screw presses

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These presses, able to develop high pressure, are applied exclusively to paper mill sludge and providing that this sludge contains more than 45-50% of fibres. Therefore, applications remain rare.

screw pressSecured image
Figure 52. Schematic cross-section of a screw press

The screw press follows on from a draining step (e.g. GDD/GDE) thus feeding the press section with very thick sludge. The screw rotates slowly in a perforated cylinder, gradually compressing the sludge; this compression is made possible by a plug of dewatered sludge that forms at the screw outlet.

When the sludge is sufficiently fibrous, it produces 45-55% dry solids contents. Output ranges from 600 to1 000 kg suspended solids · h–1 for 600-650 mm diameter screws.

These screw presses can also be installed behind a belt filter in order to improve dry solids content by 10 to 15 points. These appliances require rather high maintenance (screw wear).