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Paying Waste: Sewage Contains More Usable Energy Than Scientists Thought

A new study finds a previous estimate of wastewater's potential as a renewable energy source "a substantial underestimation"



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Is what you flush down the toilet wasted energy? People living in countries with flush-toilets and running water produce a huge amount of wastewater daily. This water, thanks largely to excrement, is full of organic compounds that store usable energy in their chemical bonds. Several methods can be employed to harvest it—for example, engineers can extract methane through anaerobic (oxygen-free) digestion, or produce electricity using microbial fuel cells.

In the past several years an increasing amount of research has focused on developing and improving on these methods, as harnessed sewage power could help water treatment plants produce enough power to meet all their own consumption—and even serve as a fuel source in developing countries where supplies are currently unreliable. But just how much usable energy does raw sewage hold? This was the question posed by the authors of a study published January 5 in Environmental Science & Technology. Their answer: wastewater likely holds a lot more than was previously thought.

Elizabeth Heidrich, a PhD student at Newcastle University in England and lead author of the new study, studies microbial fuel cells—devices that generate electrical current by capturing the electrons freed as bacteria break down organic matter in wastewater. As she was preparing her doctoral research project she decided to determine how much energy engineers could count on wastewater to provide. "It seemed like an obvious question to start with," she explains—which was why she was surprised that hardly anyone had previously asked it.

Heidrich found only one study, published in 2004, which had tried answer to the question. The authors had tested a sample of raw municipal sewage collected from a Toronto treatment plant and, using calorimetry (the measurement of heat absorption and emission), calculated the internal chemical energy of the sample to be 6.3 kilojoules per liter. They also correlated the amount of energy found in the sample to its chemical oxygen demand (COD), a commonly used indirect measurement of dissolved organic compounds. Based on this correlation, they estimated that, in all, the wastewater produced in 2004 by the world's 6.8 billion people contained a continuous supply of energy somewhere in the range of 70 to 140 gigawatts. (One large nuclear plant produces around 1 gigawatt).

But the results of this study—which Heidrich notes have been cited multiple times in the microbial fuel cell literature—are problematic, she says.

Before a sample can be tested in a calorimeter it has to be dry, and in this case the authors had dried their sample by leaving it overnight in an oven heated to 103 degrees Celsius,. And because the boiling points of several organic liquids—including methanol, ethanol and formic acid—found in sewage are lower than 103 degrees, Heidrich says, "I just felt like there would be stuff evaporating at that temperature." This loss would mean the authors had not accounted for all the energy contained in the sample.

So she and her colleagues collected their own samples—one from a plant that treats domestic, household wastewater and another from a facility that treats "mixed" wastewater containing chemicals disposed of by industrial facilities. Instead of using an oven they freeze-dried the samples before testing them in a calorimeter. They found that the industrial sample held about 16.8 kilojoules per liter, whereas the domestic sample contained 7.6—20 percent more than the previous study had reported for its domestic sample. Perhaps more importantly, given that wastewater samples are highly variable, Heidrich and her colleagues found that the commonly used COD measurement does not actually correlate to energy content, and thus is an unreliable metric. Had they relied on the same calculation methods employed in the previous study, the authors report, they would have found only around half the energy contained in each of their samples. Thus, the older estimate likely is a "substantial underestimation."

Heidrich's method has its own limitations: The freeze-drying step takes weeks, so it cannot be relied on as a routine testing method. And although the process preserves more organic matter than does oven drying, it still causes some energetic molecules to be lost. Regardless, Heidrich notes, the study's result has immediate real-world implications. "I think up until now domestic municipal wastewater has been seen as something that you can't really get energy from, so it's not worth the effort. Now hopefully that might change," she says.

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