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Biogas from Sewer Gas

written by: Peter Boysen•edited by: Lamar Stonecypher•updated: 10/14/2010

Learn more about how utilities may be using your sewage in the future. Energy from sewage and sludge could power your home -- and help drive your bill from the power station down.

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    Sewage to Electricity

    Paging King Midas ... Well, maybe. Getting energy from sewage and sludge has a lot in common with King Midas' ability to turn everything he touched into gold. And if you think about the ways that traditional electricity comes into our homes, there's a lot to like about sewage treatment energy.

    Too much of our country's electricity is tied into the burning of coal. While we're all focused on driving down our dependency on foreign oil, domestic coal is a source that won't tie us down to the fortunes of the Middle East, but it will damage the environment.

    So how does it work? Let's look at the example of a plant in Renton, Washington, a suburb of Seattle. Each day, about 700,000 residents flush an average of 86 million gallons of kitchen and bathroom waste down the drain, right to this plant. About a third of this waste goes into a system that extracts the methane gas and uses it to power a fuel-cell power plant that generates a full megawatt. The energy supplements the power plant's own needs, powering about 14% of the plant's electrical needs.

    The waste goes into digesters, which are giant tanks that hold it for up to a month. Bacteria feast on the filth, getting rid of the solid matter and releasing methane as a by-product. This isn't that different from the way that most treatment plants work, but what the Renton plant does is the revolutionary part.

    Instead of burning the methane, the Renton plant cycles it through a fuel-cell system, and the methane breaks down into carbon dioxide and hydrogen. The system turns the carbon dioxide into carbonate and then combines it with hydrogen to make water, carbon dioxide- and heat and electricity.

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    So Why Isn't this Everywhere?

    The Renton plant that draws energy from sewage and sludge cost about $22 million to build. That's way too much for the average sewage treatment plant. However, the lessons that the planners draw from this plant will go into mass production of cheaper plants that accomplish the same goal.

    You may also be wondering why we're not seeing more fuel cells in cars now that the fuel cell technology is available for power plants -- to date, the Honda Clarity is the only fuel cell vehicle on the road, and it's only in a few communities in Southern California. The answer is that these fuel cells work differently than the ones that power cars- these fuel cells get really hot, up to 1,200 degrees Fahrenheit. That would put a lot more burden on your air conditioner than what you would want- even if you live in Texas!

    Even as these plants become cheaper, they won't fully replace traditional power stations completely. They can't handle sudden spikes in power need- and sudden surges in sewage don't mean a higher capacity to turn that sludge into power. Still, this is definitely a trend in the right direction.