Xi = biological solids concentration at location i vi = settling velocity of biological solids at concentration Xi
The mass flux of biological solids due to sludge recirculation Nr is calculated as follows:
Therefore, the following equation calculates the total mass flux of biological solids NT:
Figure 7.29.2 plots Equations 7.29(8) to 7.29(10) as a function of biological solid concentration X. If a horizontal line is drawn tangent to the low point on the NT curve, its intersection on the y-axis yields the limiting mass flux of biological solids NL. Therefore, AT is calculated as follows:
The larger value between AC and AT is the design value. The required sludge concentration in the recycled flow is Xr. Table 7.29.1 summarizes typical design information for secondary clarifiers.
Secondary clarifiers have various geometric configurations; the most common ones are either circular (see Figure 7.16.9) or rectangular (see Figure 7.16.10). Other types include tray clarifiers, tube settlers (see Figure 7.16.11), lamella (parallel-plate) settlers, and intrachannel clarifiers (in oxidation ditches). Wastewater treatment facilities can increase existing clarifier capacity by installing inclined tubes or parallel plates.
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