Researchers at Stanford University have found a way to harvest considerable amounts of electricity from an unlikely source: poop. In a paper published this month in the Proceedings of the National Academy of Sciences, Xing Xie and his team of engineers describe a "microbial battery" capable of generating electricity from naturally occurring sewage bacteria.

Deriving energy from feces is hardly a new concept, but previous attempts have struggled to do so efficiently. The Stanford team's innovation involves their use of so-called exoelectrogenic microbes, which create electric energy as they consume organic material. The researchers clustered groups of these wired microbes around the negative node of the battery, where they attach to carbon filaments.

A potential poop powerhouse

As the microbes consume organic material, excess electrons are deposited onto the filaments and then transmitted across to the battery's positive node, made of silver oxide. The silver oxide converts into silver as more electrons gather, storing potential energy along the way. It takes about a day for the node to completely fill up and turn to silver, at which point it is removed from the battery and re-oxidized, releasing its stored electrons.

According to the researchers, their battery is currently capable of extracting about 30 percent of all potential energy stored in sewage — roughly equivalent to the rate at which solar cells harvest energy from the sun. Sewage, however, carries far less energy, though the team says their technique could at least be used to compensate for the electricity used at sewage treatment plants, or to break down organic water pollutants that deplete oxygen levels and threaten marine life. The biggest challenge going forward, they say, will be to find a less expensive, but equally efficient material for the positive node; poop may be cheap, but silver is not.