Stora Enso Duluth Paper Mill, formerly Lake Superior Paper Industries, makes supercalendered paper—high-gloss paper used in advertising inserts, catalogs and magazines. On average, the Duluth plant released 45,000 pounds of total suspended solids (TSS) to the sewer per day. The solids released were mostly fiber, clay and other fillers.“We were wasting raw materials and losing money in treatment and material costs,” said Diane Gobin, environmental supervisor at the Duluth plant. Stora Enso needed to reclaim these solids. MnTAP intern Tim Bauer, a chemical engineering student at the University of Minnesota-Duluth, worked with company staff to review the plant’s processes and identify ways to reduce the TSS released to the sewer.

The intern identified the top contributors of TSS at the plant. The top two were 1) solids in the water squeezed out of the paper in the paper machine press section, and 2) the reject pulp stream from the paper machine cleaners—cleaners used to remove contaminants from the pulp leaving the paper machine. Solids released from the press water alone cost Stora Enso about $66,000 per year for wastewater treatment.

Results
Reclaiming usable raw materials from its wastewater, Stora Enso reduced the amount of solids released to the sewer and saved $730,000 per year in raw material and waste treatment costs.

The $134,000 Solution

With the help of plant employees, the intern looked at installing a system to reclaim fiber, clay and other fillers. Based on his findings the company undertook a $134,000 capital investment project.

The project involved reclaiming wastewater from the press section of the paper machine. The press water was re-routed through an existing gravity strainer to remove unwanted debris. The contaminant-free press water is then returned to the papermaking system. And, the fiber and fillers reclaimed from the wastewater are incorporated back into the papermaking process.

Working with an equipment supplier, the intern found that adjusting pressure differentials and increasing dilution-water pressure in the paper machine cleaners would maximize efficiency.

The $730,000 Savings

Before starting projects with a big capital investment, Stora Enso uses a capital planning process to evaluate them. But, according to Gobin, “Just looking at the savings from the amount of solids recovered proved this project was worthwhile.”

Within eight months, overall TSS levels dropped 45 percent by reclaiming approximately 18,000 pounds of raw materials from its wastewater daily. This saved about $2,000 per day, or $730,000 per year in raw material and waste treatment costs. Looking only at these savings, the project paid for itself in just over two months.

Gobin said, “Having the intern take the time to put together the numbers was a huge help to show management that this would be money well spent.”