Is Composting for Methane Gas a Good Way to Conserve Energy?

Hey Mr. Green,

There are composting units for organic waste that produce methane gas, making it possible to generate methane for cooking. However, when I mentioned this to an oil man, he said it would take tons of compostable waste to create enough gas. I did not quite believe him, but then I don’t know how much waste is needed to furnish cooking gas for the average home. What do you know about these composting units? 

—Thomas in Phoenix

A lot depends on what kind of waste and how much you generate, which makes your question a bit difficult. If your family tosses out about a pound of food per person per day, which is close to our atrociously absurd national average, you might at least be able to generate enough methane to do all your cooking.

But honestly, home-grown methane seems like a roundabout way to conserve energy. You’d save a lot more simply by reducing your food waste. It is estimated that the energy consumed in growing, processing, and transporting food to our tables accounts for about 15.7 percent of total U.S. energy consumption. So the energy you’d save by regenerating discarded food is only a fraction of the total energy needed to produce that food and transport it to your table. 

However, if you are a truly minimal waster and still want to proceed with a composting project, here are some details: For each pound of composted waste, it’s possible to harvest anywhere from three to 18 cubic feet of methane—an obviously large variation dictated by what you add to your compost bin and how well you manage that complex biological mix. At the low end, a four-person family could generate about 4.4 million BTUs per year, enough to operate the kitchen stove. At the high end, you’d have 25.8 million BTUs, about enough to run a water heater for a year.

Another challenge is that you’d have to obtain a methane digester to capture all that gas. You could build one yourself if you’re fairly handy, or you could purchase a home digester for around $600 or $700. At the maximum methane production rate, it might pay for itself in two or three years, but at the lowest rate the payback would be well over 10 years.