Energy costs have caused several paper mills to look at adding a screw press to their wastewater treatment plant. This is done so that the sludge from the facility can be used as boiler fuel. Typically the screw press is fed the cake produced by an existing belt press.

Belt presses and screw presses have fundamental similarities. Both operate on a continuous basis (not batch) to separate liquid from solids. However, each machine design has different strengths and weaknesses.

Because a belt press works with a filter made of fabric, it achieves much finer filtration than is possible with a screw press. The metal screen (perforated metal, profile bar, or drilled plate) in a screw press results in far more suspended particles in the press liquor than is characteristic of a belt press. The screening area of a belt press is much greater than a screw press, so it has greater hydraulic capacity.

The other side of the coin is that a screw press can squeeze a lot tighter than a belt press. The all-metal construction of a screw press allows higher, more concentrated, pressure and higher torque. The result is that a screw press can make press cake of a lower moisture content than is possible with a belt press.

Several paper mills have taken advantage of these characteristics by adding a screw press in series with a belt press. The belt press filters out the suspended solids, and the screw press squeezes out additional water.

Other mills have been motivated to add a screw press because of trouble maintaining acceptable consistency in the cake from their belt press. With surges in secondary (biological) sludge flow, the cake can become too moist. Instances of landslides at the landfill are not unheard of. Placing a screw press after the belt press will increase the final press cake solids from a range of 20% to 35% up to 25% to 50%, depending on the amount of secondary treatment sludge that is present.

A benefit to adding a screw press is that the belt press can now be operated with minimum belt pressure and no nip. A belt press will run almost maintenance free under such conditions.

The Vincent interrupted flight design has a particular advantage in this application. It can handle a wide range of input consistencies and flow rates without adjustment, all while continuing to produce a consistently dry cake.