Clarifier Sludge

August 28, 1998                                                                                                                                                                                                    ISSUE #82

In 1997 Putney Paper in Putney, Vermont purchased a Vincent VP-16 screw press. Its function was to further dewater sludge from the primary clarifier in their wastewater treatment plant. The sludge was pre-thickened with a belt press ahead of the screw press.

The mill both recycled OCC (Old Corrugated Container) and did deinking of other waste paper. The OCC operation put fiber in the clarifier, which improved the subsequent dewatering. Deinking, on the other hand, results in a higher proportion of clay and other materials that are hard to dewater in a screw press.

Testing with a rental machine and initial operation with the VP-16 gave excellent results. Press cake solids of 55% were obtained. This addressed problems associated with landfill hauling and fees.

A year later the press would not work. Capacity had fallen off and dewatering was minimal. Examination revealed that the sludge no longer had the fiber content that had existed previously. This had occured as a result of improved fiber recovery. Nothing could be done with the mud-consistency material coming from the belt press. The mill stopped using the screw press.

Trials elsewhere prompted Vincent to make this proposal to mill management: if they would substitute the existing 1800 rpm drive motor with a 900 rpm model, Vincent would supply the belts and sheaves to cut the screw speed in half. This combination cut the speed of the press from the normal 13 rpm down to about 3 rpm.

The results were a dramatic improvement in dewatering capability. The 35% solids cake from the belt press was further dewatered to a respectable 45% solids in the Vincent press.

Unfortunately capacity of the screw press fell off with the speed reduction. At times this was a problem for the mill, so the 1800 rpm motor was re-installed. With this change, press cake solids fell to 40%, but capacity was up. Later, a frequency inverter was installed so that the press could be run at speeds from 3 to 7 rpm. The speed was set according to the nature of the sludge being pressed at any given time.