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One reader has a novel idea—turn trash and other waste into energy, thus reducing dependence on coal, natural gas and even green sources like solar and wind power. But how much of a dent can waste-to-energy make in our seemingly insatiable demand for energy?

A Conservancy energy expert has the answer. Read it below, and don’t forget to send us your questions on any conservation subject for one of the Conservancy's 550 staff scientists. (Note: We regret that we can only answer one or two questions each month and that we cannot answer the others offline.)

Cindy Haigh, of Auburn, CA, writes:

Joe Fargione, lead scientist with the Conservancy’s North America Region, replies:

Waste-to-energy is a good idea, with a major advantage being that it avoids the land use impacts associated with most other form of energy production.

The United States already converts a significant amount of waste to energy. For example, of the municipal waste that is not recycled or composted, about 18% is combusted for energy production.

The main issue is how the supply of waste matches up with our energy demand. The U.S. used about 1 billion tons of coal in 2009. But waste products contain less energy than coal. For example, wood has less than half the energy of coal, per ton. And manure, which is not combusted directly but can be used to create natural gas, has far less usable energy per ton.

So we would need a lot of waste. Even if you took all of the municipal solid waste that isn’t being recycled—roughly 116 million tons per year—it would only replace about 8% of coal demand.

But, as you astutely point out, municipal waste is not the only potential source of waste. A 2005 report from Oak Ridge National Lab estimates that we could get:

• 40 million tons annually from manure;
• 60 million tons of woody biomass annually from forest thinning for fuel load reduction; and
• 36 million tons of woody biomass annually from a combination of wastes from construction, demolition, packaging, tree thinning, and unused forest products residue. (Note that tree thinning is required for fire risk reduction and ecological restoration in millions of acres where humans have suppressed natural fires, so this is a win-win for energy production and nature restoration.)

If you add all of this up, it would take a meaningful bite out of our increasing demand for energy, on the order of 10-15% of our current coal usage.

There is no silver bullet for solving the problem of producing renewable energy, but waste-to-energy can be an important part of the solution. Waste from energy is not only renewable, it avoids putting the waste into landfills that produce methane gas, a greenhouse gas 23 times more potent than carbon dioxide. Therefore, waste-to-energy provides significant greenhouse gas emission reduction benefits.

Using more waste for energy is not without challenges. There are some air quality issues. However, the EPA has stringent rules for mercury and other toxic air emissions from waste-to-energy plants, rules which are not yet in place for coal plants, so waste energy compares very favorably to coal energy in this regard.

Additionally, after combustion, approximately 10% of the waste remains as ash that must be disposed of in landfills, representing a significant reduction, but not elimination, of the use of landfills.

And because these wastes have lower energy density than traditional forms of fossil fuel, it is not practical to truck them long distances. Fortunately, cities generate waste in a relatively small area, making it practical to collect enough waste in one place to fuel a power plant. Also, some of these waste streams are seasonally variable, while power plants require a constant supply year-round. Finally, local groups often oppose incinerators in and around cities, even though this continues our reliance on dirtier non-renewable energy sources that are produced outside cities.

With increased knowledge and political will, communities are increasingly overcoming these challenges. Check out the Database of State Incentives for Renewable Energy to see if your state has a Renewable Portfolio Standard that includes municipal solid waste.