Final settlers in activated sludge processes operate under continuous flow and load conditions. Mixed liquor flows from the aeration tank to the settler and return sludge, containing the concentrated suspended solids, is pumped back to the aeration tank, while a clarified effluent flow is discharged from the system. Fig. 6.4 shows a schematic representation of a final settler including the in- and outgoing flows. In this section the following items will be discussed:
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| Figure 6.4 | Schematic representation of the flows in an activated sludge process with final settler |
It will be demonstrated how to set-up a mass balance over the final settler, as shown in Figure 6.1. As is graphically displayed in Figure 6.5, the solids flux at any given depth in the final settler consists of three components:
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The downward velocity of the liquid in the lower part of the final settler "u" resulting from the abstraction of the sludge return flow at the settler bottom
- The zone settling velocity "v" of the sludge layer
- The sludge concentration at a particular settler depth
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| Figure 6.5 | The solids flux due to settling (a), the solids flux due to return sludge flow abstraction (b) and the resulting total solids flux (c) in a continuous settler as a function of the sludge concentration |
An essential condition for the settler to perform properly is that the solids loading rate does not exceed the solids flux at any level in the settler. If this condition is obeyed, all solids are transported from the feeding point to the abstraction point and accumulation of solids will in the settler not occur. However, if at some level between the inlet point and the abstraction point the solids loading rate exceeds the solids flux, then at that level solids will accumulate at a rate equal to the difference between the solids loading rate and the solids flux. Eventually the settler will be completely filled with solids, resulting in the discharge of sludge together with the effluent. Click here to download this section
(2) Determination of the limiting, critical and minimum sludge concentration
In order to establish if the sludge settling process in a settler will be determined by conditions of clarification or thickening, it is necessary to derive expressions for the limiting sludge concentration, the critical sludge concentration and the minimum sludge concentration. Refer also to Example 6.3 for application of the concept of limiting sludge concentration.