The US National Science Foundation reports on some forward thinking on sewage-treatment and a new design that puts bacteria on double duty - treating wastewater and generating electricity at the same time:

The key is an innovative, single-chambered microbial fuel cell. The prototype is described in the online version of the journal Environmental Science & Technology ...

A fuel cell operates akin to a battery, generating electricity from a chemical reaction. But instead of running down unless it's recharged, the cell receives a constant supply of fuel from which electrons can be released. Typical fuel cells run off of hydrogen. In a microbial fuel cell, bacteria metabolize their food—in this case, organic matter in wastewater—to release electrons that yield a steady electrical current.

[ Source: NSF - OLPA - PR 04-021: Fuel-Cell Microbes' Double Duty: Treat Water, Make Energy - ]