Sludges as generated are often dilute, typically 1-2% solids by weight. The first step in sludge processing is normally concentration. The most popular process is gravity thickening employing equipment similar to the secondary settler shown in Figures 7 and 8.
In the flotation thickening process shown in Figure 10, air is injected and dissolved into the sludge under a pressure of 275-550 kPa. The sludge then flows into an open tank where, at atmospheric pressure, much of the air comes out of solution as very fine bubbles. The bubbles attach themselves to hydrophobic particles and float them to the surface. The sludge forms a layer at the top of the tank, which is removed by a skimming mechanism for further processing.
Solid concentrations that are achievable in various thickening processes are listed in Table 13. In general, data reported by different investigators are reasonably consistent.
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