Bruce Logan, a professor of Civil Engineering at Penn St, has generated a lot of media hubaloo
recently with a pilot version, produced with Hong Liu and Ramanathan Ramnarayanan, of a microbial fuel cell (article is here
, it will require a subscription to Environmental Science and Technology). The basic idea is to run waste water through a closed fuel cell containing a consortium of microorganisms. Faced with the abundant source of organic carbon these organisms find themselves in metabolic heaven, oxidizing carbon like crazy. The key point is that the geometry of the fuel cell allows the bugs to dump the electrons from the carbon compounds onto conducting rods and to have this flow of energy removed from the cell where it can either be stored (in some other sort of battery) or immediately used to do useful work.
This is a really nice experiment. It is, however, worth noting its limitations. Basically the idea behind the work is to harvest some of the wasted energy from sewage. However, it still takes energy to produce all the things that go into sewage. Which is to say it takes energy to produce the food we eat, to heat and cool the houses we live in and to create the clothing we wear. Some of this fuel is used (e.g. as heat to keep us warm in the winter) other parts of it (e.g. food that is incompletely digested) is not. This system can, at best, just make up some small portion of this cost. It doesn't even have the potential to remove our fossil fuel dependence.