A two-stage, step-aeration activated-sludge process (see Figure 7.21.3) shows the design procedures involved (Ramalho 1983). This example assumes that the feed stream is equally divided between the two stages and the MLVSS concentration is constant throughout the tank. The steady-state mass balances on the microorganisms yield the following equations:
Q(r + 1)X - Q(r + 0.5)X = 0.5QX = YQ(Se1 - Se2) - kdXV2
A steady-state mass balance on the microorganisms around the entire system yields the following equations:
(Q - Qw)Xe + QwXr = YQ[(Si - SJ + (Se1 - SJ] - 2kdXV
(Q - Qw)Xe + QwXr = YQ(Si - Se) - 2kdXV 7.25(24) where V1 = V2 = V (equal volumes for each stage) and
The SRT and recycling ratio (r) of the process are as follows:
Equation 7.25(25) can be generalized for an n-stage, step-aeration, activated-sludge process as follows:
whereas Equation 7.25(26) is valid for any numbers of stages.
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