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Brian Westenhaus

Brian Westenhaus

Brian is the editor of the popular energy technology site New Energy and Fuel. The site’s mission is to inform, stimulate, amuse and abuse the…

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The Energy Potential of Household Sewage

Elizabeth S. Heidrich at Newcastle University in the United Kingdom is leading a team that gives new meaning to the adage, “waste not, want not.”  These scientists are reporting via an American Chemical Society online podcast that household sewage has far more potential as an alternative energy source than previously thought.

The podcast is based on a study in the ACS’ journal Environmental Science & Technology explaining discovering increases in the estimated potential energy in household wastewater by almost 20 percent, that could spur efforts to extract methane, hydrogen and other fuels from this vast and, as yet, untapped resource.  Actually it could even be more than that – the measures previously used account very poorly for the energy in wastewater.

The study also points up another matter, the energy used to treat wastewater now is quite significant, and any energy recovered for a gain would offset operating expense and the drainage system costs.  The estimate is the United States uses about 1.3 percent of the nation’s electrical energy to treat 12.5 trillion gallons of wastewater each year.
There is one other important point overlooked – wastewater holds a huge repository of organic phosphorus, potassium and nitrogen compounds that should be returned to the soil.

On with the numbers – the research team learned the mixed wastewater examined using freeze-dried of samples to minimize loss of volatiles, had 16.8 kilo joules per liter, while the domestic home type wastewater tested had 7.6 kJ/L, nearly 20% higher than previously estimated.  This compares with previous published measurements of the internal chemical energy of wastewater measured at 6.3 kJ/L.  Domestic is up 20%, the mixed example is up 260%.  That’s getting into a serious attention getting energy density plus the other resources.  It’s also a lot of easy raw material to be overlooked.

How this came to be is from mental inertia.  Wastewater isn’t something folks want to spend a lot of time thinking about.  Aside from the monthly bill or a clogged up drain wastewater is pretty much out of mind.  But wastewater contains many largely uncharacterized and unde?ned mixtures of compounds, including many organics, likely to range from small, simple chains through to more complex molecules. All organic compounds contain energy stored within their bonds.

The energy that can be obtained from wastewater by different processes varies, methane gas from anaerobic digestion, electricity from microbial fuel cells, or hydrogen in the case of microbial electrolysis cells, or a fermentation process. Large amounts of research are being undertaken in all of these areas, but there has been very little work conducted to quantify the amount of energy held in wastewater to start with.

Chemical Oxygen Demand or COD is the current metric used to assess the energy of wastewater. But there is no standard relationship between COD and energy content – the metric is an inference of the energy contained within the sample.  Amazing this has been overlooked for so long.

As the paper explains, there is quite a difference between boiling off the water to get to the solids vs. a freeze dry and holding the volatiles for measurement.

This brings us to the point, recovering the whole energy value of the wastewater stream isn’t going to be easy.  Even with far better numbers of what’s there, coming up with a recovery technique that’s efficient and cost effective is a whole new field ripe for the taking.  But if you’re interested, this paper is a must read – its likely the new baseline metric for establishing the energy in wastewater.

When one considers the 56.8 trillion liters of U.S. wastewater are worth something on the order of 954.681 trillion kilo joules (954.681 quadrillion joules) and its already conveniently piped to each municipality’s treatment plant, the mind suddenly refocuses – even when the basic contents are briefly considered.  There’s gold at the end of the sewage lines.  The raw material production, transport and delivery to a central location are already in place.  Can it get any better than this?  But . . .

Can that energy be recovered economically?  That’s a nearly new question.  With the new numbers, a metric based in energy instead of a chemical reaction draw, the foundation is much better, stronger and economically promising.  Extraction likely can work to great profit when the ingenuity, innovation and profit potential are used and applied.

Still one wonders now, what potential exists beyond the municipal resource in industry, agriculture and river flows?

By. Brian Westenhaus of New Energy and Fuel

Source: A Major Energy Source Re-Discovered




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