Many of the factors that affect the performance of BNR processes are similar to those that affect activated sludge systems. This section will discuss those factors that are of particular concern in BNR systems.
Solids retention time plays the same role in BNR processes as in activated sludge systems. In this case, however, the SRTs in the separate zones are of more interest than the total SRT, since they control what occurs in those zones,'1 as discussed in Chapters 6 and 7. In Section 6.5.2, the aerobic SRT (0t..AtR) was defined as the fraction of the total system SRT that is aerobic. Because the SRT is defined in terms of the mass of biomass in a system (Eq. 5.1), the fraction of the system SRT that is aerobic is equivalent to the fraction of the biomass in the system that is maintained under aerobic conditions, fXM u K. Consequently:
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