Giving waste water the power to clean itself

From nature magazine

Sewage could become a useful source of energy, by combining two existing technologies.

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A technique that combines two novel forms of renewable energy — one relying on bacteria and the other on salt water — generates more electricity than either one alone and cleans waste water at the same time. The work is published today in Science1.

Waste water contains a great deal of energy in the form of organic matter, says lead author Bruce Logan, director of the Hydrogen Energy Center and the Engineering Energy and Environment Institute at Pennsylvania State University in University Park. Domestic waste water contains nine times more chemical energy than the energy required to treat it — that amount, added to the energy in waste water from livestock and food production, would be nearly enough energy to maintain the entire US water infrastructure, he says.
One method for converting this energy to a useable form is to use microbial fuel cells (MFC). These generate electricity by using cultures of microorganisms to break down and oxidize organic matter, a process that releases electrons that migrate towards a positive electrode. At the same time, hydrogen ions from the water pass through a proton-exchange membrane and into a separate cathode chamber. The electrons are drawn to the cathode from the anode via a circuit, generating an electrical current, where they combine with the hydrogen ions and ambient oxygen to form clean water