which is the same as Eq. 5.3. Thus, if wastage is from each bioreactor in proportion to its volume, the SRT will be controlled solely by the total bioreactor volume and the total wastage flow rate, regardless of the MLSS concentration in each bioreactor. Because of its simplicity, this technique will be used in all simulations in this chapter, even though the wastage streams will not be shown in the schematic diagrams to simplify them.
In Section 5.1.5 we argue that for a single CSTR at steady-state the SRT is a better control parameter than the process loading factor or F/M ratio (U). This does not mean that the process loading factor is unimportant, however. Rather, for a tanks-in-series system it is quite significant because the value in each bioreactor is proportional to the specific growth rate of the biomass in that bioreactor. This can be seen by reviewing Eq. 5.38 in Section 5.1.5. For the general situation under consideration here, it can be rewritten as
If for any given bioreactor, the assumption can be made that Ss, << SSu,, then
The active fraction, fA, changes little from tank to tank and can be considered to be constant for a given system, as can YH. Consequently, the value of the process loading factor in each bioreactor (U,) tells us how the state of the microbial community is changing as it moves from bioreactor to bioreactor in the chain. This is important information that cannot be gained from the overall system SRT. In Chapter 10, we see how control of the process loading factor in individual bioreactors can be used to influence the competition between members of the microbial community to produce activated sludges with desired characteristics.
In SBRs, influent is added for only a portion of the cycle, called the fill period. Equation 7.5 can be used to define an "instantaneous" process loading factor for an SBR by letting F be the volumetric flow rate during the fill period and by ignoring Ss„ which is time dependent. Short fill periods require a large F for a given delivered volume, thereby giving a large instantaneous process loading factor. Conversely, long fill periods require a small F, giving low instantaneous process loading factors. While only an approximation, the calculated instantaneous process loading factor in an SBR can be used in a manner analogous to the individual reactor process loading factors in a tanks-in-series system.
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