Due to the high suspended solids concentration in mixed liquor, 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 are weak or inexistent and 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 with 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/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 within the range of 2 to 5 g/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.l -1, so that particle settling occurs.
The following items will be discussed in this section:
(1) Zone settling rate test
Using the zone settling rate test, it is possible to determine the zone settling velocity as a function of the sludge concentration, as is demonstrated in Figure 6.1. This relationship can be approximated with Vesilind's equation. In Example 6.1 the Vesilind settleability constants are determined from the experimental results of a zone settling rate test. Click here to download this Section.
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| Figure 6.1 | Experimental set-up used to determine the zone settling velocity (left) and a typical curve of the interface displacement with time (right) |
(2) Alternative parameters for sludge settleability
The zone settling velocity test is not particularly suitable for routine use at waste water treatment plants because it is very time consuming. For this reason many research workers have tried to find alternative ways to express sludge settleability in quantitative terms. Examples are the stirred sludge volume index (SSVI) and the diluted sludge volume index (DSVI). A number of authors have correlated those alternative parameters with the Vesilind constants and the influence of sludge concentration and composition. This is demonstrated in Example 6.2.To download this section, click here.