World’s Largest PCB Cleanup Takes Fresh Approach

Precision Dredging and Processing Saves Millions on Wisconsin’s Fox River

By Mike Larson
This article originally appeared on midwest.construction.com

Contractor uses precision dredging and sophisticated processing plant for nine-year, $600-million remediation in major northeastern Wisconsin river.

Tetra Tech EC Inc. is using a one-of-a-kind processing system to dredge, separate, and dewater contaminated silt in order to clean up 13.3 miles of the lower Fox River near Green Bay in northeastern Wisconsin.

World’s Largest Cleanup of its Type This massive cleanup will dredge and process 3.8 million cubic yards of river sediment contaminated with polychlorinated biphenyls (PCBs) that can cause severe health problems for wildlife and for humans who eat fish from PCB-contaminated spots.

To fix 600 acres of contaminated riverbed that can’t be dredged effectively, the Tetra Tech team will seal PCB-containing silt in place by capping it with combinations of sand, gravel, and rock ranging from 6 in. to 33 in. thick.

The dredged sediment slurry is pumped directly from the dredges to a one-of-a-kind computer-controlled processing plant that removes debris and sand before squeezing the water out of the contaminated silt to make a dry “filter cake” that is landfilled.

The sand that’s removed during processing is washed and tested to be sure it meets cleanliness standards, then reused as fill or sold for other beneficial reuse, such as making concrete. The water removed from the slurry during processing is highly filtered, then used in the processing plant or returned to the river.

Tetra Tech EC, Morris Plains, N.J., is the environmental division of Tetra Tech Inc., Pasadena, Calif.

Tetra Tech EC Vice President of Remediation Ray Mangrum manages the project. Says Mangrum, “The PCBs adhere to the organic silt in the river, so that’s what you need to landfill. There’s no use in excavating and processing more silt than necessary, and there’s no sense in incurring the expense to landfill any more sediment than you need to. We are dredging only the amount of sediment necessary, removing the sand for reuse, then drying out the PCB-laden silt and depositing it into a landfill.”

Tetra Tech’s plan to accurately map the pockets of contamination, precision-dredge only the amount of sediment that needs removing, cap and cover some areas, and landfill only the dried PCB-laden silt, has resulted in a total cost of about $160 per cu yd, including the landfilling.

Tetra Tech says its approach is costing $50 million to $100 million less than other cleanup methods would have.

The cleanup will take nine years to complete, with crews dredging 24 hours a day, five days a week, from April to November, depending on the length of Wisconsin’s winter. Covering and capping will run 10 hours a day, five days a week.

“This is the largest cleanup of its kind in the world,” says Bruce Baker, a Wisconsin Dept. of Natural Resources (DNR) administrator who oversees the project for the state. “For a U.S. river with contaminated sediment, nothing before has even come close in volume moved and processed. This is the first one on this scale.”

EPA and DNR Mandate Clean-up The U.S. Environmental Protection Agency (EPA) and the Wisconsin Dept. of Natural Resources (DNR) mandated the cleanup after identifying eight companies that had flushed 700,000 pounds of PCBs into the river from the 1950s to the 1970s , mostly while making and recycling carbonless paper. That was before the health hazards of PCBs were known. Three of the companies – Appleton Papers Inc., Appleton, Wis.; Georgia-Pacific, Atlanta, Ga., and NCR, Dayton, Ohio – have formed the Fox River Cleanup Group and taken the lead in funding the cleanup, expected to cost $600 million or more.

Fast-Track Startup In March, 2008, the Fox River Cleanup Group named Tetra Tech EC prime contractor for the cleanup. The contract required full-scale sediment processing to begin by the EPA and DNR’s deadline of May 1, 2009. That meant Tetra Tech had to select its team and then design, build, equip, and commission the processing plant in about a year.

Tetra Tech’s key subcontractors on the project are Boskalis Dolman bv and Stuyvesant Dredging (both subsidiaries of Royal Boskalis Westminster from the Netherlands), and J.F. Brennan Co. Inc., La Crosse, Wis. Boskalis and Stuyvesant designed and are operating the sediment-processing system. Brennan is dredging and will construct the sediment-isolating caps.

Normally, it would take 18 months to build a processing plant like the sophisticated, all-enclosed, 247,800-sq-ft facility Tetra Tech is using here. But this one was done in just nine months, thanks to design-build techniques and precise on-site coordination between more than a dozen companies involved in building the plant.

Miron Construction Co. Inc., Neenah, Wis., was general contractor for plant construction.

Eric Schmidt, of electrical subcontractor Van Ert Electric Co. Inc., Kaukauna, Wis., reports, “At times, erectors were placing new steel next to electricians who were mounting motors to beams that had been placed only 10 minutes before.”

Despite having some 300 workers operating in such close proximity, there were no lost-time accidents throughout nine months and 360,000 man-hours of construction.

Ambitious Solution Tetra Tech’s ambitious cleanup plan aims to not only reduce PCB levels to the EPA and DNR’s requirements, but also to maintain an absolutely clean operation, send no more material than necessary to landfills, maintain stringent quality testing and records, meet the budget, and maintain excellent community relations.

Precision Mapping, Dredging, and Testing The Fox River cleanup is making extensive use of computerized mapping to define areas that need remediation, guide precision contour dredging, and verify the results.

Before dredging started, testers took more than 2,000 core samples in the river to identify the location, size, and depth of PCB contamination pockets. The areas were mapped using GPS and a grid system. A computer turned that data into color-coded maps showing where to dredge and the depth of the contaminated sediment, which ranges from 6 in. to 10 ft.

As dredging continues, the information is updated daily, and color-coded screen displays guide dredge operators in cutting specific contours to remove exactly the right amount of sediment.

Greg Smith, one of dredging contractor J.F. Brennan’s project managers, says that on-board GPS systems compensate for barge movements, so the dredge operators can be accurate to within centimeters.

Clean Process from Beginning to End Tetra Tech’s sediment removal and treating process takes unusual care to control the sediment from dredging to landfill.

The process starts with three hydraulic suction dredges that excavate contaminated sediment from the river bed. A 12-in. dredge works areas that need high production and two 8-in. dredges work smaller areas and finish-dredge behind the 12-in. machine.

Continuous runs of HDPE pipe carry the dredged slurry directly from the dredges to the processing facility up to 10 miles away. Barge-mounted booster pumps every mile keep the flow going at up to 6,000 gpm, or about 150 cu yd of sediment per hour.

The pipelines from the dredges connect directly into the processing plant without a buffer pond—an unusual method that requires precise coordination between the dredges and processing plant to keep the slurry flowing smoothly.

One-of-a Kind Sophisticated Processing System The incoming slurry first runs through a scalping screen that takes out rocks, wood chips, bottles, and other debris larger than 6 mm (about ¼ in.) in diameter.

The removed debris is sent to a landfill, and the remaining sand, water, and silt move on to separator cyclones that remove the sand from the remaining slurry. The first set of separators removes larger grains that measure from 6 mm down to 150 microns in diameter. The second set of separators removes smaller grains with diameters from 150 microns 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, and reuse or return 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 cannister filters.

While the water goes for filtering, the silty sludge is pumped to eight of the world’s largest filter presses. Each of these 620,000-lb giants can squeeze 600 cubic feet of wet sludge into 18 tons of dry “filter cake” in 75 minutes.

At the end of its cycle, the press drops the filter cake onto a conveyor that stockpiles it in an inside storage area until it is loaded into covered trucks and hauled 37 miles to a landfill. About 85 truckloads (1,940 tons) of filter cake go to the landfill daily. High-pressure spray washes off each loaded truck before it leaves the building.

The entire sediment processing system is run by a sophisticated programmable logic control system monitored around the clock by Boskalis experts.

All water, sand, and filter cake are tested before leaving the plant, to be sure they meet the EPA and DNR’s specified standards. Testing of the in-plant samples and in-river sediment is done by an independent third party. The results are reviewed and approved by both the DNR and EPA.

Good Citizenship Part of Plan Mangrum says that being good citizens is also part of Tetra Tech’s plan. That includes keeping the public informed through a Web site, 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.