Disc Centrifuge with Nozzle Discharge

This centrifuge is specifically suited to the thickening of excess activated sludge or alum sludge that are free of coarse solids. It can handle high feed rates with effective clarification for very fine particles at low concentrations. Its high Gs make it effective for liquid-liquid and liquid—liquid—solid emulsion separations.

A disc centrifuge bowl can be suspended from or bottom-supported on a flexible spindle that permits the bowl to seek its natural axis of rotation under small unbalanced loads. Commercial units, generally belt-driven at speeds of 3000 to 9000 rpm, develop Gs of 2500 to 8000 with bowl diameters from 9 in for test units to 32 in for industrial units.

Because corrosion is seldom tolerated in the highly stressed bowls, stainless steel is the standard material of construction. The bowl is suspended inside a set of covers (see Figure 7.48.8) containing two decks that separately receive the clarified effluent and thickened sludge. For an emulsion, a third deck collects the second liquid phase.

FIG. 7.48.8 Cutaway diagram of a disk bowl centrifuge with nozzle discharge and recycling.

The distinguishing feature of the bowl is a stack of cones or discs arranged so that the feed stream is divided among them, reducing the particle settling distance. The angle of the cone causes aggregated solids to slide and settle on the inner walls of the bowl that slope toward a peripheral zone containing orifices, called nozzles. The number of nozzles varies with the bowl size, but the size of the nozzle orifice is usually 0.030 to 0.080 in.

For efficient performance, the solids should not be fibrous or pack into a hard cake, and the incoming feed must be screened. The solids concentration in the nozzle discharge can reach ten to twenty times that in the feed. Recycling some of the nozzle discharge stream back into the bowl can control underflow concentrations (thickened sludge).

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