0 0.04 0.08 0.12 0.16 0.20 0.24 0.28 0 32 0.36 Hydraulic Loading, m/day

Figure 20.6 Typical design correlation for soluble BOD< removal in municipal wastewater treatment.

20.2.3 Staging

As discussed above and illustrated in Section 17.4, RBC facilities are typically staged to improve overall performance. In a staged system, the SOL for the initial stage is higher than the value for the entire system. As indicated in Figure 20.4, increased SOL values result in an increased removal rate. Consequently, by staging the bio-reactor, the overall average removal rate can be increased, even though the SOL on the last stage is relatively low. Since the process effluent quality is determined by the SOL on the last stage, a good-quality effluent can be produced even with a relatively high overall average SOL. The principal constraint is that the SOL in any stage must not exceed the oxygen transfer capacity of a shaft, 32 g BOD5/(nr • day), as discussed in Section 20.2.1. The performance benefits of staging RBCs were demonstrated early in the development of this technology.4

Staging is particularly effective in systems that are required to achieve combined carbon oxidation and nitrification. Staging in the upstream portion of the train produces a high SOL, resulting in a high rate of organic matter removal. This results in reduced SOLs in subsequent stages and concentrations of soluble biodegradable organic matter less than 20 mg/L as COD. This, in turn, allows nitrifying bacteria to become established in the later stages of the RBC train. As indicated in Figure 20.5, nitrification is first-order for low ammonia-N concentrations, but it is zero-order for ammonia-N concentrations greater than 5 mg/L. Staging does not increase the ammonia-N flux into the biofilm when the ammonia-N concentration exceeds 5 mg/L, but it does increase the overall flux when the ammonia-N concentration is less than 5 mg/L. Table 20.2 summarizes recommendations from several sources concerning the number of stages for particular applications.-711

Staging also affects the nature of the biomass that develops on individual RBC shafts and their resulting pollutant removal capability. This is illustrated most graphically in an RBC system that is used for combined carbon oxidation and nitrification.

Table 20.2 Summary of Staging Recommendations Carbon oxidation Nitrification

Effluent soluble Effluent

BOD, conc. Number of NH,-N conc. Number of mg/L stages rng/L stages

15-25 1 or 2 <5 Based on kinetics

As indicated in the preceding paragraph, in the initial stages the biofilm will consist primarily of heterotrophs, making it very active and capable of high organic matter fluxes. The later stages, however, will contain biofilms with increasing proportions of nitrifying bacteria. As a result, the organic removal capability of these later stages will be reduced, although the nitrification capability will be enhanced.

20.2.4 Temperature

The effect of temperature on the performance of RBC systems is similar to that of other attached growth processes. Since the reaction rate is influenced strongly by diffusion, wastewater temperature has little effect on process performance over a wide range of temperatures. The effect of temperature is generally neglected for temperatures over about 15°C, although removal rates generally decline for lower temperatures.1 M Temperature relationships have been presented in two ways. Figure 20.7 provides one relationship. To use it, first determine the media surface area required for the particular application at a temperature above 55°F (13°C). Then enter

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