The clarification of biological sludge (excess activated) from feed concentrations of 0.3 to 1.0% SS is the most important application of these centrifuges. Figure 7.48.9 shows the capacity range of a large disc centrifuge used at six municipal plants for excess activated-sludge thickening to 51/2% solids concentration without the use of coagulants.

Contrary to some reports for midrange feed rates of disc centrifuges, varying the SVI from 40 to 137 or altering the centrifuge underflow concentration below 61/2% has little effect on the solids recovery (see Figure 7.48.10). Figure 7.48.9 also shows the direct effect of the feed rate on the quality of clarification of oil separated from an aged petroleum refinery waste emulsion.

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