...from 6 mm to 150 microns in diameter, the preferred size for making concrete. The second set removes grains down to 63 microns.

From there, the remaining silty water goes to a conditioning tank, where it is dosed with a polymer and stirred to uniform consistency before being pumped into a settling tank. There, the silt and polymer settle to the bottom as sludge, while the water rises to the top and is pumped to the plant’s water-filtration system for cleaning, testing, reusing or returning to the river.

The water-treatment system has three identical filter trains that can each process 3,000 gpm. Each train consists of eight 10,000-lb sand filters, 24 bag filters, six 20,000-lb carbon filters and 12 canister filters. While the water goes for filtering, the sludge is pumped to eight of the world’s largest filter presses. Each 620,000-lb giant can squeeze 600 cu ft of wet sludge into 18 tons of dry filter cake in 75 minutes.

“Not only are these membrane filter presses the largest in the world, they operate at up to 225 psi, the highest pressure of any press of this type,” Mangrum says. “That helps squeeze more water out of the sludge to make a drier filter cake. And that means less water goes to the landfill, which reduces landfilling cost.”

The press drops the cake onto a conveyor, which stockpiles it in a storage area until it is loaded into covered trucks and hauled 37 miles to a landfill. About 85 truckloads hauling 1,940 tons of filter cake go to the landfill daily. About 4% of the total, containing PCB levels of 50 ppm and more, are trucked to a dedicated landfill in Michigan.

The system is run by a sophisticated programmable-logic controller monitored around the clock by plant operator Boskalis Dolman, the Netherlands. Testing of plant samples and river sediment is done by an independent third party. The results are reviewed and approved by both DNR and EPA.

Mangrum says the company has built up a sophisticated community-relations program to ensure transparency and react to the public’s concerns. That program includes keeping the public informed through a Website, local media and public meetings, stopping work on weekends so as not to interfere with fishing and boating on the river and hiring local contractors and suppliers for about $200 million worth of work so far.

The project may influence future cleanups. McGee says he knows of nine more PCB remediation projects in various stages of development in the U.S. that could benefit from the dredging systems developed for the Fox River job. The precision contour dredging used on the Fox River will also be useful for cleaning up a variety of other chemicals that require dredge spoils to be managed as a regulated waste, he notes. One example is in ports and harbors that need to be dredged or expanded but have contaminants in their sediment.

“This precision contour dredging and sediment-processing system will be particularly beneficial on any project where landfilling costs are high,” Mangrum adds. “The savings should more than make up for the initial investment.”