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FIG. 7.48.3 Clarification of biological sludges in imperforate bowl knife centrifuge.

curves for four biological sludges showing the recovery of SS as a function of the feed rate averaged over a full cycle. Recovery refers to the proportion of the entering solids that are retained in the bowl for discharge as cake.

As with all centrifuge designs, increasing the feed rate decreases the residence time and recovery. These centrifuges produce recoveries of 90% or better over a range of feed rates. Industrial activated sludge is often more difficult to clarify than the consistent biological sludge generated in municipal sewage plants.

Concentration of discharged cake is a function of the sludge but is also influenced by the residence time in the bowl and the G level. Residence time, controllable within limits, is a function of centrifuge size, throughput rate, feed concentration, and recovery. Figure 7.48.4 gives typical performance data for whole cakes from several industrial activated sludges, municipal biological whole and sec-

FIG. 7.48.3 Clarification of biological sludges in imperforate bowl knife centrifuge.

o z

CAKE MEAN RESIDENCE TIME (MINUTES)

FIG. 7.48.4 Whole cake concentration from imperforate bowl knife centrifuge applied to various sludges.

ondary sludges, and raw co-settled primary plus activated sludges.

The figure shows that cake concentration increases with residence time. For sludges with particles too fine or too gelatinous for liquid drainage, concentration depends on the compaction characteristics; hydrogels typically compact to 5 to 15% concentration. Further dewatering generally requires chemical or heat treatment.

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