Due to the high suspended solids concentration, settling in secondary- or final settlers is of an entirely different nature from the process observed in more dilute suspensions like most waste waters. In dilute suspensions, the interactions between individual flocs or particles are weak or inexistent and therefore each particle acquires its own settling velocity as a result of frictional and gravitational forces. In more concentrated suspensions like mixed liquor, a matrix of interlinked particles is formed, all settling at the same rate: the zone settling velocity. This type of settling is not dictated by the forces acting on each particle alone, but rather by the forces acting on the whole interlinked sludge mass.
For zone settling to apply, the minimum sludge concentration is in the range of 0.5 to 1.0 g TSS/l. At lower concentrations the flocs are too dispersed and tend to settle as individual particles. As the suspended solids concentration of mixed liquor is normally in the range of 2 to 5 g TSS/l, zone settling is unavoidable in final settlers of the activated sludge process. In contrast, in most waste waters the suspended solids concentration is less than 0.5 g TSS/l, so that particle settling occurs.
The following items will be discussed in this section:
(1) Excess sludge quantity and composition
Equations are developed to predict the quantity of excess sludge generated in a primary settler and the quantity of excess sludge discharged from the activated sludge system. The crucial importance of sludge thickening as a means to reduce the volumetric flow to the digesters and that of sludge dewatering to reduce the quantity of stabilized sludge to be disposed-of is highlighted in Figure 8.2. Data from literature is presented on the range of compositions of both primary and secondary excess sludge. Click here to download this section
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| Figure 8.2 | Relationship between solids fraction, humidity and relative sludge volume |
(2) Thickening of secondary excess sludge
An excess sludge thickener is an auxiliary unit with the specific objective to concentrate the excess sludge, before it is sent to the sludge digester. The supernatant from the thickeners is returned to the aeration tank, so that its quality in terms of suspended solids concentration is of little importance. The volume of the sludge flow discharged into the thickener depends on the sludge age of the activated sludge process. Although a small unit, the thickener is crucial to the overall performance of the activated sludge process. An increase in solids concentration from 0.4% wt (4 g TSS/l) to 2% wt (20 g TSS/l) results in a significant reduction (80%) of the flow to be treated in the digester. To download this section, click here.