Researchers at Newcastle University in the U.K. estimate that one gallon of wastewater contains enough energy to power a 100-W light bulb for five minutes. That's enough to justify configuring future wastewater treatment facilities so they can convert organic molecules into fuels, thus transforming these facilities from energy drains to an energy sources. Right now, sewage treatment plants in the U.S. use about 1.5% of the nation's electrical energy to treat 12.5 trillion gallons of wastewater a year, they say.
Writing in a recent issue of the journal Environmental Science & Technology, the researchers say previous studies underestimated the energy content of sewage because some of the material was lost to evaporation (samples were baked in an oven overnight). Instead, the UK researchers freeze-dried their samples.
However, there is a long way between a production process able to extract energy from sewage and just being able to measure it in a bomb calorimeter as the researchers did. The freeze drying method they used to devise samples, for example, took the better part of several weeks. Nevertheless, it now looks as though the end result might be worth the effort.
The full paper can be accessed here: http://pubs.acs.org/stoken/presspac/presspac/full/10.1021/es103058w