ABSTRACT
Selection of the Solids Retention Time. The effects of SRT on activated sludge performance are discussed in Section 10.2.2, while the factors that must be considered during its selection are covered in Section 9.3.2. Consideration of the information in those sections makes it clear that selection of the SRT is a multifaceted decision requiring input from a number of sources. An important consideration of course, is the SRT needed to meet the required effluent quality. If a single completely mixed bioreactor is to be used, such as in CMAS or EAAS, the SRT required to produce a particular effluent COD is given by Eq. 9.7:
Ks+ Ss ®C=--A------- Ss(µH - bH) - KsbH (9.7)
As discussed in Section 9.4.2, the parameters associated with autotrophs can be substituted for the heterotrophic parameters and SNH can be substituted for S5 to determine the SRT required to achieve a required ammonia-N concentration through nitrification. In either case, the SRT calculated from Eq. 9.7 would not necessarily produce the required effluent quality. There are several reasons for this. One is uncertainty in the kinetic parameters, influent characteristics, natural variability in the microbial community, and other factors. Such factors cause statistical variability in the effluent quality as illustrated in Figure 9.9.58 Thus, the calculated SRT must be multiplied by an appropriate safety factor, i;u, to account for that uncertainty, or the value of Ss ( or SNH) must be chosen with the uncertainty in mind.
