Clarifier Underflow - A

October 11, 1999

An excellent installation was made in 1999 at a Weyerhaeuser paper mill. The mill installed a Model VP-16 screw press, along with a pair of sidehill screens. This equipment is used to dewater the underflow from their clarifier. Photos of the installation are available.

The clarifier is located at the waste water treatment plant. The flow into the clarifier is the combined flow of all of the waste streams in the mill. It includes the knots, shives, and screen rejects from all sources. Since the mill is a virgin fiber Kraft mill, there are negligible contaminants such as the ash (clay), dirt and ink that typify a recycle paper mill. We did note the presence lime, which comes from the lime kiln and cooking liquor preparation areas.

This waste stream dewatered and pressed in very good fashion. The cake, with over 40% solids, is added to the bark pile and sold as fuel to a local co-gen (electricity generating) plant.

The two Vincent sidehill screens, each 3' wide, were installed over the inlet to the press. These face each other so that any splash flow falls into the press. We have nearly identical installations at Bowater and Kimberly-Clark.

At Weyerhaeuser the clarifier underflow pumped to the sidehills was anticipated to be 300 gpm at 1.5% solids. The flow is thickened to an estimated 5% to 8% by the time it falls into the screw press. The press capacity was measured in excess of 40 TPD,DS (tons per day of air dry solids), which is somewhat greater than the expected load.

The original inquiry came from a direct mailing of the Tappi Journal article co-authored by Vincent. Two problems came together that made possible the justification required for the project. There was an existing belt press that was severely deteriorated and in need of major maintenance. Furthermore, the cake from the belt press was too wet to sell along with bark. This cake had to be landfilled, and the mill's landfill was nearing its maximum capacity.

Issue 98