Mud Cat™

Sydney Nova Scotia Tar Ponds Move Closer to Cleanup

The Sydney Tar Ponds are one of the largest and most hazardous chemical waste sites in eastern Canada. More than 80 years of discharges from the coke ovens of an adjacent steel plant have filled the once pristine Muggah Creek with over 500,000 m3 of contaminated sediments. By 1983, Environment Canada had pinpointed the coke ovens as the major source for pollution in the Sydney area. Subsequent studies found that the air downwind of the coke ovens had unacceptable levels of polynuclear aromatic hydrocarbon (PAH) contamination as well. In the early 1980’s a series of federal and provincial studies revealed the presence of carcinogens, especially PAHs and heavy metals, in the Sydney waterways. Scientists from the Federal Department of Fisheries and Oceans also discovered levels of PAHs in lobsters high enough to warrant indefinite closure of the Sydney lobster fishery.

In 1984, the Nova Scotia Department of Environment and Environment Canada awarded a contract to Acres International Ltd., an environmental consulting firm, to develop a plan for the long-term control and cleanup of the Tar Ponds. Acres identified three alternatives for the cleanup; encapsulation of the wastes onsite; excavation of wastes and disposal at a new site designed to contain hazardous wastes; or excavation via dredging, coupled with incineration of the wastes to generate usable electricity from steam. Acres recommended excavation by dredging and incineration because incineration would destroy 99.99 percent of the PAHs and because it was the lowest cost alternative especially with the economic benefit from sale of electrical power to be generated. Both Nova Scotia and the Federal Government agreed that this approach was preferable from both the environmental and financial perspectives.

In 1986, the two governments signed an agreement to fund a ten-year remediation project managed by Acres. The Tar Ponds cleanup was a massive undertaking. Acres had to supervise selection of subcontractors to develop new technologies both for the incineration plant and for the dredge for excavation. The Acres concept called for dredging without addition of water to the Tar Ponds and then pumping the contaminated sediments through flexible pipelines to an onshore transfer station. At the transfer station, the sludge will undergo preliminary conditioning to prepare for incineration. The incinerator, completed by Superburn Contractors JV, is capable of generating 10MW of electricity via steam without any release of PAHs to the atmosphere. The ash will be reburied onsite as part of the restoration program.

The dredging requirements are among the most challenging for any environmental project to date. Not only is the material toxic to humans, but exposure to jest its fumes can be harmful. The material itself constitutes significant problems as the in situ moisture content varies from 85 percent to as low as 37 percent of dry weight. This material is actually capable of supporting an adult’s weight. Dredging is complicated by the need to minimize the introduction of additional water to the system. While extra water would be helpful to dredge excavation, it would increase incineration costs dramatically.

Acres issued a worldwide tender to dredge suppliers and selected Mud Cat™ (a division of Mud Cat International in Baltimore, Maryland) based on an innovative dredge design proposal. Mud Cat™ designed and delivered a special dredge which floats on top of a shallow water layer and moves across the pond by winching wires anchored on the sides of the pond. Mud Cat™ developed two special technologies for this application; the first uses a vibrating auger cutter head to handle the thixotropic material. Secondly, the dredge incorporates a positive displacement pump to deal with the high solids content required. This high technology dredge is the first of its kind in the world. Mud Cat™ built and assembled the dredge in a local shipyard in Sydney, Nova Scotia. Environment Canada intends to use this project as a national showpiece of what is possible in environmental remediation.

Mud Cat International is the oldest and largest dredge manufacturer in North America. Mud Cat manufactures portable dredges from 6-inch to 36-inch size which can be outfitted with horizontal auger excavators, plain edge or pin-tooth basket, cutterheads or dual bucketwheel excavators.