Abstract: Many industries produce a waste-water stream high in heavy metals. Disposal of sludge from these wastewater treatment plants has become increasingly difficult and expensive in the US due to passage of the Resource Conservation and Recovery Act's 'land disposal ban' for hazardous wastes. Innovative methods can be found for dealing with such wastes. For example, in performing a mandated clean-up under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), Asarco's California Gulch water-treatment plant in Colorado meets CERCLA clean-up goals while using a waste water treatment sludge as a smelter feedstock, recovering incidental saleable metals, and producing non-hazardous products. In this plant, Asarco treats acidic mine-drainage water having high metal concentrations and uses the waste sludge generated as a lime replacement in lead smelting operations. -Author

Abstract: In 1991, LTV Steel Mining decided to close an open-pit taconite mine in northeastern Minnesota using a passive-treatment approach consisting of limiting infiltration into the stockpiles and wetland treatment to remove metals. More than 50 Mt (55 million st) of sulfide-containing waste had been stockpiled adjacent to the mine during its 30 years of operation. Drainage from the stockpiles contained elevated levels of copper, nickel, cobalt and zinc. Nickel is the major trace metal in the drainages. Before the closure, the annual median concentrations ranged from 1.5 to 50 mg/L. Copper, cobalt and zinc are also present but they are generally less than 5% of the nickel values. Median pH levels range from 5 to 7.5, but most of the stockpile drainages have pH levels greater than 6.5. Based on the chemical composition of each stockpile, a cover material was selected. The higher the potential that a stockpile had to produce acid drainage, the lower the permeability of the capping material required. Covers ranged from overburden soil removed at the mine to a flexible plastic liner. Predictions of the reduction in infiltration ranged from 40% for the native soil to more than 90% for the plastic liner. Five constructed wetlands have been installed since 1992. They have removed 60% to 90% of the nickel in the drainages. Total capital costs for all the infiltration reduction and wetlands exceeded $6.5 million, but maintenance costs are less than 1% of those for an active treatment plant. Because mine-drainage problems can continue for more than 100 years, the lower annual operating costs should pay for the construction of the wetland-treatment systems within seven years.