Biogas: When What You Flush Comes Back to Warm You

With the heightened interest in biofuels for clean energy, bioethanol tends to take a lot of the spotlight, though another biofuel – biogas – is making strong inroads to sustainability as well, with installations all over the world. Biogas may not be as sexy: it’s what’s produced when organic matter breaks down – rots and ferments, basically – in the absence of oxygen (the fancy word is “anaerobic”). When bacteria “consume” biodegradable materials such as biomass (things like crop wastes, grass and sawdust), kitchen scraps, manure or sewage, it results in biogas, primarily in the form of methane and carbon dioxide. Once created, this biogas becomes biofuel that can be used to run things: for starters, any type of heat engine, or generators to create either mechanical or electrical power.


Methane gas is of particular interest to the biofuel industry. It’s a powerful greenhouse gas (many times more damaging than carbon dioxide), and it’s a natural byproduct of the breakdown of human, animal and plant waste. The average municipal sewage treatment plant generates copious amounts of methane. In conventional treatment plants, the methane is usually burned off, considered a bothersome (not to mention stinky) byproduct. With the interest in renewable fuels, more municipalities are starting to wonder why they shouldn’t capture and harness the biogas from sewage treatment. Many plants have begun installing equipment to capture the gas and use it for energy, which can yield two-fold benefits: it’s a renewable source of fuel, and it reduces the amount of greenhouse gas emissions from the treatment plant.

What’s not to like?

A new sewage works plant with full capture and reuse of biogas opened for business this week in Oxfordshire, England. At the opening ceremony, UK Energy and Climate Change Secretary Chris Huhne proclaimed, “It’s not every day that a Secretary of State can announce that, for the first time ever in the UK, people can cook and heat their homes with gas generated from sewage. This is an historic day for the companies involved, for energy from waste technologies, and for progress to increase the amount of renewable energy in the UK.” (In hindsight, it’s never a good idea to use the words “cook” and “sewage” in the same sentence.)

Here’s how it works. One of the 14 million customers of Thames Water, the treatment plant, flushes the toilet. The organic waste matter that was flushed travels to the sewage works, where it begins its “treatment.” (Put your lunch down for a moment if you’re easily grossed out.) The solid part of what the customer flushed (it’s officially called “sludge”) is heated up in huge vats to make an inviting banquet for the friendly bacteria that feast on, and subsequently break down, the sludge in a process known as anaerobic digestion. During the process, those friendly bacteria excrete biogas in the form of mostly methane and carbon dioxide. The gases are purified a little, and then fed into the gas grid. It’s about a three-week journey from flush to finished product, at which time it will provide renewable gas to power nearby homes.

The UK government has offered up some calculations to determine the potential of treating human waste for the purpose of producing biogas. Assuming that the average adult produces about 66 pounds of “sludge” every year (who had the job of measuring that?), if all 9,600 waste treatment facilities in the UK also equipped themselves to process the waste, the resulting biofuel could power more than 200,000 homes. Properly pursued, as much as 15 percent of domestic gas needs in the UK could be met by biomethane by 2020.

So is it expensive to make the conversion? The Thames Water Works program (actually retrofitting the plant to capture biogas) took about six months to complete and cost just under $4 million. Considering the number of problems the revamped facility may potential solve, it doesn’t seem that high a price tag.

The UK project is hardly the first or only biogas experiment, however. In Texas, the San Antonio Water System is now the proud owner of a new biogas system that was installed in partnership with Ameresco at the Dos Rios Water Recycling Center. Rather than using it to power local homes, the biogas that is captured from the processed waste is actually resold commercially. It’s carried through a nearby gas pipeline and earns the municipality about $200,000 a year, which helps offset the costs of the retrofitted treatment facility. But it’s not only biofuel that the San Antionio facility produces: it also creates biosolids (use your imagination) which can be sold to agricultural operations as fertilizer. In addition, the resulting water from the removal and reuse of waste is clean enough to use for irrigation for San Antonio’s Riverwalk tourist area, nearby recreation facilities, parks and commercial developments.

This is all pretty cool, but it gets even cooler when paired with another promising green technology. At the South Treatment Plant in Renton, Washington, the world’s largest fuel cell demonstration project is currently in operation in conjunction with biogas production. Sewage is processed (back to those hot vats again) to capture about 154,000 cubic feet of biogas each day, which is then consumed by the fuel cell to produce about one megawatt of electricity. All in all, the experiment is currently producing enough electricity to fully power about 1,000 homes (though the energy is currently being used to help run the plant itself). The electric output of the fuel cell is expected to save the project’s operator, the Wastewater Treatment Division of King County’s Department of Natural Resources and Parks, about $400,000 a year that would otherwise be used to buy electricity off the grid. The plant is currently looking at plans to recover and reuse waste heat.

But it’s not only human waste that has potential for biogas energy. There are (you may have noticed while driving through Texas, the Midwest or New England) rather a lot of cows in the United States. Anyone with even a little eco-awareness understands the high environmental costs of raising livestock: medications and chemicals used in raising animals can wind up in the water system, enormous amounts of manure seeping into groundwater can present a threat to human health (not to mention noses), and the release of gas from the animals (cow burps, primarily) actually add significantly to cumulative greenhouse gas emissions.

Since animal manure can, similar to human waste, also be converted to biogas, many scientists have looked at the scope and

An anaerobic digester in Germany, with the fuel sources grazing nearby

potential of a project that would process manure into biofuel. By some estimates, if manure from all U.S. dairy cows and beef cattle were converted into methane via anaerobic digestion, our bovine buddies could potentially produce one hundred billion kilowatt hours of electricity each year, enough to power millions of homes all across the U.S. Apparently, a single cow can produce enough manure in a single day to generate three kilowatt hours of electricity: this is more energy than is needed to power a 100-watt light bulb for a day.

Many U.S. farmers are already there. The State of Vermont has a program in place called the Cow Power program. The program is administered by the Central Vermont Public Service Corporation, and customers can elect to pay a little more for their energy. What do they get in return? Electricity that is, in part, created from processed cow manure. The premium paid by customers on their electric bill is passed directly to the farms that participate in the program.

Bioethanol is still sexier (and undoubtedly smells better), but biogas can be produced on a smaller scale – a farm with livestock can actually generate its own renewable power. And while plant material for bioethanol is often grown at the expense of food crops, which could have a negative impact on human health, the source of biogas is generally something you’re pretty keen to get rid of in the first place. What’s not to like, indeed?

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