed when a high level of treatment is required. This can result in a high recirculation requirement.

Table E19.1 compares the trickling filter sizes required for the applications considered in Examples through The roughing application (Example achieves partial removal of biodegradable organic matter and requires a relatively small trickling filter of moderate depth, operating with a relatively high THL (3.0 m/hr) without recirculation. Removal of biodegradable organic matter to the level of secondary treatment (Example requires a larger trickling filter with a greater depth, operating at the minimum THL (1.8 m/hr) with a modest amount of recirculation. Finally, combined carbon oxidation and nitrification (Example requires an even larger trickling filter, although at the same depth, operating at the minimum THL (1.8 m/hr) with a large amount of recirculation. Capital costs increase as the degree of treatment is increased because larger trickling filters must be constructed. Operating costs also increase because the influent must be pumped to a higher elevation and because more recirculation flow must be pumped.

Next consider the design of a trickling filter for separate stage nitrification. As discussed in Section 19.2.1, separate stage nitrification conforms more closely than other trickling filter applications to the theoretical models of Chapter 15 and Sections 16.1 and 16.2. Consequently, it is best characterized by the surface loading approach. The design methodology proposed by Parker and co-workers'J,4J provides the basis for the approach discussed here.

In the upper regions of a trickling filter accomplishing separate stage nitrification the rate of ammonia-N oxidation will be controlled by the rate of oxygen transfer into the biofilm. This occurs because the ammonia-N concentration in the liquid phase is relatively high whereas the dissolved oxygen concentration is limited to the solubility of oxygen in water (in the range of 7 to 12 mg/L for typical wastewater temperatures). Because approximately 4.3 mg of 0: are required for the oxidation of one mg of ammonia-N, and because the half-saturation coefficient for oxygen is relatively high, oxygen will be the limiting reactant for ammonia-N concentrations above about 3 to 5 mg-N/L. However, in the lower regions of the trickling filter the rate of ammonia-N oxidation will become limited by the ammonia-N concentration. This situation can be characterized by the following expression:

Table E19.1 Comparison of Results for Examples



Surface area m'

Flow rate mVday




Secondary treatment Carbon oxidation

0 0

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