Bacteria power

A new twist on the established technology of using bacteria to treat wastewater could make the bugs work harder by using them to generate electricity at the same time.

Developed at Pennsylvania State University, these microbial fuel cells produce pollution-free power without the drawback of having to generate and store hydrogen.

The microbial fuel cell is a cylinder, containing eight graphite anodes and a hollow central cathode. The bacteria attach themselves to the anodes, and feed off a steady supply of waste water flowing over them. As they digest pollutants, they produce a stream of electrons which flows around an electrical circuit between anodes and cathode, and release positively-charged hydrogen ions into the wastewater (which reduces the water's oxygen demand).

A proton-exchange membrane allows these ions to reach the cathode, which is exposed to air inside its tube. The oxygen in the air, hydrogen ions from the wastewater and electrons from the electrical circuit combine at the cathode to create water.

The technology works, but needs refinement to make it useful, says environmental engineer Bruce Logan, who is leading the project. 'If power generation in these systems can be increased, microbial fuel cell technology may provide a new method to offset wastewater treatment plant operating costs, making advanced wastewater treatment more affordable for both developing and industrialised nations,' he says.

Part of the challenge is reducing the cost of the equipment. The fact that its fuel is domestic wastewater is an advantage - 'we're using something thought to be completely useless,' Logan says - but the experimental rig uses expensive materials for the cathode, anode and membrane. 'But we're already making progress on that,' says Logan. 'Substantially cheaper systems are just around the corner.'