Cleaning up wastewater and generating electricity at the same time — it might seem too good to be true, but Bruce Logan, professor of environmental engineering at PennStateUniversity, believes that microbial fuel cells (MFC) might make it possible.
The MFC uses bacteria to degrade organic matter, producing hydrogen directly for the attached fuel cells. Logan’s team has developed the technology further, adding ‘electrical assistance’ which allows the cell to function even without oxygen.
In a paper scheduled for publication in Environmental Science and Technology, Logan’s team explains that bacterial fermentation is limited by what’s known as the ‘fermentation barrier’. The bacteria can only convert carbohydrates into hydrogen and ‘dead end’ products, acetic and butyric acid, which cannot be broken down further. But adding electrical assistance — a 0.25V boost to the bacterial action from a programmable power supply — allows them to bypass this barrier and attack acetic acid, converting it to carbon dioxide and more hydrogen. This assistance voltage is only about a tenth of the voltage needed for electrolysis of water.
The bacteria produce electrons and protons while consuming biomass. The protons — hydrogen ions — go into solution, while the electrons are transferred to the cell’s anode. The electrons migrate across to the cathode, where they are joined by electrons from the power source. This boosted electron stream combines with the protons in solution to form atomic hydrogen, which in turn combine to form molecules, released at the cathode as hydrogen gas.
Logan (right) and graduate student Hong Lu check the output from an experimental MFC
The MFC is not limited to carbohydrate waste, Logan claims. ‘We can theoretically use our MFC to obtain high yields of hydrogen from any biodegradable, dissolved organic matter — human, agricultural or industrial wastewater, for example — and simultaneously clean the wastewater,’ he says. ‘While there is likely insufficient waste biomass to sustain a global hydrogen economy, this form of renewable energy production may help offset the substantial costs of wastewater treatment as well as provide a contribution to nations able to harness hydrogen as an energy source.’
