Biological solids removal is essentially accomplished by gravity settling (see Figure 7.16.5). However, biological solids can settle differently depending on their origins and characteristics. The sloughed solids produced from trickling filters and RBCs are generally large and heavy. Therefore, their settling motion is discrete (i.e., not influenced by the motion of neighboring particles, as shown in Figure 7.16.6) and can be described by Stokes' Law (Metcalf and Eddy, Inc. 1991).
On the other hand, biological flocs produced in activated-sludge processes undergo some flocculation with neighboring particles during the settling process. As floc-culation occurs, the mass of particles increases and settles faster (see Figure 7.16.7). As a result, the settling process is classified as flocculant settling (Metcalf and Eddy, Inc. 1991). Since many complex mechanisms are involved during flocculation and their interactions are difficult (if not impossible) to define, the analysis of flocculant settling requires experimental data obtained from settling tests (see Figure 7.16.8).
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