People don't usually see the ash left over from the electricity that's burned when they turn on their lights or run their air conditioners.

But at coal power plants, fly ash builds up every day, laced with heavy metals and toxins—one of the most difficult waste-management issues in the developed world.

In the United States, where a catastrophic 2008 coal ash spill sullied land, rivers, and homes over 300 acres (121 hectares) of Tennessee, government and industry are locked in a dispute over future handling of the nettlesome by-product of fossil-fueled electricity.

The good news: Waste from coal power plants doesn't have to be a waste.

It can be recycled into a wide variety of materials, from concrete to fertilizer. Fly ash, the fine, powdery silica material that is part of the coal ash waste stream, in fact, has an array of physical and chemical properties that have led to inventive ideas for new applications. Entrepreneurs are looking at ideas for using it to build lighter armored vehicles or to clean up oil spills.

But policymakers around the world face a difficult challenge: How to encourage safe reuse of coal ash, while discouraging unsafe uses and protecting people and ecosystems from the risks that have escalated as coal ash waste piles and landfills have grown.

A Wake-Up Call

Fly ash and the other residuals of burned coal add up to one of the largest waste streams in the United States: More than 136 million tons per year. In Europe, coal waste totals 100 million tons per year by some estimates. Similar figures aren't available for China, but since it is now burning more coal than the United States, the waste generation is significant. Scientists at the China Building Materials Academy and the Institute of Technical Information for Building Materials Industry calculate that their country has accumulated 2.5 billion tons of coal ash.

For years, critics and communities have warned that the typical practices for disposal of coal ash—in landfills—posed a hazard to groundwater and to people who live nearby. The problem spilled into public view on December 22, 2008, when an earthen dam holding coal ash collapsed at Tennessee Valley Authority's Kingston Fossil Fuel plant, west of Knoxville.

Some 5.4 million cubic yards (4 million cubic meters) of wet coal ash surged from the facility, polluting the Clinch and Emory rivers and surrounding landscape, destroying three homes and damaging dozens of others. It was called the worst U.S. industrial accident of its type, and forced the U.S. Environmental Protection Agency (EPA) to undertake a major reevaluation of coal ash regulation.

Last summer, the EPA proposed two possible new approaches: Either write new federal standards that leave enforcement to the states, or a impose a stricter EPA-enforced rule that would force electric utilities and other industries to treat coal ash as a hazardous waste for the first time. The matter has been stymied since then, with the agency facing a ferocious backlash from an industry that argues the "hazardous" designation will prevent the very recycling practices that would help curb the coal ash problem.

"Our premise is the best way to solve disposal problems is to quit throwing the stuff away," said John Ward, spokesman for the American Coal Ash Association, a group of energy and recycling companies.

But that's exactly what's been happening in the United States and elsewhere. About 60 percent of U.S. coal ash winds up in landfills; the rate is about 50 percent in Europe. In China, the recycling rate is about 30 percent, despite a 2005 government challenge to increase recycling to 75 percent.

But here the issue becomes even more complicated, because many environmental groups condemn fly ash recycling uses that the industry views as beneficial. Using coal ash as a road base, in embankments, as fertilizer or as a filler for abandoned mines--which together claimed about 7 million tons of the ash in the United States in 2009 alone—are all practices that environmentalists say spread the risk to communities and the environment. The coal ash contains arsenic, mercury, lead, and other heavy metals. And even before the Kingston spill, in 2007, the EPA had tracked at least 70 cases where coal ash had caused fish kills, or tainted drinking water and land.

Ward argues that with proper liners to separate the fly ash from contact with water, the material is safe.

Locking Ash in Concrete

There is one use that is more widely viewed as a safe fate for the waste: Fly ash's chemical properties make it a near-perfect substitute for Portland cement in concrete, and concrete makers have been using it for decades.

Technical barriers, rather than expense, are holding back greater use of coal ash in concrete, said Lura Schmoyer, of West Main Consultants, a group that is administering the Cool Climate Concrete program. The initiative aims to convince concrete makers to use fly ash and other waste materials, like rice hull ash, in their concrete in order to cut the massive carbon emissions that result from cement production.

While fly ash is free for the taking—you just need to cart it away—it's not so easy to obtain and use. "A concrete company who has never incorporated any recycled materials into their product would need to develop designs that would incorporate those materials," said Schmoyer. And "believe it or not, there are still lots of concrete companies out there that do not incorporate recycled product into their mixes," she said. Infrastructure is also an issue; fly ash needs to be stored separately from other materials, and a concrete maker needs a reliable, steady supply and a way to transport the ash from the power plant to the concrete factory.

"In an ideal world," Schmoyer said, "all concrete companies would max out the amount of recycled materials substituted for Portland cement in their mix." Every ton of Portland cement made—by heating limestone and clay to thousands of degrees—creates four-fifths of a ton of carbon dioxide. And with 62 million tons of cement produced in 2010 in the United States, reducing the industry's reliance on Portland cement would have significant environmental benefits.

Unfortunately, Schmoyer said, with the slump in the housing market, the demand for concrete has decreased enough that even in an ideal world, the United States would still have surplus fly ash. Perhaps another housing boom would safely lock the ash up inside concrete, but until then, what's the best way to dispose of ash?

Lighter Ships, A Mop for Spills

That's where science is getting creative. Fly ash has a number of qualities that make it perfect for some seemingly bizarre applications.

In addition to being a useful additive to concrete and a perfect replacement for clay in bricks (Wisconsin-based CalStar has shipped several million bricks made of 40 percent fly ash), the hollow particles are strong enough to make a lightweight additive to many metals.

Nikhil Gupta at Polytechnic Institute of New York University and Pradeep Rohatgi at the University of Wisconsin-Milwaukee have developed and stress-tested metals made with up to 50 percent fly ash instead of aluminum. Metal/fly ash composites made with a little less fly ash, 20-30 percent by weight, created a compound that was just as strong as the original but much lighter. Gupta has been funded by the U.S. Office of Naval Research, among other government agencies. One use being explored, he says, is whether these materials can be used to make lighter armored vehicles or ships.

Another unusual use: coal ash particles have a chemical structure that can easily be manipulated to absorb oil. Sudipta Seal of the University of Central Florida has created "functionalized" fly ash particles he calls OOPS, or Oil Optimized Particle Surfaces. His work was funded by the U.S. National Science Foundation shortly after last year's Deepwater Horizon oil spill in the Gulf of Mexico.

The technology hasn't moved out of the lab yet, but it's promising. The particles absorb oil, but repel water, so they could be placed in booms on the surface of an oil spill and collected later. The particles are superior to current oil collection technology, Seal says, because the raw material is readily available and because the oil-saturated ash could then be fed back into a power plant to use the energy in the spilled oil. Seal concedes, however, that specialized equipment may be needed for a coal plant to burn the oil-laden material. The OOPS process has been patented as well as proven to prevent leaching of toxins from the fly ash, Seal said.

Handling the Hazards

Although these ideas are clearly early stage, industry groups argue they will not get off the ground at all if the U.S. EPA regulates coal ash as a hazardous waste. The agency, anticipating these concerns and has specified that it would exempt fly ash recycling from the rules. That has not satisfied the industry critics.

"Just because EPA says they exempt recycling from their disposal regulations, they can't exempt us from the market perception that we're dealing with a hazardous waste," says Ward.

Tom Pounds, chief executive of CalStar, the fly ash brick company, agrees: "It's not difficult to imagine that people who didn't want to see the broader adoption of fly ash could create fear in the market."

But Scott Slesinger, legislative director for the Natural Resources Defense Council, says that due to simple economics, the industry's arguments don't make sense. The 30-year history of hazardous waste regulation by the EPA has shown that the rules prove to be an incentive, not a disincentive, to recycling and reuse. "If something is a hazardous waste, it has to go to a hazardous waste landfill. That is more expensive than what [the waste producers] had done before," so recycling becomes more attractive. "People say, 'Gee, I don't want to pay for a hazardous waste landfill.' The amount of recycling goes up."

As for stigma on the consumer level, the EPA's proposed regulations note that consumers are "generally comfortable" with other products considered hazardous wastes, like used motor oil, certain drain cleaners, and cathode ray tubes. Slesinger says that in the EPA's coal ash proposal, the agency asked for public comment on examples of beneficial uses declining after something was regulated as a hazardous waste. "Nobody has come forward," says Slesinger.

But the proposal remains deeply controversial, with Republican critics on Capitol Hill threatening to take away the EPA's authority to regulate coal ash through the budget process. Last month, the EPA took steps to open a new comment period on its proposal, something observers say could delay the new coal ash rules until after the 2012 election.

Meanwhile, the coal ash problem continues to build. This summer, the inspector general of the Tennessee Valley Authority—the power generator most at the center of attention on coal ash hazards—looked at the scope of the problem. The inspector general found groundwater contaminants leaching out of coal ash dumps at eight of the nine plants being monitored.