Sludge and Gold Tailings: Reconciling a Wasted Relationship

They were the unwanted, shunned by the society that made them. Regarded as the most despicable of wastes, they languished in huge piles, miles apart from one another. Then one fateful day they merged to form one, each complimenting the other as they became a strong, productive unit.

That romantic-sounding scenario was staged in Fairbanks this fall, as soil scientists and mining engineers played matchmaker with two unlikely bedfellows---sewage sludge and gold mine tailings. Judging from their results, this could be the beginning of a beautiful friendship.

The coupling was inspired by state and federal laws requiring miners to restore natural vegetation to areas they've disturbed. Tailings piles, especially those leached with sodium cyanide to extract gold as at the Ryan Lode Mine outside Fairbanks, are about as fertile as cement. The piles contain almost no clay or organic matter (any carbon-containing thing that was once living, like decaying leaves and twigs or dead insects) that create nutrients and hold moisture for plant life. Sewage sludge, known also as "biosolids," is very high in organic matter, and very available.

At Fairbanks' sewage treatment plant, waste water from such sources as toilets, showers, sinks and the occasional draining of a city swimming pool enters via a huge pipe at the rate of about 7,000 gallons a minute at midday. Incoming sewage is screened and aerated with oxygen to stimulate waste-eating bacteria. Heavier solids settle to the bottom of large tanks, and, after as much water as possible is squeezed out on a conveyor belt, sludge happens. Every day the Fairbanks plant generates 25 cubic yards, about two and a half dump truck loads, of the black, clumpy, pungent substance. In the past, the sludge had been piled outside the plant. Now it's hauled off and burned.

In an effort to make use of the sludge, Charlie Knight, a University of Alaska associate professor of agronomy, and graduate student Trish Willebrand teamed up with the mine in September 1993, mixing different concentrations of sludge into the tailings piles with a rotary harrow, a piece of farm equipment that works like a rototiller. On top of the mixture they sprinkled four varieties of native Alaska grass seed and irrigated the plots.

Willebrand checked the test plots once a week and found that grass grew best on two plots where they mixed sludge in at 15 and 50 tons per acre. Nobody mistook the plots for a putting green; the best plots sported scruffy tufts of grass that left them only half covered. But the researchers were encouraged when they compared the plots to the control, tailings-only plots, where grass sprouted and thrived briefly before turning brown and dying, probably due to the lack of moisture and nutrients.

Knight thinks sludge works better than topsoil, which is commonly used in reclamation, because sludge releases plant-feeding nutrients for much longer than does topsoil, which would have to be fertilized often.

It sounds perfect. Take a waste product that's now polluting the air as it's burned, mix it with tailings, and in a few years the land is reclaimed by nature. But sludge has one problem. It stinks.

Knight said complaints arose in the past because sludge was applied to Interior farmlands at ridiculous rates, such as 200 tons to the acre, which amounted to a foot-thick layer dumped on top of the soil. He said the Department of Environmental Conservation now sets the amount of sludge that may be applied to land according to how much nitrogen released by the sludge is available to plants growing atop it. Knight and his cohorts have found that on agricultural lands, sludge releases so much nitrogen that an application of only two and a half tons per acre works just fine for local crops.

As for the marriage of sludge and tailings piles, Willebrand, who earned a master's degree for her work, thinks they were made for each other.

"I really think this is fantastic," she said. "It's two wrongs make a right, basically."